Proceedings of The Physiological Society

Physiology 2012 (Edinburgh) (2012) Proc Physiol Soc 27, PC256

Poster Communications

Sensory feedback during interpersonal light fingertip contact stabilises human balance

C. J. Osler1, R. F. Reynolds1

1. Sport and Exercise Sciences, University of Birmingham, Birmingham, United Kingdom.

  • Figure - Group average cross-correlations (n=8) show the relationship between anteroposterior centre of pressure velocities of each individual.

Lightly touching a fixed point of reference has been shown to reduce body sway compared to normal standing (Jeka and Lackner, 1994). When touching another person the point of reference is not fixed, but constantly moving due to body sway. Despite this, recent studies have shown interpersonal light fingertip contact to be effective in stabilising balance (Johannsen et al., 2009; 2012). However, the underlying mechanisms remain unclear. Here sixteen subjects (3 all-male, 5 all-female pairs) stood facing a partner who was positioned in front and slightly to their right, such that their right shoulders were approximately aligned. Both stood in narrow bipedal stance on a separate force plate. We measured their body sway in a number of interpersonal contact (shoulder, light fingertip, no contact) and visual (both eyes closed, one subject eyes closed) conditions. Sway speed was significantly affected by the form of interpersonal contact between the pair (F2,28 = 20.9, p<0.001; repeated measures ANOVA). For instance, extending their right arms and grasping their partner's shoulder led to a 15±12% (n=16, mean±SD in all cases) reduction in sway, when both subjects had eyes closed. Furthermore, reaching out with their right forearms and lightly touching fingertips reduced sway by 9±7% (n=16). But how is this stabilisation achieved? Cross-correlations were used to analyse the relationship between subjects' anteroposterior body motions at delays of up to ±1s. When both subjects had eyes closed (see Figure, black lines) the results were as follows. During shoulder contact the pairs motion was highly correlated at approximately zero lag (peak correlation at -70±115ms, n=8), demonstrating they were strongly linked. This suggests that the reduction in sway in this condition involved mechanical stabilisation. In contrast, during light fingertip contact correlation magnitude was low at zero lag. It was, however, increased either side of zero, with two distinct peaks occurring at -478±202ms and 366±117ms (n=8). This demonstrates that each subjects' sway was correlated with their partner after a short delay, in a reciprocal manner. This suggests that stabilisation in this condition involves sensory feedback mechanisms. When one subject was permitted to open their eyes (see Figure, grey lines), thus causing asymmetrical visual conditions, the effects of light fingertip contact were also asymmetrical. That is, the relationship of the eyes-closed subject's sway leading their partner was enhanced. This was also the case in the no-contact condition, suggesting interpersonal visual inputs modulate body sway. Nonetheless, with light fingertip contact the average peak correlation continued to occur at a delay of around ±300-500ms. We therefore conclude that mechanisms of sensory feedback underlie the reduction in body sway during interpersonal light fingertip contact.

Where applicable, experiments conform with Society ethical requirements