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

BACKGROUND: Image-based muscle quality may be quantified using echo intensity (EI). EI is has been suggested as being able to detect proportions of contractile vs. non-contractile muscle tissue (i.e., muscle quality). The quality of skeletal muscle may influence performance, and associations between EI and muscle function have previously been shown. Though, this relationship is yet to be fully understood. Therefore, the purpose of this study is to examine the influence of EI on strength impairments following a fatiguing bout of muscle actions. METHODS: Eight subjects (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 maximal strength and fatigue assessments. 2-7 days later, subjects returned for additional strength testing, a bout of isokinetic fatigue, and follow-up strength testing. B-mode ultrasound was used to image the vastus lateralis (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. Maximal strength was assessed on an isokinetic dynamometer where subjects completed isometric maximal voluntary contractions (MVC). Next, subjects completed 50 maximal, isokinetic muscle actions (120°/sec) to serve as a fatigue trial. Following the fatigue trial, participants performed additional MVCs. Peak torque (PT) was calculated at the highest 500ms epoch during isometric MVCs. Isometric PT percent decline was calculated as %Decline=(final-PT-pre-PT)/pre-PT. Pearson’s correlation coefficient (r) determined the association between EI and %Decline. A linear regression model determined the effect of EI on %Decline. R2 determined the amount of shared variance. An alpha level of pr=0.66, p=0.109). The results of the linear regression model suggest that EI explains 43% of the variance in %Decline, though not significant (F=3.78, R2=0.431, p=0.109). CONCLUSION: These findings do not reveal statistically significant relationships between EI and %Decline. However, the trend seems to suggest that a meaningful relationship between muscle quality and isokinetic fatiguability may exist and may be revealed with increased statistical power.

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