Effect of joint stiffness on standing stability.

Standing balance depends on the effective control of the torques at the ankle, knee, and hip. Stiffness at each joint and feedback proportional to joint angle contributes to these torques and to postural stability. This study examines the interaction of multiple joints on the minimum effective joint stiffnesses needed to maintain quiet standing and determines the inherent patterns of sway motion based on dynamic calculations of a four-link, three-joint, sagittal plane model. The equations of motion for quiet standing are solved to obtain the limits of stability for an individual (75 kg, 1.753 m tall) considering different combinations of joint stiffness. These calculations demonstrate that the single-link inverted pendulum model provides a less conservative estimate of minimum stiffness. That is, more stiffness is required at each joint to preserve stability when rotation is permitted at the knee and hip joints. Based on these analyses, the well recognized ankle and hip balance strategies appear to correspond to variations of the inherent patterns of motion of the lowest frequency mode. Additional calculations show that the stability decreases with an increase in body mass index. The present results quantify the interaction of the combined active and passive stiffnesses at the ankle, knee, and hip, and identify the minimum conditions needed for quiet standing. These criteria define standing-balance stability thresholds needed to assess the risk of falling and to guide rehabilitation.