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Greater Electromyographic Responses Do Not Imply Greater Motor Unit Recruitment and ‘Hypertrophic Potential’ Cannot Be Inferred

DOI:10.1519/JSC.0000000000001249
Authors:
Andrew Vigotsky at Northwestern University
Andrew Vigotsky
Chris Beardsley at Strength and Conditioning Research Limited
Chris Beardsley
  • Strength and Conditioning Research Limited
Bret Contreras
Bret Contreras
James Steele at Solent University
James Steele
  • Solent University
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DOI: 10.1519/JSC.0000000000001249
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Greater electromyographic responses do not imply greater motor
1
unit recruitment and ‘hypertrophic potential’ cannot be inferred
2 3
4
We read with interest the study by Looney et al. (13), investigating the effects of load on 5
electromyography (EMG) amplitude and rating of perceived exertion (RPE) during squats taken 6
to muscular failure. There are numerous interesting takeaways from this study, including the 7
similar RPE outcomes of different loads when sets are taken to failure; however, we demur with 8
the authors’ interpretation of the findings. 9
10
In the title and the body of the article, the term motor unit (MU) recruitment is used 11
synonymously with EMG amplitude. This is an incorrect assumption, but regrettably a common 12
mistake in sports and exercise science. We find this mistake being made especially when dealing 13
with fatiguing and dynamic conditions, such as those investigated by Looney et al. (13). In fact, 14
Enoka and Duchateau (7) recently described how numerous studies have misinterpreted surface 15
EMG signals by inferring specific MU recruitment. More than two decades previously, De Luca 16
(4) stated, “To its detriment, electromyography is too easy to use and consequently too easy to 17
abuse.” Looney et al. (13) state that MU firing rate decreases with fatigue (10, 15) and 18
consequently that the increase in EMG amplitude is caused by increased MU recruitment (19-21) 19
and have applied that same logic to the subsequent interpretation of the findings, as the authors 20
repeatedly state that the greater EMG amplitude observed in the heavier conditions is indicative 21
of greater MU recruitment. Regrettably, the interpretation of EMG is not so straightforward. 22
Moreover, different quadriceps muscles may utilize different neural strategies to maintain force 23
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generation during repeated concentric contractions (6), which makes the findings of Looney et 24
al. (13) particularly difficult to interpret. 25
26
Although EMG amplitude is influenced by MU recruitment, MU recruitment cannot be inferred 27
from changes in surface EMG amplitude. The recruitment threshold of high threshold MUs is 28
reduced during sustained, fatiguing contractions (1) and the subsequent recruitment of these 29
MUs assists in the maintenance force production. However, MU cycling may momentarily de-30
recruit fatigued MUs in order to reduce fatigue (22). This means that, in scenarios that require 31
less force output, such as low-load conditions, there may be lower simultaneous MU recruitment 32
compared to high-load conditions. Ultimately, a comparable complement of the MU population 33
of a particular muscle may be recruited, but not simultaneously as in high-load conditions. This 34
would explain the observation of reduced peak EMG amplitude in low-load training, as reported 35
by Looney et al. (13). These factors, including the reduced recruitment threshold of high 36
threshold MUs, in addition to MU cycling during fatiguing contractions, may also explain other 37
recent work showing differences in peak amplitude measured during surface EMG for high- and 38
low-load conditions (12, 16). 39
40
EMG amplitude during fatiguing conditions can be extraordinarily misleading, as EMG 41
measures consist not only of multiple neural components (MU recruitment, rate coding, and 42
possibly MU synchronization), but also of multiple peripheral constituents: muscle fiber 43
propagation velocity and intracellular action potentials (5). Intracellular action potentials are of 44
particular interest during fatiguing conditions, as the ensuing increase in length of intracellular 45
action potentials may augment surface EMG signals, despite a decrease in intracellular action 46
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potential magnitude. These inherent limitations make it impossible to discern MU recruitment 47
from increases in EMG amplitude during fatiguing, dynamic conditions (2, 5, 8, 9). It may be 48
true that greater loads induce greater MU recruitment, but in order to measure this, more 49
advanced methods are needed, such as spike-triggered averaging (3) or initial wavelet analysis 50
followed by principal component classification of major frequency properties and optimization 51
to tune wavelets to these frequencies (11). 52
53
In addition to our concerns regarding the confusion of EMG amplitude with MU recruitment, we 54
note that inferring chronic adaptations from acute, mechanistic variables is very difficult. Looney 55
et al. (13) suggest that their findings support the use of heavier loads for hypertrophy. Such a 56
conclusion is unwarranted, as the literature does not currently differentiate between the long-57
term effects of heavy and light loads on increases in muscular size (18). Data from Mitchell et al. 58
(14) also demonstrated comparable growth of type I and II fibers following 10 weeks of strength 59
training at either low (30%-1RM) or high-loads (80%-1RM). If the differential EMG amplitude 60
between high and low-load training observed by Looney et al. (13) and others (12, 16) is 61
representative of greater recruitment of presumably high threshold MUs, then one would expect 62
a differential hypertrophic response between low and high threshold MUs, which is presently not 63
supported. In fact, from an evidence-based perspective, Schoenfeld et al. (18), in their meta-64
analysis, showed no difference between studies that have employed lighter or heavier loads to 65
induce hypertrophy. A recent study by the same author confirmed that this was true even in well 66
trained participants (17). Thus, longitudinal trials are clearly needed to elucidate these 67
mechanisms, in addition to comparing individual loading with combined loading schemes. 68
69
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The findings of Looney et al. (13) provide more data that unequal EMG amplitudes are obtained 70
during fatiguing contractions with low- and high-load conditions and the novel finding that both 71
conditions elicit similar RPE. What these data do not provide, however, is evidence that heavier 72
