['To examine the effect of unweighting as a possible contributory factor to a reduced calf muscle volume on postural sway during quiet standing, changes in postural sway following bed rest with or without strength training were investigated.', 'Twelve young men participated in a 20-day bed-rest study. Subjects were divided into a non-training group (BR-Con) and a strength training group (BR-Tr). For the BR-Tr group, training was comprised of dynamic calf-raise and leg-press exercises to maintain the muscle volume of the plantar flexors. Before and after bed rest, subjects maintained quiet standing in a barefoot position on a force platform with their eyes open or closed. During the quiet stance, foot centre-of-pressure (CoP) and the mean velocity of CoP was calculated. Muscle volume of the plantar flexors was computed using axial magnetic resonance images of the leg.', 'After the bed-rest period, the muscle volume decreased in the BR-Con group but not in the BR-Tr group. The mean velocity of CoP as an assessment of postural sway, however, increased in both groups. These results indicate that the strength training during bed rest cannot counteract the increase in postural sway.'] Do effects of 20-day bed rest with and without strength training on postural sway during quiet standing?