IDENTIFICATION OF INTRINSIC AND REFLEX CONTRIBUTIONS TO HUMAN ANKLE STIFFNESS DYNAMICS

We have examined dynamic stiffness at the human ankle using position p erturbations which were designed to provide a wide-bandwidth input wit h low average velocity. A parallel-cascade, nonlinear system identific ation technique was used to separate overall stiffness into intrinsic and reflex components. Intrinsic stiffness was described by a linear, second-order system similar to that demonstrated previously. Reflex st iffness dynamics were more complex, comprising a delay, a unidirection al rate-sensitive element and then lowpass dynamics. Reflex mechanisms were found to be most important at frequencies of 5-10 Hz. The gain a nd dynamics of reflex stiffness varied strongly with the parameters of the perturbation, the gain decreasing as the mean velocity of the per turbation increased. Under some conditions, torques generated by refle x mechanisms were of the same magnitude as those from intrinsic mechan isms. It is concluded that reflex stiffness can be large enough to be important functionally, but that its effects will depend strongly upon the particular conditions.