Kathryn Southall, McKenzie M. Hare, Kealey J. Wohlegemuth, Abbey T. Jordan, Katherine L. Ryan, Mackenzie S. Kennedy, Jacob A. Mota. The University of Alabama, Tuscaloosa, AL.

BACKGROUND: Skeletal muscle fatigue is a complicated process with mechanisms deriving from both central and peripheral factors. Ultrasound echo intensity has been proposed as a unique metric of image-based muscle quality. Previous works have suggested associations may exist between muscle function and quality, but this relationship is poorly understood. The purpose of this study was to examine the influence of echo intensity on performance during a bout of isokinetic muscle actions.METHODS: Eight subjects (four women, mean±SD age = 20±2 yrs; BMI = 25.5±3.7 kg/m2), completed 2 visits to the laboratory. On the first visit, subjects completed musculoskeletal ultrasound imaging and were familiarized with our fatigue assessment. 2-7 days later, subjects returned for a bout of isokinetic fatigue testing. B-mode ultrasound was used to image the vastus laterals (VL) at 50% muscle length. Using ImageJ, the VL cross-sectional area was traced. EI was quantified as the mean pixel brightness within the selected portion of the image. Subjects completed 50 repeated, maximal, isokinetic muscle actions (120°/sec) to serve as a fatigue trial. Isokinetic peak torque was analyzed offline using custom LabVIEW software by selecting individual torque peaks from each muscle action. initial isokinetic peak torque was calculated by averaging the highest three of the first five contractions. Final isokinetic peak torque was calculated by averaging the highest three of the last five contractions. Isokinetic peak torque % decline was calculated by %Decline=(initial-PT-final-PT)/initial-PT. Pearson’s correlation coefficient (r) determined the association between EI and %Decline. A linear regression model determined the effect of EI on %Decline. An additional linear model was used to control for sex. R2 determined the amount of shared variance. An alpha level of p<0.05 determined statistical significance. RESULTS: There were no associations between EI and %Decline (r=0.401, p=0.373). The results of the linear regression model suggest that EI explains 16% of the variance in %Decline, though not statistically significant (F=0.958, R2=0.161, p=0.373). When accounting for sex, EI explained 66% of the variance in percent decline (F=3.841, R2=0.658, p=0.117). CONCLUSION: Our findings suggest that echo intensity is not associated with %Decline of isokinetic peak torque. However, when adjusting for sex, a significant relationship is found.

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