Stepping from a narrow support.

The study addresses postural preparation to stepping. In particular, it tests a hypothesis that such preparation involves adjustments in the activity of ankle plantarflexors to produce shifts of the center of pressure. We investigated the initiation of a step from quiet stance when the subjects stood on boards with a decreased dimension of the support area in the anterior-posterior direction ("unstable boards"). Stepping from an unstable board was associated with decreased preparatory shifts of the center of pressure (COP) in the anterior-posterior direction from about 3 cm to 0.9 cm and further to 0.1cm when the support narrowed from comfortable standing to 3.3 cm and to nearly 0 cm. There was a smaller decrease in the COP shift in the medio-lateral direction. When the subjects stood on a board which rested on a very narrow ridge ("zero-support"), they showed an increase in the magnitude of changes in the horizontal force immediately prior to making a step. There was a general increase in the level of activation of leg and trunk muscles during stepping from unstable boards. The modulation of the activity of ankle plantarflexors increased during stepping from unstable boards. We conclude that, to initiate a step, COP shifts and changes in shear force can be modulated independently of each other in a constraint-specific manner. The results speak against the hypothesis that modulation of ankle plantarflexor activity during postural adjustments is directly related to the production of COP shifts.