For maintenance of metabolic equilibrium physical activity, like nutrition, is basic; yet, unlike the latter, interest in the relation of exercise to health and disease has been slow in developing. It is true that from the time of Hippocrates, and even before, exercise was held important for the well as for the sick, but its use was intuitive or empirical and often postulated on erroneous "systems" irreconcilable with modern scientific knowledge. Only recently have studies begun to elucidate the relationship of activity to physiologic processes. Such investigations as those of Deitrick and Keyes indicate the deleterious effect of immobilization upon calcium and nitrogen balance, circulation, and behavior.1,2 During World War II clinicial evaluation of inactivity in convalescence led to the use of early ambulation, with consequent decrease in time of hospitalization and in incidence of secondary complications, such as pain, constipation, insomnia, and the more serious pneumonic and thromboembolic
GORDON EE. Energy Costs of Activities in Health and Disease. AMA Arch Intern Med. 1958;101(4):702–713. doi:10.1001/archinte.1958.00260160024004
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