Metabolic characteristics of skeletal muscle during detraining from competitive swimming.

After 5 months of intense training, eight male swimmers were studied during 4 wk of inactivity. Biopsy specimens from the deltoid muscle revealed that its respiratory capacity (QO2) decreased by 50% (5174 to 2559 microliter X h-1 X g-1) after 1 wk of inactivity. Subsequent weeks of detraining did not change the QO2. Although the trained swimmers' muscle phosphofructokinase and phosphorylase activities were significantly higher (P less than 0.05) than those from a group (N = 8) of untrained men, 4 wk of detraining had no effect on these enzyme activities. Mean (+/-SE) resting muscle glycogen concentrations were significantly higher (P less than 0.05) for the trained swimmers (153 +/- 3 mmol X kg-1) than for the untrained men (85 +/- 7.5 mmol X kg-1). Over the 4 wk of inactivity, the swimmers' muscle glycogen progressively decreased from 153 (+/- 3) to 93 (+/-7) mmol X kg-1. After a standard 183-m swim at 90% of the swimmer's best time for that distance, blood lactate rose from a mean of 4.2 (+/-0.8) at week 0 to 9.7 (+/-0.8) mmol X 1(-1) at week 4. These observations demonstrate dramatic changes in the metabolic characteristics of the swimmer's muscle with a 1-4-wk interruption in training.