During most exercises and physical activities, a combination of isometric, concentric, and eccentric muscle contractions generate force. In doing so, all muscle contractions across the exercise continuum produce some amount of mechanical and/or metabolic stress. In his review of over one hundred studies on exercise-induced oxidative stress Pyne (1994) described how mechanical and metabolic stress cause muscle damage. Evidence of muscle damage following novel types of exercise, eccentric muscle contractions, and high-intensity exercise using isometric, eccentric or concentric contractions have been reported in many studies. Electron microscopy studies have reported Z-line breaks (Belcastro et al., 1998), biomechanical studies have reported reduced range of motion in the affected joints (Dutto et al., 2004), biochemical studies have reported increased creatine kinase (CK) (Clarkson, 1997), 3-methyl-hystidine (Chevion et al., 2003), proinflammatory cytokines (Chan et al., 2004), tumor necrosis factor (TNF) (Home et al., 1997), interleukins (Ostrowski et al., 1998), and many studies have reported muscle soreness and fatigue using a variety of scales (Thompson et al., 1999; Clarkson and Newham, 1995).
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