Is coordination of two-joint leg muscles during load lifting consistent with the strategy of minimum fatigue?

Control of Mammalian Locomotion by Somatosensory Feedback

When animals walk overground, mechanical stimuli activate various receptors located in muscles, joints, and skin. Afferents from these mechanoreceptors project to neuronal networks controlling locomotion in the spinal cord and brain. The dynamic interactions between the control systems at different levels of the neuraxis ensure that locomotion adjusts to its environment and meets task demands. In this article, we describe and discuss the essential contribution of somatosensory feedback to locomotion. We start with a discussion of how biomechanical properties of the body affect somatosensory feedback. We follow with the different types of mechanoreceptors and somatosensory afferents and their activity during locomotion. We then describe central projections to locomotor networks and the modulation of somatosensory feedback during locomotion and its mechanisms. We then discuss experimental approaches and animal models used to investigate the control of locomotion by somatosensory feedback before providing an overview of the different functional roles of somatosensory feedback for locomotion. Lastly, we briefly describe the role of somatosensory feedback in the recovery of locomotion after neurological injury. We highlight the fact that somatosensory feedback is an essential component of a highly integrated system for locomotor control. © 2021 American Physiological Society. Compr Physiol 11:1-71, 2021.

Biarticular muscles in light of template models, experiments and robotics: a review

Leg morphology is an important outcome of evolution. A remarkable morphological leg feature is the existence of biarticular muscles that span adjacent joints. Diverse studies from different fields of research suggest a less coherent understanding of the muscles' functionality in cyclic, sagittal plane locomotion. We structured this review of biarticular muscle function by reflecting biomechanical template models, human experiments and robotic system designs. Within these approaches, we surveyed the contribution of biarticular muscles to the locomotor subfunctions (stance, balance and swing). While mono- and biarticular muscles do not show physiological differences, the reviewed studies provide evidence for complementary and locomotor subfunction-specific contributions of mono- and biarticular muscles. In stance, biarticular muscles coordinate joint movements, improve economy (e.g. by transferring energy) and secure the zig-zag configuration of the leg against joint overextension. These commonly known functions are extended by an explicit role of biarticular muscles in controlling the angular momentum for balance and swing. Human-like leg arrangement and intrinsic (compliant) properties of biarticular structures improve the controllability and energy efficiency of legged robots and assistive devices. Future interdisciplinary research on biarticular muscles should address their role for sensing and control as well as non-cyclic and/or non-sagittal motions, and non-static moment arms.

Biarticular muscles are most responsive to upper-body pitch perturbations in human standing

Balancing the upper body is pivotal for upright and efficient gait. While models have identified potentially useful characteristics of biarticular thigh muscles for postural control of the upper body, experimental evidence for their specific role is lacking. Based on theoretical findings, we hypothesised that biarticular muscle activity would increase strongly in response to upper-body perturbations. To test this hypothesis, we used a novel Angular Momentum Perturbator (AMP) that, in contrast to existing methods, perturbs the upper-body posture with only minimal effect on Centre of Mass (CoM) excursions. The impulse-like AMP torques applied to the trunk of subjects resulted in upper-body pitch deflections of up to 17° with only small CoM excursions below 2 cm. Biarticular thigh muscles (biceps femoris long head and rectus femoris) showed the strongest increase in muscular activity (mid- and long-latency reflexes, starting 100 ms after perturbation onset) of all eight measured leg muscles which highlights the importance of biarticular muscles for restoring upper-body balance. These insights could be used for improving technological aids like rehabilitation or assistive devices, and the effectiveness of physical training for fall prevention e.g. for elderly people.

Neuromuscular adaptations in shoulder function and dysfunction

Neuromuscular activity, organized in coordinated patterns, forms the basis of task-specific function in sports and exercise. The content and extent of these patterns may be variable, but include elements of activation/inhibition, co-activation, concentric/eccentric activation, proximal-to-distal activation, plyometric activation, and preactivation stiffness. They may be based on inherent neuromuscular architecture, but are commonly affected by positive or negative adaptations to imposed functional demands. Positive neuromuscular adaptations improve the efficiency of performing the task, which can result in less energy expenditure, maximum force delivered to the task, and protection of involved joints from excessive loads/motions, and improve the effectiveness of task performance. They frequently result from specific training in task mechanics and optimal conditioning of the neuromuscular structures involved in the task. Negative neuromuscular maladaptations can affect the efficiency of performing the task, increase energy expenditure and loads, decrease the effectiveness of task performance, and can be associated with clinical presentation of injury symptoms. They can result from overload, injury, and/or limited recovery. This chapter will focus specifically on shoulder joint function to provide examples of positive adaptations and negative maladaptations. It will then provide guidelines for clinical evaluation, treatment of clinical injury, and training/conditioning, based on understanding the neuromuscular activation.

