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Vertical stiffness asymmetries during drop jumping are related to ankle stiffness asymmetries
Abstract
Asymmetry in vertical stiffness has been associated with increased injury incidence and impaired performance. The determinants of vertical stiffness asymmetry have not been previously investigated. Eighteen healthy men performed three unilateral drop jumps during which vertical stiffness and joint stiffness of the ankle and knee were calculated. Reactive strength index was also determined during the jumps using the ratio of flight time to ground contact time. "Moderate" differences in vertical stiffness (t17 = 5.49; P < 0.001), "small" differences in center of mass displacement (t17 = -2.19; P = 0.043), and "trivial" differences in ankle stiffness (t17 = 2.68; P = 0.016) were observed between stiff and compliant limbs. A model including ankle stiffness and reactive strength index symmetry angles explained 79% of the variance in vertical stiffness asymmetry (R(2) = 0.79; P < 0.001). None of the symmetry angles were correlated to jump height or reactive strength index. Results suggest that asymmetries in ankle stiffness may play an important role in modulating vertical stiffness asymmetry in recreationally trained men.
... However, the differences among RSI variants could be explained by different muscle contributions. For example, DJ performance is predominantly determined by the strength and stiffness of the ankle joints (16,24), whereas a higher contribution of knee and hip joints is typical for the CMJ (41). Moreover, movement velocity is lower although the forces are larger in unilateral compared with bilateral jumps (37). ...
... In this study, we found no association between interlimb asymmetries in unilateral RSI mod or triple hop RSI and CoD or 10-m sprint performance. Similarly, no association was reported between jumping performance and interasymmetries in vertical stiffness, assessed through unilateral DJ (24). By contrast, unilateral DJ interlimb asymmetries have been reported to be associated with slower 10-and 30-m sprint times as well as slower 505 test times (7). ...
This study sought to investigate the inter-relationship between different vertical and horizontal variants of reactive strength index (RSI) and change of direction performance. Thirty-one male volleyball players (age: 22.4 ± 3.9 years), performed bilateral drop jumps (DJ), bilateral and unilateral countermovement jumps (CMJ), and triple hops for distance. The RSI was calculated as the ratio of jump height and contact time (DJ), jump height and time to take off (CMJ), and flight time or hop distance and contact time (triple hop), and 505 change of direction test. RSI obtained from DJ and CMJ tasks exhibited excellent trial-to-trial reliability (ICC = 0.91-0.94), while triple hop based RSI had only moderate reliability (ICC = 0.67-0.74). The relationships among different RSI variants were moderate to high (i.e. DJ to CMJ: r = 0.57-0.69; p ≤ 0.004; DJ to triple hop: r = 0.54-0.66; p ≤ 0.021 and CMJ to triple hop: r = 0.42-0.63; p ≤ 0.037). For the triple hop, the associations between RSI based on hop flight time and RSI based on hop distance were high for hop 1-2 (r = 0.77-0.83; p < 0.001) and very high for hop 2-3 (r = 0.91-0.92; p < 0.001). All RSI variants were in small to moderate negative correlation with 505 test performance (r = -0.38 to -0.45; p ≤ 0.042). The agreement in inter-limb asymmetry direction between in RSI from unilateral CMJ and triple hop RSI variables was slight to moderate (Kappa coefficient = 0.06-0.36). In conclusion, although inter-relationships between RSI variants were moderate to high, the direction of inter-limb asymmetry was inconsistent, highlighting the notion of movement variability in limb dominance.
... With hands fixed on hips throughout the duration of the test, subjects stood on the designated test leg and were instructed to step off the box and land on the contact mat below, with either both legs or one leg depending on the test measure in question. Upon landing, subjects were then instructed to "jump as high as you can, while spending as little time on the ground as possible" in line with previous research (13,14). Each trial was separated by 45second rest, with the recorded metric being reactive strength index (RSI), quantified using the equation: flight time/ground contact time (14). ...
... Upon landing, subjects were then instructed to "jump as high as you can, while spending as little time on the ground as possible" in line with previous research (13,14). Each trial was separated by 45second rest, with the recorded metric being reactive strength index (RSI), quantified using the equation: flight time/ground contact time (14). Two trials were performed for each test and the trial with the greatest RSI was considered for further analysis. ...
The aim of the present study was to compare the effects of bilateral and unilateral-biased strength and power training programs on measures of physical performance in male youth soccer players. Twenty-three elite youth players (age: 17.6 1.2 years) were randomly assigned to either a unilateral (n = 11) or a bilateral (n = 12) group, who completed a strength and power intervention, twice per week for 6-weeks. The unilateral group completed rear foot elevated split squats (RFESS), single leg countermovement jumps (SLCMJ), single leg drop jumps (SLDJ) and single leg broad jumps (SLBJ). The bilateral group intervention performed back squats, countermovement jumps (CMJ), drop jumps (DJ) and broad jumps (BJ). A 2 x 2 repeated measures ANOVA showed no between-group differences. However, within-group differences were evident. The bilateral training group showed significant (p < 0.05) improvements in back squat strength (d = 1.27; %Δ = 26.01), RFESS strength (d = 1.64; %Δ = 23.34), BJ (d = 0.76; %Δ = 5.12), 10 m (d =-1.17; %Δ = 4.29) and 30 m (d =-0.88; %Δ = 2.10) performance. The unilateral group showed significant (p < 0.05) improvements in RFESS strength (d = 1.40; %Δ = 33.29), SLCMJ on the left leg (d = 0.76; %Δ = 9.84), SLBJ on the left leg (d = 0.97; %Δ = 6.50), 10 m (d =-1.50; %Δ = 5.20), and 505 on the right leg (d =-0.78; %Δ = 2.80). Standardised mean differences showed that bilateral training favoured improvements in back squat strength and unilateral training favoured improvements in RFESS strength, SLDJ on the right leg and 505 on the right leg. These results show that although both training interventions demonstrated trivial to large improvements in physical performance, the notion of training specificity was evident with unilateral training showing greater improvements in unilateral test measures.
... Greater asymmetries were found in the SLCMJ compared to the SEBT and CODS tests, and were also found when differentiating between groups both ways (i.e., maturational and chronological). This is in agreement with recent studies in other sporting populations [8,25,26]. The greater asymmetries of this test were found in circa-PHV (~19%), similar to the maturation stages comparison found in elite youth soccer [24]. ...
... The greater asymmetries of this test were found in circa-PHV (~19%), similar to the maturation stages comparison found in elite youth soccer [24]. Although the SLCMJ test appears to be the most sensitive test at detecting asymmetries, and is in agreement with previous research [8,25,26], it is necessary to have a critical perspective if we want to consider this value as a risk factor for injury or reduced sporting performance. Given the prevalence of lateral movements in tennis [4], tests which aim to determine functional deficits in performance in this plane of motion must also be considered, such as CODS tests. ...
Neuromuscular asymmetries have been previously associated with reduced performance. Similarly, maturation has shown that youth athletes may experience a loss of motor control, which could also lead to compromised physical performance. The present study aimed to evaluate and quantify the level of asymmetry among chronological and maturational groups. Forty-one youth tennis players performed the single leg countermovement jump (SLCMJ), star excursion balance test (SEBT) and a change of direction speed (CODS) test. Differences were found between the strongest and weakest limbs across all tests (p < 0.001), and also for SEBT in the posteromedial direction (p = 0.02), SEBT composite score (p < 0.01) in maturation groups, and for SEBT posterolateral direction (p = 0.03) and SEBT composite score (p = 0.01) in chronological groups. The SLCMJ showed the largest inter-limb asymmetries for the circa peak height velocity (PHV) group (19.31 ± 12.19%) and under-14 (U14) group (17.55 ± 9.90%). Chronological and maturation groups followed similar trends for inter-limb asymmetries, but the biological index showed larger asymmetry scores in the jumping test at PHV compared to that found in the chronological group (U14). These results show that inter-limb differences may be heightened during PHV. Practitioners can use this information to inform the decision-making process when prescribing training interventions in youth tennis players.
... Vertical stiffness has been modelled during drop jumping in a handful of investigations 14,17,18,22,23 and analyses of the force-deformation profiles during these tasks would suggest them to be an appropriate representation of the spring-mass model. 14,22,23 However, the reliability of drop jump derived stiffness measures has not been evaluated directly. ...
... Vertical stiffness has been modelled during drop jumping in a handful of investigations 14,17,18,22,23 and analyses of the force-deformation profiles during these tasks would suggest them to be an appropriate representation of the spring-mass model. 14,22,23 However, the reliability of drop jump derived stiffness measures has not been evaluated directly. Moreover, bilateral and unilateral drop jumping has not been compared across multiple testing sessions to evaluate inter-session reliability. ...
The current study sought to compare vertical stiffness during bilateral and unilateral drop jumping. Specifically, the inter-session reliabilities and force-deformation profiles associated with each task were to be examined. On three occasions, following familiarisation, fourteen healthy males (age: 22 ± 2 years; height: 1.77 ± 0.08 m; body mass: 73.5 ± 8.0 kg) performed three bilateral, left leg and right leg drop jumps. All jumps were performed from a drop height of 0.18 m on to a dual force plate system. Vertical stiffness was calculated as the ratio of peak ground reaction force (GRF) to the peak centre of mass (COM) displacement. Unilateral drop jumping was associated with higher GRF and greater COM displacement (both p < 0.001), but vertical stiffness was not different between tasks when considering individual limbs (p = 0.98). A coefficient of variation of 14.6% was observed for bilateral vertical stiffness during bilateral drop jumping; values of 6.7% and 7.6% were observed for left and right limb vertical stiffness during unilateral drop jumping. These findings suggest that unilateral drop jumps may exhibit greater reliability than bilateral drop jumps whilst eliciting similar vertical stiffness. It is also apparent that higher GRFs during unilateral drop jumping are mitigated by increased COM displacement.
... Hopping provides the simplest locomotive task by which to assess stiffness and is proposed to provide a strong representation of musculoskeletal stiffness given the efficiency of hopping gait (26). Drop jumps have also been utilized to measure Kvert values (59,60,61), as well as Kvert asymmetries (59,60,61). ...
... Hopping provides the simplest locomotive task by which to assess stiffness and is proposed to provide a strong representation of musculoskeletal stiffness given the efficiency of hopping gait (26). Drop jumps have also been utilized to measure Kvert values (59,60,61), as well as Kvert asymmetries (59,60,61). ...
Brazier, J, Maloney, S, Bishop, C, Read, PJ, and Turner, AN. Lower extremity stiffness: considerations for testing, performance enhancement, and injury risk. J Strength Cond Res XX (X): 000–000, 2017—Force-deformation characteristics of the lower limb have been associated with athletic performance and may modulate the risk of injury. Despite these known associations, measurements of lower extremity stiffness are not commonly administered by strength and conditioning coaches. This review provides an overview of the available literature pertaining to the effects of lower extremity stiffness on physical performance and injury risk. Practical methods of monitoring and training stiffness are also discussed. The cumulative body of evidence indicates that increases in lower extremity stiffness are associated with heightened performance in athletic tasks such as hopping, jumping, throwing, endurance running, sprinting, and changing direction. Relationships with injury are less conclusive because both excessive and insufficient limb stiffness have been postulated to increase risk. Thus, the “optimal” level of stiffness seems to be dependent on the anthropometry and physical capabilities of the athlete, in addition to sport- specific activity demands. Training interventions can positively enhance lower extremity stiffness, including isometric, eccentric, and isotonic strength training and plyometrics. Complex training also seems to provide a potent stimulus and may be more effective than the use of singular training modes. For plyometric activities, it is recommended that coaches use a developmental sequence of exercises with increasing eccentric demand to provide an appropriate stimulus based on the training age and technical competency of the athlete.
... Vertical and joint stiffness of the left and right limbs were assessed before and after the stiffness intervention ( Figure 2) using a single leg drop jump protocol. This protocol has been described in detail in a previous manuscript (19). Participants performed two drop jumps, without footwear, for each limb at each time point. ...
... Torsional stiffness of the ankle and knee joints were calculated as the ratio of the change in net muscle moment (N) to joint angular displacement (rad) between the initial ground contact phase and instant of peak angular displacement (12); these were also averaged over the two jumps. Data for hip stiffness were excluded as the phase shift for the moment displacement curve of the hip was > 10%, previously specified as exclusion criteria (12,15,19). the order in which directions were tested was randomized and counterbalanced. ...
