Gravity, thixotropy and human position sense

Abstract

This Opinion Article is about human position sense, the influences on its measurement by changes in gravity and by thixotropic conditioning. It is widely agreed that position sense is one of the more important proprioceptive senses. It provides us with information about where our different body parts are in relation to each other and to their surroundings, as well as whether they are moving or not. Knowledge of the position of our limbs is also important in motor control, for example, in order to carry out accurate reaching movements (Sarlegna & Sainsburg, 2009). In this age of an increased interest in space travel and, indeed, its commercial exploitation, growing attention has focussed on the physiological changes the body undergoes in conditions of weightlessness. We have maintained a continuing interest in human position sense (Proske & Gandevia, 2012) and were therefore fascinated by reports of disturbances to position sense evoked by weightlessness. Lackner & DiZio (2000) quoted reports by Schmitt & Reid (1985) of astronauts awakening in the dark, unable to feel the location of their arms; they could see a luminous dial floating in mid-air, but be unaware it was the watch on their own arm. These casual observations prompted us to consider making some measurements of our own.