Forward Dynamics Simulation Research Articles

Humanoid Motion Control by Compliance Optimization Explicitly Considering its Positive Definiteness

This article discusses a compliance optimization approach that satisfies positive definiteness. Physical human–robot interactions are an important topic in robotics, for which force or compliance control is a key technology. Operational space control (OSC) is one of the most common approaches for robot force control with redundant degrees of freedom. By linearizing OSC, we can derive joint stiffness and viscosity matrices equivalent to the OSC. For an appropriate control, it is important that these matrices are positive definite. However, the stiffness matrix equivalent to the OSC is not always positive definite. In this case, a high kinetic energy is required, which is a problem in terms of the control performance. Therefore, the control performance can be improved by explicitly considering the positive definiteness of the stiffness or compliance. In this article, the authors derive a dynamically consistent compliance formulation and propose a compliance optimization that satisfies positive definiteness. The space of the symmetric positive definite matrix is a Riemannian manifold. We show that minimizing the Riemaniann geodesic distance results in a better performance compared with using OSC. The proposed method is validated via forward dynamics simulations and experiments using a hydrostatically driven humanoid Hydra.

Fuzzy forward dynamics of distinct gait phases with a prosthetic foot

This work considers epistemic uncertainty in the form of fuzzy parameters in a multibody forward dynamics simulation of the human leg with a prosthetic foot. The thigh and shank are modelled as rigid bodies while the prosthetic foot, modelled after a carbon spring prosthesis, is represented by a predeformed geometrically exact beam model. A variational integrator is used to solve the equations of motion in the forward dynamics simulation and the Graph Follower algorithm is used to include epistemic uncertainty. Two cases are considered. Large movements are examined using a pendulum swing, similar to the swing phase during human gait. To validate the deformation of the prosthesis, a second case is examined, where the prosthesis is fixed in space and deforms under the weight of the body segments. Both cases consider a fuzzy Young’s modulus and determine the envelopes of the resulting uncertain target output. The Graph Follower algorithm was modified to substantially increase computational efficiency, enabling the propagation of fuzzy uncertainty through the complex multibody model with rigid and flexible bodies.

Design of a control framework for lower limb exoskeleton rehabilitation robot based on predictive assessment

BackgroundPatients suffering from lower limb dyskinesia, especially in early stages of rehabilitation, have weak residual muscle strength in affected limb and require passive training by the lower limb rehabilitation robot. Anatomy indicates that the biceps femoris short head muscle has a strong influence on knee motion at the swing phase of walking. We sought to explore how it would influence on gait cycle in optimization framework. However, the training trajectory of conventional rehabilitation robots performing passive training usually follows gait planning based on general human gait data, which cannot simultaneously ensure both effective rehabilitation of affected limbs with varying severity pathological gait and comfort of the wearer within a safe motion trajectory. MethodsTo elucidate the effects of weakness and contracture, we systematically introduced isolated defects into the musculoskeletal model and generated walking simulations to predict the gait adaptation due to these defects. An impedance control model of the rehabilitation robot is developed. Knee joint parameters optimized by predictive forward dynamics simulation are adopted as the expected values for the robot controller to achieve customized adjustment of the robot motion trajectory. FindingsSevere muscle contracture leads to severe knee flexion; severe muscle weakness induces a significant posterior tilt of the upper trunk, which hinders walking speed. InterpretationOur simulation results attempt to reveal pathological gait features, which may help to reproduce the simulation of pathological gait. Furthermore, the robot simulation results show that the robot system achieves a speedy tracking by setting a larger stiffness value. The model also allows the implementation of different levels of damping or elasticity effects.Trial registration: The method proposed in this paper is an initial basic study that did not reach clinical trials and therefore retains retrospectively registered.

Clarify Sit-to-Stand Muscle Synergy and Tension Changes in Subacute Stroke Rehabilitation by Musculoskeletal Modeling.

