Time course of sympathoadrenal adaptation to endurance exercise in man

Today's blog post is on a paper published before we were born. Well, most of us anyway :-P

One of the first documented physiological effects of endurance exercise training was that trained subjects exhibited lower heart rates during exercise as compared to untrained. In this study, the authors wanted to determine the time course of development of this phenomenon as well as the relationship to whole body sympathetic nervous system activity. They took six normal dudes and put them on a seven week exercise training program involving both cycling and running. The intensity of training was increased at week four of the program. At one week intervals while subjects were training, they measured heart rate and plasma levels of catecholamines and lactate during isolated acute bouts of exercise. As expected, heart rate during acute bouts of exercise decreased over time, with further reduction when intensity was increased. Interestingly, plasma catecholamines decreased over time as well, but were not further decreased when training intensity was increased. The authors suggest that the decrease in heart rate during the first phase of training is mediated by decreased sympathetic action on the heart. The second phase of training however, produces effects on heart rate independent of the SNS, maybe through increased parasympathetic tone alone.

This and other exercise training studies performed around that time were among the first to establish the effect that chronic exercise has on autonomic control of the cardiovascular system. Without it, we would all be just running blots and doing PCR instead of cool rat experiments.