Object size effects on initial lifting forces under microgravity conditions

Citation
I. Kingma et al., Object size effects on initial lifting forces under microgravity conditions, EXP BRAIN R, 124(4), 1999, pp. 422-428
Citations number
20
Categorie Soggetti
Neurosciences & Behavoir
Journal title
EXPERIMENTAL BRAIN RESEARCH
ISSN journal
00144819 → ACNP
Volume
124
Issue
4
Year of publication
1999
Pages
422 - 428
Database
ISI
SICI code
0014-4819(199902)124:4<422:OSEOIL>2.0.ZU;2-M
Abstract
Individuals usually report for two objects of equal mass but different volu me that the larger object feels lighter. This so-called size-weight illusio n has been investigated for more than a century. The illusion is accompanie d by increased forces, used to lift the larger object, resulting in a highe r 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 a lso counts for the enlarged initial effort in lifting a larger box. Why sho uld this happen? Under microgravity, subjects might be able to eliminate la rgely the weight-related component of the lifting force. Then, if persisten t upward scaling of the weight-related force component had been the main ca use of the elevated initial lifting force under normal gravity, this elevat ed force might disappear under microgravity. On the other hand, the elevate d 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, neces sary to accelerate the object. To test whether the elevated initial lifting effort either persists or disappears under microgravity, a lifting experim ent was carried out during brief periods of microgravity in parabolic fligh ts. Subjects performed whole-body Lifting movements with their feet strappe d to the floor of the aircraft, using two 8-kg boxes of different volume. T he 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, suggest ing a rapid adaptation to the loss of weight. Though the overall speed of t he lifting movement decreased under microgravity, the mean initial accelera tion of the box over the first 200 ms of the lifting movement remained high er (P=0.030) in the large box (1.87+/-0.127 m/s(2)) compared with the small box (1.47+/-0.122 m/s(2)). Under normal gravity these accelerations were 3 .30+/-0.159 m/s(2) and 2.67+/-0.159 m/s(2), respectively (P=0.008). A compa rable trend was found in the initial lifting forces, being significant in t he pooled gravity conditions (P=0.036) but not in separate tests on the nor mal 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 t he force component, required to accelerate the larger box, is an important factor in the elevated initial lifting effort land the associated size-weig ht illusion) under normal gravity.