BACKGROUND: Muscle unloading such as bed rest or space flight decreases strength and cross-sectional area of anti-gravitational muscles, such as the quadriceps. Muscle anisotropic properties - assessed by diffusion tensor imaging (DTI) - correlate with muscle fiber type and contractile function. PURPOSE: To assess the changes in quadriceps’ size, anisotropy, and function following unilateral limb suspension (ULLS). METHODS: ULLS is the gold-standard ground-based analog for ambulatory simulation of muscle unloading where one limb is unloaded and the contralateral limb acts as an internal (loaded) control. Healthy, physically active subjects (18-50y M and F) ambulate using forearm crutches and shoes (Brooks Revel 5) modified with a 5cm rocker-style stack (R side only) for 13d under free-living conditions to unload the left leg. Bilateral interpolated twitch (60° knee flexion) is completed before and after 13d ULLS to assess maximal voluntary contractions and electrically-evoked twitch. DTI of the quadriceps is conducted at the end of the study to assess Physiological Cross-Sectional Area (PCSA), Fractional Anisotropy (FA), Mean Diffusivity (MD), Axial Diffusivity (AD), and Radial Diffusivity (RD). RESULTS: To date, 3 subjects have been enrolled in the study (1 complete, 1 dropped, 1 in progress) and data collection is ongoing. Data for one subject (20y F) are as follows. Compared to the loaded limb (R), quadriceps PCSA of the unloaded limb (L) was lesser (R: 58.28 v. L: 53.37). Each anisotropic property was adjusted by relative percent of total quadriceps PCSA and summed; all were greater in unloaded limb (L). FA (R: 0.178 v. L: 0.184), MD (R: 1.49 v. L: 1.53), AD (R: 1.80 v. L: 1.86), and RD (R: 1.33 v. L: 1.34). Maximal Voluntary Isometric torque was 20% lesser in the unloaded limb (R: 228.4 v. L: 183.1 Nm). Interestingly, Peak twitch torque increased for both the loaded (pre: 47.9 v. post: 60.5Nm) and unloaded limbs (pre: 44.0 v. post: 64.0Nm), but this effect was greater for the unloaded limb. Electromechanical delay was lesser following unloading in both the loaded (pre: 27.0 v. post: 24.7ms) and unloaded (pre: 25.0 v. 23.0ms) limbs. Time to peak twitch tension was not appreciably altered in either the loaded (pre: 100.7 v. post: 98.0ms) or unloaded (pre: 102.5 v. post: 100.1ms) limbs. Rate of Torque Development increased in both the loaded (pre: 1064.0 v. post: 1300.5Nm/s) and unloaded (pre: 949.6 v. post: 1405.7Nm/s) limbs. CONCLUSIONS: ULLS induces voluntary weakness which may be due to impaired muscle recruitment since contractile performance during electrically evoked twitch appears to be greater following unloading. Faster rates of torque development and greater peak twitch torque, as well as changes in anisotropic properties are consistent with a slow-to-fast fiber type change that occurs with prolonged unloading.

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