Comparison of eccentric knee extensor muscle actions at two muscle lengths on indices of damage and anglespecific force production in humans

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Abstract
In this study, we investigated the effects of knee extensor length during eccentric exercise on indices of muscle damage and adaptation. Subjects (n=7) performed two bouts of 75 maximal voluntary eccentric muscle actions at a knee joint angular velocity of 1.57 rad s-1. One bout was performed at a short muscle length (bout S) with a knee joint range of motion of 2.79 to 1.40 radians (160" to 80°), and a second with the contralateral knee extensors at a long muscle length (bout L) with a range of motion of 2.01 to 0.7 radians (120" to 40"). The maximum voluntary contractile force (MVC) was measured before and 5 min after exercise, and again on days 3, 5,7,10 and 12, at knee angles of 1 60°, 120" and 80". Muscle soreness was measured before exercise and on each day after exercise. Serum creatine kinase activity was measured before exercise and on days 3, 5, 7, 10 and 12 post-exercise. The MVC declined after each bout (P<0.01), with a greater decline after bout L (P<0.05). Muscle soreness was higher relative to bout S on days 1,2,3, 5 and 6 (P<0.05). Although serum creatine kinase activity was elevated after both exercise bouts (P<0.01), there was no difference between bouts. Functional muscle damage markers and muscle soreness suggest greater damage after bout L. Post-exercise angle-specific force decrements suggest a transient increase in muscle length after bout L but not bout S.