Physical Training and Intrinsic Cardiac Adaptations

T HERE IS MUCH evidence that physical training programs are beneficial to integrated cardiovascular function. It is also suggested that training may afford protection against some of the effects of coronary artery disease. Training results in the ability to achieve an increased maximum exercise performance. This is accomplished by a greater cardiac output, increased oxygen extraction by peripheral tissues, and greater total oxygen consumption that can be achieved with maximum exercise. Part of the cardiac adaptation to training is by a decrease in heart rate and an increase in stroke volume for any level of physical exertion. This presumably permits a lower myocardial energy expenditure for a given cardiac output. Patients who have had myocardial infarctions, and who undergo retraining progra-ms, demonstrate the same types of cardiovascular adaptations as persons with normal hearts. Early reports also indicate that such patients may have a reduction in symptoms of myocardial ischemia and may have decreased recurrence rates from myocardial infarction. Although the apparent cardiovascular benefits of training might be ascribed in part to the effects of the training program on heart rate and stroke volume, this editorial addresses itself to evidence that training also leads to adaptations that are intrinsic to the myocardium, and proposes that these may be significant factors in the beneficial effects of training. The first question that might be asked is whether physical training changes the intrinsic capacity of the myocardium to function as a muscle or of the heart to function externally as a pump. When rats were made to.swim 6 hours daily for several weeks, heart rate was reduced, and isometric tension development by myocardial wall was increased.' In other experiments hearts from rats made to swim only 150 min/ day for 8 weeks were studied in an isolated working rat heart apparatus and compared with hearts from sedentary animals.2 At equal heart