Post-stroke patients exhibit distinct muscle activation electromyography (EMG) features in sit-to-stand (STS) due to motor deficiency. Muscle activation amplitude, related to muscle tension and muscle synergy activation levels, is one of the defining EMG features that reflects post-stroke motor functioning and motor impairment. Although some qualitative findings are available, it is not clear if and how muscle activation amplitude-related biomechanical attributes may quantitatively reflect during subacute stroke rehabilitation. To better enable a longitudinal investigation into a patient's muscle activation changes during rehabilitation or an inter-subject comparison, EMG normalization is usually applied. However, current normalization methods using maximum voluntary contraction (MVC) or within-task peak/mean EMG may not be feasible when MVC cannot be obtained from stroke survivors due to motor paralysis and the subject of comparison is EMG amplitude. Here, focusing on the paretic side, we first propose a novel, joint torque-based normalization method that incorporates musculoskeletal modeling, forward dynamics simulation, and mathematical optimization. Next, upon method validation, we apply it to quantify changes in muscle tension and muscle synergy activation levels in STS motor control units for patients in subacute stroke rehabilitation. The novel method was validated against MVC-normalized EMG data from eight healthy participants, and it retained muscle activation amplitude differences for inter- and intra-subject comparisons. The proposed joint torque-based method was also compared with the common static optimization based on squared muscle activation and showed higher simulation accuracy overall. Serial STS measurements were conducted with four post-stroke patients during their subacute rehabilitation stay (137 ± 22 days) in the hospital. Quantitative results of patients suggest that maximum muscle tension and activation level of muscle synergy temporal patterns may reflect the effectiveness of subacute stroke rehabilitation. A quality comparison between muscle synergies computed with the conventional within-task peak/mean EMG normalization and our proposed method showed that the conventional was prone to activation amplitude overestimation and underestimation. The contributed method and findings help recapitulate and understand the post-stroke motor recovery process, which may facilitate developing more effective rehabilitation strategies for future stroke survivors.

Combined Effects of External Moments and Muscle Activations on ACL Loading during Numerical Simulations of a Female Model in OpenSim

Background: Anterior cruciate ligament (ACL) injuries have been studied using a variety of methods and tools. However, each is hindered by specific limitations with respect to its application. Aim: To assess the combined effects of external moments and muscle activations on ACL loading using serial, forward dynamics (FD) simulations of single leg, hyperextension landings in OpenSim. Methods: The FD tool of OpenSim was iteratively run using different combinations of knee-spanning muscle activation levels, internal rotation and valgus knee moment magnitudes. A regression was conducted on the data in order to predict ACL loading under different conditions. Results: A purely abduction moment leads to greater mean ACL loading than a purely internal rotation moment or any combination of the two. Additionally, the generalized boosted regression model using both external moments and certain knee muscles identified the internal rotation moment as the most important variable in predicting the ACL load (R2 = 0.9; p < 0.0001). Conclusion: This study demonstrated a novel and practical application of an OpenSim musculoskeletal model that supports the ACL injury mechanism of landing with low knee flexion angles, high muscle forces of the Quadriceps muscles and an external knee valgus moment, though further investigation is needed.

DATSURYOKU: designing environment–body interaction for implicit muscle relief*

Decreased daily activities and tension frequently cause excessive muscle co-contractions implicitly, resulting in decreased performance and injury in sports. In this study, we modify the interaction between the environment and human body freely and temporarily, using environmental kinodynamic intervention. We target the load reduction during walking, which is realized by walking on a treadmill, where the acceleration/deceleration of the treadmill acts as the intervention. A forward dynamics simulation using a kinodynamic body model is used to design the intervention pattern. The designed intervention pattern is implemented in a real environment for verification, using a subject experiment with six subjects. The results demonstrate that the vertical component of the ground reaction force decreases by 2.14% and gastrocnemius and tibialis anterior activities decrease by 1.89% and % when the treadmill speed is low at heel contact and accelerates gradually. Hence, we conclude that the intervention decreases the change in the muscle length during walking, suppresses the stretch reflex, and reduces muscle activity. These results demonstrate the possibility to modify human motor control systems using non-invasive kinodynamic interventions, thereby realizing implicit muscle relief (DATSURYOKU) in sports and daily life, as well as to prevent their injuries.

