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Morphological and functional criteria permitting the prediction of exercise performance were identified for two ethnic groups of Yakutia (Sakha). Yakuts with a low body height, normotonic autonomic nervous regulation, a relatively high weight-height index, eukinetic blood circulation, and a high specific exercise performance are best adapted to the environment. A high exercise performance in sympathicotonic subgroups cannot be a criterion of good adaptation because it is accompanied by cardiovascular strain even at rest. A high prevalence of sympathicotonia and body weight deficit in the ethnic groups studied reflect maladaptations caused by a set of adverse natural and social environmental factors.