CONTEXT: The exact relationship between the bed rest-induced loss of skeletal muscle and reductions in muscle strength and physical performance in the older individuals is still unclear. OBJECTIVE: We examined the effect of 10 days of bed rest on changes in regional body composition, muscle strength, and functional status, and the relationship between these variables in older individuals. DESIGN, PARTICIPANTS, AND INTERVENTION: Regional body composition was measured using dual energy x-ray absorptiometry. We also determined changes in leg strength and several indices of functional status, including walking speed. RESULTS: Body weight, body mass index, and total and lower extremity lean mass decreased with bed rest. There were also significant reductions in knee extension one repetition maximum, isometric knee extension, knee extension 60° concentric, stair ascent time, stair ascent power, stair descent time, VO2 max, floor transfer test, 5-minute walk time, and chair stand. The overall change in total and lower extremity lean mass was also directly related to bed rest-induced reductions in one repetition maximum knee extension. CONCLUSIONS: Bed rest promoted overall declines in muscle mass, muscle strength, and physical function in older individuals. The changes in lean tissue were closely correlated with the bed rest-induced decline of muscle strength.