load contractions recruit more MUs and that this can be inferred to result in greater hypertrophy. 73
We hope that our letter helps put these findings into a clearer perspective. 74
75 Andrew D. Vigotsky, BS 76 Arizona State University 77 Phoenix, AZ 78 79 Chris Beardsley, MSc 80 Strength and Conditioning Research Limited 81 London, UK 82 83 Bret Contreras, MA 84 Auckland University of Technology 85 Auckland, New Zealand 86 87 James Steele, PhD 88 Southampton Solent University 89 Southampton, UK 90 91 Dan Ogborn, PhD 92 McMaster University 93 Hamilton, Ontario 94 95 Stuart M. Phillips, PhD 96 McMaster University 97 Hamilton, Ontario 98 99 100 References 101 1. Adam A and De Luca CJ. Recruitment order of motor units in human vastus lateralis 102 muscle is maintained during fatiguing contractions. J Neurophysiol 90: 2919-2927, 2003. 103 2. Behm DG, Leonard AM, Young WB, Bonsey WA, and MacKinnon SN. Trunk muscle 104 electromyographic activity with unstable and unilateral exercises. J Strength Cond Res 105 19: 193-201, 2005. 106 3. Boe SG, Stashuk DW, and Doherty TJ. Motor unit number estimation by decomposition-107 enhanced spike-triggered averaging: control data, test-retest reliability, and contractile 108 level effects. Muscle Nerve 29: 693-699, 2004. 109
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4. De Luca CJ. The use of surface electromyography in biomechanics. J Appl Biomech 13: 110 135-163, 1997. 111 5. Dimitrova NA and Dimitrov GV. Interpretation of EMG changes with fatigue: facts, 112 pitfalls, and fallacies. J Electromyogr Kinesiol 13: 13-36, 2003. 113 6. Ebersole KT, O'Connor KM, and Wier AP. Mechanomyographic and electromyographic 114 responses to repeated concentric muscle actions of the quadriceps femoris. J 115 Electromyogr Kinesiol 16: 149-157, 2006. 116 7. Enoka RM and Duchateau J. Inappropriate interpretation of surface EMG signals and 117 muscle fiber characteristics impedes progress on understanding the control of 118 neuromuscular function. J Appl Physiol (1985): jap 00280 02015, 2015. 119 8. Ertas M, Stalberg E, and Falck B. Can the size principle be detected in conventional 120 EMG recordings? Muscle Nerve 18: 435-439, 1995. 121 9. Freund HJ. Motor unit and muscle activity in voluntary motor control. Physiol Rev 63: 122 387-436, 1983. 123 10. Harwood B, Choi I, and Rice CL. Reduced motor unit discharge rates of maximal 124 velocity dynamic contractions in response to a submaximal dynamic fatigue protocol. J 125 Appl Physiol (1985) 113: 1821-1830, 2012. 126 11. Hodson-Tole EF and Wakeling JM. Variations in motor unit recruitment patterns occur 127 within and between muscles in the running rat (Rattus norvegicus). J Exp Biol 210: 2333-128 2345, 2007. 129 12. Jenkins ND, Housh TJ, Bergstrom HC, Cochrane KC, Hill EC, Smith CM, Johnson GO, 130 Schmidt RJ, and Cramer JT. Muscle activation during three sets to failure at 80 vs. 30 % 131 1RM resistance exercise. Eur J Appl Physiol, 2015. 132 13. Looney DP, Kraemer WJ, Joseph MF, Comstock BA, Denegar CR, Flanagan SD, 133 Newton RU, Szivak TK, DuPont WH, Hooper DR, Hakkinen K, and Maresh CM. 134 Electromyographical and Perceptual Responses to Different Resistance Intensities in a 135 Squat Protocol: Does Performing Sets to Failure With Light Loads Recruit More Motor 136 Units? J Strength Cond Res, 2015. 137 14. Mitchell CJ, Churchward-Venne TA, West DW, Burd NA, Breen L, Baker SK, and 138 Phillips SM. Resistance exercise load does not determine training-mediated hypertrophic 139 gains in young men. J Appl Physiol (1985) 113: 71-77, 2012. 140 15. Mottram CJ, Jakobi JM, Semmler JG, and Enoka RM. Motor-unit activity differs with 141 load type during a fatiguing contraction. J Neurophysiol 93: 1381-1392, 2005. 142 16. Schoenfeld BJ, Contreras B, Willardson JM, Fontana F, and Tiryaki-Sonmez G. Muscle 143 activation during low- versus high-load resistance training in well-trained men. Eur J 144 Appl Physiol 114: 2491-2497, 2014. 145 17. Schoenfeld BJ, Peterson MD, Ogborn D, Contreras B, and Sonmez GT. Effects of Low- 146 Versus High-Load Resistance Training on Muscle Strength and Hypertrophy in Well-147 Trained Men. J Strength Cond Res, 2015. 148 18. Schoenfeld BJ, Wilson JM, Lowery RP, and Krieger JW. Muscular adaptations in low-149 versus high-load resistance training: A meta-analysis. European journal of sport science: 150 1-10, 2014. 151 19. Smilios I, Hakkinen K, and Tokmakidis SP. Power output and electromyographic activity 152 during and after a moderate load muscular endurance session. J Strength Cond Res 24: 153 2122-2131, 2010. 154
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20. Stock MS, Beck TW, and Defreitas JM. Effects of fatigue on motor unit firing rate versus 155 recruitment threshold relationships. Muscle Nerve 45: 100-109, 2012. 156 21. Toigo M and Boutellier U. New fundamental resistance exercise determinants of 157 molecular and cellular muscle adaptations. Eur J Appl Physiol 97: 643-663, 2006. 158 22. Westad C, Westgaard RH, and De Luca CJ. Motor unit recruitment and derecruitment 159 induced by brief increase in contraction amplitude of the human trapezius muscle. J 160 Physiol 552: 645-656, 2003. 161 162
... Gains in type I muscle fiber areas in already highly strength-trained elite athletes can likely enhance strength performance at low velocities, such as powerlifting, but would be less effective than type II fiber hypertrophy to enhance force at higher velocities. Specific force (force/CSA) of single muscle fibers does not vary much between fiber types during isometric contractions (28), however, shortening velocity, maximal power output, rate constant of tension rise are often 5-10-fold higher in type II fibers compared to type I fibers (28). Indeed, Aagaard et al. (1) observed that fiber type distribution correlates well with isokinetic force at velocities ≥120 degrees per second during knee-extension, but no relationship was observed at lower velocities. ...
... Gains in type I muscle fiber areas in already highly strength-trained elite athletes can likely enhance strength performance at low velocities, such as powerlifting, but would be less effective than type II fiber hypertrophy to enhance force at higher velocities. Specific force (force/CSA) of single muscle fibers does not vary much between fiber types during isometric contractions (28), however, shortening velocity, maximal power output, rate constant of tension rise are often 5-10-fold higher in type II fibers compared to type I fibers (28). Indeed, Aagaard et al. (1) observed that fiber type distribution correlates well with isokinetic force at velocities ≥120 degrees per second during knee-extension, but no relationship was observed at lower velocities. ...
