Neuromechanical adaptations in joint stiffness when hopping with a SL-AFO

Abstract

Humans hopping on different elastic surfaces in series adjust leg stiffness to maintain system stiffness primarily by modulating ankle joint stiffness. When a parallel elastic element is applied at the ankle to assist plantar flexion, leg stiffness and total ankle joint stiffness is conserved. Moreover, biological ankle joint stiffness decreases to offset the added stiffness of the spring. We studied the effects of adding a resistive plantarflexion torque in parallel with the ankle. We hypothesized leg stiffness and total ankle joint stiffness would remain invariant and that biological ankle joint stiffness would increase to compensate.

We ran a repeated measures study on 10 subjects hopping in place under three conditions: AFO (control), Plantarflexion Assist Spring-Loaded AFO (PA-SLAFO), and Plantarflexion Resist Spring-Loaded AFO (PR-SLAFO). We collected kinematic, kinetic, and EMG data at each condition for three frequencies. We analyzed statistics using a Three-Way ANOVA (subject, condition, frequency) with a Bonferroni Post-Hoc test.

Different AFO conditions had no effect on leg stiffness. Total ankle joint stiffness was maintained for PA-SLAFO, with biological ankle joint stiffness decreasing to perfectly compensate. In the PR-SLAFO condition, total ankle joint stiffness was greater than expected. Biological ankle joint stiffness increased with PR SL-AFO, but was unable to completely compensate for the added resistance. This implies that despite adequate global compensation to maintain leg stiffness, subjects could not completely compensate for added resistive torque solely at the ankle and had to enlist a multi-joint compensation strategy. We are currently investigating these multi-joint compensation strategies.