Maloney, SJ, Richards, J, Jelly, L, and Fletcher, IM. Unilateral stiffness interventions augment vertical stiffness and change of direction speed. J Strength Cond Res 33(2): 372-379, 2019-It has previously been shown that preconditioning interventions can augment change of direction speed (CODS). However, the mechanistic nature of these augmentations has not been well considered. The current study sought to determine the effects of preconditioning interventions designed to augment vertical stiffness on CODS. Following familiarization, 10 healthy males (age: 22 ± 2 years; height: 1.78 ± 0.05 m; body mass: 75.1 ± 8.7 kg) performed 3 different stiffness interventions in a randomized and counterbalanced order. The interventions were: (a) bilateral-focused, (b) unilateral-focused, and (c) a control of CODS test practice. Vertical stiffness and joint stiffness were determined preintervention and postintervention using a single-leg drop jump task. Change of direction speed test performance was assessed postintervention using a double 90° cutting task. Performances following the unilateral intervention were significantly faster than control (1.7%; p = 0.011; d = -1.08), but not significantly faster than the bilateral intervention (1.0% faster; p = 0.14; d = -0.59). Versus control, vertical stiffness was 14% greater (p = 0.049; d = 0.39) following the unilateral intervention and 11% greater (p = 0.019; d = 0.31) following the bilateral intervention; there was no difference between unilateral and bilateral interventions (p = 0.94; d = -0.08). The findings of the current study suggest that unilateral preconditioning interventions designed to augment vertical stiffness improve CODS within this experimental cohort.
... For instance, it would seem that the ankle and knee are most relevant when looking at vertical stiffness, as hip displacement is limited. 31 However, the intensity of tasks may impact the joint stiffness patterns. Hobara et al 18 showed that an increase in knee stiffness was linked to slower hopping frequencies, with longer ground contacts and greater hopping heights, whereas increases in ankle stiffness were associated with faster hopping frequencies and shorter ground contact times. ...
The general concept of the stretch and recoil of elastic tissue during ground contact – storing and releasing energy to enhance the propulsive phase of an action – is well understood. However, different stiffness measures are frequently used incorrectly and interchangeably, leading to ineffective monitoring of stiffness changes, limiting the impact of training designed to enhance stiffness. The aim of this narrative review is to discuss how different structures react to ground contacts, how this behaviour can be modelled and how stiffness impacts performance. Hill’s three-compartment model highlights the need for tendon compliance and muscle stiffness in efficient force generation. However, this does not really explain slow stretch shortening cycle (SSC) actions, where both muscle and tendon stretch and recoil. Different models are used to describe a body’s ground impact behaviour: these include the spring-mass model, which describes centre of mass movement, and the torsional spring model, which describes leg function, with three torsional springs representing the ankle, knee and hip. These models generally link an increase in stiffness to an increase in performance in high intensity action, with vertical stiffness a predictor of high intensity sporting actions, independent of sex, age or maturation. Leg stiffness initially increases with running velocity, before remaining constant at high running velocities. When joint function is reviewed, ankle stiffness is linked to fast SSC actions, with knee stiffness linked to slow SSC actions. It is concluded that different measures of stiffness should not be used interchangeably as different aspects of stiffness impact performance independently.
... Additionally, in DJ and CMJ higher forces can be developed in eccentric phase with greater average force output compared to SJ (Bobbert & Casius, 2005;Bobbert, Gerritsen, Litjens, & Van Soest, 1996), while SJ is performed from static position, thus only concetric contraction is present. Moreover, while in DJ ankle strength and stiffness are main determinants for performance (Maloney, Richards, Nixon, Harvey & Fletcher, 2017), higher contribution of the knee joints is typical for the CMJ and SJ (Tsiokanos, Kellis, Jamurtas & Kellis, 2002 Although the literature supports the usefulness of RSI and RSImod metrics to guide training related decision making, RSI, as being a ratio, should be interpreted together with both underlying variables to provide more information about individual's neuromuscular function (Bishop, Shrier & Jordan, 2023). Although strong correlations between RSI variations indicate that somewhat similar information is provided, we should not neglect the principle of specificity when testing for performance. ...
Jump tests are commonly used in gymnasts as jump height is important
for the successful execution of gymnastics elements. For this reason, the
evaluation of jumping characteristics can provide a way to make training-related decisions. The purpose of this study was to assess the differences between reactive strength index (RSI) and reactive strength index modified (RSImod) calculations and to investigate the relationships between RSI and RSImod from different jump variations. Thirty-two men
and women gymnasts performed a drop jump (DJ) from a 35 cm high
box, a countermovement jump (CMJ), and a squat jump (SJ). RSI and
RSImod were calculated as the ratio between jump height or flight time
and ground contact time in DJ, whereas for CMJ and SJ, ground contact
time was replaced with the time to take-off. Our results showed significant differences between RSI and RSImod calculations, with very strong
associations between them, suggesting that both values can be used but
are not interchangeable. Furthermore, our results showed moderate to
strong associations between RSI and RSImod variations, suggesting that
both RSI and RSImod provide similar information and can be used in
similar ways to guide an individual‘s training. Furthermore, while RSI
and RSImod from CMJ are thought to reflect stretch-shortening cycle
ability, RSImod from SJ could be seen as concentric RFD ability. Given
the strong correlations between all three metrics, it could be concluded that RSI and RSImod from CMJ are not only determined by reactive
strength but also depend to some extent on concentric RFD ability.
... Therefore, TQCS was likely to support the flight height of slam dunk-related MVJ. The possible reason refers to that TQCS was likely to reflect the ground contact time (GCT) in take-off phase of slam dunk, because both sprinting time (0-30 meter) [33] and the GCT of vertical jump [34,35] were controlled by reactive strength, also representing the transitional time between eccentric contraction and concentric contraction during SSC [29], and the stiffness of ankle during take-off phase [36]. As the result of different level on reactive strength, long GCT was likely to dissipate more stored elastic energy in eccentric contraction into heat,while short GCT may minimal the energy loss [37]. ...
Little is known about the difference of anthropometry and physical fitness between the finalist and eliminated player in the NBA all star slam dunk contest. This study aimed to compare the difference on anthropometric and physical fitness indicator in the combine draft between finalist and eliminated player in the national basketball association all star slam dunk contest. Draft data of 32 basketball players (N = 32, age in draft year: 20.69±2.28 years old, height without shoes: 196.75±8.68 cm, weight: 96.85±10 kg, body fat percentage: 6.07±1.23%) participating in the 2000–2015 draft and 2003–2023 slam dunk contest was selected from national basketball association database. It was classified into finals group (FG) (N = 16) and elimination group (EG) (N = 16). Independent sample t-test with cohen’s d was adopted for evaluating the statistical significance of intergroup difference and its effect size. The result indicates that Finalist group was significant less than elimination group on height without shoes (FG vs EG: 193.43±9.47 cm vs 200.06±6.52 cm, P<0.05), standing reach (FG vs EG: 257.66±12.32 cm vs 268.29±10.03 cm, P<0.05) and weight (FG vs EG: 93.38±7.37 kg vs 100.33±11.25 kg, P<0.05). Conversely, compared to elimination group,finalist group has significant better performance on three quarter court sprint (FG vs EG: 3.15±0.1 s vs 3.26±0.12 s, P<0.05), standing vertical jump (FG vs EG: 84.88±5.13 cm vs 78.83±4.9 cm, P<0.05) and max vertical jump (FG vs EG: 102.39±6.47 cm vs 94.79±8.34 cm, P<0.05). However, effect size analysis indicated that height without shoes,standing reach,weight (cohen’s d = 0.73–0.959, 0.7≤cohen’s d<1.3, moderate) from the anthropometric indicator and three quarter court sprint, standing vertical jump, and max vertical jump (cohen’s d = 0.97–1.21, 0.7≤cohen’s d<1.3, moderate) from physical fitness indicator has moderate effect size, whereas effect size of body fat percentage, wingspan and lane agility time (cohen’s d = 0.31–0.67, 0.3≤cohen’s d<0.7, small) was small. To conclude, specific anthropometric and physical fitness indicator shows clear difference between finals group and elimination group. Height without shoes, standing reach,weight in anthropometry and three quarter court sprint, standing vertical jump, and max vertical jump in physical fitness are key indicator to slam dunk performance. In line with the result in the study, NBA staff can select suitable rookies for slam dunk contest. Similiarly, coach from NBA or other basketball league, who want to improve the player’s slam dunk performance, should use specific training programs to develop the slam dunk-related indicator.
... Leg stiffness during bilateral hopping tasks and vertical stiffness during single leg drop jumps has been shown to primarily depend on ankle joint stiffness during hopping tasks (Farley & Morgenroth, 1999;Kuitunen et al., 2011;Maloney et al., 2017), and therefore, the ankle joint stiffness capacity of participants would likely have contributed substantially to the vertical stiffness (repeated contact times) achieved in the repeated jumps. This is of interest since attenuating the degree of ankle dorsiflexion and achieving higher levels of ankle joint stiffness during the negative power phase of the ground contact in the initial steps, have been shown to be advantageous to initial acceleration performance (Bezodis et al., 2015;Charalambous et al., 2012). ...
Biomechanics and motor control of early acceleration: Enhancing the initial sprint performance of professional rugby union backs Sprint acceleration is an important performance feature in many sports. For professional rugby union backs, short distance sprints are frequently carried out in training and competition, but how technique and strength-based characteristics contribute to their acceleration performance during these initial steps is not currently well understood. A series of investigations were therefore undertaken to, firstly, advance the understanding of this area and, secondly, to apply this information by prescribing individual-specific interventions to enhance initial acceleration performance. Three initial investigations sought to determine how technical features and strength-based qualities of professional rugby union backs related to their sprint performance (quantified as normalised average horizontal external power) during the initial steps. Findings from these investigations highlighted that focussing on the contribution of discrete technical variables to acceleration performance in isolation is an overly reductionist approach which overlooks how complex systems achieve high sprint performance. Findings also highlighted how important information on individuals can be lost using group-based study designs, since different inter-athlete strategies were adopted to achieve similar performance outcomes. In the fourth investigation, four subgroups of participants were identified, using cluster analysis, based on their whole-body kinematic strategies. At the intra-individual level, the variables which portrayed their individual strategies remained stable (CV: 1.9% to 6.7%) across multiple separate occasions. This characterisation of whole-body strategies was used to develop a novel and rigorous approach to longitudinally assess the efficacy of technical-based acceleration interventions. Demonstrating the application of this approach in the final investigation, several individual-specific interventions were prescribed to professional rugby union backs based on within-individual relationships of their technique strategies and strength-based capabilities with acceleration performance. Changes in within-individual technique and acceleration performance were measured at multiple time points across an 18-week intervention period where meaningful enhancements in acceleration were observed. This demonstrated that individual-specific technical interventions were effective in manipulating aspects of acceleration technique and performance. The outcome of these investigations provides a novel approach for practitioners working to individualise sprint-based practices.
... However, it should be mentioned, that there are some differences in jump characteristics between DJ and CMJ. While in DJ ankle strength and stiffness are main determinants for performance [30], higher contribution of the knee joints is typical for the CMJ [31]. Approach jump is a combination of DJ and CMJ [5], thus positive correlations between approach jump height and both RSI and RSI mod were expected. ...
Jumping performance is one of the key components of volleyball game, thus evaluating jumping ability through different biomechanical variables offers opportunity for performance optimization. The aim of this study was to assess the associations between reactive strength index (RSI), reactive strength index modified (RSImod) and approach jump performance in male volleyball players. Forty volleyball players performed drop jump (DJ) form 40 cm high box, bilateral and unilateral countermovement jumps (CMJ) and approach jump. RSI in DJ was calculated as the ratio between jump height and ground contact time, while the RSI in CMJ tasks (RSImod) was calculated as ratio between jump height and jump time. Our results indicate that the relationships among different RSI variants and approach jump in volleyball players are moderate to strong (r = 0.42–0.73), with the highest correlations being observed for RSImod from bilateral CMJ (r = 0.676–0.727). Those observations are in line with the principle of movement specificity, which suggests that the best performance indicator should be the task that best resembles the demands of the sport-specific movements. Further research is needed to reveal more about the potential of implementing these findings for training optimization through monitoring RSI and RSImod values.
... However, it should also be mentioned that there are some additional differences in jump characteristics between DJ and CMJ. In DJ, ankle strength and stiffness are the main determinants of performance [57], whereas a higher contribution of the knee joints is typical for CMJ [58]. Similar to RSI [59], the RSI mod is considered a reliable measure and was reported to discriminate between different groups of athletes [54,60]. ...