Biarticular elements as a contributor to energy efficiency: biomechanical review and application in bio-inspired robotics

Despite the increased interest in exoskeleton research in the last decades, not much progress has been made on the successful reduction of user effort. In humans, biarticular elements have been identified as one of the reasons for the energy economy of locomotion. This document gives an extensive literature overview concerning the function of biarticular muscles in human beings. The exact role of these muscles in the efficiency of human locomotion is reduced to three elementary functions: energy transfer towards distal joints, efficient control of output force direction and double joint actuation. This information is used to give an insight in the application of biarticular elements in bio-inspired robotics, i.e. bipedal robots, exoskeletons, robotic manipulators and prostheses. Additionally, an attempt is made to find an answer on the question whether the biarticular property leads to a unique contribution to energy efficiency of locomotion, unachievable by mono-articular alternatives. This knowledge is then further utilised to indicate how biarticular actuation of exoskeletons can contribute to an increased performance in reducing user effort.

Task-dependent inhibition of slow-twitch soleus and excitation of fast-twitch gastrocnemius do not require high movement speed and velocity-dependent sensory feedback

Although individual heads of triceps surae, soleus (SO) and medial gastrocnemius (MG) muscles, are often considered close functional synergists, previous studies have shown distinct activity patterns between them in some motor behaviors. The goal of this study was to test two hypotheses explaining inhibition of slow SO with respect to fast MG: (1) inhibition occurs at high movement velocities and mediated by velocity-dependent sensory feedback and (2) inhibition depends on the ankle-knee joint moment combination and does not require high movement velocities. The hypotheses were tested by comparing the SO EMG/MG EMG ratio during fast and slow motor behaviors (cat paw shake responses vs. back, straight leg load lifting in humans), which had the same ankle extension-knee flexion moment combination; and during fast and slow behaviors with the ankle extension-knee extension moment combination (human vertical jumping and stance phase of walking in cats and leg load lifting in humans). In addition, SO EMG/MG EMG ratio was determined during cat paw shake responses and walking before and after removal of stretch velocity-dependent sensory feedback by self-reinnervating SO and/or gastrocnemius. We found the ratio SO EMG/MG EMG below 1 (p < 0.05) during fast paw shake responses and slow back load lifting, requiring the ankle extension-knee flexion moment combination; whereas the ratio SO EMG/MG EMG was above 1 (p < 0.05) during fast vertical jumping and slow tasks of walking and leg load lifting, requiring ankle extension-knee extension moments. Removal of velocity-dependent sensory feedback did not affect the SO EMG/MG EMG ratio in cats. We concluded that the relative inhibition of SO does not require high muscle velocities, depends on ankle-knee moment combinations, and is mechanically advantageous for allowing a greater MG contribution to ankle extension and knee flexion moments.

Analytical and numerical analysis of inverse optimization problems: conditions of uniqueness and computational methods

One of the key problems of motor control is the redundancy problem, in particular how the central nervous system (CNS) chooses an action out of infinitely many possible. A promising way to address this question is to assume that the choice is made based on optimization of a certain cost function. A number of cost functions have been proposed in the literature to explain performance in different motor tasks: from force sharing in grasping to path planning in walking. However, the problem of uniqueness of the cost function(s) was not addressed until recently. In this article, we analyze two methods of finding additive cost functions in inverse optimization problems with linear constraints, so-called linear-additive inverse optimization problems. These methods are based on the Uniqueness Theorem for inverse optimization problems that we proved recently (Terekhov et al., J Math Biol 61(3):423-453, 2010). Using synthetic data, we show that both methods allow for determining the cost function. We analyze the influence of noise on the both methods. Finally, we show how a violation of the conditions of the Uniqueness Theorem may lead to incorrect solutions of the inverse optimization problem.

An optimization approach to inverse dynamics provides insight as to the function of the biarticular muscles during vertical jumping

Traditional inverse dynamics approaches to calculating the inter-segmental moments are limited in their ability to accurately reflect the function of the biarticular muscles. In particular they are based on the assumption that the net inter-segmental moment is zero and that total joint moments are independent of muscular activity. Traditional approaches to calculating muscular forces from the inter-segmental moments are based on a consideration of joint moments which do not encapsulate the potential moment asymmetry between segments. In addition, traditional approaches may artificially constrain the activity of the biarticular muscles. In this study, an optimization approach to the simultaneous inverse determination of inter-segmental moments and muscle forces (the 1-step method) based on a consideration of segmental rotations was employed to study vertical jumping and contrasted with the more traditional 2-step approach of determining inter-segmental moments from an inverse dynamics analysis then muscle forces using optimization techniques. The 1-step method resulted in significantly greater activation of both the monoarticular and biarticular musculature which was then translated into significantly greater joint contact forces, muscle powers, and inter-segmental moments. The results of this study suggest that traditional conceptions of inter-segmental moments do not completely encapsulate the function of the biarticular muscles and that joint function can be better understood by recognizing the asymmetry in inter-segmental moments.

