Reduced training frequency effects on aerobic power and muscle adaptations in rats

Female rats were exercised by swimming up to 4 h/day either 2, 4, or 6 days/wk. After 7 wk they continued to train at these frequencies or had their training reduced from 6 to 4, 2, or 0 days/wk for an additional 9 wk. Ventricular weights and maximum O2 uptake (VO2max) were increased by 5-10% after training 2 days/wk, 15-17% after 4 days/wk, and 25-30% after 6 days/wk. Following reduced training, VO2max was similar when the 4- or 2-day/wk reduced training groups are compared with their 4- or 2-day/wk continued training counterparts. In contrast, VO2max was greater in the 0-day reduced than in the sedentary control group. No differences in mitochondrial markers or myoglobin content in red or mixed skeletal muscles were found between training 2 or 4 days/wk vs. reduced training at comparable frequencies. O2 uptake capacity of plantaris muscles and myoglobin concentration in fast-twitch red vastus lateralis muscles were greater in the 0-day reduced group than in the sedentary controls. These data show that VO2max and certain markers of aerobic metabolism in skeletal muscles of rats are lost at a slower rate than their rate of increase from the untrained state. However, a reduction of swimming frequency from 6 to 4 or 2 days/wk is not a sufficient stimulus to maintain VO2max, cardiac enlargement, or the increased aerobic potential of skeletal muscle at the 6-day/wk levels.(ABSTRACT TRUNCATED AT 250 WORDS)