Traditional neuromuscular tests (e.g., jumping and sprinting tasks) are useful to assess athletic performance, but the basic outcomes (e.g., jump height, sprint time) offer only a limited amount of information, warranting a more detailed approach to performance testing. With a more analytical approach and biomechanical testing, neuromuscular function can be assessed in-depth. In this article, we review the utility of selected biomechanical variables (eccentric utilization ratio, force–velocity relationship, reactive strength index, and bilateral deficit) for monitoring sport performance and training optimization. These variables still represent a macroscopic level of analysis, but provide a more detailed insight into an individual’s neuromuscular capabilities, which can be overlooked in conventional testing. Although the aforementioned “alternative” variables are more complex in biomechanical terms, they are relatively simple to examine, with no need for additional technology other than what is already necessary for performing the conventional tests (for example, even smartphones can be used in many cases). In this review, we conclude that, with the exception of the eccentric utilization ratio, all of the selected variables have some potential for evaluating sport performance.
... This in turn will depend on the range of motion in the knee and ankle joints. Furthermore, Maloney et al. emphasized the complexity of the relationship of muscle stiffness with the range of motion and jump height [27]. ...
The relationship of muscle strength symmetry with the symmetry of dynamic activities such as vertical jump (CMJ) and with symmetry of ranges of motion is ambiguous. Therefore, the aim of the current investigation was to assess the relationship between the flexor/extensor strength ratio and the range of motion of the knee joints and VGRF during jump in a sample of young healthy men. The sample consisted of 64 healthy, active men. The flexor and extensor torques and ranges of active and passive mobility of both knee joints were measured. Vertical ground reaction force (VGRF) was measured using two platforms. There were no significant correlations between the symmetry of muscle strength and the symmetry of ranges of motion and the symmetry of the VGRF. Therefore, using the cluster analysis, 4 subgroups were distinguished due to the size of the flexor/extensor ratio (H/Q ratio). Significant differences were found in the level of strength of the knee flexors between the clusters. Detailed analysis of muscle strength symmetry and VGRF symmetry seem to confirm the Leading Joint Hypothesis theory of motion control in multi-joint systems. It was found that the process of controlling movement by the nervous system compensates for the small asymmetries in the level of muscle strength in the knee flexors and extensors, as well as small asymmetries of VGRF.
... Hay autores que han puesto de manifiesto que niveles altos de stiffness de las extremidades inferiores están asociados a mejores resultados del salto vertical, del sprint y de la economía de carrera, mientras otros autores han indicado que valores elevados del stiffness de las extremidades inferiores pueden incrementar el riesgo de lesiones óseas por estrés (Maloney, Richards, Nixon, Harvey, & Fletcher, 2016;Waxman, et al., 2018;Goodwin, et al., 2019). Parece ser, por tanto, que debe existir un rango óptimo de stiffness que ayude al rendimiento deportivo sin incrementar el riesgo de lesión (Waxman, et al., 2018). ...
El patinaje artístico es una disciplina deportiva con varias modalidades y, en casi todas ellas, los elementos de mayor dificultad suelen ser los saltos, que deben ser altos para permitir varias rotaciones. El objetivo del presente estudio fue el de analizar las variables cinéticas y el stiffness vertical de un grupo de ocho patinadoras de artístico andaluzas de nivel regional de edades comprendidas entre 14 y 18 años, comparándolas con un grupo de ocho chicas sedentarias del mismo rango de edad. Se analizaron la altura de vuelo y las siguientes variables de la batida de un salto vertical con contramovimiento (CMJ): máximo descenso del centro de masas, pico de fuerzas, fuerza media, impulso de aceleración, duración del impulso de aceleración, potencia media, potencia pico, stiffness vertical, stiffness vertical normalizado a la masa corporal y velocidad máxima de descenso. Los resultados mostraron que las patinadoras tenían alturas de vuelo significativamente mayores que las sedentarias y presentaban valores de potencia y de impulso de aceleración significativamente mayores, lo que indica que eran más explosivas. Sin embargo, no realizaron una mayor fuerza media durante la fase concéntrica de la batida. Son necesarios nuevos trabajos que comparen a las patinadoras con otros grupos de deportistas, analizando especialmente variables poco estudiadas como el stiffness vertical y la máxima velocidad de descenso, para valorar si las patinadoras realizan una buena utilización del ciclo estiramiento-acortamiento y cómo aumentar la fuerza media durante la fase concéntrica de la batida. Abstract. Figure skating is a sport with different modalities. In most of them, jumps are usually the most difficult elements and need to be high enough so to complete several rotations. The aim of the present study was to analyse the kinetic variables and the vertical stiffness of eight andalusian female figure skaters aged 14-18 years old, comparing them to a group of eight sedentary female subjects of the same age. Flight height and the following kinetic variables during a countermovement jump (CMJ) were analyzed: maximal displacement of the centre of mass, peak force, average force, acceleration impulse, length of the acceleration impulse, average power, peak power, vertical stiffness, vertical stiffness normalized to body mass and maximum downward velocity during the eccentric phase. Results showed that figure skaters had flight heights greater than sedentary subjects and they also exhibited greater values of power and acceleration impulse, which means that they have higher explosive strength. However, average force developed during the concentric phase was not significantly higher in figure skaters. Further research comparing figure skaters with other athletes is needed, with special focus on the analysis of vertical stiffness and maximum downward velocity, so to clarify whether figure skaters make a good use of the stretch-shortening cycle; also to understand how to develop higher average force during the concentric phase of jumps.
... performance. However, in subsequent investigations, levels of force asymmetry during a CMJ were not associated with CMJ height (3) and asymmetry in unilateral DJ height was not associated with unilateral DJ performance (35). More recent data would suggest that unilateral CMJ asymmetry may be more strongly associated with performance. ...
The influence of asymmetry on performance in sports with highly asymmetric demands is yet to be properly determined. This investigation sought to examine jump-based asymmetries within skateboarders and determine any possible relationships with subjective skating ranking and jumping performance. Twelve recreational skateboarders (age: 24 ± 7 years; height: 1.76 ± 0.12 m; body mass: 66.7 ± 39.2 kg) volunteered to participate in the study. All participants had at least ten years of skateboarding experience. Participants completed one familiarisation trial followed by a single experimental trial. In the experimental trial, skaters performed three trials of three different unilateral jump tests (squat jump [SJ], countermovement jump [CMJ] and drop jump [DJ]) following a standardised dynamic warm-up. Jump heights and DJ reactive strength index (RSI) were determined via the MyJump2 smartphone application. Large asymmetries were reported across all jump tests. However, effect sizes suggested a smaller magnitude of asymmetry for CMJ height (13.6%) versus SJ height (20.5%; d = 0.66) and DJ height (20.2%; d = 0.50). A large positive correlation was reported between SJ asymmetry and subjective ranking (ρ = 0.60; P = 0.04), such that skaters with larger asymmetries ranked higher. However, SJ asymmetry was associated with a lower DJ RSI (ρ =-0.68; P = 0.01). No asymmetries were associated with CMJ height which has been previously liked to Ollie performance in skateboarders. These data suggest that asymmetries are highly prevalent in experienced recreational skateboard athletes but are not associated with skating performance.
... Hence, an overall decrease in variables of interest during right single CMJ is likely, at least partially, responsible for the weaker associations with unilateral agility compared to the left limb. Although the present study is unable to determine the exact cause of the discrepancies observed between left and right CMJ performance; leg dominance, joint stiffness, and balance may be the cause (Bishop, Read, McCubbine, & Turner, 2018;Maloney, Richards, Nixon, Harvey, & Fletcher, 2017;McElveen, Riemann, & Davies, 2010). ...
Introduction: Many sporting activities require both vertical jumping in combination with agility. Yet, both vertical jumping and agility can be executed either bilaterally or unilaterally.Problem Statement and Approach:There exist no literature exploring the association between unilateral agility with vertical jump performance variables.Thus, the purpose of this study was to determine associations and predictive ability between performance measures during unilateral and bilateral vertical countermovement jumpswith unilateral agility measures.Material and Method: Thirty recreationally active adults participated in two non-consecutive sessions. During the first session,participants completed three trials of right-sideunilateral countermovement jumps, left-side unilateral countermovement jumps, and bilateral countermovement jumpsin a randomized order. All jumps were performed akimbo, on force platforms, withthirty seconds of rest between trials. During the second session,participantscompleted two distinct unilateral agilitymaneuvers:single leg up three-back one and single leg cross hops, and were given two trials for each maneuver with thirty seconds rest between trials. The average of all completed trials for all countermovement jumps and agility maneuvers were used for statistical analysis. Spearman'sR correlation were used to find significant associations between completion time for the agility maneuvers and jump height, peak force, relative force, peak power, relative peak power, and landing force for all countermovement jumpconditions. Results:There were significant correlations between the cross hop and up three-back one agility maneuver completion time with countermovement jump height, peak force, peak power, relative peak power, and landing force during both unilateral and bilateral jumps. Conclusions:There appears to be an association between certain performance measures during bilateral and unilateral countermovement jumps and unilateral agility. Peak power and landing force assist in predicting unilateral agility completion time. Therefore, coaches may desire to implement unilateral jumping with individuals necessitating single leg agility to complete their desired exercise or sport activity.
... Maloney et al (15) investigated unilateral drop jumps from 0.18 m and showed that ankle stiffness asymmetries can explain 79% of the variance of vertical stiffness asymmetries between the two limbs. Peng (20) examined the biomechanical changes during drop jumps of incremental heights from 0.2 to 0.6 m and reported that peak vertical GRF and landing impulse increased with drop height but found no difference in takeoff impulse. ...
This study investigated the between-limb asymmetry in kinetic and temporal characteristics during bilateral plyometric drop jumps from different heights. Seventeen male basketball players performed drop jumps from 3 heights on two platforms in randomized orders. Vertical ground reaction force data were analyzed with respect to the lead limb (i.e. the limb stepping off the raised platform first) and trail limb. Peak forces and loading rates of each limb were calculated. The absolute time differential between the two limbs at initial ground contact and takeoff were determined. The frequency of symmetrical landing and taking off with ‘both limbs together’ were counted using 3 time windows. Results showed that the lead limb displayed higher peak forces and loading rates than the trail limb across all heights (p < .05). As drop height increased, the absolute time differentials decreased at initial ground contact (p < .001) but increased at takeoff (p = .035). The greater the preset time window, the more landings and takeoffs were classified as bilaterally symmetrical. In conclusion, higher drop heights allowed subjects to become more bilaterally symmetrical in the timing of landing but this reduction in temporal asymmetry did not accompany with any reduction in kinetic asymmetry.
... Participants were instructed to step, not jump, off a 0.2 m box on a force plate (MuscleLabTM, ErgoTest Innovation, Porsgrunn, Norway). Drop height was considered appropriate to facilitate short ground contact times during each jump (Maloney, Richards, Nixon, Harvey, & Fletcher, 2017). Participants dropped single legged from the box onto the force plate with the dominant leg, jumped immediately on ground contact and then landed from the jump with both feet. ...
Foam rolling and eccentric exercise interventions have been demonstrated to improve range of motion (ROM). However, these two modalities have not been directly compared. Twenty-three academy soccer players (age: 18 ± 1; height: 1.74 ± 0.08 m; body mass: 69.3 ± 7.5 kg) were randomly allocated to either a foam rolling (FR) or eccentric exercise intervention designed to improve dorsiflexion ROM. Participants performed the intervention daily for a duration of four weeks. Measurements of dorsiflexion ROM, isometric plantar flexion torque and drop jump reactive strength index were taken at baseline (pre-intervention) and at three subsequent time-points (30-min post, 24-hours post and 4-weeks post). A significant time x group interaction effect was observed for dorsiflexion (P = 0.036), but not for torque or reactive strength index. For dorsiflexion, there was a significant increase in both acute (30-min; P < 0.001) and chronic (4-week; P < 0.001) ROM for the eccentric group, whilst FR exhibited only an acute improvement (P < 0.001). Eccentric training would appear a more efficacious modality than foam rolling for improving dorsiflexion ROM in elite academy soccer players.