Sensitivity of kinematics-based model predictions to optimization criteria in static lifting tasks

The effect of eight different cost functions on trunk muscle forces, spinal loads and stability was investigated. Kinematics-based approach combined with nonlinear finite element modeling and optimization were used to model in vivo measurements on isometric forward flexions at approximately 40 degrees and approximately 65 degrees in sagittal plane with or without a load of 180N in hands. Four nonlinear (summation stress(3), summation stress(2), summation force(2) and muscle fatigue) and four linear (summation stress, summation force, axial compression and double-linear) criteria were considered. Predicted muscle activities were compared with measured EMG data. All predictions, irrespective of the cost function used, satisfied required kinetic, kinematics and stability conditions all along the spine. Four criteria (summation stress(3), summation stress(2), fatigue and double-linear) predicted muscle activities that qualitatively matched measured EMG data. The fatigue and double-linear criteria were inadequate in predicting greater forces in larger muscles with no consideration for their moment arms. Nearly the same stability margin was computed under these four cost functions. At the lower lumbar levels, the compression forces differed by <20% and the shear forces by <14% as various cost functions were considered. Smaller axial compression and anterior shear forces (by less than or approximately equal 6%) were computed when only the active components rather than the total muscle forces were taken as unknown in the summation stress(3) cost function. Overall, one single cost function of summation stress(2) or summation stress(3) rather than a multi-criteria one was found sufficient and adequate in yielding plausible results comparable with measured EMG activities and disc pressure.

Principal component analysis of lifting waveforms

BACKGROUND

One limiting factor in lifting research design has been the inability to effectively analyze waveform data, especially when differences in body mass, height, and load magnitude influence the derived kinetic variables. The purpose of this study was to demonstrate the sensitivity of principal component analysis to quantify clinically relevant differences in kinetic lifting waveforms over three load magnitudes and between two separate populations.

METHODS

Principal component analysis was applied to five kinetic lifting waveforms. The derived principal component scores were used as the dependent measures in a two-way (clinical status x load magnitude) MANOVA.

FINDINGS

Significant low back pain group differences (P<0.05) were found for three of the principal component scores on extension moment generation in the sacral and thoracic regions and for trunk compression. Significant differences were found for each variable with respect to the magnitude across the entire lift time between the three load conditions, as well as four significant differences related to inferred mechanical changes that resulted from lifting increasingly heavier loads.

INTERPRETATION

Principal component analysis of kinetic lifting waveforms was shown to be insensitive to a confounding factor of different load magnitudes when attempting to identify previously determined clinically relevant differences in the waveform trajectories. The analysis was able to partition the variability attributed to the direct influence of different external load magnitudes, versus those differences in spinal loading that arose from the variations in the lifting mechanics of increasing loads. The technique could be beneficial for other kinetic analyses where confounding magnitude modifiers like body size are present.

Sex differences in the rate of fatigue development and recovery

Background

Many musculoskeltal injuries in the workplace have been attributed to the repetitive loading of muscle and soft tissues. It is not disputed that muscular fatigue is a risk factor for musculoskeltal injury, however the disparity between gender with respect to muscular fatigability and rate of recovery is not well understood. Current health and safety guidelines do not account for sex differences in fatiguability and may be predisposing one gender to greater risk. The purpose of this study was to quantify the sex differences in fatigue development and recovery rate of lower and upper body musculature after repeated bouts of sustained isometric contractions.

Methods

Twenty-seven healthy males (n = 12) and females (n = 15) underwent bilateral localized fatigue of either the knee extensors (male: n = 8; female: n = 8), elbow flexors (male: n = 8; female: n = 10), or both muscle groups. The fatigue protocol consisted of ten 30-second sub-maximal isometric contractions. The changes in maximum voluntary contraction (MVC), electrically evoked twitches, and motor unit activation (MUA) were assessed along with the ability to control the sustained contractions (SLP) during the fatigue protocol using a mixed four-factor repeated measures ANOVA (gender × side × muscle × time) design with significance set at p < 0.05.

Results

There was a significant loss of MVC, MUA, and evoked twitch amplitude from pre- to post-fatigue in both the arms and legs. Males had greater relative loss of isometric force, a higher rate of fatigue development, and were less capable of maintaining the fatiguing contractions in the legs when compared to the females.

Conclusion

The nature of the induced fatigue was a combination of central and peripheral fatigue that did not fully recover over a 45-minute period. The results appear to reflect sex differences that are peripheral, and partially support the muscle mass hypothesis for explaining differences in muscular fatigue.

