Mohammad S. Shourijeh

Forward Dynamic Optimization of Human Gait Simulations: A Global Parameterization Approach

This study presents a 2D gait model that uses global parameterization within an optimal control approach and a hyper-volumetric foot contact model. The model is simulated for an entire gait stride, i.e., two full steps. Fourier series are utilized to represent muscle forces to provide a periodic gait with bilateral symmetry. The objectives of this study were to develop a predictive gait simulation and to validate the predictions. The comparison of simulation results of optimal muscle activations, joint angles, and ground reaction forces against experimental data showed a reasonable agreement.

An approach for improving repeatability and reliability of non-negative matrix factorization for muscle synergy analysis.

The aim of this study was to evaluate non-negative matrix factorization (NMF) and concatenated NMF (CNMF) to analyze and reliably extract muscle synergies. NMF and CNMF were used to extract knee joint muscle synergies from surface EMGs collected during a weight bearing, force matching task. Repeatability and between subject similarity were evaluated for each method using intra-class correlation coefficients (ICCs). High repeatability was found for CNMF (>0.99; 0.99-1.0) compared to NMF (>0.26; range 0.26-0.98). Reasonable consistency across subjects was improved using the CNMF over the NMF approach. CNMF was found to be a more reliable approach than NMF and suitable for between subject comparison of muscle synergies.

Use of muscle synergies and wavelet transforms to identify fatigue during squatting

The objective of this study was to supplement continuous wavelet transforms with muscle synergies in a fatigue analysis to better describe the combination of decreased firing frequency and altered activation profiles during dynamic muscle contractions. Nine healthy young individuals completed the dynamic tasks before and after they squatted with a standard Olympic bar until complete exhaustion. Electromyography (EMG) profiles were analyzed with a novel concatenated non-negative matrix factorization method that decomposed EMG signals into muscle synergies. Muscle synergy analysis provides the activation pattern of the muscles while continuous wavelet transforms output the temporal frequency content of the EMG signals. Synergy analysis revealed subtle changes in two-legged squatting after fatigue while differences in one-legged squatting were more pronounced and included the shift from a general co-activation of muscles in the pre-fatigue state to a knee extensor dominant weighting post-fatigue. Continuous wavelet transforms showed major frequency content decreases in two-legged squatting after fatigue while very few frequency changes occurred in one-legged squatting. It was observed that the combination of methods is an effective way of describing muscle fatigue and that muscle activation patterns play a very important role in maintaining the overall joint kinetics after fatigue.

A forward-muscular inverse-skeletal dynamics framework for human musculoskeletal simulations

This study provides a forward-muscular inverse-skeletal dynamics framework for musculoskeletal simulations. The simulation framework works based on solving the muscle redundancy problem forward in time parallel to a torque tracking between the musculotendon net torques and joint moments from inverse dynamics. The proposed framework can be used by any musculoskeletal modeling software package; however, just to exemplify, here in this study it is wrapped around OpenSim and the optimization is done in MATLAB. The novel simulation framework was highly robust for repeated runs and produced relatively high correlations between predicted muscle excitations and experimental EMGs for level gait trials. This simulation framework represents an efficient and robust approach to predict muscle excitation, musculotendon unit force, and to estimate net joint torque.

Efficient Hyper-Volumetric Contact Dynamic Modelling of the Foot Within Human Gait Simulations

This study describes the development of a multi-body foot contact model consisting of spherical volumetric models for the surfaces of the foot. The developed model is two-dimensional, and consists of two segments, the hind-foot, mid-foot, and forefoot as one rigid body and the phalanges collectively as the second rigid body. The model has four degrees of freedom: ankle x, y, hind-foot orientation, and metatarsal joint angle. Both ankle and metatarsal joints are assumed to be revolute joints. Three different types of contact elements are targeted: Kelvin-Voigt, linear volumetric, and nonlinear volumetric. The models are kinematically driven at the ankle and the metatarsal joints, and simulated horizontal and vertical ground reaction forces as well as center of pressure location are compared against the measured quantities within a complete human gait cycle. The hyper-volumetric foot contact model was found to be a suitable choice for foot/ground interaction modelling within human gait simulations.Copyright

Knee joint kinematics and kinetics during the hop and cut after soft tissue artifact suppression: Time to reconsider ACL injury mechanisms?

