Prior resistance training and sex influence muscle responses to arm suspension.

PURPOSE: The magnitude of atrophy and strength loss induced by partial or complete disuse of skeletal muscles varies greatly among individuals. Factors predisposing some individuals to more extreme responses have not been identified. The purpose of this investigation was to determine whether 1) level of activity before disuse or 2) sex differences influence the magnitude of atrophy and changes in muscle strength, endurance, and sense of muscular effort in response to 21 d of arm suspension.
METHODS: Thirty-one individuals (18 women, 13 men) completed either 8 wk of resistance training (TRAINED group) or no training (UNTRAINED group) before 21 d of elbow muscle arm suspension achieved by having one arm in a sling tethered to the body by a swathe. Muscle volume was measured using serial magnetic resonance imaging (MRI) cross-sectional images. Functional measurements included maximal isometric force (MIF) for elbow flexion, one-repetition maximum (1RM) for biceps curl, number of repetitions to fatigue at 50% of 1RM, and sense of muscular effort measured using a force-matching task.
RESULTS: Flexor muscle volume decreased (P < 0.001) by -7.7 +/- 7.3% across all subjects. The decrease (P < 0.001) in flexor muscle volume was significant in men but not women. Arm suspension induced decreases (P < 0.001) in MIF and 1RM that did not differ across sex or training groups. The number of repetitions to fatigue decreased (P < 0.05) in the UNTRAINED but not TRAINED groups. No changes in sense of muscular effort were measured.
CONCLUSION: The smaller initial muscle size or sex-specific factors attenuated muscle loss but not strength or endurance losses in females during disuse. Resistance training before disuse may attenuate the loss in muscular endurance.