Lower body negative pressure: the second manned Skylab mission.

Results of orthostatic evaluations of the crew of Skylab 3 with lower body negative pressure (LNBP) stress tests during their 59-d mission are reported. The test protocol was identical to that used in the first manned Skylab mission and the latter Apollo flights. Except for an inflight increase (rather than a decrease) in resting heart rates, results were essentially parallel to those observed in crewmen of the shorter Skylab 2 mission. Exaggerated elevations in heart rate and decreases in pulse pressure during LBNP stress inflight and immediately postflight corresponded to lowered orthostatic tolerance. Large decrements in resting calf size inflight and in total leg volume postflight indicated significant headward fluid shifts as had already been seen in the Skylab 2 crewmen. In addition, decreases in calf circumference gave no certain indication of a plateau over the 59 d inflight. On the other hand, percentage volume increase in calf size during LBNP stress inflight was greater than those in either preflight or postflight tests. Hypotheses elaborated after the Skylab 2 mission seem to have been substantiated, but several enigmas await data from the last and longer mission for clarification.