Kawakami Y, Muraoka Y, Kubo K, Akima H, Suzuki Y, Gunji A, Fukunaga T

Proceedings of the 4th symposium of "Inactivity (prolonged bed-rest), Health and Aging", 2000 (in press)

Fourteen healthy men (18-28 yr) carried out head-down bed rest (BR) for 20 days. The subjects were divided into three groups: Group A performed unilateral dynamic knee extension exercise (n=5), Group B performed bilateral isometric leg extension exercise (n=5), and Group C underwent only BR (control, n=4). 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. For 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 Group C and in control limbs of Group A, but the decrease was much smaller in the trained legs of Groups A and B (~5%). Decrease in muscle force after BR was greater than that of PCSA in Group C, who showed a decrease in neural activation. Pennation angles of the vastus lateralis muscle, 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.