... The respective stiffness of the ankle, knee and hip joints is most commonly determined through the estimation of net joint moments, determined by principles of inverse mechanics, and by the measurement of joint angular displacement (Equation 6). As it has been noted that the phase shift for the moment-displacement curve of the hip commonly exceeds 10% (Farley and Morgenroth, 1999;Kuitunen, et al., 2011;Maloney, Richards, Nixon, Harvey, & Fletcher, 2017b), previously alluded to as exclusion criteria by Farley, et al. (1998), the determination of hip stiffness may not be appropriate. Given also that Farley, et al. (1998) and Farley and Morgenroth (1999) have observed hip stiffness to be unaffected by changes in vertical stiffness, these findings are likely to explain why hip stiffness is not commonly determined alongside ankle and knee stiffness. ...
Stiffness describes the resistance of a body to deformation. In regard to athletic performance, a stiffer leg-spring would be expected to augment performance by increasing utilisation of elastic energy. Two-dimensional spring-mass and torsional spring models can be applied to model whole-body (vertical and/or leg stiffness) and joint stiffness. Various tasks have been used to characterise stiffness, including hopping, gait, jumping, sledge ergometry and change of direction tasks. Appropriate levels of reliability have been reported in most tasks, although they vary between investigations. Vertical stiffness has demonstrated the strongest reliability across tasks and may be more sensitive to changes in high-velocity running performance than leg stiffness. Joint stiffness demonstrates the weakest reliability, with ankle stiffness more reliable than knee stiffness. Determination of stiffness has typically necessitated force plate analyses; however, validated field-based equations permit determination of whole-body stiffness without force plates. Vertical, leg and joint stiffness measures have all demonstrated relationships with performance measures. Greater stiffness is typically demonstrated with increasing intensity (i.e., running velocity or hopping frequency). Greater stiffness is observed in athletes regularly subjecting the limb to high ground reaction forces (i.e., sprinters). Careful consideration should be given to the most appropriate assessment of stiffness on a team/individual basis.
El objetivo del presente estudio fue analizar las variables cinéticas y el stiffness vertical de jóvenes bailarinas de ballet en la realización de un salto vertical. 16 adolescentes andaluzas participaron en este estudio. Ocho de ellas eran bailarinas de ballet (15,00 ± 1,07 años) que se encontraban cursando las enseñanzas profesionales de Danza Clásica y las otras ocho eran chicas sedentarias (15,13 ± 1,36 años). Ejecutaron dos tipos distintos de saltos verticales, salto vertical con contramovimiento (CMJ) y salto Abalakov (ABK), y se analizaron las variables cinéticas y el stiffness vertical. Los resultados muestran que las bailarinas no tienen una buena utilización de las extremidades superiores, pues su altura del salto aumentó únicamente un 5,96% en el salto ABK, a diferencia de las sedentarias en las que aumentó un 22,25%. En el CMJ se encontraron diferencias significativas entre ambos grupos en la altura del salto, la fuerza media, el pico de fuerza, el impulso de aceleración, la potencia media, el pico de potencia y el stiffness vertical normalizado (p < 0,01), así como en la duración del impulso de aceleración y el stiffness vertical (p < 0,05). Estos resultados demuestran que las bailarinas tienen mayor capacidad impulsiva que las sedentarias. Además, su alto valor del stiffness vertical puede ser indicador de una buena utilización del ciclo estiramiento-acortamiento y podría ser la causa o el efecto de esa mayor capacidad impulsiva. Serían interesantes futuros trabajos que comparen las bailarinas con otros grupos de deportistas incluyendo valores del stiffness vertical.
Leg stiffness is a quantitative measure of the elastic properties that determine the ability to accumulate potential elastic energy in a vertical jump. The conceptual and methodological confusion surrounding leg stiffness makes it difficult to organize the knowledge and compare the results obtained in past research. This study attempts to standardize the concepts found in the literature and presents a methodology for determining leg stiffness during vertical jumps based on measurements of the slope of the ground reaction force curve with respect to length changes. Therefore, the aim of this study was to determine normative values for leg stiffness for young, healthy non-athletes during single vertical jumps to maximal and specific heights. Before it will be possible to provide answer to the questions about relationships between leg stiffness and sport movement performance and injury risk, it seems necessary to first estimate a leg stiffness baseline value for average representatives of the population. With this approach, it will be possible to truly compare the leg stiffness values obtained from non-athletes with those obtained from athletes. Leg (quasi-) stiffness is understood as a ratio of changes in ground reaction forces to respective changes in “spring length” that represents both lower limbs.
Human locomotion is similar to the motion of a bouncing ball. Therefore, the term “bouncing gait” has been used to describe locomotion in which the lower limbs perform the role of “springs” responsible for the movement of the general centre of mass. “Spring-mass model” is used to describe a bouncing gait and contains a material point that represents a total body mass and massless lower limb used as a “spring”, which performs the supporting function. An estimation of the stiffness of this spring determines the value of leg stiffness; this stiffness is a ratio of changes in the ground reaction force to the respective change in the “spring length” representing both lower limbs. Leg stiffness is a quantitative measure of elastic properties and determines the ability to accumulate potential elastic energy. The conceptual and methodological confusion regarding the concept of leg stiffness have made it difficult to organize and compare the results obtained by different authors. Due to the substantial roles of inertia and damping on force-displacement relationships, especially during transient states, leg stiffness in terms of human continuous movement should be considered as “quasi-stiffness”.
The conceptual and methodological confusions around the concept of leg stiffness make it difficult to organize the existing knowledge and compare the results obtained by different authors. There are several computation methods, but they do not necessarily yield the same values of leg stiffness. However, these methods provide a more general estimation of quasi- or apparent stiffness, rather than actual stiffness, consistent with strict definitions originating from mechanics. The substantially different values of leg stiffness are likely to have been caused by calculations inconsistent with force-displacement curve profiles. The leg stiffness examined in this study should be viewed as quasi-stiffness, similar to other studies that have examined human continuous motion, due to the contribution of inertia and damping forces. The vast majority of authors should use the term vertical quasi-stiffness of the human body instead of using the concept of leg stiffness. This study seems to be one of the few reports that actually consider leg quasi-stiffness. Correctly estimated leg stiffness values allow for wider use of these values for various needs, including maximizing sports performance, optimizing human movement and designing man-like robots.
The relationship between inter-limb strength asymmetry and athletic performance is not clearly well-known. Different results in the literature makes it difficult to form a common consensus on this issue. The aim of this study was to investigate the relationship between contralateral inter-limb strength asymmetry (ILLSA) values and jump and sprint performances. Fortyeight individuals (21.5±2.8 years, 1.74±0.1 m, 67.3±9.7 kg) voluntarily participated in this study. All participants participated in squat (SJ), Countermovement jump (CMJ), 40m sprint run tests and right and left isokinetic concentric knee flexion and
extension strength (IS) test at low (60°.s-1) and high (300°.s-1) angular velocities. The symmetry angle formula was used in order to determine the isokinetic knee strength asymmetry values between inter-lower limb. According to the results it was revealed that there was no relation (p>0.05) between ILSSA at low and high angular velocities and squat jump (r = -0.106 0.200), countermovement jump (r = -0.11 0.087) and 40m sprint (r = 0.012 0.810) performance. In conclusion, there is no negative or positive relationship between inter limb isokinetic knee strength symmetry angle and jump and sprint performances in physically active individuals.
Maloney, SJ. The relationship between asymmetry and athletic performance: A critical review. J Strength Cond Res XX(X): 000-000, 2018-Symmetry may be defined as the quality to demonstrate an exact correspondence of size, shape, and form when split along a given axis. Although it has been widely asserted that the bilateral asymmetries are detrimental to athletic performance, research does not wholly support such an association. Moreover, the research rarely seeks to distinguish between different types of bilateral asymmetry. Fluctuating asymmetries describe bilateral differences in anthropometric attributes, such as nostril width and ear size, and are thought to represent the developmental stability of an organism. There is evidence to suggest that fluctuating asymmetries may be related to impaired athletic performance, although contradictory findings have been reported. Sporting asymmetries is a term that may better describe bilateral differences in parameters, such as force output or jump height. These asymmetries are likely to be a function of limb dominance and magnified by long-standing participation within sport. Sporting asymmetries do not seem to carry a clear influence on athletic performance measures. Given the vast discrepancy in the methodologies used by different investigations, further research is warranted. Recent investigations have demonstrated that training interventions can reduce sporting asymmetries and improve performance. However, studies have not sought to determine whether the influence of sporting asymmetry is independent of improvements in neuromuscular parameters. It may be hypothesized that the deficient (weaker) limb has a greater potential for adaptation in comparison to the strong limb and may demonstrate greater responsiveness to training.
Whilst the measurement and quantification of vertical leg stiffness (Kvert) asymmetry is of important practical relevance to athletic performance, literature investigating bilateral asymmetry in Kvert is limited. Moreover, how the type of task used to assess Kvert may affect the expression of asymmetry has not been properly determined. Twelve healthy males performed three types of performance tasks on a dual force plate system to determine Kvert asymmetries; the tasks were (a) bilateral hopping, (b) bilateral drop jumping and (c) unilateral drop jumping. Across all the three methods, Kvert was significantly different between compliant and stiff limbs (P < 0.001) with a significant interaction effect between limb and method (P = 0.005). Differences in Kvert between compliant and stiff limbs were -5.3% (P < 0.001), -21.8% (P = 0.007) and -15.1% (P < 0.001) for the bilateral hopping, bilateral drop jumping and unilateral drop jumping methods, respectively. All the three methods were able to detect significant differences between compliant and stiff limbs, and could be used as a diagnostic tool to assess Kvert asymmetry. Drop jumping tasks detected larger Kvert asymmetries than hopping, suggesting that asymmetries may be expressed to a greater extent in acyclic, maximal performance tasks.
The purpose of this investigation was to determine the existence of bilateral strength and force production asymmetry and evaluate possible differences based on sex as well as strength level. Asymmetry was assessed during weight distribution (WtD) testing, unloaded and lightly loaded static (SJ) and countermovement jump (CMJ) testing, and isometric mid-thigh pull (IMTP) strength testing. Subjects included 63 athletes (31 male, 32 female) for WtD, SJ and CMJ tests, while 129 athletes (64 male, 65 female) participated in IMTP testing. Independent samples t tests were used to determine possible differences in asymmetry magnitude between males and females as well as between strong and weak athletes. Cohen's d effect size estimates were also used to estimate difference magnitudes. Statistically different asymmetry levels with moderate to strong effect sizes were seen between males and females in WtD, 0 kg SJ (peak force; PF), 20 kg SJ (peak power; PP), 0 kg CMJ (PF, PP, net impulse), 20 kg CMJ (PF), but no statistical differences were observed in IMTP variables. Dividing the sample into strong and weak groups produced statistically significant differences with strong effect size estimates in IMTP PF and rate of force development and many effect sizes in jump symmetry variables increased. The results of this investigation indicate that females may be more prone to producing forces asymmetrically than males during WtD and jumping tasks. Similarly, weaker athletes displayed more asymmetry than stronger athletes. This may indicate that absolute strength may play a larger role in influencing asymmetry magnitude than sex.
Greater levels of lower-body stiffness have been associated with improved outcomes for a number of physical performance variables involving rapid stretch-shorten cycles. The aim of this study was to investigate the relationship between several measures of lower-body stiffness and physical performance variables typically evident during team sports in female athletes. Eighteen female athletes were assessed for quasi-static stiffness (myometry) for several isolated muscles in lying and standing positions. The muscles included the medial gastrocnemius (MedGast), lateral gastrocnemius, soleus, and Achilles tendon. Dynamic stiffness during unilateral hopping was also assessed. Participants were separated into relatively stiff and compliant groups for each variable. A number of significant differences in performance were evident between stiff and compliant subjects. When considering the quasi-static stiffness of the MedGast in lying and standing positions, relatively stiff participants recorded significantly superior results during agility, bounding, sprinting, and jumping activities. Stiffness as assessed by hopping did not discriminate between performance ability in any test. Relationships highlighted by MedGast results were supported by further significant differences in eccentric utilisation ratio and drop jump results between stiff and compliant groups for the lateral gastrocnemius and soleus in lying and standing positions. Higher levels of lower-body stiffness appear to be advantageous for females when performing rapid and (or) repeated stretch-shorten cycle movements, including sprinting, bounding, and jumping. Further, the stiffness of the MedGast is of particular importance during the performance of these activities. It is important for practitioners working with athletes in sports that rely upon these activities for success to consider stiffness assessment and modification.
