title: EFFECTS OF UNILATERAL LOWER LIMB SUSPENSION ON QUADRICEPS’ STRUCTURE AND CONTRACTILE FUNCTION
Creators: name: Elmore, Eve
Publisher: name: University of Kentucky Libraries
Year: 2024
Subjects: classificationCode: 5.3
subject: FOS: Educational sciences

description: BACKGROUND: Muscle unloading such as bed rest or space flight decreases strength and cross-sectional area concomitant with a slow-to-fast fiber type shift, of anti-gravitational muscles such as the quadriceps. Diffusion tensor imaging (DTI) quantifies tissue anisotropic properties such as Fractional Anisotropy, Mean Diffusivity, Axial Diffusivity, and Radial Diffusivity, as well as muscle volume, fascicle length, and physiological cross-sectional area, and has recently been adapted to assess skeletal muscle structure. In skeletal muscle, anisotropic properties correlate with muscle fiber type and contractile function and may be responsive to unloading-induced atrophy.

PURPOSE: To assess the changes in quadriceps’ size, anisotropy, and contractile function following 13 days of unilateral limb suspension (ULLS).

METHODS: Healthy, physically active subjects (18-50y M and F) ambulated 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) was completed before and after 13d ULLS to assess maximal voluntary strength, voluntary activation, and electrically-evoked twitch in both the unpotentiated and potentiated states. Peak Twitch Torque (PTT), Rate of Torque Development (RTD), Time to Peak Tension (TTPT), and Electromechanical Delay (EMD) were assessed from electrically-evoked femoral nerve stimulation before and after a potentiating contraction. DTI of the quadriceps was 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). PCSA of all quadriceps muscles were summed. Similarly, anisotropic properties were adjusted by relative percent of total quadriceps PCSA and summed to reflect total quadriceps anisotropy.

RESULTS: Seven subjects enrolled in the study (5 completed, 2 dropped). A 2x2 repeated measures ANOVA was used to compare loaded versus unloaded limb pre and post ULLS. Following 13d ULLS, MVIC torque tended to be 15% lesser in the unloaded limb (R: 254.2 ± 45.8 v. L 215.7 ± 66.4 Nm; p = 0.080). Compared to the loaded limb, quadriceps PCSA of the unloaded limb (L) tended to be lesser (R: 74.8 ± 11.8 v. L: 58.39 ±6.4 cm2), but a paired t-test demonstrated not significant difference between limbs (p =0.077). Interestingly, Peak twitch torque increased following ULLS for both the loaded (pre: 64.3 ± 10.9 v. post 68.2 ± 9.4 Nm) and unloaded limbs (pre: 62.5 ± 14.0 v. post: 68.9 ± 7.7 Nm), but this effect was greater for the unloaded limb. There was no main effect of limb (p = 0.930), time (p = 0.220), or limb x time interaction (p = 0.585) on PTT. Rate of Torque Development increased in both the loaded (pre: 1310.7 ± 271.9 v. post 1474.6 ± 229.8 Nm/s) and unloaded (pre: 1261.2 ± 234.4 v. post: 1476.6 ± 143.0 Nm/s) limbs. Despite a significant main effect of time (p = 0.019), there was no main effect of limb ( p = 0.663) or limb x time interaction on RTD.. There was no significant main effect of limb (p = 0.939) or time (p = 0.233) on TTPT, but there was a trend toward a limb x time interaction (p = 0.074) where TTPT decreased in the unloaded limb, but increased in the loaded limb. There was no main effect of limb (p = 0.845), time (p = 0.641), or limb x time interaction (p = 0.254) on EMD. Quadriceps anisotropic properties tended to be lesser in unloaded limb (L). FA (R: 0.154 v. L: 0.151), MD (R: 1.83 v. L: 1.78), AD (R: 2.14 v. L: 2.07), and RD (R: 1.68 v. L: 1.64), but paired t-tests revealed no significant between-limb differences (p=0.082-0.483).

CONCLUSIONS: ULLS induces voluntary weakness which may be due to impaired muscle recruitment since contractile performance during electrically evoked twitch appears to be unchanged following 13d unloading. Faster rates of torque development, greater peak twitch torque, and lesser FA (albeit non-significant) are consistent with a slow-to-fast fiber type change that occurs with prolonged unloading. With a low sample size, it is possible this study is underpowered to detect differences in quadriceps structure and function seen in previous ULLS studies.
description: 2024 Eve N. Elmore