Relative contribution of ankle and hip muscles in regulation of the human orthograde posture in a frontal plane.

It was investigated, whether the postural regulation in the frontal plane takes place mainly at the hip or at the ankle level. The elimination of ankle torque was achieved by providing a point support in the frontal plane. (Two boards were attached to subject feet, below each board a metal pipe 2 cm diameter was fixed, so, the subject stood on contrivances resembling 'skates'). The lateral displacements of breast and hip, the angle of 'skates' tilt and the characteristics of frontal stabilogram and electromyogram of two ankle muscles (m. peroneus and m. soleus) were compared in two situations: (1) during normal standing; (2) under the conditions of the exclusion of ankle torque from postural control. During normal standing the body behaved as two-link inverted pendulum. Transition from normal standing to standing on free 'skates' produced changes in the kinematics of body movement. Under the conditions of ankle torque exclusion (free 'skates') breast and hip of a subject moved in a frontal plane as a single unit (one-link inverted pendulum). During standing on free 'skates' the electromyographic activity of m. peroneus and m. soleus was the same as during normal standing (approximately 70-100 microV).