The recent development of a soft tissue artifact (STA) suppression method allows us to re-evaluate the tibiofemoral kinematics currently linked to non-contact knee injuries. The purpose of this study was therefore to evaluate knee joint kinematics and kinetics in six degrees of freedom (DoF) during the loading phases of a jump lunge and side cut using this in silico method. Thirty-five healthy adults completed these movements and their surface marker trajectories were then scaled and processed with OpenSims inverse kinematics (IK) and inverse dynamics tools. Knee flexion angle-dependent kinematic constraints defined based on previous bone pin (BP) marker trajectories were then applied to the OpenSim model during IK and these constrained results were then processed with the standard inverse dynamics tool. Significant differences for all hip, knee, and ankle DoF were observed after STA suppression for both the jump lunge and side cut. Using clinically relevant effect size estimates, we conclude that STA contamination had led to misclassifications in hip transverse plane angles, knee frontal and transverse plane angles, medial/lateral and distractive/compressive knee translations, and knee frontal plane moments between the NoBP and the BP IK solutions. Our results have substantial clinical implications since past research has used joint kinematics and kinetics contaminated by STA to identify risk factors for musculoskeletal injuries.

A practical solution to reduce soft tissue artifact error at the knee using adaptive kinematic constraints

Musculoskeletal modeling and simulations have vast potential in clinical and research fields, but face various challenges in representing the complexities of the human body. Soft tissue artifact from skin-mounted markers may lead to non-physiological representation of joint motions being used as inputs to models in simulations. To address this, we have developed adaptive joint constraints on five of the six degree of freedom of the knee joint based on in vivo tibiofemoral joint motions recorded during walking, hopping and cutting motions from subjects instrumented with intra-cortical pins inserted into their tibia and femur. The constraint boundaries vary as a function of knee flexion angle and were tested on four whole-body models including four to six knee degrees of freedom. A musculoskeletal model developed in OpenSim simulation software was constrained to these in vivo boundaries during level gait and inverse kinematics and dynamics were then resolved. Statistical parametric mapping indicated significant differences (p<0.05) in kinematics between bone pin constrained and unconstrained model conditions, notably in knee translations, while hip and ankle flexion/extension angles were also affected, indicating the error at the knee propagates to surrounding joints. These changes to hip, knee, and ankle kinematics led to measurable changes in hip and knee transverse plane moments, and knee frontal plane moments and forces. Since knee flexion angle can be validly represented using skin mounted markers, our tool uses this reliable measure to guide the five other degrees of freedom at the knee and provide a more valid representation of the kinematics for these degrees of freedom.

Estimation of Maximum Finger Tapping Frequency Using Musculoskeletal Dynamic Simulations

A model for forward dynamic simulation of the rapid tapping motion of an index finger is presented. The finger model was actuated by two muscle groups: one flexor and one extensor. The goal of this analysis was to estimate the maximum tapping frequency that the index finger can achieve using forward dynamics simulations. To achieve this goal, each muscle excitation signal was parameterized by a seventh-order Fourier series as a function of time. Simulations found that the maximum tapping frequency was 6 Hz, which is reasonably close to the experimental data. Amplitude attenuation (37% at 6 Hz) due to excitation/activation filtering, as well as the inability of muscles to produce enough force at high contractile velocities, are factors that prevent the finger from moving at higher frequencies. Musculoskeletal models have the potential to shed light on these restricting mechanisms and help to better understand human capabilities in motion production. [DOI: 10.1115/1.4036288]