Patterns of Muscle Activation and Modulation of Ankle Intrinsic Stiffness in Different Postural Operating Conditions.

Intrinsic stiffness describes the dynamic relationship between imposed angular perturbations to a joint and the resulting torque response, due to intrinsic mechanical properties of muscles and joint, and inertia of the limbs. Recently, we showed that ankle intrinsic stiffness changes substantially with sway in normal standing. Here, we document how ankle intrinsic stiffness changes with postural operating conditions. Subjects stood on an apparatus, while subjected to ankle position perturbations, in five conditions: normal standing, toe-up and toe-down standing, backward and forward lean. In each condition, ankle intrinsic stiffness was estimated while accounting for its modulation with sway. The results demonstrated that ankle intrinsic stiffness varies widely, from 0.08 to 0.75 of critical stiffness, across postural operating condition; however, it is always smaller than the critical stiffness. Therefore, other contributions are necessary to ensure stable standing. The mean intrinsic stiffness was highest in forward lean and lowest in backward lean. Moreover, within each operating condition, intrinsic stiffness changed with center of pressure position, in one of three ways, each associated with a distinct muscle activation pattern; these include: 1) monotonically increasing stiffness-center of pressure relation, associated with a progressive increase in triceps surae activation, 2) decreasing-increasing stiffness-center of pressure relation, associated with initial activation of tibialis anterior and later activation of triceps surae, and 3) monotonically decreasing stiffness-center of pressure relation, associated with decreasing activation of tibialis anterior. Thus, intrinsic stiffness varies greatly within and across postural operating conditions and a correct understanding of postural control requires accounting for such variations.