... However, several recent studies have shown that performing high-volume BFRRE close to voluntary failure increases markers of muscle damage and cellular stress (10, 12, 53, 57, 60, 61). In fact, rhabdomyolysis and excessive breakdown of muscle tissue have been reported in some individuals (9,28,53,56). Furthermore, Hain et al. (25), using a rodent model, observed that detrimental effects may occur after 5 days of very high-frequent blood flow-restricted exercise (four times per day), as evidenced by a 35% reduction in MFA (~1,800 m 2 ). ...
PhD Thesis Thomas Bjørnsen
Thesis
Full-text available
  • Aug 2019
SUMMARY Purpose: The overarching aim of this thesis was to investigate the effect of short-term blocks with high-frequency low-load blood flow restricted resistance exercise (BFRRE) on muscular adaptations in untrained individuals, recreationally trained individuals and elite strength athletes. Three independent studies with four original papers have been completed towards this objective. High-frequency BFRRE has been shown to induce rapid muscle growth accompanied by increased numbers of satellite cells and myonuclei. However, the satellite cell and myonuclear responses appears to plateau after an initial block of training and it may be speculated that a rest period can reset the responsiveness of the system after the initial training response. Thus, the aims of Study I and II were to investigate the effect and time-course of changes in fiber and whole muscle areas, myonuclear and satellite cell numbers and muscle strength during two five-day blocks of high-frequency low-load BFRRE, separated by 10 days of rest. In addition, the importance of performing BFRRE sets to failure on cellular adaptations has not been investigated. Therefore, Study II compared the effect of a failure- vs. a non-failure high-frequency BFRRE protocol. Despite the impressive rates of muscle growth reported in some studies on high-frequency BFRRE, several recent studies have shown that BFRRE increases markers of muscle damage and cellular stress. To shed light on possible mechanisms for myocellular stress and damage after strenuous high-frequency BFRRE, heat shock protein (HSP) responses, glycogen content and inflammatory markers were investigated in Study I (paper II). Finally, the impact of low-load BFRRE has not yet been investigated in highly specialized strength athletes, such as powerlifters. Thus, the aim of Study III was to investigate the effect of implementing two five-day blocks of high-frequency low-load BFRRE during six weeks of periodized strength training in elite powerlifters, on the changes in number of satellite cells, myonuclei and muscle size and strength. METHODS: A total of 47 healthy men and women participated in the studies. Thirteen recreationally trained sports students in Study I (24±2 yrs [mean±SD], 9 men) and 17 untrained men in Study II (25±6 yrs), completed two 5-day-blocks of seven BFRRE sessions, separated by a 10-day rest period. A failure BFRRE protocol consisting of four sets with knee extensions to voluntary failure at 20% of one-repetition maximum (1RM) was performed with both legs in Study I, and randomized to one of the legs in Study II. The other leg in Study II performed a non-failure BFRRE protocol (30, 15, 15, 15 repetitions). In Study I, muscle samples from m. vastus lateralis (VL) obtained before and 1h after the first session in the first and second block (“Acute1” and “Acute2”), after three sessions (“Day4”), during the “Rest Week”, and at three (“Post3”) and ten days post-intervention (“Post10”), were analyzed for muscle fiber area (MFA), myonuclei, satellite cells, mRNA, miRNA, HSP70, αB-crystallin, glycogen (PAS staining), CD68+ (macrophages) and CD66b+ (neutrophils) cell numbers. Muscle strength (1RM knee-extension) and whole muscle size (ultrasonography and magnetic resonance imaging) was measured up until 20 days after the last exercise session (Post20). In Study II, muscle samples obtained before, at midtraining, and 10 days post-intervention (Post10) were analyzed for muscle fiber area (MFA), myonuclei, and satellite cells. Muscle thickness, cross-sectional area and echo intensity were measured by ultrasonography, and knee-extension strength with 1RM and maximal isometric contraction (isomMVC) up until Post24. In Study III, seventeen national level powerlifters (25±6 yrs, 15 men) were randomly assigned to either a BFRRE group (n=9) performing two blocks (week 1 and 3) of five BFRRE front squat sessions within a 6.5-week training period, or a conventional training group (Con; n=8) performing front squats at ~70% of 1RM. The BFRRE consisted of four sets (first and last set to voluntary failure) at ~30% of 1RM. Muscle biopsies were obtained from VL and analyzed for MFA, myonuclei, satellite cells and capillaries. Cross-sectional areas (CSA) of VL and m. rectus femoris (RF) were measured by ultrasonography. Strength was evaluated by maximal voluntary isokinetic torque (dynMVC) in knee-extension and 1RM in front squat. RESULTS: With the first block of BFRRE in Study I (paper I), satellite cell number increased in both fiber types (70-80%, p<0.05), while type I and II MFA decreased by 6±7% and 15±11% (p<0.05), respectively. No significant changes were observed in number of myonuclei or strength during the first block of training. With the second block of training, muscle size increased by 6-8%, while the number of satellite cell (type I: 80±63%, type II 147±95%), myonuclei (type I: 30±24%, type II: 31±28%) and MFA (type I: 19±19%, type II: 11±19%) peaked 10 days after the second block of BFRRE. Strength peaked after 20 days of detraining (6±6%, p<0.05). Pax7- and p21 mRNA expression were elevated during the intervention, while myostatin, IGF1R, MyoD, myogenin, cyclinD1 and -D2 mRNA did not change until 3-10 days post intervention. In paper II of Study I, αB-crystallin was reported to translocate from the cytosolic to the cytoskeletal fraction after Acute1 and Acute2 (p<0.05), and immunostaining revealed larger responses in type 1 than type 2 fibers (Acute1, 225±184% vs. 92±81%, respectively, p=0.001). HSP70 was increased in the cytoskeletal fraction at Day4 and Post3, and immunostaining intensities were more elevated in type 1 than in type 2 fibers (Day4, 206±84% vs. 72±112%, respectively, p<0.001). Glycogen content was reduced in both fiber types; but most pronounced in type 1, which did not recover until the Rest Week (-15-29%, p≤0.001). Intramuscular