Aerobic Capacity Using Graded Bicycle Ergometer

Objectives

Assessment of aerobic exercise metabolism during space flight has been limited. Aerobic exercise responses during long-duration space flight have not been measured in the U.S. since the Skylab Program. The primary objective of the Skylab investigation was to determine whether man's metabolic effectiveness during mechanical work was progressively altered by exposure to microgravity. Skylab results of the exercise tests showed decreases in aerobic condition of the first crew during and after flight. The second crew increased the amount of inflight exercise and showed no change in aerobic condition during the flight, but showed a decrease postflight. The third crew, which exercised with both ergometer and treadmill, showed results different than the first two crews. The inflight exercises produced a physiologically significant increase (22%) of inflight peak oxygen consumption (VO_2peak) and peak work rate (20%) for one crewmember, while the other two showed a slight upward trend (3-6% and 0-2%, respectively) during the flight. This data, in conjunction with an insignificant change in postflight submaximal heart rate (3 to 9 bpm higher than preflight), may suggest an uncoupling between aerobic capacity and cardiovascular adjustments to exercise postflight. However, there were no studies that compared the inflight and postflight submaximal and peak responses of these linked systems. Two hypotheses were drawn based on the Skylab results:

1. The inflight cardiorespiratory responses to supine cycling will not be different from pre- and postflight values where posture is not a part of the testing.

2. The inflight cardiorespiratory responses to treadmill exercise will be different from pre- and postflight values where posture is a part of ground-based testing.

A 1977 Russian study evaluated a Russian "microcycle" countermeasure program. This program provided the equivalent volume of exercise as running 12.9 km (8 miles) per day for three consecutive days for a 70 kg person (no exercise was done on the fourth day). Exercising subjects maintained their baseline work capacity levels after 45 days at -4° head-down tilt bed rest (the effects of head down tilt bed rest is a close approximation to effects of weightlessness). A 1980 Russian study used a slightly reduced countermeasure protocol. The average volume of exercise for this study was equivalent to running 10 km (6.2 miles) per day, for approximately 6 days per week for a 70 kg person. Exercising U.S. subjects reported this amount of exercise to be greater than the average amount of preflight exercise, which is equivalent to approximately 3-4 miles per day for 3-4 days per week. The subjects showed a decrease in heart rates (91% of the maximal heart rate to 47%) when evaluated at a constant work rate. This data suggested that the subject showed an increase in aerobic capacity. This microcycle countermeasure program was used during a year-long Mir flight. The heart rate response to standard exercise increased during flight, suggesting a decrease in exercise conditioning. Adherence to the microcycle program, as well as postflight aerobic responses, were not reported. A 1992 report by Siconolfi and Charles reported that astronauts were able to maintain the VO_2peak after 7 to 11 days of space flight even when the volume of inflight exercise was equivalent to only 25-75% of their preflight exercise volumes. The ability to maintain aerobic responses on short-duration flights might not require the higher than normal volumes or intensities used on the longer duration Skylab 4 and Russian flights. Given the long-duration of the Mir flight, and the high volume of exercise in the Russian countermeasure program, a third hypothesis was proposed:

3. The Russian countermeasure program will increase aerobic capacity during the flight and this increase will be evident after the flight.

The main objective of this investigation was, therefore, to evaluate whether exercise countermeasures would mitigate the normally observed decreases in cardiorespiratory responses to exercise associated with long-duration exposure to microgravity. Aerobic capacity was measured before, during and after flight.

Shuttle-Mir Missions
Mir-18

Approach
Aerobic capacity was assessed from a graded exercise test performed on a cycle ergometer (CYCLE), as well as a modification of the Russian treadmill protocol (SPORT). Subjects cycled at several different stages:

ˆ 2 minutes at 50 watts
ˆ 5 minutes at 100 watts
ˆ 5 minutes at 125 watts
ˆ 5 minutes at 150 watts

The last stages of exercise were one minute in duration at 25 watt increments until the subject reached volitional fatigue or the maximum work rate of the cycle ergometer. Subjects then pedaled for 3 to 5 minutes after the test for a cool-down.

The SPORT protocol allowed subjects to select speeds (of walking, jogging, and running) based on standard descriptions of desired exercise level. Each subject completed the following:

ˆ 3 minutes of walking
ˆ 5 minutes of slow jogging
ˆ 2 minutes of moderate jogging
ˆ 1 minute of running at maximal speed

Heart rate (EKG) and metabolic gas analysis were recorded. The gas analysis included measurement of oxygen consumption (VO₂), carbon dioxide output (VCO₂), expired volume (VE), and respiratory rate (RR). Cardiac output (CO₂ rebreathing gas technique) was measured once during each 5-minute stage (3 times during CYCLE; once during SPORT). All measurements were repeated during the first minute of recovery. Resting (supine) heart rate and blood pressure were assessed before exercise. Aerobic capacity (VO_2peak) was the largest VO₂ recorded during the graded exercise test.

Preflight baseline data collection (BDC) occurred at 120, 32 and 31 before flight for the Mir-18 crew. Postflight BDC occurred at 5, 6, 12, and 15 days after landing. In-flight data collection occurred on FD5 of STS-71. All inflight data collection sessions were canceled due to time constraints. The aerobic responses to graded exercises will be assessed during the flight at regular intervals while on the Mir and Shuttle's Spacelab module. Postflight assessments will be conducted immediately before and immediately after rehabilitation if crewmembers have not returned to preflight levels by R+29.

Results
The sample size of this investigation was too small to make general inferences for long-duration space flight. However, the following results apply to the three subjects: (1) subjects completing a greater percentage of the prescribed exercise were closer to preflight levels of performance, (2) maintenance of submaximal performance, regardless of modality, appears to be linked to stroke volume, and (3) both CYCLE and SPORT protocols should be continued because they independently assess different aspects (cardiorespiratory and neuromuscular) of performance.

Publications
None available at this time.

Principal Investigators
Steven F. Siconolfi, Ph.D.
NASA/Johnson Space Center

Valeriy M. Mikhaylov, M.D.
Institute of Biomedical Problems

Co-Investigators
Eugene Kobsov, M.D.
Suzanne Fortney, Ph.D.
John H. Gilbert III, Ph.D.
Alexander Kotov, Ph.D.

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