- 1. Serum creatine kinase (CK) and muscle soreness, common indicators of muscle damage, were assessed in 16 subjects after performance of two bouts of eccentric exercise spaced 7 days apart. The purpose was (1) to examine rapid muscle adaptation when the ipsilateral limb was exercised on the second bout and (2) to investigate possible central adaptations when the contralateral limb was exercised on the second bout. 2. The LSL group (n = 7) and the LOL group (n = 9) performed the exercise with the ipsilateral and the contralateral limb, respectively. Careful attention was paid to stabilization of the subject so that the contralateral limb was not active in isometric stabilization. 3. For the LSL group, a lower CK and soreness response was found on about 2 compared with bout 1. For the LOL group, no significant difference in CK response and soreness was found between bout 1 and bout 2. 4. No repeat bout effect was found when the contralateral limb was exercised; therefore central adaptation from performance of the first exercise was minimal. Because a lower repeated bout effect was found after bout 2 using the ipsilateral limb, it was concluded that an experimental design using ipsilateral muscle groups should provide a good model to study rapid muscle adaptation to exercise damage.