A secondary objective of the experiment was to evaluate the bicycle ergometer as an inflight exercise device for long-duration missions.
The exercise levels were based upon the aerobic capacity (VO2 max) of each individual crewmember, determined at approximately 12 months and again at six months prior to launch. Based upon the aerobic capacity of each individual crewmember, a three-step workload protocol was established; it consisted of a rest period, exercise at 25%, 50%, and 75% max VO2, and a recovery period, each lasting for 5 minutes. The test was performed 5 to 8 times preflight. The inflight experiment protocol was scheduled to be repeated every 5 to 6 days by each crewmember during all three Skylab missions. There were 19 inflight tests on Skylab 2, 27 on Skylab 3, and 36 on Skylab 4. Thus, a total of 82 tests were performed inflight on the nine Skylab astronauts. Postflight, eight M171 tests were conducted on each crewmember: on landing day (designated R+0) and on days 1, 2, 3, 5, 11, 17, and 31 following landing.
Observations made on the earlier Skylab missions led to the decision to perform a modified protocol on Skylab 4, involving preflight and postflight tilt ergometry exercise tests (30 degrees from the horizontal). This decision was made in an attempt to better understand the previously observed postflight decrements in response to exercise. Astronauts on Skylab 4 followed the standard protocol described above, along with the modified tilt ergometer protocol, during the preflight and postflight data collection. The modified protocol consisted of supine rest, upright rest, upright exercise, supine exercise, and supine recovery, each performed for a period of 5 minutes. The exercise level used was identical to the first level at 25% VO2 max of each crewman's standard protocol. On Skylab 4, the modified tilt ergometer protocol was accomplished 15 and 5 days prior to launch, and again on landing day, on the first day after landing prior to performance of the M-171 standard protocol, and on 17 and 31 days after landing.
A secondary experiment objective was to evaluate the bicycle ergometer as an inflight personal exercise device used to maintain cardiovascular and muscular health. Although not required, the Skylab 4 crew agreed to periodically use instrumentation to measure heart rate, blood pressure, and respiratory parameters during personal exercise.
Inflight and postflight responses to exercise by the crews of all three missions were similar. Inflight, some subtle isolated differences were seen. However, no trends were observed, which would indicate a degradation in the exercise response. Six of the nine crewmembers had elevated resting ventilation inflight that was maintained in five of these individuals during the immediate postflight period. The exercise diastolic blood pressures were significantly decreased inflight in five of the crewmembers while exercising inflight oxygen consumption was slightly decreased in six crewmembers.
All three crewmembers of Skylab 4 displayed a decreased inflight heart rate during the 5 minute recovery period, as well as elevated resting and exercising heart rate immediately postflight. All three crewmembers exhibited a significantly elevated inflight resting minute volume which continued during the postflight testing for two of the astronauts. All crewmembers had decreased inflight resting and exercising diastolic blood pressure pattern. Also, inflight resting minute volume was significantly elevated, which continued during the postflight testing for two of the astronauts. On the first day following landing, one astronaut from Skylab 4 exhibited a decrease in cardiac output and stroke volume which returned to within 15% normal values one month postflight. This was believed to be associated with an increase in the arterial-venous oxygen difference.
The six astronauts of the Skylab 2 and 3 missions exhibited large decreases in both cardiac output and stroke volume after flight as compared to preflight values. In addition, for both crews, the cardiac output values returned to within 15% of the preflight level by the second day after landing, while the stroke volume deficit required 8 to 16 days to return to within 15% of preflight values.
Tilt ergometry demonstrated that resting HR increased when subjects were placed in an upright position after being tilt or supine. Preflight, the average increase was 12.6% while immediately postflight, the increase was 27%. On Skylab 4, supine exercising stroke volumes immediately postflight were within 5 ml of preflight values, whereas in the upright position, SV decreased 24 ml in all 3 crewmembers.
The conclusion is that, postflight, all crewmembers from Skylab missions 2, 3, and 4 showed a significant decrement in submaximal exercise response. The degradation was, in large part, evidenced by decreases in oxygen consumption, cardiac output, and stroke volume. These responses resulted from decreased venous return due to readjustments in fluid balance or vascular tone. Although not assessed experimentally, inflight exercise reduced the time required for readaptation postflight. Therefore, the bicycle ergometer was considered successful in terms of influencing cardiovascular fitness.