Cellular adaption to repeated eccentric exercise-induced muscle damage

Eccentrically biased exercise results in skeletal muscle damage and stimulates adaptations in muscle, whereby indexes of damageare attenuated when the exercise is repeated. We hypothesizedthat changes in ultrastructural damage, inflammatory cell infiltration,and markers of proteolysis in skeletal muscle would come aboutas a result of repeated eccentric exercise and that gender mayaffect this adaptive response. Untrained male (<i>n</i> = 8) and female(<i>n</i> = 8) subjects performed two bouts (<i>bout 1</i> and <i>bout 2</i>), separatedby 5.5 wk, of 36 repetitions of unilateral, eccentric leg pressand 100 repetitions of unilateral, eccentric knee extension exercises(at 120% of their concentric single repetition maximum), the subjects'contralateral nonexercised leg served as a control (rest). Biopsieswere taken from the vastus lateralis from each leg 24 h postexercise.After <i>bout 2</i>, the postexercise force deficit and the rise in serumcreatine kinase (CK) activity were attenuated. Women had lowerserum CK activity compared with men at all times (<i>P</i> < 0.05), butthere were no gender differences in the relative magnitude ofthe force deficit. Muscle Z-disk streaming, quantified by usinglight microscopy, was elevated vs. rest only after <i>bout 1</i> (<i>P</i> <0.05), with no gender difference. Muscle neutrophil counts weresignificantly greater in women 24 h after <i>bout 2</i> vs. rest and<i>bout 1</i> (<i>P</i> < 0.05) but were unchanged in men. Muscle macrophageswere elevated in men and women after <i>bout 1</i> and<i>bout 2</i> (<i>P</i> < 0.05).Muscle protein content of the regulatory calpain subunit remainedunchanged whereas ubiquitin-conjugated protein content was increasedafter both bouts (<i>P</i> < 0.05), with a greater increase after <i>bout2</i>. We conclude that adaptations to eccentric exercise are associatedwith attenuated serum CK activity and, potentially, an increasein the activity of the ubiquitin proteosome proteolyticpathway.<br />