macrophage numbers were increased by ~65% postintervention, but no changes were observed in muscle neutrophils. Both protocols in Study II increased myonuclear numbers in type-1 (12- 17%) and type-2 fibers (20-23%), and satellite cells in type-1 (92-134%) and type-2 fibers (23-48%) at Post10 (p<0.05). RF and VL size increased by 7-10% and 5-6% in both legs at Post10 to Post24, whereas the MFA of type-1 fibers in Failure was decreased at Post10 (-10±16%; p=0.02). Echo intensity increased by ~20% in both legs during Block1 (p<0.001) and was ~8-11% below baseline at Post24 (p=0.001-0.002). IsomMVC decreased by 8-10% in both legs and 1RM by 5% in the failure leg after Block1 (p=0.01-0.02). IsomMVC and 1RM were increased in both legs by 6-7% and 9-11% at Post24, respectively (p<0.05). In Study III, BFRRE in powerlifters induced selective type I fiber increases in MFA (BFRRE: 12% vs. Con: 0%, p<0.01) and myonuclear number (BFRRE: 17% vs. Con: 0%, p=0.02). Type II MFA was unaltered in both groups. BFRRE induced greater changes in VL CSA than control (7.7% vs. 0.5%, p=0.04), and the VL CSA changes correlated with the increases in MFA of type I fibers (r=0.81, p=0.02). No significant group differences were observed in SC and strength changes. CONCLUSIONS: High-frequency low-load BFRRE in Study I and II induced pronounced responses in satellite cell proliferation, delayed myonuclear addition and increases in muscle size, concomitantly with delayed increases in strength in untrained and recreationally trained individuals. While the gains in satellite cell and myonuclear numbers as well as muscle size and strength were similar between non-failure and failure BFRRE protocols in Study II, perceptions of exertion, pain and muscle soreness were lower in the non-failure leg. Hence, nonfailure BFRRE may be a more feasible and safe approach. However, we report that short-term strenuous high-frequency BFRRE can induce elevations in multiple markers of cellular stress and damage in non-strength trained individuals. We showed that low-load BFRRE stressed both fiber types, but the fiber type-specific HSP-responses and prolonged glycogen depletion strongly indicated that type 1 fibers were more stressed than type 2 fibers. It appears that the first block of unaccustomed BFRRE exceeded the capacity for recovery in both Study I and II, and may have induced muscle damage in some of our participants. In accordance with our hypothesis, our participants seemed to recover during the rest week and to respond well to the second block of BFRRE. It is intriguing that BFRRE induced preferential type I hypertrophy after the second block of training in Study I. This indicates that although the initial stress may be too high (and cause damage), adaptive responses will occur and later the same exercise stress will be the important stimuli for adaptation. Our findings from Study I and II may provide insights into some of the physiological mechanisms underpinning overreaching and subsequent recovery and supercompensation after periods of very strenuous exercise. Finally, in Study III, two one-week blocks with high-frequency low-load BFRRE implemented during six weeks of periodized strength training induced a significant increase in muscle size and myonuclear addition in elite powerlifters. Preferential hypertrophy and myonuclear addition of type I fibers appears to explain most of the overall muscle growth. Intriguingly, these responses are in contrast to heavy-load strength training, that typically induces a greater type II fiber hypertrophy. Consequently, BFRRE appears to have complementary effects to heavyresistance training and the combination of these two methods may optimize adaptations of both fiber types in highly strength-trained individuals. However, despite the increases in muscle size, we could not observe any group differences in maximal strength.
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... As such it is perhaps unsurprising that high effort aerobic modalities also increase muscle water content (Mora-Rodriguez et al., 2016) and result in muscle swelling (Ozaki et al., 2013b). In consideration of Henneman's size principle, motor unit recruitment should also be similar between muscular actions when they are performed to a near maximal effort (Potvin & Fuglevand, 2017) and indeed this has been argued to be the case for resistance training whether performed at high or low loads (Fisher, Steele & Smith, 2017;Vigotsky et al., 2017). Thus, it might be expected that low force/load 'cardio' modalities might also produce high motor unit recruitment if performed with close proximity to momentary failure and thus high effort. ...
... Further, normalised electromyographic amplitudes appear to be greater during typical resistance training (single leg knee extension) compared with 'cardio' mode (single leg cycle ergometry) exercise performed to volitional failure (Noble et al., 2017). Although, time to task failure in Noble et al. (2017) was unclear and amplitude based analyses may not reflect the entirety of motor units recruited where task durations differ, particularly if differing recruitment patterns are occurring (i.e., sequential recruitment of low to high threshold during low force tasks, and simultaneous recruitment of both low and high threshold motor units during high force tasks; Fisher, Steele & Smith, 2017;Vigotsky et al., 2017;Potvin & Fuglevand, 2017;Enoka & Duchateau, 2015). However, where resistance training modes (knee extension) and 'cardio' modes (cycling) have been performed to momentary failure (thus controlling effort), and frequency based analyses applied, evidence suggests that similar recruitment of motor units may occur (Kuznetsov et al., 2011). ...
... This is thought to be due to Henneman's size principle, and recent modelling studies suggest that motor unit recruitment should be similar between muscular actions when they are performed to a near maximal effort (Potvin & Fuglevand, 2017). Motor unit recruitment patterns are difficult to discern specifically from amplitude based surface EMG analyses (Fisher, Steele & Smith, 2017;Vigotsky et al., 2017;Enoka & Duchateau, 2015). Indeed, if tasks being compared differ considerably in effort and/or durations, motor unit recruitment patterns may differ (e.g., synchronous vs sequential) such that, even though similar numbers of total motor units are recruited, EMG amplitudes may differ (Fisher, Steele & Smith, 2017;Vigotsky et al., 2017). ...
