Individual Muscle Contributions Research Articles

Ultrasound-tensiometry: A new method for measuring differential loading within a tendon during movement

BackgroundTendons transmit tensile load from muscles to the skeleton. Differential loading across a tendon can occur, especially when it contains subtendons originating from distinct muscles. Tendon shear wave speed has previously been shown to reflect local tensile stress. Hence, a tool that measures spatial variations in wave speed may reflect differential loading within a tendon during human movement. Research questionDo wave speeds measured via high-framerate ultrasound-based tensiometry correspond with differential loading across a tendon? MethodsUltrasound-tensiometry uses an external mechanical actuator to induce waves and high-framerate plane wave ultrasound imaging (20 kHz) to track tissue displacements arising from wave propagation within a tendon. Local tissue displacements are temporally and spatially filtered to remove high-frequency noise and reflected waves. A Radon transform of the spatio-temporal displacement data is used to compute the shear wave speed across the tendon.We evaluated ultrasound-tensiometry’s ability to measure differential loading across a tendon using in silico, ex vivo and in vivo approaches. The in silico approach used a finite element model to simulate wave propagation along two adjacent subtendons undergoing differential loading. The ex vivo experiment measured wave speed in adjacent porcine flexor subtendons subjected to differential loading. In vivo, we tracked wave speed across the Achilles tendon while a participant performed calf stretches to differentially load the subtendons, and while walking on a treadmill at 1.5 m/s. ResultsWave speeds modulated with local tendon stress under both in silico and ex vivo conditions, with higher wave speeds observed in subtendons subjected to higher loads (6–16 m/s higher at 1.5× load differential). Spatial variations in in vivo Achilles tendon wave speeds were consistent with differential subtendon loading arising from distinct muscle loads (maximum range: 0–137 m/s, resolution: 0.1 mm×0.2 mm, precision: ±0.2 m/s). SignificanceHigh-framerate ultrasound-tensiometry tracks spatial variations in tendon wave speed, which may be useful to investigate local tissue loading and to delineate individual muscle contributions to movement.

Analysis of Abduction Moment Arms after Rotator Cuff Tear and Acellular Dermal Matrix Allograft Reconstruction.

Biomechanical testing of abduction moment arms presents a useful method to assess the contribution of individual rotator cuff muscles to glenohumeral function. This study aimed to investigate the changes in abduction moment arms after a supraspinatus tear treated with Superior Capsule Reconstruction (SCR), Bursal Acromial Reconstruction (BAR), and a combined SCR-plus procedure, all with human dermal allograft. 7 fresh-frozen cadaveric specimens were tested in 6 conditions: (1) intact, (2) 50% supraspinatus tear (partial tear), (3) 100% supraspinatus tear, (4) SCR, (5) SCR+BAR, (6) BAR. In each condition, moment arms for the individual muscles of teres minor, subscapularis, and infraspinatus were calculated throughout 90º of abduction using Optotrak motion capture. ANOVA and post-hoc Tukey testing were performed to determine significance. In teres minor, the moment arms of the SCR (11.92mm), BAR (10.05mm), and SCR+BAR (11.87mm) conditions were greater than those in the intact (8.51mm, p=0.001, p=0.001, p=0.001, respectively), partial tear (9.09mm, p=0.001, p=0.128, p=0.001, respectively), and complete tear (8.75mm, p=0.001, p=0.011, p=0.001, respectively) conditions. Similarly in subscapularis, moment arms in the SCR (13.36mm), BAR (13.84mm), and SCR+BAR (13.53mm) conditions were greater than those in the intact condition (10.59mm, p=0.006, p=0.001, p=0.003, respectively) and partial tear condition (10.43mm, p=0.006, p=0.001, p=0.003, respectively). In teres minor, SCR (11.92mm) and SCR+BAR (11.87mm) were also found to have significantly increased moment arms compared to BAR (10.05mm, p=0.001, p=0.001, respectively). In Infraspinatus, BAR (13.77mm) was found to have a significantly decreased moment arm compared to the partial tear condition (15.78mm, p=0.026), with no other significant findings between conditions. These results suggest that the moment arm contributions from individual muscles comprising the rotator cuff can change after reconstruction to compensate for tears. SCR and SCR+BAR increase moment arms in teres minor and subscapularis, potentially allowing for increased abduction ability.

Functional anatomy of mystacial active sensing in rats.

