EFFECTS OF GUAYAKI YERBA MATE CONSUMPTION ON COGNITIVE PERFORMANCE IN COLLEGE STUDENTS MUSCLE FATIGUE REDUCES ACTIVE STIFFNESS

J.D. Cooper, G.E. Privett, A.W. Ricci, D.M. Callahan University of Oregon, Eugene, OR Muscle stiffness impacts, flexibility, force production, and soft tissue injury risk. Past studies suggest fatiguing exercise reduces whole muscle passive stiffness, but the origins of fatigue-induced compliance are unclear. Furthermore, similar measures of stiffness in actively contracting muscle are non-existent. Evidence from pre-clinical studies of skeletal and cardiac muscle suggest fatigue modifies the muscle protein titin, and these modifications reduce mechanical stiffness. However, these possibilities have not been explored in human skeletal muscle. Therefore, the PURPOSE of this study was to evaluate how acute fatigue modifies active cellular mechanics in humans and to test the hypothesis that fatigue reduces stiffness while not affecting active force generation. METHODS: Two males and two females completed repeated maximum voluntary knee extensions with their dominant leg until task failure. Immediately following, the volunteers underwent a bilateral percutaneous needle muscle biopsy of the Vastus Lateralis, providing a fatigued and non-fatigued sample. Individual fibers were dissected from the biopsy samples and mounted to a permeabilized single fiber force measurement apparatus (Aurora Scientific, ON Canada). Active stiffness was measured by submerging single fibers in activating solution (pCa 4.5) and measuring peak isometric force at steady-state. Immediately after, fiber length was briefly shortened then returned to original length. Stiffness was defined by the resultant change in force per unit length change (mN/mm). RESULTS: Fatigue reduced stiffness in activated single fibers (12.9 ± 4.8 mN/mm and 11.1 ± 4.8 mN/mm; p \u3c 0.01, n= 175). There was no main effect of sex, nor a sex/fatigue interaction. There was no difference in active isometric tension between non-fatigued and fatigued fibers (139.3 ± 29.3 mN/mm2 and 136.2 ± 32.0 mN/mm2). CONCLUSION: Our findings suggest fatigue reduces active stiffness, but not isometric force generating capacity, at the cellular level. Active stiffness reduction may be due to intracellular protein mechanisms. Therefore, these data could influence future research into the mechanisms explaining fatigue induced musculotendinous stiffness reduction. This work was supported by the Wu Tsai Human Performance Alliance