Objectives: The purpose of this investigation was to determine what effect a bilateral strength training regimen has on isometric force production symmetry and if changes in force production symmetry can be accounted for by differences in pre-intervention strength levels. Design: Sixteen recreationally trained males (1-RM squat: 146.8 ± 23.0 kg.) were assigned to two groups for the 7-week training intervention: strong (S) and weak (W) based on pre-training squat isometric peak force allometrically scaled (IPFa) at 120° knee angle. Methods: Subjects completed a 7-week training intervention following a block-periodized model and were tested on measures of dynamic (1RM squat) and isometric (isometric squat at 90° and 120° knee angle) strength pre- and post-intervention. The degree of bilateral lower limb asymmetry was calculated as a percentage where 0% symmetry index (SI) indicates perfect symmetry on the isometric squat. Results: ANCOVA results showed no statistical difference between groups for all dependent variables when pre-intervention IPFa 120° scores were used as the covariate. Paired t-tests results showed both groups statistically improved 1RM squat and IPFa 120° (p<0.05). IPFa 120° SI decreased statistically from pre-training in the W group (p=0.03). Independent t-test results showed the W group had statistically larger pre-intervention SI scores for IPFa 90° (p=0.045) and IPFa 120° (p=0.007); however this difference was no longer present following strength training. There was a strong inverse relationship between pooled IPFa 120° and IPFa 120° SI (r=-0.64, p=0.004). Conclusions: The findings of the current study support the notion weaker individuals can augment lower limb symmetry with strength training. The same does not seem to be true for stronger individuals who already have a low symmetry index score. These findings indicate that strength training improves force production symmetry in relatively weak males, which may be important for bilateral tasks and injury potential reduction.
Differential loading patterns during game-based participation may produce or exacerbate strength imbalances between the lower limbs. It is currently unknown whether such imbalances are functionally beneficial or detrimental to performance. This study assessed the influence of lower limb strength and lean mass symmetry on kicking accuracy in Australian Football. Thirty-one Australian footballers were required to perform a kicking assessment, producing ten drop punt kicks over twenty metres to a player target. Athletes were subsequently separated into accurate (n = 15) and inaccurate (n = 16) groups, with lower-body lean mass assessed using whole body DXA scans, and lower-body strength assessed using an isometric protocol. Accurate kickers demonstrated significantly higher relative lean mass (~8% to 16%; p = 0.001 to 0.004) and significantly lower relative fat mass (~21% to 40%; p = 0.001 to 0.024) than inac-curate kickers. Accurate kickers did not contain any significant difference in lean mass or unilateral strength between lower limbs. Inaccurate kickers displayed significant asymmetry in lean mass (~3%; p ≤ 0.003), producing significant imbalances in strength (~8%; p ≤ 0.002) highlighting a deficiency in their support leg. Greater relative strength and improved lower limb symmetry in strength and muscularity could increase the capacity of an athlete to be technically proficient in favour of greater accuracy.
Objectives: The purpose of this study was to identify the relationship between isometric force production symmetry and jumping performance in weighted and un-weighted static and countermovement jumps (SJ and CMJ).
Design: Bivariate correlation between isometric force production symmetry and vertical jump performance variables.
Methods: Collegiate athletes were evaluated for this study (n=36). Subjects performed SJ, CMJ, and isometric mid-thigh pulls (IMTP). Jumps were analyzed for jump height (JH) and peak power (PP). IMTP was analyzed for peak force (PF) for left and right sides, and values were calculated to produce a peak force symmetry index (PF-SI) score. Correlational statistics were performed examining the relationship between PF-SI and jump variables.
Results: Moderate statistically significant negative correlations were observed between PF-SI and all jump variables,indicating that as asymmetry increases jump performance decreases. SJ correlations weakened in weighted conditions (JH r=-0.52 @ 0 kg/r=-0.39 @ 20 kg, PP r=-0.43 @ 0 kg/r=-0.34 @ 20 kg), but CMJ produced similar correlations for both conditions (JH r=-0.47 @ 0 kg/r=-0.49 @ 20 kg, PP r=-0.28 @ 0 kg/r=-0.34 @ 20 kg). Unlike the SJ, which only contains the propulsive or concentric portion of the jump, the CMJ also contains the eccentric portion and performance contributions of the stretch-shortening cycle (SSC). The addition of the SSC may play a role in the maintaining the magnitude of asymmetry in the CMJ weighted condition.
Conclusions: The results indicate that force production asymmetry may be detrimental to bilateral vertical jumping
performance. The findings should be considered for further investigation on sport-specific tasks.
The purpose of this study was to identify the relationship between isometric force production symmetry and jumping performance in weighted and un-weighted static and countermovement jumps (SJ and CMJ). Design: Bivariate correlation between isometric force production symmetry and vertical jump performance variables. Methods: Collegiate athletes were evaluated for this study (n=36). Subjects performed SJ, CMJ, and isometric mid-thigh pulls (IMTP). Jumps were analyzed for jump height (JH) and peak power (PP). IMTP was analyzed for peak force (PF) for left and right sides, and values were calculated to produce a peak force symmetry index (PF-SI) score. Correlational statistics were performed examining the relationship between PF-SI and jump variables. Results: Moderate statistically significant negative correlations were observed between PF-SI and all jump variables, indicating that as asymmetry increases jump performance decreases. SJ correlations weakened in weighted conditions (JH r=-0.52 @ 0 kg/r=-0.39 @ 20 kg, PP r=-0.43 @ 0 kg/r=-0.34 @ 20 kg), but CMJ produced similar correlations for both conditions (JH r=-0.47 @ 0 kg/r=-0.49 @ 20 kg, PP r=-0.28 @ 0 kg/r=-0.34 @ 20 kg). Unlike the SJ, which only contains the propulsive or concentric portion of the jump, the CMJ also contains the eccentric portion and performance contributions of the stretch-shortening cycle (SSC). The addition of the SSC may play a role in the maintaining the magnitude of asymmetry in the CMJ weighted condition. Conclusions: The results indicate that force production asymmetry may be detrimental to bilateral vertical jumping performance. The findings should be considered for further investigation on sport-specific tasks.
The purpose of this study was to compare the biomechanics of single- and double-legged drop jumps (SDJ vs. DDJ) with changes in drop height. Jumping height, ground contact time, reactive strength index, ground reaction force, loading rate of ground reaction force, joint power and stiffness were measured in 12 male college students during SDJ from 20-, 30-, 40-, and 50-cm heights and DDJ from of 20- and 40-cm heights. The peak impact force was increased with the incremental drop height during SDJs. The jumping height and leg and ankle stiffness of SDJ30 were greater than those of SDJ40 and SDJ50. The knee and hip stiffnesses of SDJ30 were greater than those of SDJ50. The impact forces of SDJ30-50 were greater than those of DDJ40. The leg, ankle, knee and hip joint stiffnesses of SDJ20-30 were greater than those of DDJ20 and DDJ40. The propulsive forces of SDJ20-50 were greater than those of DDJ20 and DDJ40. The jumping height of SDJ30 was greater than that of DDJ20. Drop height of 30 cm was recommended during single-legged drop jump with the best biomechanical benefit. Single-legged drop jump from 20-30 cm could provide comparable intensity to double-legged drop jump from 40 cm.
Understanding the degree of leg stiffness during human movement would provide important information that may be used for injury prevention. In the current study, we investigated bilateral differences in leg stiffness during one-legged hopping. Ten male participants performed one-legged hopping in place, matching metronome beats at 1.5, 2.2, and 3.0 Hz. Based on a spring-mass model, we calculated leg stiffness, which is defined as the ratio of maximal ground reaction force to maximum center of mass displacement at the middle of the stance phase, measured from vertical ground reaction force. In all hopping frequency settings, there was no significant difference in leg stiffness between legs. Although not statistically significant, asymmetry was the greatest at 1.5 Hz, followed by 2.2 and 3.0 Hz for all dependent variables. Furthermore, the number of subjects with an asymmetry greater than the 10% criterion was larger at 1.5 Hz than those at 2.2 and 3.0 Hz. These results will assist in the formulation of treatment-specific training regimes and rehabilitation programs for lower extremity injuries.
This study examined whether ground reaction force (GRF) asymmetry of two-legged countermovement jumps (CMJ) is related to one-legged CMJ asymmetry. The GRF asymmetry of a two-legged CMJ has been suggested as a preferred test to the one-legged CMJ for functional strength and power deficit assessment. Twenty-eight men and 30 women performed 5 trials each of a one-legged CMJ with the right limb, a one-legged CMJ with the left limb, and a two-legged CMJ. Vertical GRFs were collected from each lower limb using two force platforms. While several GRF variables were calculated, vertical impulse correlated most strongly with jump height in all conditions (p < .05), and there was used in subsequent analyses. A moderate correlation was found for impulse asymmetry between the one-legged and two-legged CMJs for women (r = .45, p < .05), but not for men (r = .06, p = .76). In contrast, cross-tabulation analyses of subjects presented with the same dominant characteristics in the one-legged and the two-legged CMJs revealed poor associations for both men (Freeman-Halton Exact P = .61) and women (Freeman-Halton Exact P = .19). Only 11 women recorded the same dominant limb for both one-legged and two-legged CMJs. This suggests that impulse asymmetries found in the one-legged and two-legged CMJ were unrelated. As the one-legged CMJ relies on the extension forces generated entirely from one limb, variations in jump heights and GRF impulses by left and right limbs separately were more indicative of functional strength differences between sides. Hence, it is recommended that the one-legged CMJ is used when examining functional strength asymmetry in the lower limbs. In contrast, factors causing asymmetry in GRF impulses during two-legged CMJs are more complicated and require further investigation.
When a human being runs, muscles, tendons, and ligaments together behave like a single linear spring. This “leg spring” can be described remarkably well by spring/mass models. Although leg-stiffness during running (and logically, therefore, in hopping) has been shown to be adjusted in line with the individual characteristics of the external contact surface, the relative contribution of each of the sub-components of the leg spring to the mechanics of running is unclear. We proposed the three-degree-of-freedom leg spring chain in a position of stable equilibrium under the action of the leg stiffness. If the leg spring receives a displacement in hopping, the forces will no longer equilibrate, but the system will be exposed to the action of a force on a leg spring chain. We thus have two corresponding sets of modes, one set being the mode about which the chain is displaced, the other set for the forces which are evoked in consequence of the displacement. We found that if the leg has been displaced from
THIS ARTICLE REVIEWS RESEARCH RELATING TO THE STRETCH-SHORTENING CYCLE AND PLYOMETRICS. THE ARTICLE INSTRUCTS STRENGTH AND CONDITIONING PRACTITIONERS IN THE USE OF GROUND CONTACT TIMES AND THE REACTIVE STRENGTH INDEX IN PLYOMETRIC TRAINING. DOCUMENTATION ON HOW THESE MEASUREMENTS CAN BE USED TO OPTIMIZE PLYOMETRICS AND TO IMPROVE ATHLETES' FAST STRETCH SHORTENING CYCLE PERFORMANCE IS PROVIDED. RECOMMENDATIONS ARE MADE REGARDING THE USE OF GROUND CONTACT TIMES TO IMPROVE TRAINING SPECIFICITY AND THE USE OF THE REACTIVE STRENGTH INDEX TO OPTIMIZE PLYOMETRICS, TO MONITOR TRAINING PROGRESS, AND TO SERVE AS A MOTIVATIONAL TOOL. A 4-STEP PROGRESSION OF IMPLEMENTATION IS DETAILED.
Limb stiffness or musculotendinous stiffness (MTS) has previously been examined in relation to performance and characterized using a number of different methods. However, the fact that MTS shows only low to moderate correlations to performances may indicate a lack of understanding of this parameter. In addition to this, variation is seen between studies examining the same factors. To date, our understanding of MTS and its components are not complete and thus it is unclear which characteristic value represents the ideal index of stiffness as it relates to performance. Moreover, it is uncertain how MTS stiffness as a functional measure relates to performance, and also if there is an optimal amount of MTS stiffness for specific functions or tasks. The knowledge of the interplay of MTU stiffness as it relates to performance and injury risk is also poorly understood in that there is likely a disparity between levels of stiffness required to optimize performance and those required to minimize injury risk. The aim of this article is to review the literature as it describes the components of MTS and to discuss these in terms of their relationship to functional performance; consider adaptations of the MTU with training along with associated performance changes; highlight and discuss how stiffness may affect loading of the soft and bony tissues in terms of the MTU components and gender, with respect to risk of injury; discuss the apparent differences in the literature regarding associations of the various forms of stiffness index to function; suggest recommendations for training in light of adaptation of the muscle and tendon and injury risk in context of gender; and, finally, to highlight potential limitations of current methodologies and suggest further work to gain insight into the mechanisms of stiffness. It is hoped that by suggesting future work, a more detailed and comprehensive understanding of MTS will be gained, thus enabling appropriate interventions to optimally modify this parameter for specific requirements.