Similar acute physiological responses from effort and duration matched leg press and recumbent cycling tasks
Article
Full-text available
  • Feb 2018
The present study examined the effects of exercise utilising traditional resistance training (leg press) or 'cardio' exercise (recumbent cycle ergometry) modalities upon acute physiological responses. Nine healthy males underwent a within session randomised crossover design where they completed both the leg press and recumbent cycle ergometer conditions. Conditions were approximately matched for effort and duration (leg press: 4 × 12RM using a 2 s concentric and 3 s eccentric repetition duration controlled with a metronome, thus each set lasted 60 s; recumbent cycle ergometer: 4 × 60 s bouts using a resistance level permitting 80-100 rpm but culminating with being unable to sustain the minimum cadence for the final 5-10 s). Measurements included VO 2 , respiratory exchange ratio (RER), blood lactate, energy expenditure, muscle swelling, and electromyography. Perceived effort was similar between conditions and thus both were well matched with respect to effort. There were no significant effects by 'condition' in any of the physiological responses examined (all p > 0.05). The present study shows that, when both effort and duration are matched, resistance training (leg press) and 'cardio' exercise (recumbent cycle ergometry) may produce largely similar responses in VO 2 , RER, blood lactate, energy expenditure, muscle swelling, and electromyography. It therefore seems reasonable to suggest that both may offer a similar stimulus to produce chronic physiological adaptations in outcomes such as cardiorespiratory fitness, strength, and hypertrophy. Future work should look to both replicate the study conducted here with respect to the same, and additional physiological measures, and rigorously test the comparative efficacy of effort and duration matched exercise of differing modalities with respect to chronic improvements in physiological fitness.
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... Although it is not possible to International Journal of Exercise Science http://www.intjexersci.com 270 establish a simple relationship between sEMG amplitude and force production for several reasons (62), previous studies reported higher mean sEMG amplitudes with heavy loads compared to light loads (22,37). It has also been suggested that sEMG amplitude decreases when fatigue affects the ability to exert force (2,31). ...
... The present study has limitations. Assessing sEMG activity during different exercises helps generate hypotheses and gain insight into the neuromuscular system, but EMG activity does not necessarily imply greater motor unit recruitment, changes in force development or fatigue, neuromuscular adaptations, or differentiation between muscle fiber types (22,62). Nevertheless, it allowed us to perform a preliminary comparison of muscle recruitment during IVR cable resistance exercises and better understand the adaptive resistance. ...
Muscle Activity During Immersive Virtual Reality Exergaming Incorporating an Adaptive Cable Resistance System
Article
  • Jan 2022
The purpose of this exploratory study was to characterize muscle activation via surface electromyography (sEMG), user-perceived exertion, and enjoyment during a 30-minute session of immersive virtual reality (IVR) cable resistance exergaming. Ten healthy, college-aged males completed a signature 30-minute exergaming session using an IVR adaptive cable resistance system that incorporated six traditional compound exercises. Muscle activation (sEMG) was captured during the session with a wearable sEMG system. Rated of Perceived Exertion (RPE) and Physical Activity Enjoyment Scale (PACES) were recorded following the session. Pectoralis major showed the highest activation during chest press, deltoids showed the highest activation on overhead press, latissimus dorsi showed the highest activation during lat pulldown and row exercises, hamstrings were the most activated muscles during Romanian deadlift, and glutes showed the highest activity during squats. RPE and PACES mean scores were 14 (1) and 4.27 (0.38), respectively. IVR exergaming with resistance cable training provides an enjoyable experience and distracts practitioners from exertion while exercising at a high intensity. Results from this study suggest similar muscle activation responses compared to traditional resistance exercises as demonstrated with prior evidence. This novel form of exercise might have important repercussions for improving health outcomes among those who find it challenging to adhere to and enjoy exercise routines, as well as with little knowledge on how to progress in their resistance training. Further investigations are needed to explore long-term adaptations and to assess if IVR exergaming has additional benefits compared to traditional resistance training.
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... 1,16,17 Low-load training could elicit hypertrophic gains similar to those observed in high-load training as a result of a subject's proximity to failure, as recruitment thresholds of high-threshold motor units tend to decrease as set duration and fatigue levels increase. 18,19 In addition, as fatigue begins to accumulate during a long-duration RT set, contractile velocities of muscle fibers are slowed, even if the intent is to move a load quickly. 20 According to the force-velocity relationship, 21 low muscle-fiber-shortening velocities allow for greater force production potential and higher levels of mechanical tension due to the greater number of attached cross-bridges. ...
... If this were the case, the muscle fibers that were a part of the high-threshold motor units would experience greater mechanical loads and could potentially experience an increase in the cross-sectional area as a result. Vigotsky et al 19 suggest a substantial number of motor units might be recruited during low-load fatiguing contractions in order to provide the force necessary to complete the final repetitions of an RT set, but the simultaneous recruitment of motor units is likely less when compared with high-load training. In addition, Schoenfeld et al 27 demonstrated that well-trained men achieved superior gains in skeletal muscle hypertrophy while using lower loads (as low as 67% 1RM) to failure over a period of 8 weeks when compared with those that used high loads (as high as 95% 1RM) to failure. ...
The Effect of Low-Load Resistance Training on Skeletal Muscle Hypertrophy in Trained Men: A Critically Appraised Topic
Article
Full-text available
Clinical Scenario : Resistance training (RT) programs promote skeletal muscle hypertrophy through the progressive physiological stress applied to an individual. Currently, the vast majority of studies regarding the hypertrophic response to RT have focused on either sedentary or untrained individuals. This critically appraised topic focuses on the hypertrophic response to high- and low-load RT in resistance-trained men. Clinical Question : In experienced male weightlifters, does high-load RT lead to greater increases in muscle mass than low-load RT? Summary of Key Findings : Six studies met the inclusion criteria, while 4 studies were included in the analysis. Each of the 4 studies showed that low-load RT elicited hypertrophic gains similar to high-load RT when sets were taken to failure. Three of the studies were not volume equated, indicating a dose–response relationship between training volume-load and skeletal muscle hypertrophy. One of the studies was volume equated, indicating that skeletal muscle hypertrophy could be achieved at levels comparable to those observed in high-load protocols as a result of high levels of metabolic stress and the concomitant recruitment of high-threshold motor units that can occur during fatiguing contractions. Clinical Bottom Line : Evidence suggests that low-load training produces hypertrophic gains similar to those observed in high-load RT protocols when sets are taken to failure in resistance-trained men. Strength of Recommendation : There is moderate to strong evidence to suggest that low-load RT elicits hypertrophic gains similar to those observed in high-load RT protocols when sets are taken to failure in resistance-trained men.