Control of ground reaction forces by hindlimb muscles during cat locomotion

It has been proposed that biarticular muscles are primarily responsible for the control of the direction of external forces, as their activation is closely related and highly sensitive to the direction of external forces. This functional role for biarticular muscles has been supported qualitatively by experimental evidence, but has never been tested quantitatively for lack of a mathematical/mechanical formulation of this theory and the difficulty of measuring individual muscle forces during voluntary movements. The purposes of this study were: (1) to define rules for muscular coordination based on the control of external forces; (2) to develop a model of the cat hindlimb that allows for the calculation of the magnitude and direction of the ground reaction forces (GRFs) produced by individual hindlimb muscles; and (3) to test if the coordination of mono- and biarticular cat hindlimb muscles is related to the control of the resultant GRF. We measured the GRF, hindlimb kinematics, selected muscle forces and activations during cat locomotion. Then, the measured muscle forces were used as input to the hindlimb model to compute the muscle-induced GRF. We assume that if activation (and possibly force) increased as the muscle-induced component of GRF approximated the resultant GRF, then that muscle was used by the central nervous system (CNS) to help control the direction of the external GRF. During cat walking, medial gastrocnemius (MG) and plantaris (PL) forces increased with increasing proximity to the GRF, while soleus (SOL) forces and vastus lateralis (VL) activations did not. SOL and VL activation were most strongly related to the vertical and parallel (braking/accelerating) component of the GRF, respectively. We concluded from these results that MG and PL are primarily responsible for the control of the direction of the GRF, while SOL primarily functions as an anti-gravity muscle, and VL as an acceleration/deceleration muscle.

Multi-functionality of the cat medical gastrocnemius during locomotion

The functional role of biarticular muscles was investigated based on direct force measurement in the cat medial gastrocnemius (MG) and analysis of hindlimb kinematics and kinetics for the stance phase of level, uphill, and downhill walking. Four primary functional roles of biarticular muscles have been proposed in the past. These functional roles have typically been discussed independently of each other, and biarticular muscles have rarely been assigned more than one functional roles for different phases of the work cycle. The purpose of this study was to elucidate the functional role of the biarticular cat MG during locomotion. It was found that MG forces were primarily associated with the moment requirements at the ankle for most of the stance phase, but also helped to satisfy the moments at the knee in the initial phase of stance. In the second half of stance, MG transferred mechanical energy from the knee to the ankle from the knee to the ankle, while simultaneously producing a substantial amount of mechanical work. Based on these results, we hypothesize that MG's primary function is that of an ankle extensor. However, because of the coupling of the ankle extensor moment with a knee flexor moment in the initial, and a knee extensor moment in the final phase of stance, MG satisfies two joint moments in early stance, and transfers mechanical energy from the knee to the ankle in late stance. We conclude that cat MG has multiple functional roles during the stance phase of locomotion, and speculate that such multi-functionality also exists in other bi- and multi-articular muscles.

Static single-arm force generation with kinematic constraints

Smooth, frictionless, kinematic constraints on the motion of a grasped object reduce the motion freedoms at the hand, but add force freedoms, that is, force directions that do not affect the motion of the object. We are studying how subjects make use of these force freedoms in static and dynamic manipulation tasks. In this study, subjects were asked to use their right hand to hold stationary a manipulandum being pulled with constant force along a low-friction linear rail. To accomplish this task, subjects had to apply an equal and opposite force along the rail, but subjects were free to apply a force against the constraint, orthogonal to the pulling force. Although constraint forces increase the magnitude of the total force vector at the hand and have no effect on the task, we found that subjects applied significant constraint forces in a consistent manner dependent on the arm and constraint configurations. We show that these results can be interpreted in terms of an objective function describing how subjects choose a particular hand force from an infinite set of hand forces that accomplish the task. Without assuming any particular form for the objective function, the data show that its level sets are convex and scale invariant (i.e., the level set shapes are independent of the hand-force magnitude). We derive the level sets, or "isocost" contours, of subjects' objective functions directly from the experimental data.

Electromechanical delay estimated by using electromyography during cycling at different pedaling frequencies

The purpose of this study was to propose a new method that can be used to calculate electromechanical delay (EMD) without the measurement of forces. A secondary purpose, as an example of the importance of measuring EMD, was to predict muscle force development events based on the EMG activity of selected muscles during cycling at different pedaling frequencies. EMD was estimated using newly derived equations based on activation dynamics hypothesis. Tibialis anterior (TA) and soleus (SL) muscles of 16 male participants were studied while subjects pedaled at targeted cadences of 60, 80, and 100 revolutions per minute. The estimated EMDs of TA and SL were significantly different from each other with means of 68.1 and 88.7 ms, respectively. The average crank angle for the initiation and time to peak TA contraction was estimated at 75 +/- 35 degrees and 26 +/- 15 degrees before the crank reached top-dead-center (TDC), while the contraction ended at 31 +/- 19 degrees after the TDC on average. The projected starting, peak and end angles of SL contraction activity were 45 +/- 18 degrees , 123 +/- 13 degrees, and 218 +/- 35 degrees after the TDC, respectively. There was no difference among different pedaling cadences observed for these mechanical events. The proposed method was proven to be effective in studying EMD and estimate muscle contraction patterns during cycling.