The aim of the study was to assess the reliability of a mobile contact mat in measuring a range of stretch-shortening cycle parameters in young adolescents. Additionally, vertical leg stiffness using contact mat data was validated against a criterion method using force-time data. The reliability study involved 18 youths completing a habituation and three separate test sessions, while 20 youths completed a single test session for the validity study. Participants completed three trials of a squat jump, countermovement jump, and maximal hopping test and a single trial of repeated sub-maximal hopping at 2.0 Hz and 2.5 Hz. All tests were performed on the contact mat. Reliability statistics included repeated-measures analysis of variance, intraclass correlation coefficient, and coefficient of variation (CV), while the correlation coefficient (r) and typical error of estimate (TEE) were reported for the validity study. Squat jump height was the most reliable measure (CV = 8.64%), while leg stiffness during sub-maximal hopping, and reactive strength index produced moderate reliability (CV = 10.17-13.93% and 13.98% respectively). Measures of leg stiffness obtained from contact mat data during sub-maximal hopping were in agreement with the criterion measure (r = 0.92-0.95; TEE = 6.5-7.5%), but not during maximal hopping (r = 0.59; TEE = 41.9%). The contact mat was deemed a valid tool for measuring stretch-shortening cycle ability in sub-maximal but not maximal hopping. Although reliability of performance was generally moderate, the tests offer a replicable assessment method for use with paediatric populations.
When humans hop in place or run forward, leg stiffness is increased to offset reductions in surface stiffness, allowing the global kinematics and mechanics to remain the same on all surfaces. The purpose of the present study was to determine the mechanism for adjusting leg stiffness. Seven subjects hopped in place on surfaces of different stiffnesses (23-35,000 kN/m) while force platform, kinematic, and electromyographic data were collected. Leg stiffness approximately doubled between the most stiff surface and the least stiff surface. Over the same range of surfaces, ankle torsional stiffness increased 1.75-fold, and the knee became more extended at the time of touchdown (2.81 vs. 2.65 rad). We used a computer simulation to examine the sensitivity of leg stiffness to the observed changes in ankle stiffness and touchdown knee angle. Our model consisted of four segments (foot, shank, thigh, head-arms-trunk) interconnected by three torsional springs (ankle, knee, hip). In the model, an increase in ankle stiffness 1.75-fold caused leg stiffness to increase 1.7-fold. A change in touchdown knee angle as observed in the subjects caused leg stiffness to increase 1.3-fold. Thus both joint stiffness and limb geometry adjustments are important in adjusting leg stiffness to allow similar hopping on different surfaces.
The purpose of the present study was to investigate the interaction between the pre-landing activities and the stiffness regulation of the knee joint musculoskeletal system and the takeoff speed during a drop jump (DJ). Nine healthy male subjects performed a DJ test from the height of 50 cm. The surface electromyographic (EMG) activity of the vastus lateralis (VL) muscle was recorded to evaluate both the pre-landing and post-landing muscle activation levels. Simultaneous recording of the jumping motion and ground reaction force was performed by a high-speed video camera (100 frames x s(-1)), and a force platform was employed to allow joint moment analysis. Joint stiffness was calculated by a linear regression of the knee joint moment/angle relationship. Elasticity of the knee extensor muscle during DJ was estimated by means of a four-element muscle model consisting of a parallel elastic component, a series elastic component (SEC), a viscous damper, and a contractile element. DJ performance correlated positively with the positive peak power of the knee joint (P < 0.01) and with the moment of the knee joint at the end of stretch (P < 0.01). However, there was no significant relationship between DJ performance and the positive peak power of the ankle joint. The knee joint moment at the end of stretch correlated with the SEC stiffness during the transmission phase from the end of the initial impact to the onset of the concentric action (P < 0.01) and with the maximum rate of isometric force development of the knee extensors (P < 0.01). Multiple regression analysis showed that the SEC stiffness during the transmission phase of the knee joint can be explained by a combination of the pre-activity of the VL muscle and the knee joint angular velocity at touchdown (F = 5.76, P < 0.05). These results seem to emphasize the functional significance of the pre-programmed activity for controlling the subsequent stiffness regulation and then contributing to the performance in DJ. Thus, it can be suggested that the centrally pre-programmed activity and the associated elastic behavior of the SEC in the knee extensor muscle in conjunction with the muscle contractile property play a major role in regulating the performance in DJ.
Alternating a resistance exercise with a plyometric exercise is referred to as "complex training". In this study, we examined the effect of various resistive loads on the biomechanics of performance of a fast stretch-shortening cycle activity to determine if an optimal resistive load exists for complex training. Twelve elite rugby players performed three drop jumps before and after three back squat resistive loads of 65%, 80%, and 93% of a single repetition maximum (1-RM) load. All drop jumps were performed on a specially constructed sledge and force platform apparatus. Flight time, ground contact time, peak ground reaction force, reactive strength index, and leg stiffness were the dependent variables. Repeated-measures analysis of variance found that all resistive loads reduced (P < 0.01) flight time, and that lifting at the 93% load resulted in an improvement (P < 0.05) in ground contact time and leg stiffness. From a training perspective, the results indicate that the heavy lifting will encourage the fast stretch-shortening cycle activity to be performed with a stiffer leg spring action, which in turn may benefit performance. However, it is unknown if these acute changes will produce any long-term adaptations to muscle function.
A mathematical model for terrestrial running is presented, based on a leg with the properties of a simple spring. Experimental force-platform evidence is reviewed justifying the formulation of the model. The governing differential equations are given in dimensionless form to make the results representative of animals of all body sizes. The dimensionless input parameters are: U, a horizontal Froude number based on forward speed and leg length; V, a vertical Froude number based on vertical landing velocity and leg length, and KLEG, a dimensionless stiffness for the leg-spring. Results show that at high forward speed, KLEG is a nearly linear function of both U and V, while the effective vertical stiffness is a quadratic function of U. For each U, V pair, the simulation shows that the vertical force at mid-step may be minimized by the choice of a particular step length. A particularly useful specification of the theory occurs when both KLEG and V are assumed fixed. When KLEG = 15 and V = 0.18, the model makes predictions of relative stride length S and initial leg angle θ0 that are in good agreement with experimental data obtained from the literature.
A number of methods are used to measure lower extremity musculoskeletal stiffness, but there is a paucity of research examining the reliability of these techniques. Therefore, we investigated the reliability of vertical, leg, knee, and ankle stiffness during overground running and hopping in 20 active men. Participants were required to run on a 10 m overground runway at 3.83 m/s (actual; 3.35 +/- 0.12 m/s) and to hop in place at 2.2 Hz (actual; 2.37 +/- 0.03 Hz), and at a self-selected frequency (actual; 2.05 +/- 0.12 Hz) and at 2.2 Hz (actual; 2.39 +/- 0.04 Hz). Reliability was determined using the intraclass correlation coefficient, coefficient of variation, mean differences, and Cohen's effect sizes. There was good reliability for vertical stiffness, moderate reliability for leg stiffness, and poor reliability for knee and ankle stiffness during the running task. Similar results were observed during the 2.2 Hz hopping tasks, with good reliability displayed for vertical stiffness and poor reliability for ankle and knee stiffness. In conclusion, our results suggest that vertical stiffness is a reliable measure when running at 3.83 m/s and hopping at 2.2 Hz.
Each of the four muscles shown in Figure 36.1 (a to d) consists of muscle fascicles (bundles of muscle fibers) connected at either end to tendons, but they show striking differences of architecture. Most authors would describe (a) and (c) as pen- nate, but (b) and (d) as parallel-fibered. It often seems convenient to use these adjectives, but the distinction that they make is not a sharp one: it is easy to imagine a continuous series of intermediates between (a) and (b) or between (c) and (d). It is sometimes suggested that the diagnostic feature of a pennate muscle is that its fascicles attach obliquely to the tendons. However, the cross- sectional areas of the tendons are always much less than the total of the cross-sectional areas of the muscle fascicles, so geometry requires that the attachment be oblique even in muscles such as (b) and (d) that would generally be described as parallel-fibered. Muscle (e) has fascicles that attach at one end directly to a bone rather than to a tendon.
The present study aimed to clarify the asymmetry between the dominant (DL) and non-dominant takeoff legs (NDL) in terms of lower limb behavior during running single leg jumps (RSJ) in collegiate male basketball players in relation to that of the jump height. Twenty-seven players performed maximal RSJ with a 6 m approach. Three-dimensional kinematics data during RSJ was collected using a 12 Raptor camera infrared motion analysis system (MAC 3D system) at a sampling frequency of 500 Hz. The symmetry index in the jump heights and the kinematics variables were calculated as {2 × (DL - NDL) / (DL + NDL)} × 100. The run-up velocity was similar between the two legs, but the jump height was significantly higher in the DL than in the NDL. During the takeoff phase, the joint angles of the ankle and knee were significantly larger in the DL than the NDL. In addition, the contact time for the DL was significantly shorter than that for the NDL. The symmetry index of the kinematics for the ankle joint was positively correlated with that of jump height, but that for the knee joint was not. The current results indicate that, for collegiate basketball players, the asymmetry in the height of a RSJ can be attributed to that in the joint kinematics of the ankle during the takeoff phase, which may be associated with the ability to effectively transmit run-up velocity to jump height. Key pointsAsymmetry of height during running single leg jump between two legs is due to the behavior of the ankle joint (i.e. stiffer the ankle joint and explosive bounding).The dominant leg can transmit run-up velocity into the vertical velocity at takeoff phase to jump high compared with the non-dominant leg.Basketball players who have a greater asymmetry of the RSJ at the collegiate level could be assessed as non-regulars judging by the magnitude of asymmetry.
The purpose of this study was to determine the relationships between selected physical qualities, change-of-direction speed and defensive agility performance in Australian Rules football players. Twenty-four male community-level players were assessed on sprint acceleration (10-m time), maximum strength (3 repetition-maximum half squat), leg power (countermovement jump), reactive strength (drop jump), as well as a single change-of-direction speed test and a defensive agility test. Change-of-direction speed was correlated with reactive strength (r = -0.645, p = 0.001) and sprint acceleration (r = 0.510, p = 0.011). Multiple regression indicated the combined physical qualities explained 56.7% of the variance associated with change-of-direction speed (adjusted R = 0.567, p < 0.05). Participants were median-split into faster and slower change-of-direction speed groups, and these were compared by independent t-tests. The faster group was significantly better (p < 0.05) on the sprint acceleration and reactive strength tests (large effect size). The correlations between physical qualities and agility were trivial to small (r = - 0.101 to 0.123, p > 0.05) and collectively explained only 14.2% of the variance associated with agility performance (adjusted R = - 0.142, p > 0.05). When faster and slower agility groups were compared, there were trivial to moderate differences (p > 0.05) in all physical qualities. It was concluded that reactive strength and sprint acceleration are important for change-of-direction speed but the physical qualities assessed are not associated with defensive agility performance. For agility tasks similar to this study, sprint and resistance training should not be emphasized, and training other factors such as the development of sport-specific technique and cognitive skill is recommended.
Cutting ability is an important aspect of many team sports however the biomechanical determinants of cutting performance are not well understood. This study aimed to address this issue by identifying the kinetic and kinematic factors correlated with the time to complete a cutting maneuver. In addition, an analysis of the test-retest reliability of all biomechanical measures was performed. Fifteen (n = 15) elite multi-directional sports players (Gaelic hurling) were recruited, and a 3D motion capture analysis of a 75° cut was undertaken. The factors associated with cutting time were determined using bi-variate Pearson correlations. Intraclass correlation coefficients (ICC) were used to examine the test-retest reliability of biomechanical measures. Five biomechanical factors were associated with cutting time (2.28 ± 0.11s): peak ankle power (r = 0.77), peak ankle plantar flexor moment (r = 0.65), range of pelvis lateral tilt (r = -0.54), maximum thorax lateral rotation angle (r = 0.51) and total ground contact time (r = - 0.48). ICC scores for these five factors, and indeed for the majority of the other biomechanical measures, ranged from good-to-excellent (ICC > 0.60). Explosive force production about the ankle, pelvic control during single limb support and torso rotation toward the desired direction of travel, were all key factors associated with cutting time. These findings should assist in the development of more effective training programs aimed at improving similar cutting performances. In addition, test-retest reliability scores were generally strong therefore motion capture techniques seem well placed to further investigate the determinants of cutting ability.
