MUSCLE FATIGUE REDUCES STIFFNESS AT THE CELLULAR LEVEL BUT NOT IN COMPOSITE MUSCLE TISSUE

G.E. Privett, A.W. Ricci, D.M. Callahan University of Oregon, Eugene, OR Skeletal muscle stiffness influences locomotor function, predicting risk for soft tissue injury in athletes and falls risk in older adults. Chronic effectors of musculotendinous stiffness (resistance training and age) are well characterized but acute modifiers are poorly understood. Recently, fatiguing exercise was reported to acutely reduce whole muscle stiffness. It is possible that intracellular mechanisms may contribute to whole muscle compliance but it is unknown if fatigue differentially affects cellular or whole tissue stiffness. Therefore, the PURPOSE of this study is to examine the effects of acute fatigue on inherent stiffness of skeletal muscle measured at the cellular and tissue level. METHODS: 1 female volunteer completed a bout of repeated maximum voluntary knee extensions with her dominant leg until task failure. Immediately following task failure, percutaneous needle muscle biopsy was performed on the vastus lateralis of the fatigued limb. Thereafter, the biopsy procedure was repeated on the contralateral (not fatigued) limb. Fibers (10 control, 12 fatigued) and bundles of 5 fibers including the extracellular matrix (ECM) and non-contractile tissue (3 control, 3 fatigued) were dissected from biopsy samples, mounted in relaxing solution, and stretched to 156% of initial sarcomere length (2.4 μM) in 0.19 μM increments. Passive stiffness was measured as the linear relationship between stress (force per unit area; kPa) and strain (% length). RESULTS: Fatigued and control fiber bundles generated similar active tension (117±21 kPa and 90±20 kPa, respectively, p=0.93), consistent with single fibers (144±33 kPa and 141±35 kPa, respectively, p=0.83). Passive stiffness differed between fatigued (0.11± 0.08 kPa/%Lo) and control (0.20± 0.06 kPa/%Lo) single fibers (p\u3c0.01), but was not significantly different between fatigued (0.34 ± 0.13 kPa/%Lo) and control (0.42 ± 0.05 kPa/%Lo) fiber bundles (p=0.37). CONCLUSION: Our initial findings suggest that although acute fatigue reduces stiffness in single fibers, it does not alter fiber bundle stiffness. This surprisingly divergent response suggests that fatigue induced compliance is more related to intracellular than extracellular mechanisms. This work was supported by the Wu Tsai Human Performance Alliance and the Joe and Clara Tsai Foundation