[Effects of 6-degree head-down bed rest on physical fitness]

We investigated changes in enduring physical fitness during a ground-based weightlessness simulation of 6-degree head-down bed rest (HDBR). Ten healthy volunteers participated in the 6 days of HDBR. In each, their physique, muscular strength, physical fitness, cardiovascular function, and hematocrit were measured before and after HDBR. The results are as follows: (1) Thigh extensor power: the cross-sectional areas of the thigh and leg muscles were significantly decreased after HDBR, but recovered immediately afterwards. (2) Hand grip power and instantaneous power: the body response time showed no significant change before and after HDBR. (3) High physical capacity was correlated with a greater decrease in physical capacity after HDBR. (4) Remarkable functional depression appeared in the cardiovascular system compared with the gas exchange system as measured by the enduring physical fitness test. (5) The functional depression in the cardiovascular system lasted for more than 3 days after HDBR. (6) Six days of HDBR had little influence on physical fitness attributable to the inactivity and seemed to effectively simulate weightlessness for the cardiovascular system.