The purpose of this investigation was to: (1) examine how asymmetry in lower extremity lean mass influenced force and power asymmetry during jumping, (2) determine how power and force asymmetry affected jump height, and (3) report normative values in collegiate athletes. Force and power were assessed from each limb using bilateral force plates during a countermovement jump in 167 Division 1 athletes (mass=85.7±20.3kg, age=20.0±1.2years, 103M/64F). Lean mass of the pelvis, thigh, and shank was assessed via dual-energy X-ray absorptiometry. Percent asymmetry was calculated for lean mass at each region (pelvis, thigh, and shank) as well as force and power. Forward stepwise regressions were performed to determine the influence of lean mass asymmetry on force and power asymmetry. Thigh and shank lean mass asymmetry explained 20% of the variance in force asymmetry (R=0.20, P<0.001), while lean mass asymmetry of the pelvis, thigh and shank explained 25% of the variance in power asymmetry (R=0.25, P<0.001). Jump height was compared across level of force and power asymmetry (P>0.05) and greater than 10% asymmetry in power tended to decrease performance (effect size>1.0). Ninety-five percent of this population (2.5 to 97.5 percentile) displayed force asymmetry between -11.8 to 16.8% and a power asymmetry between -9.9 to 11.5%. A small percentage (<4%) of these athletes displayed more than 15% asymmetry between limbs. These results demonstrate that lean mass asymmetry in the lower extremity is at least partially responsible for asymmetries in force and power. However, a large percentage remains unexplained by lean mass asymmetry.
Limb stiffness or musculotendinous stiffness (MTS) has previously been examined in relation to performance and characterized using a number of different methods. However, the fact that MTS shows only low to moderate correlations to performances may indicate a lack of understanding of this parameter. In addition to this, variation is seen between studies examining the same factors. To date, our understanding of MTS and its components are not complete and thus it is unclear which characteristic value represents the ideal index of stiffness as it relates to performance. Moreover, it is uncertain how MTS stiffness as a functional measure relates to performance, and also if there is an optimal amount of MTS stiffness for specific functions or tasks. The knowledge of the interplay of MTU stiffness as it relates to performance and injury risk is also poorly understood in that there is likely a disparity between levels of stiffness required to optimize performance and those required to minimize injury risk.
The aim of this article is to review the literature as it describes the components of MTS and to discuss these in terms of their relationship to functional performance; consider adaptations of the MTU with training along with associated performance changes; highlight and discuss how stiffness may affect loading of the soft and bony tissues in terms of the MTU components and gender, with respect to risk of injury; discuss the apparent differences in the literature regarding associations of the various forms of stiffness index to function; suggest recommendations for training in light of adaptation of the muscle and tendon and injury risk in context of gender; and, finally, to highlight potential limitations of current methodologies and suggest further work to gain insight into the mechanisms of stiffness. It is hoped that by suggesting future work, a more detailed and comprehensive understanding of MTS will be gained, thus enabling appropriate interventions to optimally modify this parameter for specific requirements.
Beck, TW. The importance of a priori sample size estimation in strength and conditioning research. J Strength Cond Res 27(8): 2323-2337, 2013-The statistical power, or sensitivity of an experiment, is defined as the probability of rejecting a false null hypothesis. Only 3 factors can affect statistical power: (a) the significance level (α), (b) the magnitude or size of the treatment effect (effect size), and (c) the sample size (n). Of these 3 factors, only the sample size can be manipulated by the investigator because the significance level is usually selected before the study, and the effect size is determined by the effectiveness of the treatment. Thus, selection of an appropriate sample size is one of the most important components of research design but is often misunderstood by beginning researchers. The purpose of this tutorial is to describe procedures for estimating sample size for a variety of different experimental designs that are common in strength and conditioning research. Emphasis is placed on selecting an appropriate effect size because this step fully determines sample size when power and the significance level are fixed. There are many different software packages that can be used for sample size estimation. However, I chose to describe the procedures for the G*Power software package (version 3.1.4) because this software is freely downloadable and capable of estimating sample size for many of the different statistical tests used in strength and conditioning research. Furthermore, G*Power provides a number of different auxiliary features that can be useful for researchers when designing studies. It is my hope that the procedures described in this article will be beneficial for researchers in the field of strength and conditioning.
Figure A.1 Walking Trial—Marker Locations and Mass and Frame Rate Information Table A.1 Raw Coordinate Data (cm) Table A.2(a) Filtered Marker Kinematics—Rib Cage and Greater Trochanter (Hip) Table A.2(b) Filtered Marker Kinematics—Femoral Lateral Epicondyle (Knee) and Head of Fibula Table A.2(c) Filtered Marker Kinematics—Lateral Malleolus (Ankle) and Heel Table A.2(d) Filtered Marker Kinematics—Fifth Metatarsal and Toe Table A.3(a) Linear and Angular Kinematics—Foot Table A.3(b) Linear and Angular Kinematics—Leg Table A.3(c) Linear and Angular Kinematics—Thigh Table A.3(d) Linear and Angular Kinematics—½ HAT Table A.4 Relative Joint Angular Kinematics—Ankle, Knee, and Hip Table A.5(a) Reaction Forces and Moments of Force—Ankle and Knee Table A.5(b) Reaction Forces and Moments of Force—Hip Table A.6 Segment Potential, Kinetic, and Total Energies—Foot, Leg, Thigh, and ½ HAT Table A.7 Power Generation/Absorption and Transfer—Ankle, Knee, and Hip
When a human being runs, muscles, tendons, and ligaments together behave like a single linear spring. This ''leg spring'' can be described remarkably well by spring/mass models. Although leg-stiffness during running (and logically, therefore, in hopping) has been shown to be adjusted in line with the individual characteristics of the external contact surface, the relative contribution of each of the sub-components of the leg spring to the mechanics of running is unclear. We proposed the three-degree-of-freedom leg spring chain in a position of stable equilibrium under the action of the leg stiffness. If the leg spring receives a displacement in hopping, the forces will no longer equilibrate, but the system will be exposed to the action of a force on a leg spring chain. We thus have two corresponding sets of modes, one set being the mode about which the chain is displaced, the other set for the forces which are evoked in consequence of the displacement. We found that if the leg has been displaced from a position of equilibrium about one of harmonic modes, then a vibration about this harmonic mode evokes a system of forces in the leg spring which in its turn tends to produce a motion on the original harmonic mode, and thus produce oscillation about the same harmonic mode. Our results suggest that the desired harmonic mode can be explained in terms of the natural shock absorption ability of the leg.
Certain strength measures represent specific or independent qualities of neuromuscular performance that can be assessed and trained independently. Strength diagnosis is the process of determining an athlete's level of development in each of these distinct strength qualities. Application of strength diagnosis has the potential to increase training efficiency, reduce injury, and facilitate rehabilitation by determining the relative assets and deficits of strength qualities. More research in this field is required, but several papers allude to the promise of strength diagnosis in making training program design more objective, more individualized, and safer.
The present study investigated the regulation of leg and joint stiffness in hopping at different intensity levels. Eight male subjects performed bilateral hopping at various intensity levels that were determined by peak vertical ground reaction force (GRF). In addition to the GRF, the measurements included hopping kinematics and electromyography (EMG) of selected leg muscles. While the leg and ankle joint stiffness remained invariant, the knee joint stiffness increased significantly (P<0.01) with the hopping intensity. EMG analysis revealed a significant increase in averaged EMG for all the measured muscles before and during the early phase of ground contact (P<0.05-0.001) with increasing hopping intensity. However, only the vastus lateralis muscle showed significant increase in stretch reflex EMG with increasing hopping intensity (P<0.01). The present study indicates that in hopping with short contact time the leg stiffness modulation is sensitive to changes in ankle joint stiffness and the role of knee joint stiffness is to regulate the jumping performance (height). Furthermore, our results suggest that leg and joint stiffness in hopping is mainly adjusted by centrally programmed motor commands and the contribution of stretch reflexes to muscle force output is muscle-dependent.
Leg stiffness is a modifiable mechanical property that may be related to soft tissue injury risk. The purpose of this study was to examine mean leg stiffness and bilateral differences in leg stiffness across an entire professional Australian Football League (AFL) season, and determine whether this parameter was related to the incidence of lower body soft tissue injury. The stiffness of the left and right legs of 39 professional AFL players (age 24.4 ± 4.4 years, body mass 87.4 ± 8.1 kg, stature 1.87 ± 0.07 m) was measured using a unilateral hopping test at least once per month throughout the season. Injury data were obtained directly from the head medical officer at the football club. Mean leg stiffness and bilateral differences in leg stiffness were compared between the injured and non-injured players. There was no difference between the season mean leg stiffness values for the injured (219.3 ± 16.1 N x m(-1) x kg(-1)) and non-injured (217.4 ± 14.9 N x m(-1) x kg(-1); P = 0.721) groups. The injured group (7.5 ± 3.0%) recorded a significantly higher season mean bilateral difference in leg stiffness than the non-injured group (5.5 ± 1.3%; P = 0.05). A relatively high bilateral difference in leg stiffness appears to be related to the incidence of soft tissue injury in Australian football players. This information is of particular importance to medical and conditioning staff across a variety of sports.
The acute effects of manipulating the volume and load of back squats on subsequent countermovement vertical jump performance were investigated in the present study. Eleven National Collegiate Athletic Association division II female volleyball players performed 10 countermovement vertical jumps (CMJs) on a force platform 2 minutes after the last squat repetition of a high-load (HL) or high-volume (HV) squat protocol. Two minutes of rest was provided between each CMJ. The HL protocol culminated in the subjects having to perform 3 repetitions with a load equivalent to 90% 1 repetition maximum (1RM) back squat, whereas 12 repetitions with a load equivalent to 37% 1RM were performed in the HV protocol. During an initial familiarization session, knee angles were recorded during a series of CMJs, and these angles were used to control the depth of descent during all subsequent back squats. Jump height (JH) and vertical stiffness (VStiff) were calculated during each of the 10 CMJ, and the change in these variables after the 2 squat protocols was assessed using an analysis of variance model with repeated measures on 2 factors (Protocol [2-levels]; Time [2-levels]). There was no significant difference in JH after the HL and HV protocols (p > 0.05). A significant Protocol × Time interaction for VStiff resulted from the increase after the HL protocol being greater than that after the HV protocol (p = 0.03). The knee angles before the HL and HV protocols were significantly greater than those measured during the initial familiarization session (p = 0.001). Although neither squat protocol provided any benefit in improving JH, the heavy squat protocol produced greater increases in VStiff during the CMJ. Because of the increased VStiff caused by the HL protocol, volleyball coaches may consider using such protocols with their players to improve performance in jumps performed from a run such as the spike and on-court agility.
Hamstring strains remain one of the most prevalent injuries in Australian Rules football. The authors prospectively examined the relationship between musculotendinous stiffness of the hamstring and leg stiffness with hamstring injury in professional Australian Rules footballers during the 2006 season.
Higher hamstring stiffness and leg stiffness are related to noncontact, soft tissue hamstring injury risk in professional Australian Rules footballers.
Case-control study; Level of evidence, 3.
Unilateral hamstring stiffness and leg stiffness were assessed in 136 professional footballers in the month before the commencement of the competitive season. This information was then investigated relative to players who suffered noncontact, soft tissue hamstring injuries during either matches or training throughout the season to identify whether preseason stiffness was related to injury occurrence.
Fourteen tested players recorded acute, noncontact hamstring injuries, resulting in 3.3 ± 2.8 weeks of missed match play per injury. At preseason testing, the players who ended up sustaining a hamstring injury during the season recorded significantly higher mean hamstring stiffness (11%, P = .04) and leg stiffness (5%, P = .03). When considering the injured players, the leg stiffness of the involved limb was significantly higher than the noninjured players (P = .02), whereas hamstring stiffness was significantly higher on the noninvolved limb (P = .01). Further, those players who suffered a hamstring injury were significantly older than the noninjured players (P = .01).
