Delta efficiency defined as increase in work over the corresponding increase in energy liberation (delta work/delta energy) may be used to express the efficiency of working muscles under standard conditions where work is performed with similar changes in muscle length, identical pedal revolution frequencies, and contraction-to-relaxation ratios. The Delta efficiency is probably the most valid measure of the efficiency of muscular work, so it may be influenced by the difference in distribution and/or density of muscle fiber types in exercising muscles. It has been reported that after bed rest of 7-14 days, not only maximum oxygen uptake (VO2 max) but also oxygen uptake (VO2) at 3-min submaximal upright exercise decreased. However, the decrease might be apparent, and the mechanical efficiency might be unchanged. On the other hand, muscle mass of bicycling legs was decreased after continuous horizontal bed rest of 10 days and 20 days. Because the decreased muscle mass is probably related to decrease in the density of the slow twitch muscle fibers (ST-fiber), the decrease in submaximal VO2 during bicycle exercises after bed rest may result from a decrease in ST-fiber mass. Therefore, it could be hypothesized that the mechanical efficiency should increase during upright exercise because of the relative increase in amount of more efficient fast twitch muscle fibers (FT-fiber) than ST fibers in exercising muscles. The purpose of the present study was to investigate whether delta efficiency in working muscle is influenced by the decrease in muscle mass after 20 days horizontal bed rest in young males.