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... Caution is needed when extrapolating an acute effect to a chronic change. Although the association of muscle activation to muscle strength has been consistently shown [30][31][32][33][34], some authors suggest that muscle activation is not necessarily associated with hypertrophy [34]. However, it has been suggested that mechanotransduction, the translation of mechanical tension into a chemical signal that initiates the cascade responsible for muscle hypertrophy, is likely to occur only in the activated muscle fibers during exercise [35]. ...
... Caution is needed when extrapolating an acute effect to a chronic change. Although the association of muscle activation to muscle strength has been consistently shown [30][31][32][33][34], some authors suggest that muscle activation is not necessarily associated with hypertrophy [34]. However, it has been suggested that mechanotransduction, the translation of mechanical tension into a chemical signal that initiates the cascade responsible for muscle hypertrophy, is likely to occur only in the activated muscle fibers during exercise [35]. ...
"NO LOAD" Resistance Training Promotes High Levels of Knee Extensor Muscles Activation-A Pilot Study
Article
Full-text available
  • Jul 2020
The present article aims to compare electromyographic (EMG) activity of the knee extensors during traditional resistance training (TRT) and no load resistance training with or without visual feedback (NL-VF and NL-NF). Sixteen healthy men (age: 25.2 ± 3.6) volunteered to participate in the study. Participants visited the laboratory on three occasions involving: (1) a 10 repetition maximum test (10 RM test), (2) familiarization and (3) performance of knee extensions using TRT, NL-VF and NL-NF in a random order, with 10 min of rest between them. TRT involved the performance of a set to momentary muscle failure using the 10 RM load. NL-NF involved the performance of 10 repetitions with no external load, but with the intention to maximally contract the muscles during the whole set. NL-VF involved the same procedure as NL-NF, but a monitor was positioned in front of the participants to provide visual feedback on the EMG activity. Peak and mean EMG activity were evaluated on the vastus medialis (VM), vastus lateralis (VL) and rectus femoris (RF). Results: there were no significant differences in VM and VL peak EMG activity among different situations. There was a significant difference for peak EMG activity for RF, where TRT resulted in higher values than NL-VF and NL-NF (p < 0.05). Higher values of mean EMG activity were found for VM, VL and RF during TRT in comparison with both NL-VF and NL-NF. Conclusions: resistance training with no external load produced high levels of peak muscle activation, independent of visual feedback, but mean activation was higher during TRT. These results suggest that training with no external load might be used as a strategy for stimulating the knee extensors when there is limited access to specialized equipment. Although the clinical applications of no load resistance training are promising, it is important to perform long-term studies to test if these acute results will reflect in muscle morphological and functional changes.
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... This is an important finding considering previous assumptions that squat exercise does not sufficiently target RF among knee extensors [4,5,10]. However, it should be noted that the foundation of this concept has been preferably based on the lower electrical activity of RF compared to the vasti muscles during squats although a greater electromyographic response does not imply a greater hypertrophic potential [23]. ...
Does Back Squat Exercise Lead to Regional Hypertrophy among Quadriceps Femoris Muscles?
Article
Full-text available
  • Dec 2022
The present study investigated the effects of squat resistance training on intermuscular hypertrophy of quadriceps femoris muscles (i.e., rectus femoris, RF; vastus intermedius, VI; vastus medialis, VM; and vastus lateralis, VL). Eighteen university students (age: 24.1 ± 1.7 years, 9 females) underwent 7 weeks of parallel squat training (2 days/week) preceded by a 2-week familiarization period. Squat strength (1RM) and cross-sectional area (CSA) of four quadriceps muscles were assessed at baseline and at the end of the study. At posttest, 1RM and CSA of quadriceps muscles significantly increased (p < 0.01), with moderate-to-large effect (ES = 1.25-2.11) for 1RM (8.33 ± 6.64 kg), VM CSA (0.12 ± 0.08 cm 2), and VL CSA (0.19 ± 0.09 cm 2) and small effect (ES = 0.89-1.13) for RF CSA (0.17 ± 0.15 cm 2) and VI CSA (0.16 ± 0.18 cm 2). No significant differences were found in the changes in CSA between muscles (F = 0.638, p = 0.593). However, the squat 1RM gain was significantly associated only with the changes in CSA of the VL muscle (r = 0.717, p < 0.001). The parallel squat resulted in significant growth of all quadriceps muscles. However, the novelty of this study is that the increase in strength is associated only with hypertrophy of the VL muscle.
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... However, several points must be considered when using sEMG including: (a) comparison of exercises in different planes and muscle lengths; (b) technique of signal capture (e.g., electrode diameter, distance between poles, etc.), processing and analysis of signal amplitude (i.e., mean rectified value, moving average, linear envelope, median frequency, and root mean square) 4 , and; (c) standardization to a parameter (e.g., maximum isometric voluntary contraction). It is important to note that although there is an association between hypertrophy and muscle activation 5 , hypertrophy is a multifactorial phenomenon 2 and is influenced by more than simply neuromuscular activation; thus, greater electromyography responses does not necessarily translate to greater muscle hypertrophy 6 . ...
Resistance training exercise selection: efficiency, safety and comfort analysis method
Article
Full-text available
  • Jan 2021
Manipulation of resistance training variables has been shown to have a substantial effect on muscular adaptations. A major variable in this process is exercise selection. In addition to the effectiveness of a given exercise to recruit the target muscle groups, safety considerations and individual comfort during execution of a lift should be considered. The correct biomechanics of the chosen exercise will assist in promoting desired muscle adaptations, while proper safety procedures will reduce risk of injury. Lifting comfort will facilitate enjoyment and foster adherence to the program. Therefore, the purpose of this paper was to offer guidelines for selection of resistance training exercises based on the Efficiency, Safety, and Comfort Analysis Method (ESCAM).
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... We acknowledge that caution is required when extrapolating an acute effect onto a chronic change. Whilst some studies suggest that activation might be important for muscle adaptations [30], others suggest that electromyography cannot be necessarily linked to gains in muscle size and strength [55]. However, it has been suggested that mechanotransduction is likely to occur only in muscle fibers activated during exercise [31]. ...
Multi-and Single-Joint Resistance Exercises Promote Similar Plantar Flexor Activation in Resistance Trained Men
Article
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The present study aimed to compare soleus, lateral, and medial gastrocnemius muscles activation during leg press and calf raise exercises in trained men. The study involved 22 trained men (27.1 ± 3.6 years, 82.7 ± 6.6 kg, 177.5 ± 5.2 cm, 3.6 ± 1.4 experience years) who performed one set of each exercise using a 10-repetition maximum (10RM) load in a counterbalanced randomized order and separated by 10 min of rest. The electromyographic signal was measured for the three major plantar flexors: soleus, medial, and lateral gastrocnemius. A comparison between exercises showed that the mean adjusted by peak values during the leg press were 49.20% for the gastrocnemius lateralis, 51.31% for the gastrocnemius medialis, and 50.76% for the soleus. Values for calf raise were 50.70%, 52.19%, and 51.34% for the lateral, medial gastrocnemius, and soleus, respectively. There were no significant differences between exercises for any muscle (lateral gastrocnemius (p = 0.230), medial gastrocnemius (p = 0.668), and soleus (p = 0.535)). The present findings suggest that both leg press and calf raises can be used with the purpose to recruit triceps surae muscles. This bring the suggestion that one can chose between exercises based on personal preferences and practical aspects, without any negative impact on muscle activation.
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... First, greater electromyography (EMG) amplitudes have been demonstrated with HL-RT than LL-BFR both acute (5,19,25) and chronically (19), indicating a greater neural drive during heavy load RT. However, it is important to note that EMG is not a direct measure of MU recruitment, and these findings may be impacted by other factors (8,43). Moreover, it has been recently proposed that strength gains are more dependent on test specificity and training load, whereby the closer the load of the training protocol is to the load of the exercise test, the greater observed strength gains (2,22). ...
Perceptual and Neuromuscular Responses Adapt Similarly Between High-Load Resistance Training and Low-Load Resistance Training With Blood Flow Restriction
Article
Full-text available
  • Sep 2020
Teixeira, EL, Painelli, VdS, Schoenfeld, BJ, Silva-Batista, C, Longo, AR, Aihara, AY, Cardoso, FN, Peres, BdA, and Tricoli, V. Perceptual and neuromuscular responses adapt similarly between high-load resistance training and low-load resistance training with blood flow restriction. J Strength Cond Res XX(X): 000-000, 2020-This study compared the effects of 8 weeks of low-load resistance training with blood flow restriction (LL-BFR) and high-load resistance training (HL-RT) on perceptual responses (rating of perceived exertion [RPE] and pain), quadriceps cross-sectional area (QCSA), and muscle strength (1 repetition maximum [RM]). Sixteen physically active men trained twice per week, for 8 weeks. One leg performed LL-BFR (3 sets of 15 repetitions, 20% 1RM), whereas the contralateral leg performed HL-RT (3 sets of 8 repetitions, 70% 1RM). Rating of perceived exertion and pain were evaluated immediately after the first and last training sessions, whereas QCSA and 1RM were assessed at baseline and after training. Rating of perceived exertion was significantly lower (6.8 ± 1.1 vs. 8.1 ± 0.8, p = 0.001) and pain significantly higher (7.1 ± 1.2 vs. 5.8 ± 1.8, p = 0.02) for LL-BFR than that for HL-RT before training. Significant reductions in RPE and pain were shown for both protocols after training (both p < 0.0001), although no between-protocol differences were shown in absolute changes (p = 0.10 and p = 0.48, respectively). Both LL-BFR and HL-RT were similarly effective in increasing QCSA (7.0 ± 3.8% and 6.3 ± 4.1%, respectively; both p < 0.0001) and 1RM (6.9 ± 4.1% and 13.7 ± 5.9%, respectively; both P < 0.0001), although absolute changes for 1RM in HL-RT were greater than LL-BFR (p = 0.001). In conclusion, LL-BFR produces lower RPE values and a higher pain perception than HL-RT. However, consistent application of these approaches result in chronic adaptations so that there are no differences in perceptual responses over the course of time. In addition, muscle strength is optimized with HL-RT despite similar increases in muscle hypertrophy between conditions.
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Blood Flow Restriction Does Not Promote Additional Effects on Muscle Adaptations When Combined With High-Load Resistance Training Regardless of Blood Flow Restriction Protocol
Article
Full-text available
  • Feb 2021
Teixeira, EL, Ugrinowitsch, C, de Salles Painelli, V, Silva-Batista, C, Aihara, AY, Cardoso, FN, Roschel, H, and Tricoli, V. Blood flow restriction does not promote additional effects on muscle adaptations when combined with high-load resistance training regardless of blood flow restriction protocol. J Strength Cond Res 35(5): 1194-1200, 2021-The aim of this study was to investigate, during high-load resistance training (HL-RT), the effect of blood flow restriction (BFR) applied during rest intervals (BFR-I) and muscle contractions (BFR-C) compared with HL-RT alone (no BFR), on maximum voluntary isometric contraction (MVIC), maximum dynamic strength (one repetition maximum [1RM]), quadriceps cross-sectional area (QCSA), blood lactate concentration ([La]), and root mean square of the surface electromyography (RMS-EMG) responses. Forty-nine healthy and untrained men (25 ± 6.2 years, 178.1 ± 5.3 cm and 78.8 ± 11.6 kg) trained twice per week, for 8 weeks. One leg of each subject performed HL-RT without BFR (HL-RT), whereas the contralateral leg was randomly allocated to 1 of 2 unilateral knee extension protocols: BFR-I or BFR-C (for all protocols, 3 × 8 repetitions, 70% 1RM). Maximum voluntary isometric contraction, 1RM, QCSA, and acute changes in [La] and RMS-EMG were assessed before and after training. The measurement of [La] and RMS-EMG was performed during the control sessions with the same relative load obtained after the 1RM test, before and after training. Similar increases in MVIC, 1RM, and QCSA were demonstrated among all conditions, with no significant difference between them. [La] increased for all protocols in pre-training and post-training, but it was higher for BFR-I compared with the remaining protocols. Increases in RMS-EMG occurred for all protocols in pre-training and post-training, with no significant difference between them. In conclusion, despite of a greater metabolic stress, BFR inclusion to HL-RT during rest intervals or muscle contraction did not promote any additive effect on muscle strength and hypertrophy.
