A.W. Ricci, A.D. Donovan, A.O. Popoola, D.M. Callahan

University of Oregon

Musculotendinous stiffness is important for muscle function, related to soft tissue injury risk in athletes and falls risk in older adults. A few studies suggest acute muscle fatigue may reduce musculotendinous stiffness. Female athletes experience significantly higher risk for soft tissue injury, and fatigue differently than males, suggesting the intriguing possibility that fatigue-induced differences in muscle stiffness may predispose them to enhanced injury risk. The PURPOSE of this study was to assess the changes in musculotendinous stiffness of the vastus lateralis (VL) muscle and patellar tendon (PT) following fatiguing exercise using two different non-invasive measurement tools, ultrasound (US) and digital palpation (DP) in healthy young men and women. METHODS: We recruited 14 young healthy men (9) and women (5). Subjects performed 3 maximum voluntary isometric contractions (MVIC) of the knee extensors (KE) to assess strength (MVIC) and contractile speed (RTD). Active muscle stiffness was measured at the VL and PT with simultaneous DP and US at 25%, 50%, and 100% MVIC during ramped isometric contractions after subjects achieved steady state target torque. Subjects then performed a fatiguing isotonic KE at 30% MVIC until task failure. Immediately following, passive and active stiffness measures were repeated. Results were assessed two factor (sex/fatigue) repeated measures ANOVA. RESULTS: US based measurements were not different following fatigue, therefore DP stiffness is reported. Passive VL stiffness increased ~20% following fatigue (p=0.039). Passive VL stiffness was also different between men and women following fatigue (p=0.038), but not at baseline (p=0.49). Active PT stiffness increased ~17% with fatigue (p=0.024) and showed a trend towards a difference in response between men and women (p=0.06). Active VL stiffness was not different between sexes at baseline (p=0.41), but were different after fatigue (p=.004). CONCLUSION: Our data suggest fatigue increases passive stiffness at VL and PT, a response driven primarily by men. Methodological issues may have contributed US measurement variability that obscured other potential differences.

This work was supported by the Wu Tsai Human Performance Alliance and the Joe and Clara Tsai Foundation.


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