Human standing posture: multi-joint movement strategies based on biomechanical constraints.

We developed a theoretical framework for studying coordination strategies in standing posture. The framework consists of a musculoskeletal model of the human lower extremity in the sagittal plane and a technique to visualize, geometrically, how constraints internal and external to the body affect movement. The set of all feasible accelerations (i.e., the "feasible acceleration set" or FAS) that muscles can induce at positions near upright were calculated. We found that musculoskeletal mechanics dictate that independent control of joints is relatively difficult to achieve. When muscle activations are constrained so the knees stay straight, to approximate the typical postural response to perturbation, the corresponding subset of the feasible acceleration set greatly favors a combination of ankle and hip movement in the ratio 1:3 (called the "hip strategy"). Independent control of these two joints remains difficult to achieve. When near the boundary of instability, the orientation and shape of this subset show that the movement strategy necessary to maintain stability, without taking a step, is quite restricted. Hypothesizing that regulation of center-of-mass position is crucial to maintaining balance, we examined the feasible set of center-of-mass accelerations. When the knees must be kept straight, the acceleration of the center of mass is severely limited vertically, but not horizontally. We also found that the "ankle strategy", involving rotation about the ankles only, requires more muscle activation than the "hip strategy" for a given amount of horizontal acceleration. Our model therefore predicts that the hip strategy is most effective at controlling the center of mass with minimal muscle activation ("neural effort").