Concurrent exercise and muscle protein synthesis: implications for exercise countermeasures in space.

Introduction: Exercise countermeasures for long duration space crews incorporate aerobic and resistance exercise (i.e., concurrent training). Microgravity simulation studies show that resistance exercise alone can be completely effective at preventing muscle loss, yet crews return from missions with reduced muscle mass and function. Some Earth-based studies show human skeletal muscle size and strength increases typically noted with resistance training are blunted when aerobic and resistance training are performed concurrently. The purpose of this study was to determine if the addition of aerobic exercise prior to performing resistance exercise blunts the acute anabolic response to resistance exercise.

Methods: Myofibrillar protein synthesis rates were determined in 12 individuals (6 men, 6 women, 26 +/- 2 yr) by measuring the incorporation of [2H5]phenylalanine into vastus lateralis muscle protein following resistance exercise (4 sets of 10 repetitions of both leg press and leg extension) and following resistance exercise that had been preceded by 90 min of strenuous aerobic cycling exercise.

Results: Myofibrillar protein synthesis following resistance exercise (0.092 +/- 0.006% x h(-1)) was not suppressed (p > 0.05) compared with following concurrent exercise (0.100 +/- 0.007% x h(-1)).

Discussion: It appears the anabolic response to resistance exercise cannot explain the attenuated muscle mass and strength gains imposed by chronic concurrent aerobic and resistance training, nor the response of space crews to in-flight concurrent countermeasures. These findings have important implications for future optimization of the concurrent aerobic and resistance exercise countermeasures for long duration space crews.