Rats' whisking motion and objects' palpation produce tactile signals sensed by mechanoreceptors at the vibrissal follicles. Rats adjust their whisking patterns to target information type, flow, and resolution, adapting to their behavioral needs and the changing environment. This coordination requires control over the activity of the mystacial pad's intrinsic and extrinsic muscles. Studies have relied on muscle recording and stimulation techniques to describe the roles of individual muscles. However, these methods lack the resolution to isolate the mystacial pad's small and compactly arranged muscles. Thus, we propose functional anatomy as a complementary approach for studying the individual and coordinated effects of the mystacial pad muscles on vibrissae movements. Our functional analysis addresses the kinematic measurements of whisking motion patterns recorded in freely exploring rats. Combined with anatomical descriptions of muscles and fascia elements of the mystacial pad in situ, we found: (1) the contributions of individual mystacial pad muscles to the different whisking motion patterns; (2) active touch by microvibrissae, and its underlying mechanism; and (3) dynamic position changes of the vibrissae pivot point, as determined by the movements of the corium and subcapsular fibrous mat. Finally, we hypothesize that each of the rat mystacial pad muscles is specialized for a particular function in a way that matches the architecture of the fascial structures. Consistent with biotensegrity principles, the muscles and fascia form a network of structural support and continuous tension that determine the arrangement and motion of the embedded individual follicles.

Sonomyography for Control of Upper-Limb Prostheses: Current State and Future Directions.

ABSTRACT Problem Statement Despite the recent advancements in technology, many individuals with upper-limb loss struggle to achieve stable control over multiple degrees of freedom in a prosthesis. There is an ongoing need to develop noninvasive prosthesis control modalities that could improve functional patient outcomes. Proposed Solution Ultrasound-based sensing of muscle deformation, known as sonomyography, is an emerging sensing modality for upper-limb prosthesis control with the potential to significantly improve functionality. Sonomyography enables spatiotemporal characterization of both superficial and deep muscle activity, making it possible to distinguish the contributions of individual muscles during functional movements and derive a large set of independent prosthesis control signals. Using sonomyography to control a prosthesis has shown great promise in the research literature but has not yet been fully adapted for clinical use. This article describes the implementation of sonomyography for upper-limb prosthesis control, ongoing technological development, considerations for deploying this technology in clinical settings, and recommendations for future study. Clinical Relevance Sonomyography may soon become a clinically viable modality for upper-limb prosthesis control that could offer prosthetists an additional solution when selecting optimal treatment plans for their patients.

Individual Muscle Contributions to the Acceleration of the Center of Mass During the Barbell Back Squat in Trained Female Subjects.

Goodman, WW, Helms, E, and Graham, DF. Individual muscle contributions to the acceleration of the center of mass during the barbell back squat in trained female subjects. J Strength Cond Res 37(10): 1947-1954, 2023-The squat is used to enhance performance and rehabilitate the lower body. However, muscle forces and how muscles accelerate the center of mass (CoM) are not well understood. The purpose was to determine how lower extremity muscles contribute to the vertical acceleration of the CoM when squatting to parallel using 85% one-repetition maximum. Thirteen female subjects performed squats in a randomized fashion. Musculoskeletal modeling was used to obtain muscle forces and muscle-induced accelerations. The vasti, soleus, and gluteus maximus generated the largest upward accelerations of the CoM, whereas the muscles that produced the largest downward acceleration about the CoM were the hamstrings, iliopsoas, adductors, and tibialis anterior. Our findings indicate that a muscle's function is task and posture specific. That is, muscle function depends on both joint position and how an individual is interacting with the environment.

Ex vivo experimental strategies for assessing unconstrained shoulder biomechanics: A scoping review.

Biomechanical studies of the shoulder often choose an ex vivo approach, especially when investigating the active and passive contribution of individual muscles. Although various simulators of the glenohumeral joint and its muscles have been developed, to date a testing standard has not been established. The objective of this scoping review was to present an overview of methodological and experimental studies describing ex vivo simulators that assess unconstrained, muscular driven shoulder biomechanics. All studies with ex vivo or mechanical simulation experiments using an unconstrained glenohumeral joint simulator and active components mimicking the muscles were included in this scoping review. Static experiments and humeral motion imposed through an external guide, e.g., a robotic device, were excluded. Nine different glenohumeral simulators were identified in 51 studies after the screening process. We identified four control strategies characterized by: (a) using a primary loader to determine the secondary loaders with constant force ratios; (b) using variable muscle force ratios according to electromyography; (c) calibrating the muscle path profile and control each motor according to this profile; or (d) using muscle optimization. The simulators with the control strategy (b) (n=1) or (d) (n=2) appear most promising due to its capability to mimic physiological muscle loads.

