Lower limb skeletal muscle function after 6 wk of bed rest.

Force, electromyographic (EMG) activity, muscle mass, and fiber characteristics were studied in seven healthy men before and after 6 wk of bed rest. Maximum voluntary isometric and concentric knee extensor torque decreased (P < 0.05) uniformly across angular velocities by 25-30% after bed rest. Maximum quadricep rectified EMG decreased by 19 +/- 23%, whereas submaximum (100-Nm isometric action) EMG increased by 44 +/- 28%. Knee extensor muscle cross-sectional area (CSA), assessed by using magnetic resonance imaging, decreased by 14 +/- 4%. Maximum torque per knee extensor CSA decreased by 13 +/- 9%. Vastus lateralis fiber CSA decreased 18 +/- 14%. Neither type I, IIA, and IIB fiber percentages nor their relative proportions of myosin heavy chain (MHC) isoforms were altered after bed rest. Because the decline in strength could not be entirely accounted for by using decreased muscle CSA, it is suggested that the strength loss is also due to factors resulting in decreased neural input to muscle and/or reduced specific tension of muscle, as evidenced by decreased torque/EMG ratio. Additionally, it is concluded that muscle unloading in humans does not induce important changes in fiber type or MHC composition or in vivo muscle contractile properties.