Controlled lengthening or shortening contraction-induced damage is followed by fiber hypertrophy in rat skeletal muscle

To study the hypothesis that more severe damage, caused by controlled lengt hening (L) contractions, results in greater myofiber hypertrophy compared t o increase in fiber size followed shortening (S) contractions, tibialis ant erior muscles of anesthesized male Wistar rats were subjected to 240 either L or S contractions. The highest increase in muscle beta-glucuronidase act ivity, an indicator of muscle damage, was observed in L (7.1-fold) 4 days a nd in S (2.6-fold) 8 days postexercise. Dystrophin- and desmin-negative as well as fibronectin-positive fibers (signs of the early phase of damage) we re observed immediately after exercise in the L group. At 4 days, massive m yofiber injury was visible, and internally localized nuclei were present at 15-80 days after exercise in the L group. The shift towards more glycolyti c fiber types (p < 0.05 in L and S) and an increased mean cross-sectional a rea of type IIX/B fibers (p < 0.001 in L and S) at 80 days were observed in both groups. The observed minor damage with unchanged myofiber structures following S induced, however, an increase in myofiber cross-sectional area of nearly the same magnitude as that following L, which was more damaging. The results do not support the hypothesis that fiber hypertrophy depends on the extent of the myofiber damage upon the exercised muscles.