Individual muscle contributions to the acceleration of the centre of mass during gait in people with mild-to-moderate hip osteoarthritis

BackgroundPeople with mild-to-moderate hip osteoarthritis (OA) exhibit hip muscle weakness, alterations in hip kinematics and kinetics and hip contact forces during gait compared to healthy controls. However, it is unclear if those with hip OA use different motor control strategies to coordinate the motion of the centre of mass (COM) during gait. Such information could provide further critical assessment of conservative management strategies implemented for people with hip OA. Research questionDo muscle contributions to the acceleration of the COM during walking differ between individuals with mild-to-moderate hip OA and controls? MethodsEleven individuals with mild-to-moderate hip OA and 10 healthy controls walked at a self-selected speed while whole-body motion and ground reaction forces were measured. Muscle forces during gait were obtained using static optimisation and an induced acceleration analysis was performed to determine individual muscle contributions to the acceleration of the COM during single-leg stance (SLS). Between-group comparisons were made using independent t-tests via Statistical Parametric Modelling. ResultsThere were no between-group differences in spatial-temporal gait parameters or three-dimensional whole-body COM acceleration. The rectus femoris, biceps femoris, iliopsoas and gastrocnemius muscles in the hip OA group contributed less to the fore-aft accelerations of the COM (p < 0.05), and more to the vertical COM acceleration with the gluteus maximus (p < 0.05), during SLS, compared to the control group. SignificanceSubtle differences exist in the way people with mild-to-moderate hip OA use their muscles to accelerate the whole-body centre of mass during the SLS phase of walking relative to healthy controls. These findings improve understanding of the complex functional consequences of hip OA and enhance our understanding of how to monitor the effectiveness of an intervention on biomechanical changes in gait in people with hip OA.

Individual muscle responses to mediolateral foot placement perturbations during walking

Walking requires active control of frontal plane balance through adjustments to mediolateral foot placement and ground reaction forces. Previous work on mediolateral balance perturbations and control of foot placement has often focused on the bilateral gluteus medius muscles. However, additional leg and trunk muscles can influence foot placement by transferring power to the foot and pelvis during swing. Thus, the purpose of this study was to determine individual muscle contributions to balance control following medial and lateral foot placement perturbations. Ten participants performed treadmill walking trials which included perturbations immediately before randomized heel strikes. Muscle contributions to foot placement, ground reaction forces, trunk power and frontal plane external moments during representative perturbed and unperturbed gait cycles were estimated using musculoskeletal modeling and simulation. Net muscle contributions to foot placement were 61 ± 50% more medial during the first recovery step following lateral perturbations and 28 ± 14% less medial in the second recovery step following medial perturbations. Following lateral perturbations, the swing gluteus medius performed 57 ± 50% more lateral work and the stance gluteus medius performed 61 ± 50% more medial work on the foot. Following medial perturbations, the erector spinae performed 39 ± 33% less lateral work on the foot. Changes in net muscle work on the foot were inconsistent with changes in step width, suggesting that changes in step width were not due to active muscle control but rather the mechanical effect of the perturbation. These outcomes provide a foundation for future studies analyzing balance control in populations at risk of falling.

Muscle contributions to pre-swing biomechanical tasks influence swing leg mechanics in individuals post-stroke during walking

