Autonomic control of exercise tolerance was comparatively analyzed in three groups of apparently healthy adolescents and young people aged 16.5±2 years: 1) juvenile convicted persons; 2) schoolchildren and students; 3) sportsmen in cyclic sports. Original procedures for mathematical simulation of a cardiac rhythmogram obtained during exercise testing were used to study of autonomic heart rhythm regulation. The characteristic properties that are determined by lifestyle and that are a basis for mixed (aerobic/anaerobic) endurance and a cause of exercise-induced extrasystole were revealed. At the same time, forced hypodynamia (as in Group 1 examinees) mediates suppressed autonomic regulation (reduced duration of control and activity of both branches), by decreasing exercise tolerance with the higher probability of exercise-induced cardiac arrhythmias. Autonomic control improvement as in Groups 2 and 3 in the formation of mixed endurance reduces and even eliminates the probability of extrasystole, by increasing the potentially dangerous exercise threshold and the time of manifestation (the duration of sinus rhythm until extrasystole occurs) of premature beat complexes and by decreasing their number. It is shown that studying the autonomic exercise tolerance control during an unlimited exercise test can enhance safety with preserved high prognostic and prophylactic effectiveness and promotes the correct dosage and optimal tolerance of exercise. Recommendations are proposed to enhance the adaptive capabilities of teenagers, including those in the penitentiary context.