Intrinsic and reflex contributions to human ankle stiffness: variation with activation level and position

A parallel-cascade system identification method was used to identify intrin sic and reflex contributions to dynamic ankle stiffness over a wide range o f tonic voluntary contraction levels and ankle positions in healthy human s ubjects. Intrinsic stiffness dynamics were de scribed well by a linear path way having elastic, viscous, and inertial properties. A velocity-sensitive pathway comprising a delay, a static non-linearity, resembling a half-wave rectifier. followed by a low-pass filter, described reflex stiffness dynami cs. The absolute magnitude of intrinsic and reflex stiffness parameters var ied from subject to subject but the relative changes with contraction level and position were consistent. Intrinsic stiffness increased monotonically with contraction level while reflex stiffness was maximal at low contractio n levels and then decreased. Intrinsic and reflex stiffness both increased as the ankle was dorsiflexed. As a result, reflex mechanics made their larg est relative contributions near the neutral position at low levels of activ ity. The size of the maximum reflex contribution varied widely among subjec ts, in some it was so small (ca 1%) that it would be unlikely to have any f unctional importance: however, in other subjects, reflex contributions were large enough (as high as 55% in one case) to play a significant role in th e control of posture and movement. This variability may have arisen because stretch reflexes were not useful for the torque-matching task in these exp eriments. It will be of interest to examine other tasks where stretch refle xes would have a direct impact on performance.