BackgroundSuccessful walking requires the execution of the pre-swing biomechanical tasks of body propulsion and leg swing initiation, which are often impaired post-stroke. While excess rectus femoris activity during swing is often associated with low knee flexion, previous work has suggested that deficits in propulsion and leg swing initiation may also contribute. The purpose of this study was to determine underlying causes of propulsion, leg swing initiation and knee flexion deficits in pre-swing and their link to stiff knee gait in individuals post-stroke.MethodsMusculoskeletal models and forward dynamic simulations were developed for individuals post-stroke (n = 15) and healthy participants (n = 5). Linear regressions were used to evaluate the relationships between peak knee flexion, braking and propulsion symmetry, and individual muscle contributions to braking, propulsion, knee flexion in pre-swing, and leg swing initiation.ResultsFour out of fifteen of individuals post-stroke had higher plantarflexor contributions to propulsion and seven out of fifteen had higher vasti contributions to braking on their paretic leg relative to their nonparetic leg. Higher gastrocnemius contributions to propulsion predicted paretic propulsion symmetry (p = 0.005) while soleus contributions did not. Higher vasti contributions to braking in pre-swing predicted lower knee flexion (p = 0.022). The rectus femoris had minimal contributions to lower knee flexion acceleration in pre-swing compared to contributions from the vasti. However, for some individuals with low knee flexion, during pre-swing the rectus femoris absorbed more power and the iliopsoas contributed less power to the paretic leg. Total musculotendon work done on the paretic leg in pre-swing did not predict knee flexion during swing.ConclusionsThese results emphasize the multiple causes of propulsion asymmetry in individuals post-stroke, including low plantarflexor contributions to propulsion, increased vasti contributions to braking and reliance on compensatory mechanisms. The results also show that the rectus femoris is not a major contributor to knee flexion in pre-swing, but absorbs more power from the paretic leg in pre-swing in some individuals with stiff knee gait. These results highlight the need to identify individual causes of propulsion and knee flexion deficits to design more effective rehabilitation strategies.

Quantifying Knee Strength Deficits and Muscular Contribution to Torque Generation in ACL-Injured Adolescents Using A Simple Musculoskeletal Model

Background:Surface electromyography (sEMG) is commonly used to elucidate the effect of anterior cruciate ligament (ACL) injury on neuromuscular function. To facilitate comparisons, sEMG is normalized to a known value, such as peak activation during a maximum voluntary isometric contraction (MVIC). However, voluntary muscle inhibition after an injury compromises one’s ability to achieve a true MVIC value. Musculoskeletal modeling can be used to quantify this strength deficit by comparing a theoretical MVIC torque to the actual MVIC torque.Purpose:This study sought to evaluate voluntary knee extensor and flexor strength deficits and individual muscle contribution to peak torque generation in ACL injured (ACLi) and uninjured (CON) adolescent populations using a simplified subject-specific modeling framework.Methods:Thirty-nine ACLi (25 females) and 39 CON (25 females) adolescents (12-17years) completed knee extension and flexion MVICs on an isokinetic dynamometer. Peak experimental torque (TE) was identified. A subject-specific modeling framework used normalized sEMG of the quadriceps, hamstrings, and gastrocnemius muscles to determine theoretically ideal torque (TI) for each exercise, assuming agonist muscles were fully activated. Strength deficit ratios (TE/TI) and individual muscle contribution to TE were computed. Group mean differences were compared using independent t-tests.Results:ACLi demonstrated significantly lower extension (2.53±0.94 vs 3.07±0.57Nm/kg, p<0.001) and flexion (1.14±0.50 vs 1.37±0.31Nm/kg, p<0.001) peak TE compared to CON (FIG 1.A). Significant between group differences in sEMG of antagonist muscles were observed. Both groups demonstrated similar TI values (FIG 1.B-upper). Significantly lower trends in strength ratios (TE/TI) were maintained between groups (FIG 1.B-lower). However, percent between group differences were minimized from 17.8% to 6.3% for knee extension, and 16.7% to 10.0% for knee flexion, when TE is expressed relative to TI. Significantly lower medical gastrocnemius contribution to flexion TE was also observed in the ACLi (24.7±0.08%) compared to CON (30.9±10.1%, p=0.037).Conclusion:Significant between group differences in peak extensor or flexor torques were observed. Our model confirmed a strength deficit in ACLi compared to CON, however, between group differences were less prominent when described relative to TI. This may be attributed to between group differences in antagonist contributions (i.e. co-contraction) during MVIC exercise. Simplified modeling frameworks that can incorporate muscle activations as well as torque generation may be more appropriate for evaluating functional outcomes of ACLi populations in a clinical setting.Figure 1:

The effect of virtual reality environment on spatial-temporal gait parameters during self-paced treadmill walking

