Maximal instantaneous muscular power after prolonged bed rest in humans

A reduction in lower limb cross-sectional area (CSA) occurs after bed rest (BR). This should lead to an equivalent reduction in maximal instantaneous muscular power (W-p) if the body segments' lengths remain unchanged. W-p wa s determined during maximal jumps off both feet on a force platform before and on days 2, 6, 10, 32, and 48 after a 42-day duration BR. CSA of thigh m uscles was measured by magnetic resonance imaging before and on day 5 after BR. Before BR, W-p was 3.63 +/- 0.43 kW or 48.6 +/- 3.3 W/kg. On days 2 an d 6 after BR, W-p was reduced by 23.7 +/- 6.9 and 22.7 +/- 5.4% (P < 0.01), respectively. Thigh extensors CSA (CSAEXT) was 16.7 +/- 4.7% (P < 0.01) lo wer than before. When normalized per CSAEXT, W-p was reduced by only 4.8 +/ - 4.5% (P < 0.05). By day 48 of recovery, W-p had returned to baseline valu es. Therefore, if W-p is appropriately normalized for CSA of the extensor m uscles, the reduction in CSAEXT explains most of the decrease in W-p, decre ase after BR. Other factors such as a deficit in neural activation or a dec rease in fiber-specific tension may account for only 5% of the W-p loss aft er BR.