Modeling Effects of Ankle Foot Orthoses (AFOs) in Computer Simulations of Gait

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Abstract

AFOs are prescribed for ambulatory stroke patients to eliminate foot drop, provide joint stability and reduce pain. Previous studies of AFO effects have focused on functional gains (e.g., stride length, speed, and single limb support); however, none to date have elucidated AFO effects on neuromuscular control. Therefore, the primary goals of this study were to develop an AFO torque model and apply it to existing 2-D forward dynamic computer simulations of gait. The model—derived from analysis of isolated AFO torque response—was applied to normal (symmetric) and hemiparetic (asymmetric) gait simulations to optimize input control parameters and thus, identify plausible changes in muscle excitation and function caused by the AFOs. Our torque isolation test results generally agree with previously published studies; however, we observed more pronounced rate dependencies. Simulations with the model revealed several AFO effects linked to altered muscle excitation. Compared with the non-AFO condition, we found delayed peak hip flexor moment, increased knee moments, earlier stance ankle extension moments, decreased net excitation of tibialis anterior, decreased soleus-induced ankle accelerations, and strengthening of synergy between vastii and gastrocnemius (VAS-GAS) with an AFO. In the hemiparetic case, AFO changes included altered soleus and medial gastrocnemius excitation and stronger VAS-GAS synergy. Future work will focus on further model development to enhance our understanding of the scope of AFO influences and facilitate improvements in AFO design, tuning and therapies.

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