Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, PC63
Vestibular and somatosensory influences on the position of balance during human standing
A. Butler1, R. Fitzpatrick1
1. Prince of Wales Medical Research Institute, Randwick, New South Wales, Australia.
Studies in which subjects balanced a ‘virtual body’ to exclude vestibular input suggest that proprioceptive input from the legs is sufficient to stand (Fitzpatrick et al., 1994) and that vestibular inputs normally play no part in controlling body sway (Fitzpatrick & McCloskey, 1994). However, there is no unique pattern of somatosensory input from the legs that signals the vertical alignment of the body. The implication that vestibular inputs are not involved with balance control is inconsistent with clinical experience, studies of reflex responses to balance perturbations (Allum & Pfaltz, 1985) and vestibular stimulation (Cathers et al., 2005). Thus, it seems that the vestibular system has a role in balance control but not one concerned with controlling the extent of body sway. This study uses the virtual-body method to investigate the proprioceptive and vestibular contribution to human standing. Twelve healthy adults participated in this study that was approved by the Human Research Ethics Committee of the University of New South Wales. Balance was assessed under three conditions; (i) normal standing, (ii) splinted standing to prevent rotation of joints above the ankle and (iii) balancing a virtual body (inverted pendulum), which simulated standing but excluded vestibular and graviceptive inputs. In each condition, subjects were tested with and without additional weight attached to the body or pendulum. Sway, alignment angle and ankle torque were recorded. While standing normally, the position of the centre of mass of the body was approximately over the centre of the perimeter of the feet. This was maintained when a 30 kg weight was fastened around the pelvis (Δ mean: 2.0 ± 4.4 mm). In contrast, adding 30 kg to the virtual body with vestibular input unavailable caused the centre of mass to shift back towards the ankles (Δ mean: 24.3 ± 1.5 mm; P < 0.001). With only somatosensory input available, subjects balanced a lightweight virtual body over the front of the feet, their matched virtual body over the centre of the feet and a heavyweight virtual body over the heels. These positions did not keep constant the centre of foot pressure, muscle force, ankle angle or the excursion of sway. It is concluded that vestibular and graviceptive sensory input, although not used to control sway during standing, has a unique role in aligning the body’s centre of mass above the feet, a function that cannot be achieved by proprioceptive inputs alone.
Where applicable, experiments conform with Society ethical requirements