Fourteen healthy men (18-28 yr) carried out head-down bed rest (BR) for 20 days. The subjects were divided into three groups: five subjects who performed unilateral dynamic knee extension exercise, five subjects who performed bilateral isometric leg extension exercise, and four subjects who acted as control. The first two groups trained every day at their maximal effort. Before and after BR, physiological cross-sectional areas (PCSA) of the quadriceps muscles were determined from a series cross-sectional MRI scans of the thigh. In the latter two groups, maximal isometric knee extension force was measured and neural activation was assessed by a supramaximal twitch interpolated over voluntary contraction. PCSA decreased by ~10% in the control subjects and control limbs, but the decrease was much smaller in the trained legs. Decrease in muscle force after BR was greater than that of PCSA in the control subjects, who showed a decrease in neural activation. Pennation angles of the vastus lateralis, determined by ultrasonography, did not show significant changes in any groups. The results suggest that reduction of muscle strength by BR is affected both by a decreased muscle mass and reduced ability to activate motor units, both of which can be retained by resistance training. It was also shown that static and dynamic exercises are similarly effective as countermeasure against muscle atrophy.

DESCENTE SPORTS SCIENCE Vol.21/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE