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# The influence of lower leg configurations on muscle force variability

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... In a study, 19 adults performed knee extension, knee flexion, and ankle plantarflexion isometric force contractions to target forces ranging up to 95% of maximal voluntary contraction at two angles. The force output was non-linear, and the joint angle affected the force-variability function for each muscle contraction [92]. ...
... An optimal state of variability, characterized by a chaotic structure, is required for a healthy and functional movement [93]. The variability in force during motion is associated with non-uniformity in the sarcomere length and depends on the joint angle, type of muscular contraction, muscle orientation related to the joint, and the synchrony and firing rate of motor units [79,92]. The non-linearity of muscle activity was associated with parameters that determine strength, including the speed of contraction, muscle length and composition, cross-sectional area, gender, and age [94,95]. ...
... During repetitive isokinetic exercise, with the onset of fatigue, there is a decrease in the overall limb output [97]. Isokinetic exercise is limited in the determined range and velocity, and the force produced is always in the chaotic and non-repetitive range [79,92]. The normalized mutual information between muscle pairs increases with time while the variability decreases. ...
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The plateau effect in training is a significant obstacle for professional athletes and average subjects. It evolves from both the muscle-nerve-axis-associated performance and various cardiorespiratory parameters. Compensatory adaptation mechanisms contribute to a lack of continuous improvement with most exercise regimens. Attempts to overcome this plateau in exercise have been only partially successful, and it remains a significant unmet need in both healthy subjects and those suffering from chronic neuromuscular, cardiopulmonary, and metabolic diseases. Variability patterns characterize many biological processes, from cellular to organ levels. The present review discusses the significant obstacles in overcoming the plateau in training and establishes a platform to implement subject-tailored variability patterns to prevent and overcome this plateau in muscle and cardiorespiratory performance.
... In the knee extensors, for example, as the knee joint is moved into flexion, the muscles of the quadriceps femoris increase in length and maximal torque increases in a parabolic fashion up to approximately 75° (with 0° being full extension), before decreasing with further increases in length (Rassier et al. 1999;Becker and Awiszus 2001). Joint angle does not, however, simply affect the ability to produce maximal torque; rather, it is also a critical factor in determining endurance and neuromuscular fatigue mechanisms (Fitch and McComas 1985;Kooistra et al. 2006) and torque fluctuations (Ofori et al. 2018). ...
... Joint angle is a critical, though under-investigated, factor in determining the dynamics of muscle torque fluctuations (Ofori et al. 2018). Previous research on the magnitude of fluctuations has demonstrated a lower SD of fluctuations at more extended joint angles (Sosnoff et al. 2009;Ofori et al. 2018), though no difference between the linear slopes fitted to the SD-contraction intensity relationship between extended and flexed joint angles (Shinohara et al. 2006). ...
... Joint angle is a critical, though under-investigated, factor in determining the dynamics of muscle torque fluctuations (Ofori et al. 2018). Previous research on the magnitude of fluctuations has demonstrated a lower SD of fluctuations at more extended joint angles (Sosnoff et al. 2009;Ofori et al. 2018), though no difference between the linear slopes fitted to the SD-contraction intensity relationship between extended and flexed joint angles (Shinohara et al. 2006). Only one study to date has investigated muscle torque complexity at different joint angles (Ofori et al. 2018), indicating that more extended joint angles are associated with greater complexity. ...
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Purpose Joint angle is a significant determinant of neuromuscular and metabolic function. We tested the hypothesis that previously reported correlations between knee-extensor torque complexity and metabolic rate ( $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 ) would be conserved at reduced joint angles (i.e. shorter muscle lengths). Methods Eleven participants performed intermittent isometric knee-extensor contractions at 50% maximum voluntary torque for 30 min or until task failure (whichever occurred sooner) at joint angles of 30º, 60º and 90º of flexion (0º = extension). Torque and surface EMG were sampled continuously. Complexity and fractal scaling of torque were quantified using approximate entropy (ApEn) and detrended fluctuation analysis (DFA) α. $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 was determined using near-infrared spectroscopy. Results Time to task failure/end increased as joint angle decreased ( P < 0.001). Over time, complexity decreased at 90º and 60º (decreased ApEn, increased DFA α , both P < 0.001), but not 30º. $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 increased at all joint angles ( P < 0.001), though the magnitude of this increase was lower at 30º compared to 60º and 90º (both P < 0.01). There were significant correlations between torque complexity and $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 at 90º (ApEn, r = − 0.60, P = 0.049) and 60º (ApEn, r = − 0.64, P = 0.035; DFA α , ρ = 0.68, P = 0.015). Conclusion The lack of correlation between $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 and complexity at 30º was likely due to low relative task demands, given the similar kinetics of $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 and torque complexity. An inverse correlation between $${\text{m}\dot{\text{V}}\text{O}}_{{2}}$$ m V ˙ O 2 and knee-extensor torque complexity occurs during high-intensity contractions at intermediate, but not short, muscle lengths.
... The muscle activity has a nonlinear pattern along the range of motion. [1] There are several hypotheses on the reason of variability in the motor system. A prominent theory is the Generalized Motor Program Theory that suggested the error in the prediction of the motor program causes variation in a movement pattern. ...
... [2,5] During the repetitive isokinetic exercise, the output work by limb decline and the fatigue can be created. [6] Whenever the isokinetic exercise is limited in the determined range and velocity, the amount of produced force is always in the chaotic and nonrepetitive pattern [1] and fatigue influences the variability of muscle activity. [7] In the case of motor system control, the force produced with muscles and the pattern is too nonlinear and this is affected by condition [8] and disease. ...
... In addition, the recruitment threshold and synchrony of motor units and firing rate of motor unit also change the force variability. [1] In the current study, whenever the muscles and type of motion used are the same, then the differences seen between groups might be due to the effect of T2DM on muscle morphology and function. On the other hand, the differences between genders could be as a result of different muscle properties. ...
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Background Force variability is related to many kinesiological and neuromuscular properties of the body. This study was conducted to evaluate the effect of type 2 diabetes mellitus (T2DM) and sex on the several fractal and entropy indices of force changing during the repetitive isokinetic exercise of knee flexion-extension. Methods Fifty individuals were allowed to participate in the study, and they consist of 18 patients with short-term T2DM, 12 patients with long-term T2DM, and 20 gender/body mass index/ankle imposed to brachial pressure index and physical activity index-matched healthy control (HC) individuals. Torque of knee flexion-extension was recorded for each cycle of 40 isokinetic repetitions at a velocity of 150°/s. The slope across the peak of torques and nonlinear fractal and entropy features in the time series was calculated. Two-way univariate analysis of variance was used to analyze the effect of the groups and gender on the variables. Results The slope of flexor peak torques was significantly less in the long-term T2DM than the other groups. However, the fractal features such as SD1 and 2 of Poincare plot and fractal dimension katz were significantly decreased in the T2DM groups than the HC and in the women than men. Alpha detrended fluctuation analysis and empirical hurts exponent increased in women of short-term T2DM than men. Conclusion The force variability decreased in the T2DM as compared to HC and in women as compared to men. However, the randomness of force was significantly increased in women of short-term T2DM.
... First, existing studies have not controlled joint position during force control tasks [5]. Changes in joint position enables engagement of non-specific muscle groups, which alters force characteristics and proprioceptive feedback to the motor system, subsequently making it difficult to reliably determine force variability [9][10][11]. Second, available studies have used predefined absolute force targets (e.g., 0.5 N, 1 N, 10 N), which do not account for the innate differences in muscle strength between participants [5,6]. ...
... In particular, people with prodromal HD demonstrated significantly greater torque variability at 10% and 30% MVIC values compared to healthy controls. These findings are relevant as the present study controlled for limb position and interindividual differences in maximal isometric strength of participants, which have been shown to influence torque control [9][10][11]13]. Together, these findings suggest that torque variability may be a useful measure of disease onset, however this needs to be explored further in longitudinal studies. (Spearman rho =0.44; P= 0.049, not significant after correction). ...
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Background: Torque steadiness can be impaired in people with Huntington's disease (HD) and worsen with disease advancement. However, existing studies have several methodological oversights. Studies have used absolute torque targets, which do not account for differences in maximal torque capacity between people. Furthermore, despite its known influence on torque steadiness, previous studies in HD have not controlled for visual feedback. This study evaluated torque variability at relative intensities with and without visual feedback between people with prodromal HD and healthy controls. Methods: Twenty-four people with prodromal HD and twenty-seven age- and sex-matched healthy controls were recruited for this study. Torque variability was evaluated, with and without visual feedback, in the right plantar flexors at 10% and 30% of each participant's maximum voluntary isometric contraction (MVIC). Measures of disease burden included the CAG age product, diagnostic confidence level and Unified Huntington's Disease Rating Scale - Total Motor Score. Results: Significant differences in torque variability were observed, though not in overall MVIC, between people with prodromal HD and healthy controls. Significantly higher torque fluctuations were observed for both groups when visual feedback was removed. No associations were observed between torque variability and disease burden in people with prodromal HD. Torque variability measurements showed higher reliability in healthy controls. Conclusions: People with prodromal HD exhibited greater torque variability than healthy controls. Torque variability worsened for both groups when visual feedback was removed. These findings support further investigation into the utilisation of torque variability measurements as markers of disease progression in people with prodromal HD.
... Most notably, the human-driven legged exoskeleton considered in this paper requires a special type of robot leg that could change its stiffness without affecting the energy stored by the spring [22], [53]. The function of such variable stiffness spring leg is twofold: first, it could enable the human to supply a large amount of energy by circumventing the force-velocity tradeoff [54], [55] and configuration-dependent force limitations of the human limb [56]; and second, it could enable the robot limb to amplify the human leg force and power to support and accelerate a heavy load beyond the capability of the human limb alone. Research on developing spring legs that incorporate these novel features is underway [22]. ...
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Reaching and maintaining high walking speeds is challenging for a human when carrying extra weight, such as walking with a heavy backpack. Robotic limbs can support a heavy backpack when standing still, but accelerating a backpack within a couple of steps to race-walking speeds requires limb force and energy beyond natural human ability. Here, we conceive a human-driven robot exoskeleton that could accelerate a heavy backpack faster and maintain top speeds higher than what the human alone can when not carrying a backpack. The key components of the exoskeleton are the mechanically adaptive but energetically passive spring limbs. We show that by optimally adapting the stiffness of the limbs, the robot can achieve near-horizontal center of mass motion to emulate the load-bearing mechanics of the bicycle. We find that such an exoskeleton could enable the human to accelerate one extra body weight up to top race-walking speeds in ten steps. Our finding predicts that human-driven mechanically adaptive robot exoskeletons could extend human weight-bearing and fast-walking ability without using external energy.
... The contractile characteristics of muscle vary as a function of its length (Gordon, Huxley, & Julian, 1966). At short lengths, the twitch time course shortens (Bigland-Ritchie, Furbush, Gandevia, & Thomas, 1992;Marsh, Sale, McComas, & Quinlan, 1981) and greater motor unit discharge rates are needed to achieve a given relative torque (Christova, Kossev, & Radicheva, 1998;Krishnan, Allen, & Williams, 2011;Ofori, Shim, & Sosnoff, 2018;Shinohara, Yoshitake, Kouzaki, & Fukunaga, 2006;Tax, Denier van der Gon, & Erkelens, 1990;Vander Linden, Kukulka, & Soderberg, 1991). For example, Pasquet, Carpentier, and Duchateau (2005) found that the recruitment and discharge rates of motor units in tibialis anterior (TA) was greater at short compared with long fascicle lengths during submaximal contractions with the dorsiflexor muscles at the same relative and absolute target torques. ...
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The purpose of the study was to assess the influence of short, intermediate, and long muscle lengths on dorsiflexor force steadiness and the discharge characteristics of motor units in tibialis anterior during submaximal isometric contractions. Steady contractions were performed at 5 target forces (5, 10, 20, 40, and 60% maximal voluntary contraction, MVC) for 3 ankle angles (75ο, 90ο, and 105o). MVC force was less (p=0.043) at the smallest joint angle compared with the other two angles. The absolute (standard deviation) and normalized amplitudes (coefficient of variation) of the force fluctuations were similar for all 3 ankle angles at each target force. The coefficient of variation for force decreased progressively from 5% to 20% MVC force and then it plateaued at 40% and 60% MVC force. At all target forces, the mean discharge rate (MDR) of the motor units at 75o was greater than at 90o (p=0.006) and 105o (p=0.034). Moreover, the MDR was similar for 5% and 10% MVC forces and then increased gradually until 60% MVC force (p<0.005). The variability in discharge times (coefficient of variation for interspike interval) and variability in neural drive (coefficient of variation of filtered cumulative spike train) were similar at all ankle angles. Variability in neural drive had a greater influence on force steadiness than did the variability in discharge times. Changes in ankle-joint angle did not influence either the normalized amplitude force fluctuations during steady submaximal contractions or the underlying modulation of the discharge characteristics of motor units in tibialis anterior.
... Alternatively, multiple-angle maximal voluntary isometric contractions allow for reliable and robust length-tension assessments and the quantification of the rate of force development and impulse (269). Additionally, isometric assessments are commonly utilized to examine muscular activation and the variability of maximal and submaximal motor outputs (195,258,332). However, obtaining a full isometric length-tension profile requires an extensive number of contractions (250, 268). ...
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... Data showing the influence of muscle length on force steadiness primarily focuses on the lower limb [18,29,33], while less is known on this topic for the upper limb [4]. The present results are in line with the majority of the published literature on this topic. ...
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PurposeThe purpose of this study was to examine the influence of muscle length and fatigue on maximal force, submaximal force steadiness, and electromyographic (EMG) activity of the biceps brachii.Methods Force and EMG responses were measured before and after a fatigue protocol consisting of maximal intermittent contractions of the elbow flexors until task failure (n = 20). The protocol was performed on two separate occasions in a randomized order. During one visit, the elbow joint was at 90° (EF90) and for the other, it was extended to 120° (EF120).ResultsThe results show a large effect size for greater force loss following fatigue at long muscle length (P = 0.067, $$\eta_{p}^{2}$$ = 0.166). The fatigue-based decreases in force steadiness were not different between muscle lengths (P = 0.502, $$\eta_{p}^{2}$$ = 0.024). Force steadiness was lower at long muscle length before and after fatigue (P < 0.01, d = 0.691). Following fatigue, muscle excitation decreased and increased during maximal and submaximal force tasks, respectively, yet there were no length-dependent EMG responses.Conclusions The novel findings show fatigue at long muscle length likely affects force loss to a greater degree than fatigue-based decreases in force steadiness. These data show lower elbow flexion force steadiness when the biceps brachii is in a lengthened position.
... Alternatively, multiple-angle maximal voluntary isometric contractions allow for reliable and robust length-tension assessments and the quantification of the rate of force development and impulse (Oranchuk et al 2019c). Additionally, isometric assessments are commonly utilized to examine muscular activation and the variability of maximal and submaximal motor outputs (Sosnoff et al 2010, Ofori et al 2018, Lanza et al 2019. However, obtaining a full isometric length-tension profile requires an extensive number of contractions (Noorkoiv et al 2014, Oranchuk et al 2019b. ...
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Objective: Length-tension relationships are widely reported in research, rehabilitation and performance settings; however, several isometric contractions at numerous angles are needed to understand these muscular outputs. Perhaps a more efficient way to determine torque-angle characteristics is via isokinetic dynamometry; however, little is known about the variability of isokinetic measurements besides peak torque and optimal-angle. This paper examines the variability of angle-specific isokinetic torque and impulse measures. Approach: Three sessions of concentric (60º.s-1) knee extensions were performed by both limbs of 32 participants. Assessments were repeated on three occasions, separated by 5-8 days. To quantify variability, the standardized typical error of measurement (TEM) was doubled and thresholds of 0.2-0.6 (small), 0.6-1.2 (moderate), 1.2-2.0 (large), 2.0-4.0 (very large) and >4.0 (extremely large) were applied. Additionally, variability was deemed large when the intraclass correlation coefficient (ICC) was <0.67 and coefficient of variation (CV) >10%; moderate when ICC>0.67 or CV<10% (but not both); and small when both ICC>0.67 and CV<10%. Main Results: Isokinetic torque and angular impulse show small to medium variability (ICC=0.75-0.96, CV=6.4-15.3%, TEM=0.25-0.53) across all but the longest (100°) and shortest (10°) muscle lengths evaluated. However, moderate to large variability was found for the optimal-angle (ICC=0.58-0.64, CV=7.3-8%, TEM=0.76-0.86), and torque and impulse at the beginning and end of the range of motion (ICC=0.57-0.85, CV=11-42.9%, TEM=0.40-0.89). Intersession variability of isokinetic torque and impulse were small to moderate at medium (90-20°) joint angles. Significance: Researchers and practitioners can examine the muscle torque-angle relationship and activity-specific torque outputs within these ranges, without resorting to more strenuous and time-consuming isometric evaluations
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Context In the elderly, weak lower limb muscles impair functional tasks' performance. Objective To evaluate the healthy elderly's ankle dorsiflexion and plantarflexion maximum torque and its variability in two sets of 5 RM isokinetics evaluation. Method 50 women (68.0 ± 4.6 years old) and 50 men (72.7 ± 8.5 years old) did two sets of ankle plantar flexor and dorsiflexor isokinetic tests at 30°/s. Peak torque, total work, and coefficient of variation were analyzed. Results Men did the strongest plantarflexion torque (p < 0.05) and dorsiflexion torque (p < 0.05); their highest peak torque occurred at set 2 (p < 0.05), while the largest plantarflexion torque variability (p < 0.05), dorsiflexion torque variability (p < 0.05), and the largest plantarflexion torque variability occurred at set 1 (p < 0.05). Men did the highest plantarflexion and dorsiflexion total work (p < 0.05) at set 2 (p < 0.05). Conclusion Older men are stronger than older women. The torque variability, in men, was higher during the first set, suggesting an adaptation to the isokinetics evaluation. Clinicians and researchers should consider that different muscles might need different numbers of sets and trials to measure their maximal muscle strength.
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Prevention of weight regain following successful weight loss is a major challenge in the treatment of obesity, irrespective of the weight reduction method used. The majority of individuals regain the lost weight over time; thus, achieving long-term sustainability in weight loss remains an unresolved issue. A compensatory adaptation to the weight loss methods occurs in several body organs and partly explains the lack of sustainable effect. Variability is inherent in many biological systems, and patterns of variability constitute a body mechanism that is active at several levels, starting from the genes and cellular pathways through to the whole-organ level. This study aimed to describe a platform that introduces individually tailored variability in vagal nerve stimulation and dietary regimen to ensure prolonged and sustainable weight loss and prevent weight regain. The platform is intended to provide a method that can overcome the body’s compensatory adaptation mechanisms while ensuring a prolonged beneficial effect.
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The equilibrium control hypothesis (λ model) is considered with special reference to the following concepts: (a) the length-force invariant characteristic (IC) of the muscle together with central and reflex systems subserving its activity; (b) the tonic stretch reflex threshold (λ) as an independent measure of central commands descending to alpha and gamma motoneurons; (c) the equilibrium point, defined in terms of λ, IC and static load characteristics, which is associated with the notion that posture and movement are controlled by a single mechanism; and (d) the muscle activation area (a reformulation of the “size principle”)— the area of kinematic and command variables in which a rank-ordered recruitment of motor units takes place. The model is used for the interpretation of various motor phenomena, particularly electromyographic patterns. The stretch reflex in the λ model has no mechanism to follow-up a certain muscle length prescribed by central commands. Rather, its task is to bring the system to an equilibrium, load-dependent position. Another currently popular version defines the equilibrium point concept in terms of alpha motoneuron activity alone (the α model). Although the model imitates (as does the λ model) spring-like properties of motor performance, it nevertheless is inconsistent with a substantial data base on intact motor control. An analysis of α models, including their treatment of motor performance in deafferented animals, reveals that they suffer from grave shortcomings. It is concluded that parameterization of the stretch reflex is a basis for intact motor control. Muscle deafferentation impairs this graceful mechanism though it does not remove the possibility of movement.
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Criticisms of the equilibrium point (EP) hypothesis have recently appeared that are based on misunderstandings of some of its central notions. Starting from such interpretations of the hypothesis, incorrect predictions are made and tested. When the incorrect predictions prove false, the hypothesis is claimed to be falsified. In particular, the hypothesis has been rejected based on the wrong assumptions that it conflicts with empirically defined joint stiffness values or that it is incompatible with violations of equifinality under certain velocity-dependent perturbations. Typically, such attempts use notions describing the control of movements of artificial systems in place of physiologically relevant ones. While appreciating constructive criticisms of the EP hypothesis, we feel that incorrect interpretations have to be clarified by reiterating what the EP hypothesis does and does not predict. We conclude that the recent claims of falsifying the EP hypothesis and the calls for its replacement by EMG-force control hypothesis are unsubstantiated. The EP hypothesis goes far beyond the EMG-force control view. In particular, the former offers a resolution for the famous posture-movement paradox while the latter fails to resolve it.
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The goal of this study was to improve the ability of a motor unit model to predict experimentally measured force variability across a wide range of forces. Motor unit discharge characteristics were obtained from 38 motor units of the first dorsal interosseus muscle. Motor unit discharges were recorded in separate isometric contractions that ranged from 4 to 85% of the maximal voluntary contraction (MVC) force above recruitment threshold. High-threshold motor units exhibited both greater minimal and peak discharge rates compared with low-threshold units (P < 0.01). Minimal discharge rate increased from 7 to 23 pps, and peak discharge rate increased from 14 to 38 pps with an increase in recruitment threshold. Relative discharge rate variability (CV) decreased exponentially for each motor unit from an average of 30 to 13% as index finger force increased above recruitment threshold. In separate experiments, force variability was assessed at eight force levels from 2 to 95% MVC. The CV for force decreased from 4.9 to 1.4% as force increased from 2 to 15% MVC (P < 0.01) and remained constant at higher forces (1.2-1.9%; P = 0.14). When the motor unit model was revised using these experimental findings, discharge rate variability was the critical factor that resulted in no significant difference between simulated and experimental force variability (P = 0.22) at all force levels. These results support the hypothesis that discharge rate variability is a major determinant of the trends in isometric force variability across the working range of a muscle.
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Force fluctuations during steady contractions of multiple agonist muscles may be influenced by the relative contribution of force by each muscle. The purpose of the study was to compare force fluctuations during steady contractions performed with the plantar flexor muscles in different knee positions. Nine men (25.8+/-5.1 years) performed steady contractions of the plantar flexor muscles in the knee-flexed and knee-extended (greater involvement of the gastrocnemii muscles) positions. The maximal voluntary contraction (MVC) force was 32% greater in the knee-extended position compared with the knee-flexed position. The target forces were 2.5-10% MVC force in the respective position. The amplitude of electromyogram in the medial gastrocnemius muscle was greater in the knee-extended position (10.50+/-9.80%) compared with the knee-flexed position (1.26+/-1.15%, P<0.01). The amplitude of electromyogram in the soleus muscle was not influenced by the knee position. The amplitude of electromyogram in the lateral gastrocnemius and tibialis anterior muscles was marginal and unaltered with knee position. At the same force (in Newtons), the standard deviation of force was lower in the knee-extended position compared with the knee-flexed position. These results indicate that force fluctuations during plantar flexion are attenuated with greater involvement of the medial gastrocnemius muscle.
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Understanding the origin of noise, or variability, in the motor system is an important step towards understanding how accurate movements are performed. Variability of joint torque during voluntary activation is affected by many factors such as the precision of the descending motor commands, the number of muscles that cross the joint, their size and the number of motor units in each. To investigate the relationship between the peripheral factors and motor noise, the maximum voluntary torque produced at a joint and the coefficient of variation of joint torque were recorded from six adult human subjects for four muscle/joint groups in the arm. It was found that the coefficient of variation of torque decreases systematically as the maximum voluntary torque increases. This decreasing coefficient of variation means that a given torque or force can be more accurately generated by a stronger muscle than a weaker muscle. Simulations demonstrated that muscles with different strengths and different numbers of motor units could account for the experimental data. In the simulations, the magnitude of the coefficient of variation of muscle force depended primarily on the number of motor units innervating the muscle, which relates positively to muscle strength. This result can be generalised to the situation where more than one muscle is available to perform a task, and a muscle activation pattern must be selected. The optimal muscle activation pattern required to generate a target torque using a group of muscles, while minimizing the consequences of signal dependent noise, is derived.
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This study was designed to test the hypothesis derived from information theory that increases in the variability of motor responses result from increases in perceptual-motor noise. Young adults maintained isometric force for extended periods at different levels of their maximum voluntary contraction. Force variability (SD) increased exponentially as a function of force level However, the signal-to-noise ratio (M/SD), an index of information transmission, as well as measures of noise in both the time (approximate entropy) and frequency (power spectrum) domains, changed according to an inverted U-shaped function over the range of force levels. These findings indicate that force variability is not directly related to noise but that torce output noisiness is positively correlated with the amount of information transmitted.
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Understanding the origin of noise, or variability, in the motor system is an important step towards understanding how accurate movements are performed. Variability of joint torque during voluntary activation is affected by many factors such as the precision of the descending motor commands, the number of muscles that cross the joint, their size and the number of motor units in each. To investigate the relationship between the peripheral factors and motor noise, the maximum voluntary torque produced at a joint and the coefficient of variation of joint torque were recorded from six adult human subjects for four muscle/joint groups in the arm. It was found that the coefficient of variation of torque decreases systematically as the maximum voluntary torque increases. This decreasing coefficient of variation means that a given torque or force can be more accurately generated by a stronger muscle than a weaker muscle. Simulations demonstrated that muscles with different strengths and different numbers of motor units could account for the experimental data. In the simulations, the magnitude of the coefficient of variation of muscle force depended primarily on the number of motor units innervating the muscle, which relates positively to muscle strength. This result can be generalised to the situation where more than one muscle is available to perform a task, and a muscle activation pattern must be selected. The optimal muscle activation pattern required to generate a target torque using a group of muscles, while minimizing the consequences of signal dependent noise, is derived.
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Human aging is typically accompanied by a loss of muscle mass and an associated decrease in muscle strength. The muscle atrophy is caused by the death of motoneurons in the spinal cord and, preferentially, involves the motoneurons that innervate the histochemically defined type IIb muscle fibers. Surviving motoneurons, however, are able to develop axonal sprouts and reinnervate some of the abandoned muscle fibers. The net result of this reorganization is a reduction in the number of motor units in a muscle but an increase in the innervation ratio of the motoneurons that remain in the muscle. According to the Size Principle, the order in which motor units are recruited progresses from the smallest to the largest motoneuron as the force that a muscle exerts is graded. In this scheme, differences in motoneuron size and those properties that co-vary with size are the major determinants in the spinal-cord control of the muscle force. Therefore, disturbances in the relative size of motor units within a population, such as the reorganization that occurs with aging, may have significant consequences for the ability of humans to perform tasks requiring subtle variations in the force exerted by the muscle. The studies described in this chapter characterize the ability of older adults to perform steady submaximal contractions and determine the role of changes in motor unit size on the observed impairments in performance.
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The purpose of this study was to investigate the mechanisms contributing to the different scaling functions between force and force variability in continuous and discrete isometric forces. Muscle forces were simulated with the Fuglevand et al. (1993) model of motor unit recruitment and rate coding, and a range of recruitment and firing properties were manipulated. The influence of time-to-peak force on the discrete force variability was also examined. The results revealed that the peak firing rate, the synchrony between motoneurons, and the recruitment range contributed to the different variability functions in continuous and discrete forces. The shorter time-to-peak force led to higher variability in the peak force. The findings show that the model can produce the distinct properties of the force variability scaling functions in continuous and discrete forces. The simulation results provide preliminary insight into the neuromuscular mechanisms of the different force variability functions in continuous and discrete isometric forces.
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The purpose of the author's investigation was to examine the effect of knee joint angle on torque control of the quadriceps muscle group. In all, 12 healthy adults produced maximal voluntary contractions and submaximal torque (15, 30, and 45% MVC [maximal voluntary contraction]) at leg flexion angles of 15 degrees , 30 degrees , 60 degrees , and 90 degrees below the horizontal plane. As expected, MVC values changed with respect to joint angle with maximum torque output being greatest at 60 degrees and least at 15 degrees . During the submaximal tasks, participants appropriately scaled their torque output to the required targets. Absolute variability (i.e., standard deviation) of torque output was greatest at 60 degrees and 90 degrees knee flexion. However, relative variability as indexed by coefficient of variation (CV) decreased as joint angle increased, with the greatest CV occurring at 15 degrees . These results are congruent with the hypothesis that joint angle influences the control of torque.
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1. Magnetic stimulation was applied over the motor cortex in forty-five normal human subjects and peristimulus time histograms (PSTHs) of the discharges of single motor units were used to record changes in the firing probability of individual spinal motoneurones of contralateral upper limb muscles. Recordings were obtained from 153 motor units from fourteen upper limb muscles. 2. For the majority of motor units the initial effect was a short latency facilitation. The estimated central conduction velocities and the rise times of the underlying excitatory postsynaptic potentials (EPSPs) were compatible with monosynaptic facilitation by a fast corticospinal pathway. In some motor units the initial effect was a short latency inhibition. Other units showed no statistically significant changes in firing probability. The proportion of the tested motor units in each of these categories depended on the muscle. All of the sampled units of first dorsal interosseous (1DI) showed short latency facilitation, as did the majority of units in the forearm and the biceps brachii. More than half of the sampled motor units of triceps brachii and deltoid showed either no effect or were inhibited. 3. To compare the net short latency actions of the neurones activated by magnetic stimulation on various motoneurone pools, the magnitude of the short latency facilitation or inhibition in a given motor unit was normalized to the magnitude of the short latency facilitation in the 1DI motor unit of the same subject at the same stimulus intensity, and these data were pooled for a number of subjects. 4. 1DI motoneurones received strong net facilitation (estimated mean EPSP amplitude 2.9 +/- 0.2 mV), the motoneurones of forearm muscles and biceps brachii received weaker net facilitation and triceps brachii and deltoid received no net effect. 5. It is concluded that the short latency corticospinal projections to upper limb motoneurones in humans have a distinct pattern which is similar to that in other primates.
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The lengths of the sarcomeres of some muscles of the human leg were determined for the anatomical position, using a method based on diffraction. Measurements were made of the muscle lengths and angles of pennation from cadavers, and these were used to predict sarcomere lengths at other limb positions. The measured and predicted sarcomere lengths were compared with the length-tension curve for human muscle, which showed the range of sarcomere length from both extremes of muscle length to cover the entire range of the length-tension curve.
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1. The variation of isometric tetanus tension with sarcomere length in single fibres from frog striated muscle has been re-investigated with special precautions to ensure uniformity of sarcomere length within the part of the fibre being studied.2. In most respects the results of Ramsey & Street (1940) were confirmed, but (a) the peak of the curve was found to consist of a plateau between sarcomere lengths of 2.05 and 2.2 mu, (b) the decline of tension above this plateau is steeper than found by Ramsey & Street, and (c) the decline of tension below the plateau becomes suddenly steeper at a sarcomere length of about 1.67 mu.3. Many features of this length-tension relation are simply explained on the sliding-filament theory.4. It is concluded that, in the plateau and at greater lengths, the tension on each thin filament is made up of equal contributions from each bridge which it overlaps on adjacent thick filaments.5. Internal resistance to shortening is negligible in this range but becomes progressively more important with shortening below the plateau.
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The relationship between muscle length, integrated electromyographic activity, and torque of the biceps femoris muscle was investigated while the line of action of the muscle at the knee was held constant. Muscle length was changed by varying the hip joint angle. Sixteen subjects produced 1) maximal isometric contractions, 2) contractions with constant submaximal torque, and 3) contractions with constant submaximal muscle activity at four different hip positions (0, 45, 90, and 135 degrees of flexion). Simultaneous readings of hip angle, muscle torque, and raw and integrated electromyographic activity revealed that changes in muscle length influence production of integrated muscle activity and development of torque differently. During maximal isometric contraction, an increase in integrated electromyographic activity and a decrease in torque occurred as the muscle was shortened; the opposite occurred when the muscle was at lengthened positions. A greater difference in this relationship was noted when the respective electromyographic activity and torque were held constant. Some clinical questions were raised.
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The purpose of this study was to compare the steadiness and discharge rate of motor units during submaximal contractions performed by young and old adults. Subjects performed isometric and slow shortening and lengthening contractions with the first dorsal interosseous muscle. The steadiness of the isometric and slow anisometric contractions was less for the old subjects compared with young subjects, especially at the lower target forces and with the lightest loads. Furthermore, the steadiness of the lengthening contractions was less compared with the shortening contractions for the old subjects. Although the mean discharge rates of motor units were not different for the two groups of subjects, the variability of the discharge rates was greater for the old subjects during the isometric and anisometric contractions. We conclude that a more variable discharge by single motor units probably contributes to the reduced ability of old adults to perform steady muscle contractions.
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The purpose of this study was to investigate the influence of different angles of the knee joint on voluntary activation of the quadriceps muscle, estimating the ability of a subject to activate a muscle maximally by means of voluntary contraction. Isometric torque measurement was performed on 6 healthy subjects in 5 degrees intervals between 30 degrees and 90 degrees of knee joint flexion. Superimposed twitches at maximal voluntary contraction (MVC) and at a level of 60% and 40% of the MVC were applied and the voluntary activation estimated. At between 30 degrees and 75 degrees of knee flexion, the maximal extension torque increased at an average rate of 2.67 +/- 0.6 Nm/degree, followed by a decline with further flexion. However, throughout the joint-angle range tested, voluntary activation increased on average by 0.37%/degree with a maximum at 90 degrees of flexion. Due to the influence of joint position it is not possible to generalize results obtained at the knee joint angle of 90 degrees of flexion, which is usually used for the quadriceps twitch-interpolation technique. Consequently, it is useful to investigate voluntary activation deficits in knee joint disorders at a range of knee joint angles that includes, in particular, the more extended joint angles used frequently during daily activity.
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The authors modeled variability of force during continuous isometric contractions of the quadriceps femoris. Twenty adults (aged 25 +/- 6 years old) performed isometric leg extensions. Target forces were 11 percentages of maximum voluntary contraction (%MVC), ranging from 2 to 95%, and 5 absolute levels, from 25 to 225 N. The authors used standard deviation of absolute force, coefficient of variation, and signal-to-noise ratio as measures of variability. The results suggested a nonlinear relationship between variability and level of force, which could best be expressed as %MVC and not as absolute force. Variability for continuous isometric contractions was described best by a sigmoidal logistic function. The sigmoidal pattern of variability as a function of %MVC is consistent with physiological mechanisms.
Article
We investigated force enhancement following stretching in the in situ cat soleus muscle on the ascending and descending limb of the force-length relationship by varying the amount and speed of stretching and the frequency of activation (5 Hz, 30 Hz). There was a small but consistent (P<0.05) amount of force enhancement following muscle stretching on the ascending limb of the force-length relationship for both stimulation frequencies. The steady-state active isometric forces following stretches of 9 mm on the descending limb of the force-length relationship were always equal to or greater than the corresponding forces from the purely isometric contractions at the length at which the stretch was started. Therefore, force production for these trials showed positive stiffness and was associated with stable behavior. Following active stretching of cat soleus on the descending limb of the force-length relationship, the passive forces at the end of the test were significantly greater than the corresponding passive forces for purely isometric contractions, or the passive forces following stretching of the passive muscle. This passive force enhancement following active stretching increased with increasing magnitude of stretch, was not associated with structural damage, and only disappeared once the muscle was shortened. For stretches of 6 mm and 9 mm, the passive force enhancement accounted for more than 50 % of the total force enhancement, reaching a peak contribution of 83.7 % for the stretches of 9 mm at a speed of 3 mm s(-1). The results of this study suggest that a passive structural element provides a great part of the force enhancement on the descending limb of the force-length relationship of the cat soleus. Furthermore, the results indicate that mechanisms other than sarcomere length non-uniformity alone are operative.
Article
The purpose of the current experiment was to investigate the amount (standard deviation (S.D.) and coefficient of variation (CV)) and structure (approximate entropy (ApEn)) of force variability at very low force levels. Participants produced isometric force output of index finger abduction at five levels (0.4, 0.8, 1.0, 2.0, and 4.0 N) with high and low visual feedback gain. The findings showed that: subjects scaled their force output to the targets; S.D. increased non-linearly with force level and decreased with visual gain; and CV decreased with force level as well as visual gain. ApEn of the force output did not change as a function of force level, although the high gain increased ApEn in contrast to low gain. It is proposed that the recruitment of additional motor units at very low force levels does not significantly alter the structure of the force output, although it does increase the magnitude of force and its amount of variability. Overall, the findings provide evidence that the amount and structure of motor variability can be influenced by separate control processes at low force levels.
Article
This study examines the effect of a change in fascicle length on motor unit recruitment and discharge rate in the human tibialis anterior during shortening and lengthening contractions that involved a similar change in torque. The dorsiflexor torque and the surface and intramuscular electromyograms (EMGs) from the tibialis anterior were recorded in eight subjects. The behaviour of the same motor unit (n=63) was compared during submaximal shortening and lengthening contractions performed at a constant velocity (10 deg s-1) with the dorsiflexor muscles over a 20 deg range of motion around the ankle neutral position. Muscle fascicle length was measured non-invasively using ultrasonography. Motor units that were active during a shortening contraction were always active during the subsequent lengthening contraction. Furthermore, additional motor units (n=18) of higher force threshold that were recruited during the shortening contraction to maintain the required torque were derecruited first during the following lengthening contraction. Although the change in fascicle length was linear (r2>0.99), and similar for both shortening and lengthening contractions, modulation of discharge rate differed during the two contractions. Compared with an initial isometric contraction at short (11.9+/-2.4 Hz) or long (11.7+/-2.2 Hz) muscle length, discharge rate increased only slightly and stayed nearly constant throughout the lengthening contraction (12.6+/-2.0 Hz; P<0.05) whereas it augmented progressively and more substantially during the shortening contraction, reaching 14.5+/-2.5 Hz (P<0.001) at the end of the movement. In conclusion, these observations indicate a clear difference in motor unit discharge rate modulation with no change in their recruitment order between shortening and lengthening contractions when performed with a similar change in muscle fascicle length and torque.
Article
Neural mechanisms contribute significantly to the gains that occur in the range of motion about a joint with stretching exercises. In the acute condition, lengthening of a muscle-tendon unit decreases spinal reflex excitability, which reduces passive tension and increases joint range of motion. Similarly, participation in a stretch-training program decreases tonic reflex activity and increases flexibility.
Article
This experiment examined the magnitude and structure of force variability in isometric index finger force production tasks at 5, 15, 25, 35, 45, 55, 65, 75, 85, and 95% of maximal force in two different finger orientations. In the finger flexion task, the participants generated a downward isometric force through index finger flexion. In the finger abduction task, isometric force was generated by adducting the index finger (mediolateral motion of the middle finger and forearm were restricted). The task-related, normal force (Fz) and tangential forces (Fx and Fy) were collected with a three-dimensional force transducer. The standard deviation (SD) of the task-related force output (Fz) increased exponentially with force level. With increasing force level, approximate entropy (ApEn, a measure of irregularity) of Fz followed an inverted-U function for finger flexion, but decreased linearly in finger abduction. However, changes in the ApEn of the tangential forces were generally opposite to that of Fz, revealing compensations in the irregularity of force output between force dimensions. The findings provide evidence that force variability is related to muscle force-length characteristics (Feldman, 1966; Gottlieb %%% Agarwal, 1988).
Neural aspects of muscle stretching. [Review]
• Guissard
Signal-dependent noise determines motor planning
• Harris
Steadiness is reduced and motor unit discharge is more variable in old adults
• Laidlaw