Effects of microgravity on maximal power of lower limbs during very short efforts in humans

The maximal power of the lower limbs was determined in four astronauts (age 37-53 yr) 1) during maximal pushes of similar to 250 ms on force platforms ["maximal explosive power" (MEP)] or 2) during all-out bouts of 6-7 s on a n isokinetic cycloergometer [pedal frequency 1 Hz: maximal cycling power (M CP)]. The measurements were done before and immediately after spaceflights of 31-180 days. Before flight, peak and mean values were 3.18 +/- 0.38 and 1.5 +/- 0.13 (SD) kW for MEP and 1.17 +/- 0.12 and 0.68 +/- 0.08 kW for MCP , respectively. After reentry, MEP was reduced to 67% after 31 days and to 45% after 180 days. MCP decreased less, attaining similar to 75% of preflig ht level, regardless of the flight duration. The recovery of MCP was essent ially complete 2 wk after reentry, whereas that of MEP was slower, a comple te recovery occurring after an estimated time close to that spent in flight . In the same subjects, the muscle mass of the lower limbs, as assessed by NMR, decreased by 9-13%, irrespective of flight duration (J. Zange, K. Mull er, M. Schuber, H. Wackerhage, U. Hoffmann, R. W. Gunther, G. Adam, J. M. N euerburg, V. E. Sinitsyn, A. O. Bacharev, and O. I. Belichenko. Int. J. Spo rts Med. 18, Suppl. 4: S308-S309, 1997). The larger fall in maximal power, compared with that in muscle mass, suggests that a fraction of the former ( especially relevant for MEP) is due to the effects of weightlessness on the motor unit recruitment pattern.