Influence of knee angle and individual flexibility on the flexion-relaxation response of the low back musculature

In many occupational settings (e.g. agriculture and construction) workers are asked to maintain static flexed postures of the low back for extended periods of time. Recent research indicates that the resulting strain in the viscoelastic, ligamentous tissues may have a deleterious effect on the stability of the spine and the normal reflex response of spinal tissues. The purpose of this study was to evaluate the previously described flexion-relaxation response in terms of the interactive effect of trunk flexion angle (30 degrees, 50 degrees, 70 degrees, 90 degrees ), knee flexion angle (0 degrees (straight knees), 20 degrees, 40 degrees ) and individual flexibiliteky (low, medium, and high). These conditions were tested under two levels of loading: no load (just supporting the weight of the torso) and trunk extension moment equal to 50% of the subject's posture-specific maximum voluntary trunk extension capacity. Surface electromyographic (EMG) data were collected from the multifidus, the longissimus, the iliocostalis, the vastus medialis, the rectus femoris, the vastus lateralis, the biceps femoris, and the gastrocnemius-soleus group from a sample of eight male participants as they performed isometric weight holding tasks in the postures defined by the combinations of trunk angle and knee angle. The results of this study showed that knee angle did have a significant effect on the lumbar extensor muscle activity but only consistently at the 90 degrees trunk angle. Participant flexibility showed a consistent trend of decreasing lumbar extensor muscle activity with decreased flexibility across all trunk angle values. Most interesting was the interactive response of flexibility and knee angle, wherein the flexibility of the participant influenced the trunk angles at which the knee flexion angle affected the flexion-relaxation response. Highly flexible subjects showed an effect of knee angle on the flexion-relaxation response only at the 90 degrees trunk angle; subjects in the medium flexibility category showed a similar response in both the 70 degrees and 90 degrees trunk angles; subject in the low flexibility group showed no knee angle effect on the flexion-relaxation response. Overall the results confirm previous results with regard to the contribution of the passive tissues to the overall trunk extension moment but also show that the tension in the bi-articular biceps femoris, which was influenced by knee flexion angle and flexibility, affects the ratio of active extensor moment contributions of the lumbar extensor musculature to passive extensor moment contributions from the muscular and ligamentous tissues. The results of this study provide empirical data describing this complicated, interactive response.

Neuromuscular objectives of the human masticatory apparatus during static biting

OBJECTIVE

The central nervous system controls the muscles of mastication and may dictate muscle outputs according to a biologically important objective. This study tested the hypotheses that (a) the effective sagittal TMJ eminence morphology, and (b) the outputs of the masticatory muscles during static biting, are consistent with minimisation of joint loads or minimisation of muscle effort.

DESIGN

Numerical modelling predicted effective eminence morphology (from sagittal plane directions of TMJ force for centred loading over a range from molar to incisor biting) and TMJ and muscle forces during static unilateral biting in seven subjects. In vivo effective eminence morphology was measured from jaw tracking recorded from each subject. Muscle activities during biting tasks on first molar and incisor teeth were measured by electromyography using surface or indwelling electrodes.

RESULTS

Subject-specific predicted effective eminence morphology correlated with in vivo data (0.85< or =R2< or =0.99). Mixed and random coefficient analysis of covariance indicated good agreement between predicted and measured muscle outputs for all muscles of mastication investigated. Individual linear regression analysis showed that modelled muscle outputs accurately predicted EMG data, with average errors of 8% for molar and 15% for incisor biting.

CONCLUSIONS

Effective sagittal eminence morphology was consistent with minimisation of joint loads for all subjects. Masticatory muscle outputs during unilateral biting were consistent with minimisation of joint loads or minimisation of muscle effort, or both, depending on the subject. These results are believed to be the first to test model predictions of muscle output during biting for all muscles of mastication.

