Additional afferent signals in the human upright posture control system

Abstract

Equilibrium maintenance was estimated in a subject standing with the eyes closed while holding a small weight. The experiments were performed on a movable platform in the form of a seesaw. Loads of 200, 500, and 1000 g were held by the subjects between the forefinger and the thumb, with the arm bent at the elbow. The rate of change in the length of the sagittal stabilogram and the root mean square deviation of the center of pressure from the equilibrium position were less when a subject was holding a load while standing on a movable support. The rate of change in the stabilogram length was the lowest (44.5 ± 6.8 mm/s) if the subject stood holding a 1000-g load and was 52.6 ± 9.2 mm/s without a load (p < 0.05, paired T-test). At the same time, when these loads were fixed on a mechanical holder attached to the trunk and simulating an arm bent at the elbow, there was no significant change in stabilogram parameters. Apparently, postural sway reduction is associated with the fact that the system of equilibrium maintenance can control the upright posture using an uncommon afferent input, namely, modulation of afferent signals induced by inertial interaction of an object and the fingers.