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Muscle activation during three sets to failure at 80 vs. 30 % 1RM resistance exercise
Article
Full-text available
The purpose of this study was to investigate electromyographic amplitude (EMG AMP), EMG mean power frequency (MPF), exercise volume (VOL), total work and muscle activation (iEMG), and time under concentric load (TUCL) during, and muscle cross-sectional area (mCSA) before and after 3 sets to failure at 80 vs. 30 % 1RM resistance exercise. Nine men (mean ± SD, age 21.0 ± 2.4 years, resistance training week(-1) 6.0 ± 3.7 h) and 9 women (age 22.8 ± 3.8 years, resistance training week(-1) 3.4 ± 3.5 h) completed 1RM testing, followed by 2 experimental sessions during which they completed 3 sets to failure of leg extension exercise at 80 or 30 % 1RM. EMG signals were collected to quantify EMG AMP and MPF during the initial, middle, and last repetition of each set. Ultrasound was used to assess mCSA pre- and post-exercise, and VOL, total work, iEMG, and TUCL were calculated. EMG AMP remained greater at 80 % than 30 % 1RM across all reps and sets, despite increasing 74 and 147 % across reps at 80 and 30 % 1RM, respectively. EMG MPF decreased across reps at 80 and 30 % 1RM, but decreased more and was lower for the last reps at 30 than 80 % 1RM (71.6 vs. 78.1 % MVIC). mCSA increased more from pre- to post-exercise for 30 % (20.2-24.1 cm(2)) than 80 % 1RM (20.3-22.8 cm(2)). VOL, total work, iEMG and TUCL were greater for 30 % than 80 % 1RM. Muscle activation was greater at 80 % 1RM. However, differences in volume, metabolic byproduct accumulation, and muscle swelling may help explain the unexpected adaptations in hypertrophy vs. strength observed in previous studies.
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Effects of Low- Versus High-Load Resistance Training on Muscle Strength and Hypertrophy in Well-Trained Men
Article
Full-text available
The purpose of this study was to compare the effect of low- versus high-load resistance training (RT) on muscular adaptations in well-trained subjects. Eighteen young men experienced in RT were matched according to baseline strength, and then randomly assigned to 1 of 2 experimental groups: a low-load RT routine (LL) where 25-35 repetitions were performed per set per exercise (n = 9), or a high-load RT routine (HL) where 8-12 repetitions were performed per set per exercise (n = 9). During each session, subjects in both groups performed 3 sets of 7 different exercises representing all major muscles. Training was carried out 3 times per week on non-consecutive days, for 8 total weeks. Both HL and LL conditions produced significant increases in thickness of the elbow flexors (5.3 vs. 8.6%, respectively), elbow extensors (6.0 vs. 5.2%, respectively), and quadriceps femoris (9.3 vs. 9.5%, respectively), with no significant differences noted between groups. Improvements in back squat strength were significantly greater for HL compared to LL (19.6 vs. 8.8%, respectively) and there was a trend for greater increases in 1RM bench press (6.5 vs. 2.0%, respectively). Upper body muscle endurance (assessed by the bench press at 50% 1RM to failure) improved to a greater extent in LL compared to HL (16.6% vs. -1.2%, respectively). These findings indicate that both HL and LL training to failure can elicit significant increases in muscle hypertrophy among well-trained young men; however, HL training is superior for maximizing strength adaptations.
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Muscular adaptations in low- versus high-load resistance training: A meta-analysis
Article
Full-text available
Abstract There has been much debate as to optimal loading strategies for maximising the adaptive response to resistance exercise. The purpose of this paper therefore was to conduct a meta-analysis of randomised controlled trials to compare the effects of low-load (≤60% 1 repetition maximum [RM]) versus high-load (≥65% 1 RM) training in enhancing post-exercise muscular adaptations. The strength analysis comprised 251 subjects and 32 effect sizes (ESs), nested within 20 treatment groups and 9 studies. The hypertrophy analysis comprised 191 subjects and 34 ESs, nested with 17 treatment groups and 8 studies. There was a trend for strength outcomes to be greater with high loads compared to low loads (difference = 1.07 ± 0.60; CI: -0.18, 2.32; p = 0.09). The mean ES for low loads was 1.23 ± 0.43 (CI: 0.32, 2.13). The mean ES for high loads was 2.30 ± 0.43 (CI: 1.41, 3.19). There was a trend for hypertrophy outcomes to be greater with high loads compared to low loads (difference = 0.43 ± 0.24; CI: -0.05, 0.92; p = 0.076). The mean ES for low loads was 0.39 ± 0.17 (CI: 0.05, 0.73). The mean ES for high loads was 0.82 ± 0.17 (CI: 0.49, 1.16). In conclusion, training with loads ≤50% 1 RM was found to promote substantial increases in muscle strength and hypertrophy in untrained individuals, but a trend was noted for superiority of heavy loading with respect to these outcome measures with null findings likely attributed to a relatively small number of studies on the topic.
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Muscle activation during low- versus high-load resistance training in well-trained men
Article
Full-text available
  • Aug 2014
Purpose It has been hypothesized that lifting light loads to muscular failure will activate the full spectrum of MUs and thus bring about muscular adaptations similar to high-load training. The purpose of this study was to investigate EMG activity during low- versus high-load training during performance of a multi-joint exercise by well-trained subjects. Methods Employing a within-subject design, 10 young, resistance-trained men performed sets of the leg press at different intensities of load: a high-load (HL) set at 75 % of 1-RM and a low-load (LL) set at 30 % of 1-RM. The order of performance of the exercises was counterbalanced between participants, so that half of the subjects performed LL first and the other half performed HL first, separated by 15 min rest. Surface electromyography (EMG) was used to assess mean and peak muscle activation of the vastus medialis, vastus lateralis, rectus femoris, and biceps femoris. Results Significant main effects for trials and muscles were found (p
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The Use of Surface Electromyography in Biomechanics
Article
  • May 1997
This lecture explores the various uses of surface electromyography in the field of biomechanics. Three groups of applications are considered: those involving the activation timing of muscles, the force/EMG signal relationship, and the use of the EMG signal as a fatigue index. Technical considerations for recording the EMG signal with maximal fidelity are reviewed, and a compendium of all known factors that affect the information contained in the EMG signal is presented. Questions are posed to guide the practitioner in the proper use of surface electromyography. Sixteen recommendations are made regarding the proper detection, analysis, and interpretation of the EMG signal and measured force. Sixteen outstanding problems that present the greatest challenges to the advancement of surface electromyography are put forward for consideration. Finally, a plea is made for arriving at an international agreement on procedures commonly used in electromyography and biomechanics.
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