A common measure of rehabilitation effectiveness post-stroke is self-selected walking speed, yet individuals may achieve the same speed using different coordination strategies. Asymmetry in the propulsion generated by each leg can provide insight into paretic leg coordination due to its relatively strong correlation with hemiparetic severity. Subjects walking at the same speed can exhibit different propulsion asymmetries, with some subjects relying more on the paretic leg and others on the nonparetic leg. The goal of this study was to assess whether analyzing propulsion asymmetry can help distinguish between improved paretic leg coordination versus nonparetic leg compensation.Three-dimensional forward dynamics simulations were developed for two post-stroke hemiparetic subjects walking at identical speeds before/after rehabilitation with opposite changes in propulsion asymmetry. Changes in the individual muscle contributions to forward propulsion were examined.The major source of increased forward propulsion in both subjects was from the ankle plantarflexors. How they were utilized differed and appears related to changes in propulsion asymmetry. Subject A increased propulsion generated from the paretic plantarflexors, while Subject B increased propulsion generated from the nonparetic plantarflexors. Each subject's strategy to increase speed also included differences in other muscle groups (e.g., hamstrings) that did not appear to be related to propulsion asymmetry.The results of this study highlight how speed cannot be used to elucidate underlying muscle coordination changes following rehabilitation. In contrast, propulsion asymmetry appears to provide insight into changes in plantarflexor output affecting propulsion generation and may be useful in monitoring rehabilitation outcomes.

How muscles maximize performance in accelerated sprinting.

We sought to provide a more comprehensive understanding of how the individual leg muscles act synergistically to generate a ground force impulse and maximize the change in forward momentum of the body during accelerated sprinting. We combined musculoskeletal modelling with gait data to simulate the majority of the acceleration phase (19 foot contacts) of a maximal sprint over ground. Individual muscle contributions to the ground force impulse were found by evaluating each muscle's contribution to the vertical and fore-aft components of the ground force (termed "supporter" and "accelerator/brake," respectively). The ankle plantarflexors played a major role in achieving maximal-effort accelerated sprinting. Soleus acted primarily as a supporter by generating a large fraction of the upward impulse at each step whereas gastrocnemius contributed appreciably to the propulsive and upward impulses and functioned as both accelerator and supporter. The primary role of the vasti was to deliver an upward impulse to the body (supporter), but these muscles also acted as a brake by retarding forward momentum. The hamstrings and gluteus medius functioned primarily as accelerators. Gluteus maximus was neither an accelerator nor supporter as it functioned mainly to decelerate the swinging leg in preparation for foot contact at the next step. Fundamental knowledge of lower-limb muscle function during maximum acceleration sprinting is of interest to coaches endeavoring to optimize sprint performance in elite athletes as well as sports medicine clinicians aiming to improve injury prevention and rehabilitation practices.

The Effect of Step Width on Muscle Contributions to Body Mass Center Acceleration During the First Stance of Sprinting.

BackgroundAt the beginning of a sprint, the acceleration of the body center of mass (COM) is driven mostly forward and vertically in order to move from an initial crouched position to a more forward-leaning position. Individual muscle contributions to COM accelerations have not been previously studied in a sprint with induced acceleration analysis, nor have muscle contributions to the mediolateral COM accelerations received much attention. This study aimed to analyze major lower-limb muscle contributions to the body COM in the three global planes during the first step of a sprint start. We also investigated the influence of step width on muscle contributions in both naturally wide sprint starts (natural trials) and in sprint starts in which the step width was restricted (narrow trials).MethodMotion data from four competitive sprinters (2 male and 2 female) were collected in their natural sprint style and in trials with a restricted step width. An induced acceleration analysis was performed to study the contribution from eight major lower limb muscles (soleus, gastrocnemius, rectus femoris, vasti, gluteus maximus, gluteus medius, biceps femoris, and adductors) to acceleration of the body COM.ResultsIn natural trials, soleus was the main contributor to forward (propulsion) and vertical (support) COM acceleration and the three vasti (vastus intermedius, lateralis and medialis) were the main contributors to medial COM acceleration. In the narrow trials, soleus was still the major contributor to COM propulsion, though its contribution was considerably decreased. Likewise, the three vasti were still the main contributors to support and to medial COM acceleration, though their contribution was lower than in the natural trials. Overall, most muscle contributions to COM acceleration in the sagittal plane were reduced. At the joint level, muscles contributed overall more to COM support than to propulsion in the first step of sprinting. In the narrow trials, reduced COM propulsion and particularly support were observed compared to the natural trials.ConclusionThe natural wide steps provide a preferable body configuration to propel and support the COM in the sprint starts. No advantage in muscular contributions to support or propel the COM was found in narrower step widths.

