Repeated bouts of eccentric muscle contractions were used to examine indirect indices of exercise-induced muscle damage and adaptation in human skeletal muscle. Twenty-four subjects (18 females, 6 males) aged 20.0 +/- 1.4 years (mean +/- S.D.) performed an initial bout of either 10 (n = 7), 30 (n = 9) or 50 (n = 8) maximum voluntary eccentric contractions of the knee extensors, followed by a second bout of 50 contractions 3 weeks later using the same leg. Muscle soreness was elevated after all bouts (P < 0.05, Wilcoxon test), although the initial bout reduced the soreness associated with the second bout. Force loss and a decline in the 20:100 Hz percutaneous electrical myostimulation force ratio were observed after all exercise bouts (P < 0.01). Serum creatine kinase activity was elevated following the initial bouts of 30 and 50 repetitions (P < 0.01), but there was no increase following 10 repetitions. No increase in serum creatine kinase activity was observed in any group following the second bout of contractions (P > 0.05). We conclude that skeletal muscle adaptation can be brought about by a single bout of relatively few eccentric muscle contractions. Increasing the number of eccentric muscle repetitions did not result in an increased prophylactic effect on skeletal muscle.