Cellular adaptation to repeated eccentric exercise-induced muscle damage

Eccentrically biased exercise results in skeletal muscle damage and stimula tes adaptations in muscle, whereby indexes of damage are attenuated when th e exercise is repeated. We hypothesized that changes in ultrastructural dam age, inflammatory cell infiltration, and markers of proteolysis in skeletal muscle would come about as a result of repeated eccentric exercise and tha t gender may affect this adaptive response. Untrained male (n = 8) and fema le (n = 8) subjects performed two bouts (bout 1 and bout 2), separated by 5 .5 wk, of 36 repetitions of unilateral, eccentric leg press and 100 repetit ions of unilateral, eccentric knee extension exercises (at 120% of their co ncentric single repetition maximum), the subjects' contralateral nonexercis ed leg served as a control (rest). Biopsies were taken from the vastus late ralis from each leg 24 h postexercise. After bout 2, the postexercise force deficit and the rise in serum creatine kinase (CK) activity were attenuate d. Women had lower serum CK activity compared with men at all times (P < 0. 05), but there were no gender differences in the relative magnitude of the force deficit. Muscle Z-disk streaming, quantified by using light microscop y, was elevated vs. rest only after bout 1 (P < 0.05), with no gender diffe rence. Muscle neutrophil counts were significantly greater in women 24 h af ter bout 2 vs. rest and bout 1 (P < 0.05) but were unchanged in men. Muscle macrophages were elevated in men and women after bout 1 and bout 2 (P < 0. 05). Muscle protein content of the regulatory calpain subunit remained unch anged whereas ubiquitin-conjugated protein content was increased after both bouts (P < 0.05), with a greater increase after bout 2. We conclude that a daptations to eccentric exercise are associated with attenuated serum CK ac tivity and, potentially, an increase in the activity of the ubiquitin prote osome proteolytic pathway.