Ankle intrinsic stiffness is modulated by postural sway.

Dynamic ankle stiffness, which defines the relationship between joint position and the torque acting about it, determines the joint resistance to external perturbations. The contribution of joint stiffness to stance control is usually assumed to be constant; however, the stiffness may be modulated due to the ongoing sway during stance. This paper describes the results of the direct estimation of ankle intrinsic stiffness and its modulation with background torque during stance. Three subjects stood on a standing apparatus, while subjected to pulse perturbations of ankle position. The angle of center of mass, and ankle torque and angle were measured and individual torque responses to each position perturbation were analyzed. Each response lasted 90 milliseconds, where the mean of the torque record in the first 25 milliseconds was considered as its background torque. Ensemble of the responses with similar background torques were used to estimate the joint intrinsic stiffness. The estimated parameters showed that elastic element of the ankle intrinsic stiffness varied significantly (by a factor of 3) with background torque, i.e. the higher the background torque, the higher the stiffness. Thus ankle stiffness is not constant but varies greatly with postural sway.