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PENN, I-W., Z-M. WANG, K. M. BUHL, D. B. ALLISON, S. E. BURASTERO, and S. B. HEYMSFIELD. Body composition and two-compartment model assumptions in male long distance runners. Med. Sci. Sports Exerc., Vol. 26, No. 3, pp. 392–397, 1994. Long distance running increases. bone mineral mass, skeletal muscle weight, and extracellular fluid volume. Each of these changes may have an impact on classic two-compartment body composition methods that assume a constant fat-free body mass (FFM) density (1.100 g·cc-1), potassium content (65 mmol·kg-1FFM), and hydration (0.73 kg H2O·kg-1FFM). The aims of the present study were: to use newly developed multi-component methods to evaluate the validity of two-compartment methods in white male recreational long-distance runners (N= 10); and to compare the body composition of these runners to sedentary controls (N= 10) of similar age, body weight, and body mass index. Runners had a significantly smaller fraction of body weight as fat (P= 0.001) and a larger fraction of FFM as lower extremity skeletal muscle (P= 0.045) and bone (P= 0.049). Although FFM constituted a larger proportion of body weight in the runners, the fractional contributions of water, protein, and mineral were similar to those in the control group. There were no significant differences between the two groups in density of FFM, total body potassium/FFm, and total body water/FFM. New methods thus allow in-depth analyses of body composition in athletes, with results suggesting that classic two-compartment methods are valid in while recreational long-distance runners.