Reduced training intensities and loss of aerobic power, endurance, and cardiac growth

Twelve subjects participated in an exercise program of cycling and running 40 min/day, 6 days/wk. After 10 wk, they continued to train with either a one-third or two-thirds reduction in work rates for an additional 15 wk. Frequency and duration for the additional training remained the same as during the 10 wk of training. The average increases in maximum O₂ uptake (V̇O₂(max)) were between 11 and 20% when measured during cycling and treadmill running after 10 wk of training. V̇O₂(max) was not maintained at the 6-day/wk training levels with a one-third reduction in training intensity but was still higher than pretraining levels. With a two-thirds reduction in intensity, V̇O₂(max) declined to an even greater extent than with the one-third reduction. Short-term endurance (~5 min) was maintained in the one-third reduced group but was markedly reduced in the two-thirds reduced group. Long-term endurance was decreased significantly from training by 21% in the one-third reduced group (184-145 min) and by 30% in the two-thirds reduced group (202-141 min). Calculated left ventricular mass, obtained from echocardiographic measurements, increased ~15% after training but returned to control levels after reduced training in both groups. These results demonstrate that training intensity is an essential requirement for maintaining the increased aerobic power and cardiac enlargement with reduced training. However, other performance criteria such as short- and long-term endurance were not reduced in parallel with the V̇O₂(max) changes and may, therefore, be under additional regulation.