We studied five young healthy volunteers who performed a "damage protocol" consisting of 240 (24 sets x 10 repetitions/set) maximal isokinetic eccentric muscle contractions (30 degrees/s) on each leg one week apart. Biopsies were taken from the vastus lateralis on two occasions. Two biopsies were taken from within the same muscle 24h following the damage protocol. On a second occasion a single biopsy was taken from the contralateral leg at 24h following the same damage protocol. Biopsies at all three sites showed Z-band disruption, much greater (i.e., approximately 14-fold) than is typically observed in resting biopsies, with no significant differences (ANOVA) according to site location (within legs or between legs). The within-leg coefficient of variation (CV) was, however, 41 +/- 30%, and the between-leg CVs were 57 +/- 36% and 68 +/- 36%. Macrophage cells were also detected within the muscle, and cell numbers were not statistically different between biopsy sites. However, the within-biopsy CV = 52 +/- 19% and the between-biopsy CVs of 34 +/- 24% and 48 +/- 27%. We conclude that eccentric contraction-induced Z-band streaming and inflammatory cell response, as detected in muscle biopsy samples from humans, is highly variable with a CV of 40-70%.