Modulation of intrinsic and reflex contributions to dynamic ankle stiffness with the level of voluntary contraction

A parallel-cascade system identification method was used to identify intrinsic and reflex contributions to dynamic ankle stiffness at different levels of tonic voluntary contractions. Intrinsic stiffness dynamics were well modeled by a linear second-order system relating intrinsic torque to joint position. Reflex stiffness dynamics were accurately described by a linear, third-order system between half-wave rectified velocity and reflex torque. Intrinsic stiffness gain increased with the level of tonic contraction in the ankle extensors (gastrocnemius-soleus). Reflex gain decreased with the level of contraction while reflex-EMG gain increased. This suggests reflex contributions are more significant at low levels of activity.