Changes in muscle function in response to 10 days of lower limb unloading in humans

Force-generating capacity and electromyographic (EMG) activity of the knee extensor muscles were studied before and after short-term (10 d) unilateral lower limb unloading and during 4 days of recovery. Ten healthy males used crutches to prevent one of their lower limbs from weight-bearing while maintaining joint mobility as well as daily ambulatory activities. Knee extensor torque and quadriceps rectified EMG during maximal voluntary isometric contraction (MVC) was measured repeatedly before and after the intervention. Also, EMG at a fixed submaximal level (100 Nm; 30-45% MVC) and maximal angular velocity (AVmax), during unresisted knee extension, were assessed. Maximum torque decreased (P < 0.05) by 13 +/- 8% in response to unloading while maximum EMG activity did not change after unloading or during recovery (P = 0.35). Submaximum EMG increased (P < 0.05) by 25 +/- 16% after unloading. Maximum and submaximum torque/EMG ratio decreased (P < 0.05) after unloading. AVmax decreased (P < 0.05) by 9 +/- 8% after unloading. The post value, however, was not different from that of the weight-bearing limb. Torque, EMG and AVmax were recovered (P > 0.05) after 4 days of resumed weight-bearing. The pronounced decrease and the rapid recovery in maximum torque appears not to be attributed to a change in muscle mass alone. Because the findings of unchanged maximum EMG and increased EMG at a submaximal force level suggest no change in neural drive, we propose that unspecific tissue factors in part impair muscle function in response to short-term loss of weight-bearing activity. Results also indicate that recovery in muscle function after short-term unloading seems to be completed in a time span shorter than the period of preceding inactivity.