I. Absolute Physical Efficiency

The development of proper and practical tests for the estimation of the dynamic physical fitness of adolescents presents the same difficulties which accompany all attempts to study any aspect of the physiology of members of that group: the variations in degree of maturity and size which obtain for each of its chronological age groups are so great that standards of achievement and of procedure require both considerable study and considerable apology. An attempt has been made, however, to study various aspects of the response of adolescents to exercise, and to develop a simple, practical method which can be used to test their ability to perform and to recover from brief, strenuous exercise. Only physical fitness so defined is considered here. It is realized that no single test can measure fitness, in all its aspects, with great accuracy.10 There is, however, a close relation between many of the physiological changes which accompany exercise,1'46 and a reasonable estimate of fitness can be obtained by a study of a combination of such factors as maximum heart rate, maximum lactate and duration of work,7 or of duration of work and deceleration of heart rate following work.8 In our first investigations the amount of work given each subject was kept constant regardless of his size or age, as it is in most methods of testing adults, because we were interested in finding what relation exists between absolute capacity to perform muscular work and either age or body surface. Our later modifications of this preliminary technic were made possible by the information gained in that way, and more practical methods of testing fitness gradually have been developed. Method A group of 149 boys who had been declared normal following a complete medical examination and ranging in age from 13 to 19 years were chosen,; for the purposes of another study each of these individuals was selected because of membership in one or another athletic group. Each boy, regardless of his age or size, was asked to ride a bicycle ergometer at a rate of 20 miles per hour against a friction load of 5 pounds for 5 minutes. Before exercise, during the work period, and also during the subsequent 5-minute recovery period, a continuous recording of the subject's heart rate was taken on a Guillemin cardiotachometer.3 Five minutes after exercise blood for a lactate 659