Musculoskeletal Model to Predict Muscle Activity During Upper Limb Movement

Assessing biomechanics of upper limb movement is essential for guiding targeted therapy to treat conditions such as spasticity and dystonia. Targeted therapy, including injections of medications into specific muscles (e.g., lidocaine, botulinum toxin type A), requires accurate identification (activity) and contribution of as many muscles as possible. Currently, this is achieved by visual clinical assessment or using surface electromyography (sEMG). Although sEMG could provide a reasonable estimate of muscle activity for certain superficial muscles after an intense filtering process, they are unable to provide separated activity and contribution for every superficial and deep muscle. Other proposed musculoskeletal and machine learning models similarly do not provide a detailed and accurate activity of every muscle. The objective of the study is to design a subject-specific musculoskeletal model to predict the activity and contribution of each muscle pertaining to any upper limb movement with improved detail and accuracy over existing methodologies. Performance metrics were calculated for validation by comparing the predicted muscle activity with the normalized sEMG data computed from 8 superficial muscles, while the deeper muscles were not included in the validation as the sEMG is unable to provide a separated activity for those muscles. The results show that the proposed model has a mean R2 value of 0.8190 and also indicated a statistically significant correlation ( $P ) between the calculated (normalized sEMG data) and predicted activity value. Additionally, and significantly, compared to earlier studies, the proposed model predicts the individual muscle activity and contribution of deeper muscles.

Development of a Musculoskeletal Model of Hyolaryngeal Elements for Understanding Pharyngeal Swallowing Mechanics

A detailed understanding of muscle activity in human swallowing would provide insights into the complex neuromuscular coordination underlying swallowing. The purpose of this study was to introduce musculoskeletal analysis to investigate muscle activities involved in swallowing as there are limitations on studying comprehensive muscle activation patterns by conventional methods such as electromyography (EMG) measurement. A musculoskeletal model of swallowing was newly developed based on the skeletal model made from CT data of a healthy volunteer. Individual muscle forces were predicted in pharyngeal swallowing by inverse dynamics’ computations with static optimization, in which the typical trajectories of the hyoid bone and thyroid cartilage analyzed from videofluoroscopic (VF) data of the volunteer were used. The results identified the contribution of individual muscles in pharyngeal swallowing in relation to the movements of the hyoid bone and thyroid cartilage. The predicted sequence of muscle activity showed a qualitative agreement with salient features in previous studies with fine wire EMG measurements. This method, if validated further by imaging and EMG studies, enables studying a broader range of neuromuscular coordination in swallowing. The proposed method offers an avenue to understanding the physiological mechanisms of swallowing and could become useful to evaluate rehabilitation effects on dysphagia.

Muscle Contributions to Balance Control During Amputee and Nonamputee Stair Ascent.

Dynamic balance is controlled by lower-limb muscles and is more difficult to maintain during stair ascent compared to level walking. As a result, individuals with lower-limb amputations often have difficulty ascending stairs and are more susceptible to falls. The purpose of this study was to identify the biomechanical mechanisms used by individuals with and without amputation to control dynamic balance during stair ascent. Three-dimensional muscle-actuated forward dynamics simulations of amputee and nonamputee stair ascent were developed and contributions of individual muscles, the passive prosthesis, and gravity to the time rate of change of angular momentum were determined. The prosthesis replicated the role of nonamputee plantarflexors in the sagittal plane by contributing to forward angular momentum. The prosthesis largely replicated the role of nonamputee plantarflexors in the transverse plane but resulted in a greater change of angular momentum. In the frontal plane, the prosthesis and nonamputee plantarflexors contributed oppositely during the first half of stance while during the second half of stance, the prosthesis contributed to a much smaller extent. This resulted in altered contributions from the intact leg plantarflexors, vastii and hamstrings, and the intact and residual leg hip abductors. Therefore, prosthetic devices with altered contributions to frontal-plane angular momentum could improve balance control during amputee stair ascent and minimize necessary muscle compensations. In addition, targeted training could improve the force production magnitude and timing of muscles that regulate angular momentum to improve balance control.

