A study was conducted to investigate changes in muscle soreness, serum creatine kinase (CK) activity and white blood cell (WBC) count following exercise bouts spaced three weeks apart.
The subjects were six male students (aged 23-25 yr), who had not participated in any training program for over 18 months. They performed muscular exercise of the nondominant arm using elbow flexors. Twenty percent of maximum voluntary contraction was used as the exercise intensity. After three weeks, the subjects repeated the same exercise bout. Perceived muscle soreness, CK activity and WBC count were assessed before, immediately after, 6h after and over 9 days after each exercise bout.
After the first exercise bout (1 st Ex), the subjects experienced muscle sorenss for 3-7 days. Also, a large increase of CK was found in five subjects (266-763%) . When the peak CK efflux was observed (day 3-4 after exercise), soreness had almost disappeared. WBC count was increased immediately and 6 h after exercise, then returned to the resting level. However, a significant increase (p<0.05) in WBC count was observed again on day 7 after exercise when CK had returned to the resting level. After the second exercise (2 nd Ex), a significant decrease of muscle soreness and the CK response was found in comparison with the 1 st Ex (p<0.41) . One interesting feature was that the CK efflux of subjects who had shown a large increase of CK after the 1 st Ex was not increased after the 2 nd Ex.
The initial exercise bout may have induced some damage to the muscle fibers or mem. bran. This damage would induce a process of repair in the damaged tissue, which in turn would adapt the muscle to the next stimulus. However, the subjects who showed a slight increase of CK after the 1 st Ex did not show this adaptation. Therefore an adaptive threshold for fiber or membrane damage may exist.