Forward dynamics simulation of a human leg model with a geometrically exact beam model as prosthetic foot.

AbstractIn this work, we present a model for a human leg with a prosthetic foot, consisting of rigid bodies for the thigh and shank and a geometrically exact beam model for the prosthesis. The prosthesis is modelled after the Össur Vari‐Flex® carbon spring prosthetic foot. The individual bodies are connected by joints imposing constraints, mimicking the motions of human joints. A structure preserving variational integrator is used to calculate the forward dynamic simulation of the model and to analyse the energy exchange between potential, kinetic and deformation energy. Due to the variational integrator, there is no artificial energy loss introduced by the numerical integration over the simulation time [3].

Femur Abduction Associated with Transfemoral Amputation Alters the Profile of Lumbopelvic Mechanical Loads During Generalized End-Limb Loading.

Pain in the lower back is frequent problem for most individuals with transfemoral amputation, which limits their overall mobility and quality of life. While the underlying root causes of back pain are multifactorial, a contributing factor is the mechanical loading environment within the lumbopelvic joint. Specifically, this study aims to explore the upstream effects amputation has on the mechanical loading environment of the lumbopelvic joint using a 3D musculoskeletal model of transfemoral amputation. A generic musculoskeletal model was altered to represent a transfemoral amputation. Muscle parameters were adjusted to represent a myodesis amputation surgery that preserved musculotendon tension in a neutral anatomical pose. The model contained a total of 28 degrees of freedom and 76 muscles spanning the lower-limb and torso. In forward dynamics simulations, generalized external forces were applied to the distal end of the residual limb at a series of directions. Axial, oblique and transverse 10 N end-limb loads were applied. In addition, simulations were performed for 0°, 4°, and 8° of femur abduction, which are clinically observed in individuals with transfemoral amputation. In these simulations, reaction forces and moments at the lumbopelvic joint were computed. In general, femur abduction had little effect on back loading for an axial applied end-limb force. These data showed that while the individual magnitudes of lumbopelvic force and moment reactions did not significantly deviate for differing levels of femur abduction, the pattern of how these forces changes in response to different end-limb force directions (applied circumferentially along the limb) was affected by femur abduction angle.Clinical Relevance- The changes in joint reaction forces in the lumbopelvic joint from an aligned position to an abducted position reinforce the importance of avoiding hip flexion-abduction contracture during amputation surgery. This suggests that surgical techniques such as myodesis, osseointegration, or medial thighplasty, which intend to maintain anatomical alignment may have beneficial upstream effects for the patients during locomotion. Given the prevalence of lower back pain in individuals with transfemoral amputation, teasing out the causes of lower back pain could bring relief to a population that struggles with community independence.

Estimating Human Upper Limb Impedance Parameters From a State-of-the-Art Computational Neuromusculoskeletal Model.

The human neuro-musculoskeletal system constantly deploys passive (e.g., posture adjustment) and active (e.g., muscle co-contraction) control strategies to regulate upper limb impedance and stability while interacting with the outside world. Upper limb impedance has been assessed through in vivo experiments and model-based simulations. The experiments are practically limited to small samples of able-bodied subjects and few limb postures, and model-based approaches have mostly used simplified upper limb models. Our objective was to develop and validate a computational approach to estimate upper limb impedance parameters - stiffness, viscosity, and inertia - at the endpoint (i.e., hand) using a neuromusculoskeletal model with realistic geometry. We added a planar manipulandum to an existing upper limb model implemented in OpenSim (version 3.3) and used contact modeling to attach the manipulandum's handle to the musculoskeletal model's hand. The hand was placed at several locations lateral to the shoulder joint along anterior/posterior and medial/lateral axes. At each location, during forward dynamics simulations, the manipulandum applied small perturbations to the hand in eight different directions. The spatial variation of the computed, model-based impedance parameters was similar to that of experimentally measured impedance parameters. However, the overall size of the stiffness and viscosity components was larger in the model than from experiments.Clinical Relevance- Computational modeling and simulations can estimate upper limb impedance properties to complement and overcome the limitations of experiments, especially for clinical populations. The computational approach could ultimately inform new interventions and devices to restore limb stability in people with shoulder disabilities.