Gait variability using waist- and ankle-worn inertial measurement units in healthy older adults

A common measure of rehabilitation effectiveness post-stroke is self-selected walking speed, yet individuals may achieve the same speed using different coordination strategies. Asymmetry in the propulsion generated by each leg can provide insight into paretic leg coordination due to its relatively strong correlation with hemiparetic severity. Subjects walking at the same speed can exhibit different propulsion asymmetries, with some subjects relying more on the paretic leg and others on the nonparetic leg. The goal of this study was to assess whether analyzing propulsion asymmetry can help distinguish between improved paretic leg coordination versus nonparetic leg compensation.Three-dimensional forward dynamics simulations were developed for two post-stroke hemiparetic subjects walking at identical speeds before/after rehabilitation with opposite changes in propulsion asymmetry. Changes in the individual muscle contributions to forward propulsion were examined.The major source of increased forward propulsion in both subjects was from the ankle plantarflexors. How they were utilized differed and appears related to changes in propulsion asymmetry. Subject A increased propulsion generated from the paretic plantarflexors, while Subject B increased propulsion generated from the nonparetic plantarflexors. Each subject's strategy to increase speed also included differences in other muscle groups (e.g., hamstrings) that did not appear to be related to propulsion asymmetry.The results of this study highlight how speed cannot be used to elucidate underlying muscle coordination changes following rehabilitation. In contrast, propulsion asymmetry appears to provide insight into changes in plantarflexor output affecting propulsion generation and may be useful in monitoring rehabilitation outcomes.

Individual muscle contributions to hip joint-contact forces during walking in unilateral transfemoral amputees with osseointegrated prostheses

Direct skeletal attachment of prostheses in transfemoral amputees circumvents skin-interface complications associated with conventional sockets; however, joint pain and musculoskeletal disease is known to occur postoperatively. This study quantified hip contact forces and the roles of individual muscles in producing hip contact forces during walking in transfemoral amputees with osseointegrated prostheses. Musculoskeletal models were developed for four transfemoral amputees. Gluteus maximus and gluteus medius were the major contributors to the hip contact forces, and the intact limb hip muscles demonstrated greater contributions to hip contact forces than those of the residual limb. The findings may be useful for mitigating walking asymmetry.

Effects of the weight of shoes on calf muscle simulation

The current study investigated the effects of shoes of different weights on calf individual muscle contributions during a running cycle. Twenty male runners ran on a force platform with shoes of four different weights (175 g, 255 g, 335 g, and 415 g). The study evaluated runners’ lower extremity muscle forces under the four shoe weight conditions using a musculoskeletal modeling system. The system generates equality and inequality constraint equations to simulate muscle forces. The individual muscle contributions in each calf were determined using these muscle forces. Data were compared using one-way repeated-measure ANOVA. The results revealed significant differences in the contributions of the gastrocnemius lateralis. Post hoc comparisons revealed that running in the 175 g shoes resulted in a larger contribution of the gastrocnemius lateralis than running in the 415 g shoes during the braking phase. Therefore, wearing lightweight shoes while running may promote fatigue in the gastrocnemius muscle during the braking phase. The calf muscle activation results may indicate that an adaptation period is warranted when changing from heavy to lightweight shoes.

Estimation of the force-velocity properties of individual muscles from measurement of the combined plantarflexor properties.

The force-velocity (F-V) properties of isolated muscles or muscle fibers have been well studied in humans and other animals. However, determining properties of individual muscles in vivo remains a challenge because muscles usually function within a synergistic group. Modeling has been used to estimate the properties of an individual muscle from the experimental measurement of the muscle group properties. While this approach can be valuable, the models and the associated predictions are difficult to validate. In this study, we measured the in situ F-V properties of the maximally activated kangaroo rat plantarflexor group and used two different assumptions and associated models to estimate the properties of the individual plantarflexors. The first model (Mdl1) assumed that the percent contributions of individual muscles to group force and power were based upon the muscles' cross-sectional area and were constant across the different isotonic loads applied to the muscle group. The second model (Mdl2) assumed that the F-V properties of the fibers within each muscle were identical, but because of differences in muscle architecture, the muscles' contributions to the group properties changed with isotonic load. We compared the two model predictions with independent estimates of the muscles' contributions based upon sonomicrometry measurements of muscle length. We found that predictions from Mdl2 were not significantly different from sonomicrometry-based estimates while those from Mdl1 were significantly different. The results of this study show that incorporating appropriate fiber properties and muscle architecture is necessary to parse the individual muscles' contributions to the group F-V properties.