Training-induced changes in hormonal and metabolic responses to submaximal exercise

Plasma glucagon and catecholamines increase during prolonged submaximal exercise, but the magnitude of the increase is less in endurance-trained individuals than in untrained subjects. We have studied the rapidity at which this adaptation occurs. Six initially untrained healthy subjects exercised vigorously (on bicycle ergometers and by running) 30–50 min/day, 6 days/wk, for 9 wk. Prior to the beginning of training and at 3-wk intervals thereafter, participants were subjected to 90-min bicycle ergometer test work loads that elicited 58 +/- 2% of the subjects' initial maximal oxygen consumption. The major proportion of the training-induced decrement in plasma glucagon and catecholamine responses to exercise was seen after 3 wk of training. We conclude that the hormonal component of the training adaptation occurs very early in the course of a vigorous endurance training program.