The influence of intrinsic properties (mass, stiffness, and alignment) on ankle joint dynamics, along with the sensitivity of a parametric optimization model to changes in these properties, was investigated. Experiments were performed on two young adults using a robotic manipulator to apply controlled perturbations comprising multiple ramp-and-holds while recording position, torque, and electromyography (EMG) data.
Position, torque, and EMG data were subsequently used as input for the computational model, producing a modeled torque. The model, designed to parametrize neural and non-neural contributors to joint stiffness associated with spasticity, employs a 23-parameter Hill-type neuromuscular framework to estimate muscle properties by minimizing the difference between modeled and measured torque.
Experimental results revealed that mass and stiffness significantly influenced the required torque, particularly during the hold phases, while alignment had minimal effects. Model sensitivity analysis showed similar trends, with stiffness having the greatest impact on torque, followed by mass. Changes in alignment were minimal, highlighting its redundancy.
Discrepancies were observed between model predictions and experimental data in capturing the effects of mass and stiffness changes. The model predicted equivalent mass effects regardless of angle, whereas experimental data exhibited angle dependency. Furthermore, modifying stiffness did not affect the dynamic phases of the modeled torque but did influence the dynamic phases of the measured torque.
These findings highlight the complex interplay between biomechanical factors and torque generation. Future work should refine the model to capture physical nuances better, improve experimental setups to control joint angles and optimize model parameters in a staged approach. Such efforts will advance the development of the model and deepen the understanding of spasticity, paving the way for more accurate and personalized treatment strategies.

Introduction: Pes equinus with increased ankle joint stiffness is a common impairment in stroke and Cerebral Palsy (CP) patients with structural and/or neural deficits of the ankle muscles. Equinus gait is characterized by toe-strike, abnormal ankle plantarflexion and decreased ankle range of motion (ROM). A new ankle-foot orthosis (AFO) with negative stiffness was previously developed to compensate for the increased ankle joint stiffness and improve equinus gait, reducing plantarflexion and increasing ankle ROM. The goal of this project was to predict the effects of ankle stiffness compensation on equinus gait simulations.

Methods: Forward simulations of unimpaired and equinus gait were generated in
the sagittal plane with a musculoskeletal model implemented in OpenSim and a
neural controller implemented in SCONE. After validation of the unimpaired gait
simulation, a sensitivity analysis on ankle kinematics was performed, introducing
structural and/or neural alterations of the ankle muscles to achieve an equinus gait. Consecutively, equinus gait simulations were validated with previously collected data of CP patients. Finally, an AFO model was developed in OpenSim to simulate AFO-assisted gait.

Results: The unimpaired gait simulation yielded realistic results, and was robust to all alterations, generating stable gaits. Shorter Gastrocnemius fiber length and/or increased plantarflexor muscles activations, with Tibialis Anterior weakness, had the largest effect on the ankle joint kinematics and resulted in realistic CP equinus gaits. Simulations of AFO-assisted gait resulted in reduced abnormal plantarflexion, and increased ankle ROM in the condition with shorter Gastrocnemius.

Discussion and Conclusion: This study presented realistic simulations of equinus
gait, by modelling structural and/or neural alterations of the ankle muscles and
predicted improved ankle function when compensating ankle stiffness with an external force. Although higher stiffness compensation and more simulations should be obtained, this study provides a solid ground for further investigation of the AFO effects. These results can ultimately assist in the prescription and tuning of the new AFO for patients with equinus.