EFFECT OF VISION AND STANCE WIDTH ON HUMAN-BODY MOTION WHEN STANDING - IMPLICATIONS FOR AFFERENT CONTROL OF LATERAL SWAY

1. Measurements of human upright body movements in three dimensions have been made on thirty-five male subjects attempting to stand still with various stance widths and with eyes closed or open. Body motion was inferred from movements of eight markers fixed to specific sites on the body from the shoulders to the ankles. Motion of these markers was recorded together with motion of the point of application of the resultant of the ground reaction forces (centre of pressure). 2. The speed of the body (average from eight sites) was increased by closing the eyes or narrowing the stance width and there was an interaction between these two factors such that vision reduced body speed more effectively when the feet were closer together. Similar relationships were found for components of velocity both in the frontal and sagittal planes although stance width exerted a much greater influence on the lateral velocity component. 3. Fluctuations in position of the body were also increased by eye closure or narrowing of stance width. Again, the effect of stance width was more potent for lateral than for anteroposterior movements. In contrast to the velocity measurements, there was no interaction between vision and stance width. 4. There was a progressive increase in the amplitude of position and velocity fluctuations from markers placed higher on the body. The fluctuations in the position of the centre of pressure were similar in magnitude to those of the markers placed near the hip. The fluctuations in velocity of centre of pressure, however, were greater than of any site on the body. 5. Analysis of the amplitude of angular motion between adjacent straight line segments joining the markers suggests that the inverted pendulum model of body sway is incomplete. Motion about the ankle joint was dominant only for lateral movement in the frontal plane with narrow stance widths (< 8 cm). For all other conditions most angular motion occurred between the trunk and leg. 6. The large reduction in lateral body motion with increasing stance width was mainly due to a disproportionate reduction in the angular motion about the ankles and feet. A mathematical model of the skeletal structure has been constructed which offers some explanation for this specific reduction in joint motion. 7. The model demonstrates that when the knees are locked the joints of the ankles and hips become coupled together so that movement of one is accompanied by movement of the others. The strength of this coupling increases with stance width. The stiffness of the legs-pelvic structure is therefore passively increased by increasing stance width. 8. Coupling of the hips and ankles also predicts an increase in proprioceptive sensitivity to lateral motion about the ankles with increasing stance width, a factor which may contribute to the observed reduction in lateral motion. At the same time the model predicts that information from receptors in the head (visual and vestibular) become less sensitive in the detection of lateral motion about the ankles with increasing stance width. This may explain why vision was less effective in reducing lateral velocity of the body with greater stance widths.