Sedentary voluntary students were exposed to 10 (n = 8) and 20 (n = 14) days bed rest to study effects of prolonged bed rest on isometric strength of knee extension and flexion, total leg strength, right hand grip and elbow flexion using different dynamometers. Further, muscle mass of right leg and arm was measured by dual energy X-ray absorptiometry, and cross sectional area of right thigh by X-ray computed tomography before and after bed rest and after 4 or 8 weeks of recovery. After 10 days bed rest, these variables tended to decrease except for the muscle mass of right arm which increased. After 20 days of bed rest, a similar trend was observed, but at a statistically significant level of p < 0.05. However, no correlation was observed between the decrease in knee extensor strength and cross sectional area of the thigh. Nor was a correlation observed between the sum total of isometric strength of the leg and the total muscle mass measured. The 24h urinary excretion of N2 and creatinine was not changed through the experiments. During recovery, the variables measured had reached the initial levels after 4 or 8 weeks. These changes were not affected by mild supine exercise training. The results suggest that isometric muscle strength was decreased in non-antigravity muscles as well as antigravity muscles by prolonged bed rest, but the rate of changes did not correlate to the corresponding changes in muscle mass. The decreases in maximum voluntary strength are probably not only due to the decrease in muscle mass but also due to reductions of neuromuscular function.