Muscle and temporomandibular joint forces associated with chincup loading predicted by numerical modeling

Development of the components of the temporomandibular joint (TMJ) is thought to reflect joint loading. The aims of this project were to test 3 hypotheses: whether effective eminence morphology, masticatory muscle forces, and predicted TMJ forces during chincup loading of the mandible were consistent with the objectives of minimization of joint loads (MJL) or muscle effort (MME), or both. Regression relationships of MJL model-predicted versus measured eminence shapes in 9 subjects indicated a high degree of correlation (mean slope = 0.99, compared with perfect-match slope = 1.00). Model predictions of muscle output during chincup loading of the mandible were tested by comparison with data gathered in 6 subjects. Midsagittal plane chin loads were applied over a range of 60 degrees while bilateral masticatory muscle surface electromyography was quantified. The regression relationships of predicted versus measured masseter and anterior digastric muscle outputs indicated that model predictions were highly correlated (mean slope (masseter muscle) = 1.02; mean slope (digastric muscle) = 0.96). TMJ forces predicted by modeling showed intersubject differences of up to 34% for similar chincup loading conditions. Intrasubject variation in TMJ forces was as high as 57%, depending on chin load angle. The results demonstrated that TMJ eminence shape and masticatory muscle forces were consistent with objectives of both MJL and MME. Variation in TMJ forces depended on the subject and the direction of chincup loading.

Hierarchical genetic algorithm versus static optimization-investigation of elbow flexion and extension movements

The applicability of static optimization (and, respectively, frequently used objective functions) for prediction of individual muscle forces for dynamic conditions has often been discussed. Some of the problems are whether time-independent objective functions are suitable, and how to incorporate muscle physiology in models. The present paper deals with a twofold task: (1) implementation of hierarchical genetic algorithm (HGA) based on the properties of the motor units (MUs) twitches, and using multi-objective, time-dependent optimization functions; and (2) comparison of the results of the HGA application with those obtained through static optimization with a criterion "minimum of a weighted sum of the muscle forces raised to the power of n". HGA and its software implementation are presented. The moments of neural stimulation of all MUs are design variables coding the problem in the terms of HGA. The main idea is in using genetic operations to find these moments, so that the sum of MUs twitches satisfies the imposed goals (required joint moments, minimal sum of muscle forces, etc.). Elbow flexion and extension movements with different velocities are considered as proper illustration. It is supposed that they are performed by two extensor muscles and three flexor muscles. The results show that HGA is a suitable means for precise investigation of motor control. Many experimentally observed phenomena (such as antagonistic co-contraction, three-phasic behavior of the muscles during fast movements) can find their explanation by the properties of the MUs twitches. Static optimization is also able to predict three-phasic behavior and could be used as practicable and computationally inexpensive method for total estimation of the muscle forces.

Changes in the surface EMG signal and the biomechanics of motion during a repetitive lifting task

The analysis of surface electromyographic (EMG) data recorded from the muscles of the back during isometric constant-force contractions has been a useful tool for assessing muscle deficits in patients with lower back pain (LBP). Until recently, extending the technique to dynamic tasks, such as lifting, has not been possible due to the nonstationarity of the EMG signals. Recent developments in time-frequency analysis procedures to compute the instantaneous median frequency (IMDF) were utilized in this study to overcome these limitations. Healthy control subjects with no history of LBP (n = 9; mean age 26.3 +/- 6.7) were instrumented for acquisition of surface EMG data from six electrodes on the thoraco-lumbar region and whole-body kinematic data from a stereo-photogrammetric system. Data were recorded during a standardized repetitive lifting task (load = 15% body mass; 12 lifts/min; 5-min duration). The task resulted in significant decreases in IMDF for six of the nine subjects, with a symmetrical pattern of fatigue among contralateral muscles and greater decrements in the lower lumbar region. For those subjects with a significant decrease in IMDF, a lower limb and/or upper limb biomechanical adaptation to fatigue was observed during the task. Increases in the peak box acceleration were documented. In two subjects, the acceleration doubled its value from the beginning to the end of the exercise, which lead to a significant increase in the torque at L4/L5. This observation suggests an association between muscle fatigue at the lumbar region and the way the subject manipulates the box during the exercise. Fatigue-related biomechanical adaptations are discussed as a possible supplement to functional capacity assessments among patients with LBP.

Validated numerical modeling of the effects of combined orthodontic and orthognathic surgical treatment on TMJ loads and muscle forces

Investigations of the changes in the mechanics of the craniomandibular system as a result of treatment have been limited by the lack of validated models of this system. The aims of this project were to (1) validate numerical model predictions of temporomandibular joint (TMJ) eminence morphology and muscle forces produced during molar biting and (2) use the validated models to calculate the changes in TMJ and muscle forces as a consequence of treatment involving orthognathic surgery. Ten volunteers participated; their combined orthodontic and orthognathic surgical treatments were completed. Three-dimensional anatomical data from each subject were used in computer models to predict the sagittal TMJ eminence morphology and joint and muscle forces for each subject, consistent with the neuromuscular objectives of minimizing joint loads and muscle effort. The actual shape of the eminence in each subject was measured with jaw tracking. Surface electromyographic recordings were a measure of the muscle forces involved in static molar biting. Model predictions were compared with measured data from the subjects for eminence shape (R(2) = 0.96) and for muscle activity ratios (R(2) = 0.98). The strength of these relationships validated the models for use in calculating changes in joint loads and muscle forces after treatment. The results suggested that the mechanics of the masticatory system are affected by the combined treatments. The TMJ loads increased in 8 subjects. The average increases in condylar and muscle forces were 4% relative to the applied bite force, but in 1 case the increases were up to 20%. Therefore, although average increases in the forces were small, some persons may experience biologically significant increases in joint and muscle forces as a result of treatment.

