Stabilization of old and new postural patterns in standing humans

In human stance, rotations around the hips and ankles typically exhibit a relative phase close to 20°, or close to 180°. In this article, we propose a model of stance that captures these postural states and the changes between them. We also describe the results of a recent study in which participants learned a novel pattern of hip and ankle coordination (a relative phase of 135°). Participants learned this novel pattern rapidly. At the same time, learning led to a robust destabilization of pre-existing patterns of hip-ankle coordination. The rate and type of destabilization depended upon the initial stability of the pre-existing patterns. We discuss similarities and differences between the learning of postural and bimanual coordination modes.