Differential identification of passive and reflex mechanisms in human ankle stiffness dynamics

The authors have examined dynamic stiffness at the human ankle using position perturbations which were designed to provide a wide-bandwidth position input with low average velocity; and a parallel-cascade system technique for non-linear system identification. Ankle mechanics were identified in terms of a model having a linear passive pathway in parallel with a non-linear, velocity dependent reflex pathway. The reflex contribution to stiffness was most significant at low frequencies (below 10 Hz); its size relative to the passive contribution varied strongly with the parameters of the perturbation, joint position and level of activation. Thus, the role of stretch reflexes in motor control is likely to be very context sensitive