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Soldiers are expected to maintain a high degree of physical readiness as operational demands can severely degrade performance capabilities. This study examined the physiological consequences of U.S. Army Ranger training on strength, power, body composition, and somatotrophic hormones.In an intensive 8-wk military training course that included an average daily energy deficit of 1000 kcal·d−1, lower-body power output, maximal lifting strength, body composition, and serum concentrations of several somatotrophic hormones were measured in 50 male soldiers (24.6 ± 4.4 y; 176.1 ± 7.8 cm; 78.4 ± 8.7 kg; 14.7 ± 4.2% body fat) before and after the course.Vertical jump height (−16%), explosive power output (−21%), maximal lifting strength. (−20%), body mass (−13%), fat-free mass (−6%), and fat mass (−50%) declined (P < 0.05) after the training course. Circulating total testosterone and insulin-like growth factor-I (IGF-I) experienced significant (P < 0.05) declines, and cortisol was significantly increased. Lower-body power output, but not maximal lifting strength, correlated with changes in fat-free mass. IGF-I and cortisol, but not total testosterone, were correlated with losses of tissue mass.Lower-body power output, estimated from vertical jump height and body mass, is a sensitive and field expedient measure that can be used to assess the influence of caloric deficit on physical performance after 8 wk of U.S. Army Ranger training. With severe weight loss (≥13% of body mass), IGF-I and cortisol correlate more closely with soft-tissue tissue adaptations than does testosterone.