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Individuals usually report for two objects of equal mass but different volume that the larger object feels lighter. This so-called size-weight illusion has been investigated for more than a century. The illusion is accompanied by increased forces, used to lift the larger object, resulting in a higher initial lifting speed and acceleration. The illusion holds when subjects know that the mass of the two objects is equal and it is likely that this also counts for the enlarged initial effort in lifting a larger box. Why should this happen? Under microgravity, subjects might be able to eliminate largely the weight-related component of the lifting force. Then, if persistent upward scaling of the weight-related force component had been the main cause of the elevated initial lifting force under normal gravity, this elevated force might disappear under microgravity. On the other hand, the elevated initial lifting effort in the large box would be preserved if it had been caused mainly by a persistent upward scaling of the force component, necessary to accelerate the object. To test whether the elevated initial lifting effort either persists or disappears under microgravity, a lifting experiment was carried out during brief periods of microgravity in parabolic flights. Subjects performed whole-body lifting movements with their feet strapped to the floor of the aircraft, using two 8-kg boxes of different volume. The subjects were aware of the equality of the box masses. The peak lifting forces declined almost instantaneously with approx. a factor 9 in the first lifting movements under microgravity compared with normal gravity, suggesting a rapid adaptation to the loss of weight. Though the overall speed of the lifting movement decreased under microgravity, the mean initial acceleration of the box over the first 200 ms of the lifting movement remained higher (P=0.030) in the large box (1.87+/-0.127 m/s2) compared with the small box (1.47+/-0.122 m/s2). Under normal gravity these accelerations were 3.30+/-0.159 m/s2 and 2.67+/-0.159 m/s2, respectively (P=0.008). A comparable trend was found in the initial lifting forces, being significant in the pooled gravity conditions (P=0.036) but not in separate tests on the normal gravity (P=0.109) and microgravity (P=0.169) condition. It is concluded that the elevated initial lifting effort with larger objects holds during short-term exposure to microgravity. This suggests that upward scaling of the force component, required to accelerate the larger box, is an important factor in the elevated initial lifting effort (and the associated size-weight illusion) under normal gravity.

Object size effects on initial lifting forces under microgravity conditions