Sensitivity of predicted muscle forces to parameters of the optimization-based human leg model revealed by analytical and numerical analyses

There are different opinions in the literature on whether the cost functions: the sum of muscle stresses squared and the sum of muscle stresses cubed, can reasonably predict muscle forces in humans. One potential reason for the discrepancy in the results could be that different authors use different sets of model parameters which could substantially affect forces predicted by optimization-based models. In this study, the sensitivity of the optimal solution obtained by minimizing the above cost functions for a planar three degrees-of-freedom (DOF) model of the leg with nine muscles was investigated analytically for the quadratic function and numerically for the cubic function. Analytical results revealed that, generally, the non-zero optimal force of each muscle depends in a very complex non-linear way on moments at all three joints and moment arms and physiological cross-sectional areas (PCSAs) of all muscles. Deviations of the model parameters (moment arms and PCSAs) from their nominal values within a physiologically feasible range affected not only the magnitude of the forces predicted by both criteria, but also the number of non-zero forces in the optimal solution and the combination of muscles with non-zero predicted forces. Muscle force magnitudes calculated by both criteria were similar. They could change several times as model parameters changed, whereas patterns of muscle forces were typically not as sensitive. It is concluded that different opinions in the literature about the behavior of optimization-based models can be potentially explained by differences in employed model parameters.

Swing- and support-related muscle actions differentially trigger human walk–run and run–walk transitions

There has been no consistent explanation as to why humans prefer changing their gait from walking to running and from running to walking at increasing and decreasing speeds, respectively. This study examined muscle activation as a possible determinant of these gait transitions. Seven subjects walked and ran on a motor-driven treadmill for 40s at speeds of 55, 70, 85, 100, 115, 130 and 145% of the preferred transition speed. The movements of subjects were videotaped, and surface electromyographic activity was recorded from seven major leg muscles. Resultant moments at the leg joints during the swing phase were calculated. During the swing phase of locomotion at preferred running speeds (115, 130, 145%), swing-related activation of the ankle, knee and hip flexors and peaks of flexion moments were typically lower (P&lt;0.05) during running than during walking. At preferred walking speeds (55, 70, 85%), support-related activation of the ankle and knee extensors was typically lower during stance of walking than during stance of running (P&lt;0.05). These results support the hypothesis that the preferred walk–run transition might be triggered by the increased sense of effort due to the exaggerated swing-related activation of the tibialis anterior, rectus femoris and hamstrings; this increased activation is necessary to meet the higher joint moment demands to move the swing leg during fast walking. The preferred run–walk transition might be similarly triggered by the sense of effort due to the higher support-related activation of the soleus, gastrocnemius and vastii that must generate higher forces during slow running than during walking at the same speed.

Intermuscular coordination during pendulum rebound exercises

In this study, we assessed coordination during pendulum rebound jumps. To gain insight into the movement coordination strategy, nine experienced male volleyball players performed maximal rebound jumps in a pendulum swing device using three different seat arrangements (90 degrees, 135 degrees and 180 degrees). Two-dimensional filming was performed in the right sagittal plane (200 Hz) synchronized with a force platform fitted to the wall (1000 Hz). The surface electromyograms of five muscles were recorded (200 Hz), in conjunction with kinematic and kinetic assessment. During the countermovement phase, the impact forces were attenuated by eccentric contractions of most muscles, which helped to reduce the energy input into the system. The wall reaction forces, net moments and joint power profiles were comparable between conditions. The small differences found between the extreme seat arrangements were attributed to differences in muscle length and the position of the feet. The strategy used during landing was similar to that observed in unconstrained vertical and drop jumps, where the neuromuscular system attenuates the impact forces. During the push-off phase, most muscles were found to contribute to positive work generation, except the semitendinosus, which was stretching throughout the propulsive phase. Despite not being able to exert a large influence over the trunk segment, this muscle was deemed to play an important role in regulating and synchronizing the onset of knee extension, enabling hip extension to occur before extension in the other more distal joints. Our findings show that the neuromuscular system is able to produce consistent movement coordination across experimental conditions and in accordance with the specific task demands and constraints imposed in the movement structure.

Electromechanical delay in isometric actions initiated from nonresting levels

PURPOSE

The purpose of this study was to determine whether electromechanical delay (EMD) was associated exclusively with the onset of tension from a resting state and whether EMD remained constant across different rates of force development.