Simulating Human Upper and Lower Limb Balance Recovery Responses Using Nonlinear Model Predictive Control.

The ability to generate predictive dynamic simulations of human movement using optimal control has been a growing point of interest in the design of medical/assistive devices, e.g. robotic exoskeletons. Despite this, many disseminated simulations of whole-body tasks, such as balance recovery, neglect the role of the upper body instead focusing on postural joints, e.g. ankle, knees, hips. Thus, the purpose of the current study was to use a novel nonlinear model predictive control (NMPC) approach to assess how actuated upper limbs, as well as different individual performance (optimality) criteria, can shape simulated reactive balance recovery responses. A sagittal biomechanical model of a young adult standing was designed and actuated via nonlinear muscle torque generators (rotational single-muscle equivalents). Forward dynamic simulations of balance recovery (NMPCdriven) following an unexpected support-surface perturbation were generated for each unique combination of selected performance criteria (6 total), perturbation direction (forward and backward), and arm joints free/locked. The observed joint trajectories provide insight into the emergence of human elements of postural control from individual optimality criteria, e.g. hip-ankle strategies emerge from single-joint regulation. Quantitative analysis of performance improvements with the arms free suggest that whether arm responses emerge in the simulations may be dependent on the problem's initial guess. Future work should focus on testing further performance criteria and improving NMPC as a model of the nervous system.

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.

Increase in muscle tone promotes the use of ankle strategies during perturbed stance.

To maintain an upright standing posture against external disturbances, the human body mainly employs two types of postural control strategies: "ankle strategy" and "hip strategy." While it has been reported that the magnitude of the disturbance alters the use of postural control strategies, it has not been elucidated how the level of muscle tone, one of the crucial parameters of bodily function, determines the use of each strategy. We have previously confirmed using forward dynamics simulations of human musculoskeletal models that an increased muscle tone promotes the use of ankle strategies. The objective of the present study was to experimentally evaluate a hypothesis: an increased muscle tone promotes the use of ankle strategies. Do changes in the muscle tone affect the use of ankle strategies? Participants were asked to maintain their standing posture on a movable platform sliding horizontally at several accelerations. Postural reactions for support surface translations were examined under three instructions with or without handgrips: relax state, squeezing a handgrip, and an increased muscle tone of the whole body. Surface-electromyography and marker locations of joints were measured to calculate the index of muscle tone and postural control strategies. The relationship of the indexes was evaluated based on correlation coefficients. In half of the conditions, weak negative correlations were noted between the muscle tone index and postural control strategy index. In other words, an increased muscle tone rather promoted the use of the ankle strategy than the hip strategy. These findings are consistent with our previous simulation results. The results recognized a positive response to the research question. This suggests that it is crucial to take muscle tone into account to understand postural control strategies.

Embodiment of intra-abdominal pressure in a flexible multibody model of the trunk and the spinal unloading effects during static lifting tasks.

The role of intra-abdominal pressure (IAP) in spinal load reduction has remained controversial, partly because previous musculoskeletal models did not introduce the pressure generating mechanism. In this study, an integrated computational methodology is proposed to combine the IAP change with core muscle activations. An ideal gas relationship was introduced to calculate pressure distribution within the abdominal cavity. Additionally, based on flexible multibody dynamics, a muscle membrane element was developed by incorporating the muscular fiber deformation, inter-fiber stiffness, and volume constancy. This element was then utilized in discretizing the diaphragm and transversus abdominis, forming an IAP-muscle coupling system of the abdominal cavity. Based on this methodology, a forward dynamic simulation of spinal flexion was presented to examine the unloading effect of abdominal breathing. The results confirm that core muscle contraction during the abdominal breathing cycle can substantially reduce the forces of spinal compression together with trunk extensor muscles, and this effect is more pronounced when the IAP increase is produced by contraction of the transversus abdominis. This unloading effect still holds even with the co-activation of other abdominal muscles, providing a potential choice when designing trunk movements during weight-lifting tasks.