It appears that a high bilateral hamstring stiffness and leg stiffness may be a determinant in the risk of sustaining a hamstring injury. Further, relatively lower hamstring stiffness in the involved limb of injured players appears to be associated with increased injury and may be related to a lack of strength. The information from stiffness assessment may allow medical staff to determine the hamstring risk status for individual players in team sports.
The classic book on human movement in biomechanics, newly updated. Widely used and referenced, David Winter's Biomechanics and Motor Control of Human Movement is a classic examination of techniques used to measure and analyze all body movements as mechanical systems, including such everyday movements as walking. It fills the gap in human movement science area where modern science and technology are integrated with anatomy, muscle physiology, and electromyography to assess and understand human movement. In light of the explosive growth of the field, this new edition updates and enhances the text with: Expanded coverage of 3D kinematics and kinetics. New materials on biomechanical movement synergies and signal processing, including auto and cross correlation, frequency analysis, analog and digital filtering, and ensemble averaging techniques. Presentation of a wide spectrum of measurement and analysis techniques. Updates to all existing chapters. Basic physical and physiological principles in capsule form for quick reference. An essential resource for researchers and student in kinesiology, bioengineering (rehabilitation engineering), physical education, ergonomics, and physical and occupational therapy, this text will also provide valuable to professionals in orthopedics, muscle physiology, and rehabilitation medicine. In response to many requests, the extensive numerical tables contained in Appendix A: "Kinematic, Kinetic, and Energy Data" can also be found at the following Web site: www.wiley.com/go/biomechanics.
Understanding stiffness of the lower extremities during human movement may provide important information for developing more effective training methods during sports activities. It has been reported that leg stiffness during submaximal hopping depends primarily on ankle stiffness, but the way stiffness is regulated in maximal hopping is unknown. The goal of this study was to examine the hypothesis that knee stiffness is a major determinant of leg stiffness during the maximal hopping. Ten well-trained male athletes performed two-legged hopping in place with a maximal effort. We determined leg and joint stiffness of the hip, knee, and ankle from kinetic and kinematic data. Knee stiffness was significantly higher than ankle and hip stiffness. Further, the regression model revealed that only knee stiffness was significantly correlated with leg stiffness. The results of the present study suggest that the knee stiffness, rather than those of the ankle or hip, is the major determinant of leg stiffness during maximal hopping.
A mathematical model for terrestrial running is presented, based on a leg with the properties of a simple spring. Experimental force-platform evidence is reviewed justifying the formulation of the model. The governing differential equations are given in dimensionless form to make the results representative of animals of all body sizes. The dimensionless input parameters are: U, a horizontal Froude number based on forward speed and leg length; V, a vertical Froude number based on vertical landing velocity and leg length, and KLEG, a dimensionless stiffness for the leg-spring. Results show that at high forward speed, KLEG is a nearly linear function of both U and V, while the effective vertical stiffness is a quadratic function of U. For each U, V pair, the simulation shows that the vertical force at mid-step may be minimized by the choice of a particular step length. A particularly useful specification of the theory occurs when both KLEG and V are assumed fixed. When KLEG = 15 and V = 0.18, the model makes predictions of relative stride length S and initial leg angle theta o that are in good agreement with experimental data obtained from the literature.
The purpose of this study was to quantify the relationship between musculotendinous stiffness and performance in eccentric, isometric, and concentric activities. Thirteen trained subjects performed a series of maximal effort eccentric, concentric, and isometric muscular contractions in a bench press-type movement. Additionally, subjects performed a series of quasi-static muscular contractions in a bench press movement. A brief perturbation was applied to the bar while these isometric efforts were maintained, and the resulting damped oscillations provided data pertaining to each subject's musculotendinous stiffness. Musculotendinous stiffness was significantly related to isometric and concentric performance (r = 0.57-0.78) but not to eccentric performance. These results are interpreted as demonstrating that the optimal musculotendinous stiffness for maximum concentric and isometric activities was toward the stiff end of the elasticity continuum. A stiffer musculotendinous unit may facilitate such performances by improving the force production capabilities of the contractile component, due to a combination of improved length and rate of shortening, and additionally by enhancing initial force transmission.
Trotting and hopping animals use muscles, tendons and ligaments to store and return elastic energy as they bounce along the ground. We examine how the musculoskeletal spring system operates at different speeds and in animals of different sizes. We model trotting and hopping as a simple spring-mass system which consists of a leg spring and a mass. We find that the stiffness of the leg spring (kleg) is nearly independent of speed in dogs, goats, horses and red kangaroos. As these animals trot or hop faster, the leg spring sweeps a greater angle during the stance phase, and the vertical excursion of the center of mass during the ground contact phase decreases. The combination of these changes to the spring system causes animals to bounce off the ground more quickly at higher speeds.
Analysis of a wide size range of animals (0.1–140kg) at equivalent speeds reveals that larger animals have stiffer leg springs (kleg∝M0.67, where M is body mass), but that the angle swept by the leg spring is nearly independent of body mass. As a result, the resonant period of vertical vibration of the spring-mass system is longer in larger animals. The length of time that the feet are in contact with the ground increases with body mass in nearly the same way as the resonant period of vertical vibration.
When humans hop in place or run forward, they adjust leg stiffness to accommodate changes in stride frequency or surface stiffness. The goal of the present study was to determine the mechanisms by which humans adjust leg stiffness during hopping in place. Five subjects hopped in place at 2.2 Hz while we collected force platform and kinematic data. Each subject completed trials in which they hopped to whatever height they chose ("preferred height hopping") and trials in which they hopped as high as possible ("maximum height hopping"). Leg stiffness was approximately twice as great for maximum height hopping as for preferred height hopping. Ankle torsional stiffness was 1.9-times greater while knee torsional stiffness was 1.7-times greater in maximum height hopping than in preferred height hopping. We used a computer simulation to examine the sensitivity of leg stiffness to the observed changes in ankle and knee stiffness. Our model consisted of four segments (foot, shank, thigh, head-arms-trunk) interconnected by three torsional springs (ankle, knee, hip). In the model, increasing ankle stiffness by 1.9-fold, as observed in the subjects, caused leg stiffness to increase by 2.0-fold. Increasing knee stiffness by 1.7-fold had virtually no effect on leg stiffness. Thus, we conclude that the primary mechanism for leg stiffness adjustment is the adjustment of ankle stiffness.
The purposes of this study are: a) to examine the possibility of influencing the leg stiffness through instructions given to the subjects and b) to determine the effect of the leg stiffness on the mechanical power and take-off velocity during the drop jumps. A total of 15 athletes performed a series of drop jumps from heights of 20, 40 and 60 cm. The instructions given to the subjects were a) "jump as high as you can" and b) "jump high a little faster than your previous jump". The jumps were performed at each height until the athlete could not achieve a shorter ground contact time. The ground reaction forces were measured using a "Kistler" force plate (1000 Hz). The athletes body positions were recorded using a high speed (250 Hz) video camera. EMG was used to measure muscle activity in five leg muscles. The data was divided into 5 groups where group 1 was made up of the longest ground contact times of each athlete and group 5 the shortest. The leg and ankle stiffness values were higher when the contact times were shorter. This means that by influencing contact time through verbal instructions it is possible to control leg stiffness. Maximum center of mass take-off velocity the can be achieved with different levels of leg stiffness. The mechanical power acting on the human body during the positive phase of the drop jumps had the highest values in group 3. This means that there is an optimum stiffness value for the lower extremities to maximize mechanical power.
The purpose of this study was to measure the effects of varied rest interval lengths on the vertical jump heights and ground reaction forces during the execution of a depth jump from a predetermined optimal height. The subjects were 12 men with a mean (SD) age of 25.08 +/- 2.43 years. Each subject's optimal depth jump height was determined by executing depth jumps from 10-80 cm. After determining their optimal depth jump height, the subjects performed 3 sets of 10 depth jumps, each set with a different rest interval duration. The 3 rest intervals between depth jumps were 15, 30, and 60 seconds and were counterbalanced for each subject. Maximal vertical jump height and vertical ground reaction forces were calculated for each depth jump trial. The Peak Performance Motion Measurement System was used to measure vertical jump height and the Kistler force platform was used to measure ground reaction forces. Two-way analyses of variance revealed that rest interval length did not affect (p > 0.05) vertical jump height or vertical ground reaction forces. Therefore, this study demonstrated a 15-second rest interval was sufficient for recovery during the performance of depth jumps.
The influence of muscle series elasticity on the relationship between torque and joint angle during dynamic contractions was studied. The torque-angle relationship during the maximal isokinetic knee extension was determined for six male subjects (25-45 years) at 0.52, 1.05, 1.57, 2.09, 2.62, 3.49 rad/s. The knee joint angle at which peak torque was observed showed a systematic shift to more extended positions, i.e., the quadriceps muscle-tendon unit length became shorter as the velocity increased [from 1.01 (0.12) rad (0.52 rad/s) to 0.75 (0.14) rad (3.49 rad/s), mean (SD)]. The corresponding difference in muscle-tendon unit length between 0.52 and 3.49 rad/s, estimated from the angle shift at peak torque and the moment arm length change of the quadriceps muscles, was 9 (4) mm. The relationship between estimated changes in muscle-tendon unit length and muscle force of the vastus lateralis and intermedius (VLI) over the seven velocities (including isometric contraction, 0 rad/s) coincided with the load-elongation properties of the series elastic component of VLI, determined separately in vivo by ultrasonography when the same subjects performed a ramp isometric knee extension. The results suggest that the torque-angle relationship is affected by the interaction between contractile and elastic components, and that peak torque angle shift is attributable to the elongation of tendinous tissues as a function of force applied to them.
Lower extremity stiffness is thought to be an important factor in musculoskeletal performance. However, too little or too much stiffness is believed to increase the risk of musculoskeletal injury.
To provide a current update of the lower extremity stiffness literature as it pertains to both performance and injury.
It appears that increased stiffness is beneficial to performance. As well it appears that there may be an optimal amount of stiffness that allows for injury-free performance. There is some evidence that increased stiffness may be related to bony injuries and decreased stiffness may be associated with soft tissue injuries. Further investigations should evaluate the relationship between stiffness and injury prospectively. Initial reports suggest that stiffness can be modified in response to the external environment or verbal cues.
A greater understanding of the role of stiffness in both performance and injury will provide a stronger foundation for the development of optimal training intervention programs.
The authors compared leg stiffness (K(VERT)), muscle activation, and joint movement patterns between 11 men and 10 women during hopping. Physically active and healthy men and women performed continuous 2-legged hopping at their preferred rate and at 3.0 Hz. Compared with men, women demonstrated decreased K(VERT); however, after the authors normalized for body mass, gender differences in K(VERT) were eliminated. In comparison with men, women also demonstrated increased quadriceps and soleus activity, as well as greater quadriceps-to-hamstrings coactivation ratios. There were no significant gender differences for joint movement patterns (p>.05). The relationship between the observed gender differences in muscle recruitment and the increased risk of anterior cruciate ligament injury in women requires further study.
Differences in muscle dynamics between the preferred and nonpreferred jumping legs of subjects in maximal, explosive exercise were examined. Eight subjects performed nonfatiguing bouts of single-legged drop jumps and rebound jumps on a force sledge apparatus. Measures of flight time, reactive strength index, peak vertical force, and vertical leg-spring stiffness were obtained for 3 drop jumps and 3 rebound jumps on both legs. Subjects utilized a stiffer leg spring and a more explosive jumping action in the nonpreferred leg when performing a cyclical rebound jumping task in comparison to a single drop jump task (observed through differences in vertical leg-spring stiffness, peak vertical force, and reactive strength index, p < 0.05). The preferred leg performed equally well in both tasks. Between-leg analysis showed no differences in dependent variables between the preferred and the nonpreferred leg in the rebound jumping protocol. However, the drop jump protocol showed significant performance differences, with flight time and reactive strength index greater in the preferred leg than the nonpreferred leg (p < 0.05). We hypothesize that, throughout the lifespan, both legs are equally trained in cyclical rebound jumping tasks through running. However, because a preferred leg must be selected when performing any one-off, single-legged jump, imbalances in this specific task develop over time with consistent selection of a preferred jumping leg. The data demonstrate that the rebound jump protocol is representative of the symmetrical mechanics of forward running and that leg-spring stiffness is modulated depending on the demands of the specific task involved. Strength and conditioning practitioners should give careful consideration to appropriate jump protocol selection and should exercise caution when comparing laboratory results to data gathered in field testing.