EFFECT OF SHORT-TERM UNWEIGHTING ON HUMAN SKELETAL-MUSCLE STRENGTH AND SIZE

This study examined the effect of short-term unweighting on strength a nd size of lower limb muscle groups to predict probable responses to p lanned 16-d Shuttle flights. Subjects were 10 healthy males, exposed t o 16 d of lower limb suspension (LLS). All ambulatory activity wets pe rformed on crutches while wearing a shoe with a IO-cm thick sole on th e right foot. This eliminated ground contact by the left feet, and the reby, body weight bearing by the left lower limb. Biopsies of the left vastus lateralis muscle and T-1 weighted magnetic resonance (MR) imag es (1.5 Tesla, TR/TE 600/20, 4 nex, 48 cm rectangular FOV, 10 mm trans axial slices at 5-mm intervals) of both thighs were used to examine mu scle morphology. The in vivo speed-torque relation for the left and ri ght quadriceps femoris (QF) muscle group wets used to measure strength . Fiber type composition and average cross-sectional area were not alt ered by LLS. The speed-torque relation for the left QF was down-shifte d 12% (p < 0.05) after LLS. There was no effect of speed or type of mu scle action (eccentric, isometric, or concentric). The speed-torque re lation for the right QF showed no change after LLS. Average cross-sect ional area of the left QF in eight MR images of the mid-thigh decrease d (p < 0.05) 8% with LLS (70 +/- 3 to 64 +/- 4 cm(2)), while the right QF showed no change (72 +/- 4 to 72 +/- 4 cm(2)). The hamstring muscl e group showed no change in average cross-sectional area after LLS. Th e results suggest that 16 d of unweighting will evoke decreases in str ength and ''size'' of lower limb extensor muscle groups in humans of s ufficient magnitude to be operationally significant, that Is greater t han 10%. Accordingly, it is sug gested that the planned 16-d Shuttle m issions will be of sufficient duration to 1) warrant investigation of the effect of microgravity on certain aspects of the human neuromuscul ar system, and 2) potentially compromise ability of the crew to perfor m emergency egress.