The Influence of Load Carriage on Knee Joint Loading and Metabolic Cost on Walking with Lower-Limb Amputation: A Preliminary Modeling Study

ABSTRACT Introduction For able-bodied individuals, the mechanical output from the ankle muscles is modulated to meet the altered demands of load carriage. However, for individuals with a lower-limb amputation, the stiffness properties of standard-of-care prosthetic feet do not change with varying load conditions. Thus, individuals with amputation often develop gait asymmetries during load carriage that increase their risk for developing overuse injuries such as in the intact knee and increase the metabolic cost of walking relative to able-bodied individuals. The purpose of this preliminary study was to assess the influence of load carriage technique on knee joint loading and metabolic cost during gait of an individual with a below-knee amputation using a forward dynamics simulation framework. Methods Simulations were generated to track the experimental walking data of individuals with amputation for 3 loading conditions (unloaded, front load, and back load). Results These simulations showed that individuals with amputation rely on their intact limb as a compensatory strategy to meet the increased demands of carrying a load. Carrying a back load was found to increase intact knee joint loading relative to carrying a front load but reduced metabolic cost. Conclusion The tradeoff between joint loading and metabolic cost should be considered when determining the appropriate load carriage technique. Future work should focus on improving prosthetic foot designs to help reduce joint loading asymmetry and elevated metabolic cost during different loading conditions for individuals with lower-limb amputation.

Forward dynamic simulation of Japanese macaque bipedal\xa0locomotion demonstrates better energetic economy in a virtualised plantigrade posture

A plantigrade foot with a large robust calcaneus is regarded as a distinctive morphological feature of the human foot; it is presumably the result of adaptation for habitual bipedal locomotion. The foot of the Japanese macaque, on the other hand, does not have such a feature, which hampers it from making foot–ground contact at the heel during bipedal locomotion. Understanding how this morphological difference functionally affects the generation of bipedal locomotion is crucial for elucidating the evolution of human bipedalism. In this study, we constructed a forward dynamic simulation of bipedal locomotion in the Japanese macaque based on a neuromusculoskeletal model to evaluate how virtual manipulation of the foot structure from digitigrade to plantigrade affects the kinematics, dynamics, and energetics of bipedal locomotion in a nonhuman primate whose musculoskeletal anatomy is not adapted to bipedalism. The normal bipedal locomotion generated was in good agreement with that of actual Japanese macaques. If, as in human walking, the foot morphology was altered to allow heel contact, the vertical ground reaction force profile became double-peaked and the cost of transport decreased. These results suggest that evolutionary changes in the foot structure were important for the acquisition of human-like efficient bipedal locomotion.

Simulating finger-tip force using two common contact models: Hunt-Crossley and elastic foundation

Musculoskeletal models of the hand rarely include fingerpad contact mechanics, thereby limiting our ability to simulate and examine hand-object interactions. The objective of this study was to evaluate whether two common contact models (Hunt-Crossley and Elastic Foundation) can accurately represent the fingerpad. Two musculoskeletal models of the index finger were created by adding fingerpad contact geometry using either the Hunt-Crossley or Elastic Foundation contact models. Key contact parameters (target force, contact area, and stiffness) were then systematically varied through 432 forward dynamic simulations to examine how these parameters influenced estimation of finger-tip forces. Across all simulations, variation in target force, contact area, and stiffness parameters impacted the computation time required to complete the simulations and the accuracy of the predicted finger-tip force. Computation time was over three times longer in simulations with high versus low values of contact area and stiffness in both contact models. For both contact models, larger contact area and stiffness values resulted in simulations that more closely predicted target force. However, across all simulations, the Hunt-Crossley model produced a greater proportion of accurate finger-tip force simulations than the Elastic Foundation model, suggesting that the Hunt-Crossley contact model may be preferable for modeling the fingerpad. Overall, our study demonstrates how the Hunt-Crossley and Elastic Foundation contact models behave in low-force biomechanical scenarios, such as those experienced during hand-object manipulation, and provides a foundation for incorporating contact mechanics into musculoskeletal models of the hand.

A Review of Forward-Dynamics Simulation Models for Predicting Optimal Technique in Maximal Effort Sporting Movements

The identification of optimum technique for maximal effort sporting tasks is one of the greatest challenges within sports biomechanics. A theoretical approach using forward-dynamics simulation allows individual parameters to be systematically perturbed independently of potentially confounding variables. Each study typically follows a four-stage process of model construction, parameter determination, model evaluation, and model optimization. This review critically evaluates forward-dynamics simulation models of maximal effort sporting movements using a dynamical systems theory framework. Organismic, environmental, and task constraints applied within such models are critically evaluated, and recommendations are made regarding future directions and best practices. The incorporation of self-organizational processes representing movement variability and “intrinsic dynamics” remains limited. In the future, forward-dynamics simulation models predicting individual-specific optimal techniques of sporting movements may be used as indicative rather than prescriptive tools within a coaching framework to aid applied practice and understanding, although researchers and practitioners should continue to consider concerns resulting from dynamical systems theory regarding the complexity of models and particularly regarding self-organization processes.

2-D forward dynamics simulation of gait adaptation to muscle weakness in elderly gait

BackgroundAge-related changes of human gait characteristics associated with muscle weakness have been reported in previous studies. Human gait is considered as a cyclic motion adapted to individual body-characteristics and the surrounding-environment based on motion criteria. Based on this hypothesis, elderly gait characteristics may be caused by an adaptation to muscle weakness. Research questionWhat role does gait adaptation to muscle weakness play in the development of elderly gait, and what criteria are used in elderly gait adaptation? MethodsWe examined the effects of gait adaptation to muscle weakness on steady gait characteristics using computational forward dynamics simulation with a two-dimensional neuro-musculo-skeletal model. For gait adaptation, we tested two motion criteria: (i) energy cost minimization, which is a widely used criterion for healthy adults; and (ii) energy rate minimization, based on existing measurements of elderly gait characteristics. ResultsProgression of muscle weakness enhanced the reduction of joint angle motion and minimum toe clearance, and finally resulted in falling. Gait adaptation to muscle weakness successfully formed stable walking patterns regardless of motion criteria, even at muscle weakness of 30 %, which represents a moderate degree of elderly muscle weakness. When criterion (i) was used, the time courses of joint motion were similar to those of healthy adults and a relatively high level of muscle activation was found in the whole gait cycle to compensate for muscle weakness. When criterion (ii) was used, the muscle activity level was lower than that of criterion (i) to minimize the energy rate, and the constructed gait successfully captured the characteristics of elderly gait reported in previous studies. SignificanceThese findings suggest that gait adaptation to muscle weakness plays an essential role in the development of stable gait characteristics, whereas elderly people might use a different motion criterion compared with healthy adults in gait adaptation.

The Effects of the Rectus Femoris Muscle on Knee and Foot Kinematics during the Swing Phase of Normal Walking

The role of rectus femoris (RF) muscle during walking was analyzed through musculoskeletal models to understand the effects of muscle weakness and hyperactivity. Such understanding is fundamental when dealing with pathological gait, but the contribution of RF as a bi-articular muscle is particularly difficult to estimate. Anybody software was used for inverse dynamics computation, and SimWise-4D for forward dynamics simulations. RF force was changed in the range of 0 to 150%, and the resulting kinematics were analyzed. Inverse dynamics showed a short positive RF power in correspondence with the onset of knee extension in the swing phase. Forward dynamics simulations showed an increasing knee flexion and initial toe contact when the RF force was decreased, and increasing knee extension and difficult foot clearance when the RF force was increased. The step became shorter with both increased and reduced RF force. In conclusion, the RF actively contributes to the knee extension in the swing phase. RF also contributes to obtaining a proper step length and to preparing the foot for initial heel contact. So the effect of RF muscle as a bi-articular muscle seems fundamental in controlling the motion of distal segments. RF overactivity should be considered as a possible cause for poor foot clearance in some clinical cases, while RF weakness should be considered in cases with apparent equinus.

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.