Twenty astronauts on missions of 5 to 17 days' duration completed one or two exercise tests before space flight and another test on landing day. Seventeen of the astronauts exercised during the flight, using a rower (three astronauts), a cycle ergometer (10) or a treadmill (four) that had a harness and tether to simulate gravity. They followed continuous (one) or interval (16) exercise protocols with target heart rates intended to correspond to 65 to 85% of preflight maximum oxygen consumption. Some subjects had difficulty achieving these heart rates on the treadmill. A monitor displayed and recorded the heart rate. The total exercise performed during flight was quantified by multiplying the percentage of the maximal heart rate by the number of minutes per exercise session and by the number of sessions per week.

The body composition of 10 of the astronauts was analyzed preflight and on R+2 after flights lasting seven to 16 days. Extracellular water, intracellular water, total body fat, percent fat, and fat-free mass were derived from body weight, body density, and body water. Body density was determined by weighing the astronauts under water and correcting for residual lung volume. Body water was determined from a previously validated bioelectrical response spectroscopy model to avoid taking the multiple blood samples and using the radioactive tracers or other tracers that are usual for this purpose. Total body protein and mineral contents were derived from total body water and body density by using a mathematical model that assumes the body is made up of three compartments (fat, water, and dry lean mass).

The effects of changes in body composition on aerobic capacity were studied in eight astronauts from 7- to 16-day missions. Preflight and R+0 or R+2, their body composition was analyzed by the methods described above, and their aerobic capacity was determined in treadmill exercise tests on the same day, after the body composition measurements.

RESULTS:
Postflight peak oxygen consumption during the exercise tests was 13% lower than that of the astronauts who did not exercise preflight. The exercise test results from this DSO were analyzed together with those from DSO 476, and the combined results indicated that the capacity to perform aerobic exercise was reduced after space flight, but that aerobic exercise during flight attenuated this effect. The results did not indicate the optimal exercise device or protocol for maintaining aerobic capacity.

Results of the analysis of body composition showed that the astronauts lost about two percent of their fat-free mass, about three percent of their intracellular water and about four percent of their total body protein and minerals from preflight to postflight. Little change occurred in extracellular water or in fat. The decrease in intracellular water may have been largely due to the loss of proteins and minerals, because differences in the concentrations of dissolved proteins and minerals cause water to move by osmosis. These losses could affect the performance or health of the astronauts during or after space flight.

The eight astronauts who underwent both body composition analysis and exercise testing lost 12% of their aerobic capacity from preflight to postflight. Statistical analysis of covariance, which was performed to separate the effects of the various body composition variables, showed that the decreases in aerobic capacity were due at least in part to decreases in total body protein. The protein loss most likely represented a loss of muscle mass.