The contribution of foot deformation to the changes of muscular length and angle in the ankle joint during standing in man.

This study was designed to evaluate the extent of foot deformation in healthy subjects during standing on an immobile support and during slow tilts of the support platform by 1 deg. The angle in ankle joint was evaluated by two methods: as an angle of skin inclination relative to the platform and as an angle, calculated on the basis of recording of the projective length of the soleus muscle. It was shown that the real changes of the angle in ankle joint during standing on an immobile platform were up to 2 times smaller than the changes of angular position of the shin relative to vertical axis. However, considerable intersubject variability was observed in this respect. During slow tilts of the support platform a marked divergence was observed in the shape of recordings of two "ankle angles" in subjects with high foot compliance. The vertical displacements of the calcaneus recorded by means of a clamp rigidly fixed at the heel were 0.5 +/- 0.3 mm (the range 0.1-1 mm) for each degree of body deviation in the forward or backward direction. In 12 subjects, the average foot compliance was 0.04 +/- 0.03 deg/Nm (maximal value 0.1 deg/Nm). It can be assumed that the mechanical properties of the foot can appreciably influence the afferent outflow during maintenance of orthograde posture in man.