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.