METHODS

Twenty-four subjects (23.9 +/- 5.4 yr, 171.7 +/- 7.3 cm, 72.9 +/- 12.8 kg) performed isometric elbow flexion trials in the transverse plane by using the dominant arm during which isometric force data and surface EMG activity were collected. Subjects completed three trials to establish a maximal force (MF) reference. Subjects then completed trials in which pulse forces of varying magnitudes were elicited at a frequency of 1 Hz from different baseline intensities. All forces were expressed relative to MF. Three trials of the following conditions (baseline-pulse) were performed in random order: 0-25%, 25-50%, 50-75%, 0-50%, and 0-75%. EMG and force data were collected for 10 pulse cycles during these trials. EMD was defined as the temporal shift that maximized a normalized cross-correlation function.

RESULTS

EMD for a 25% pulse force developed from rest (83.5 +/- 12.9 ms) was significantly longer than that developed from 25% (66.3 +/- 11.5 ms) or 50% (60.6 +/- 16.6 ms) baselines. EMD values were not different when force was developed from 25% and 50% baselines. EMD associated with a 25% pulse force from rest was significantly longer than 50% (70.3 +/- 10.0 ms) and 75% (68.9 +/- 8.7 ms) pulse forces from rest. EMD for 50% and 75% pulse forces from rest were not statistically different.

CONCLUSION

It was concluded that EMD is present during exertions initiated from both resting and nonresting states but is reduced when exertions are initiated from non-resting states and with higher rates of force development.

A dynamic model of the forearm including fatigue

A biomechanical model of the forearm, consisting of 61 muscle-tendon systems or tendons and 8 sections, is presented. The model can be used to calculate the muscle forces when resultant of the external forces and the motion is known. Calculations are based on constraints of muscle forces, joint forces, contact forces, and tendon junctions, and a load sharing principle telling which of the feasible solutions are likely and which are not. Fatigue is accounted for by updating the upper limits of the muscle forces according to the loading history. As an example, the model is used to predict the load sharing between the fingers when they are pressed against a table with a given total force.

Theoretical considerations on cocontraction of sets of agonistic and antagonistic muscles

It is well known that static, non-linear minimization of the sum of the stress in muscles to a certain power cannot predict cocontraction of pairs of one-joint antagonistic muscles. In this report, we prove that for a single joint either all agonistic muscles cocontract or all are silent. For two-joint muscles, we show that lengthening and shortening of muscles corresponds closely to zero force and non-zero force states, respectively. This gives a new physiological interpretation of situations in which cocontraction of pairs of two-joint antagonistic muscles is predicted by these static non-linear optimization approaches.

Analysis of muscle coordination strategies in cycling

The functional significance of the stereotypical muscle activation patterns used in skilled multi-joint tasks is not well understood. Optimization methods could provide insight into the functional significance of muscle coordination. The purpose of this study was to predict muscle force patterns during cycling by pushing and pulling the pedal using different optimization criteria and compare the predictions with electromyographic (EMG) patterns. To address the purpose of the study, 1) the contribution of muscle length and velocity changes to EMG-muscle force relationships during cycling was examined by comparing joint moments calculated from EMG and inverse dynamics, 2) patterns of individual muscle forces during cycling of five subjects were predicted using 13 different optimization criteria, and 3) the properties of the criterion with the best performance in predicting the normalized EMG were used to explain the features and functional significance of muscle coordination in cycling. It was shown that the criterion that minimizes the sum of muscle stresses cubed demonstrated the best performance in predicting the relative magnitude and patterns of muscle activation. Based on this criterion, it was suggested that the functional significance of muscle coordination strategy in cycling may be minimization of fatigue and/or perceived effort.

Muscle coordination: the discussion continues

In this response, the major criticisms of the target article are addressed. Terminology from the target article that may have caused some confusion is clarified. In particular, the tasks that have the basic features of muscle coordination, as identified in the target article, have been limited in scope. A new metabolic optimization criterion suggested by Alexander (2000) is examined for its ability to predict muscle coordination in walking. Issues concerning the validation of muscle force predictions, the rules of muscle coordination, and the role of directional constraints in coordination of two-joint muscles are discussed. It is shown in particular that even in one-joint systems, the forces predicted by the criterion of Crowninshield and Brand (1981) depend upon the muscle moment arms and the physiological cross-sectional areas in much more complex ways than either previously assumed in the target article, or incorrectly derived by Herzog and Ait-Haddou (2000). It is concluded that the criterion of Crowninshield and Brand qualitatively predicts the basic coordination features of the major one- and two-joint muscles in a number of highly skilled, repetitive motor tasks performed by humans under predictable conditions and little demands on stability and accuracy. A possible functional significance of such muscle coordination may be the minimization of perceived effort, muscle fatigue, and/or energy expenditure.