Time course of the adaptive responses of aerobic power and heart rate to training

Nine subjects participated in an exercise program for 40 min/day, 6 days/wk. The training work rates were kept constant for the first 4 wk. The work rates were then increased to a higher level and kept constant for an additional 5 wk. During both training periods, maximum oxygen uptake (V _2max ) increased for the first 3 wk and then remained constant. The half times (t _1/2 ) of the increases in VO _2max during the two periods were 10.3 and 10.8 days. These results provide evidence that the adaptation to endurance exercise of the system(s) that limit VO _2max is rapid, with a t _1/2 of less than 11 days. The total increase in VO _2max in the 9 wk of training was 23%. The decreases in the heart rate and blood lactate responses to a standard submaximal exercise test also occurred within the first 2 to 3 wk of each training period. Our results show that unless the training stimulus is increased, a high intensity daily exercise program does not result in a further increase in VO _2max or further decreases in the blood lactate or heart rate responses to submaximal exercise after 3 wk.