The Duration of the Repeated-Bout Effect After Eccentric Exercise.

The duration of the repeated-bout effect following eccentric knee-extensor exercise remains undefined, despite its widespread use in muscle damage research and training. The present study examined how long this protective effect persists in the knee extensors. Forty-eight young men performed two identical bouts of maximal eccentric knee-extensor exercise (5 × 15 repetitions at 60°/s). Four independent groups performed the second bout 3, 6, 9, or 12 weeks after the first. Isometric torque, range of motion (ROM), delayed-onset muscle soreness (DOMS), muscle thickness, echo intensity, creatine kinase (CK), and high-sensitivity C-reactive protein (hs-CRP) were assessed before and 2 and 4 days after each bout. Baseline-adjusted linear mixed-effects models were used to compare responses between bouts, complemented by equivalence testing at 12 weeks. The repeated-bout effect was evident at 3 and 6 weeks across most outcomes and remained detectable at 9 weeks for selected markers. At 12 weeks, no significant differences between bouts were observed at either day 2 or day 4 for any outcome. Exploratory equivalence analyses further indicated that the effect was no longer meaningfully present at these time points for isometric torque, ROM, DOMS, muscle thickness, and echo intensity, whereas the findings for CK and hs-CRP remained inconclusive. Overall, the repeated-bout effect may persist for some outcomes for up to 9 weeks, whereas by 12 weeks no meaningful effect was detectable at the assessed post-exercise time points. This time course can guide the design of muscle damage studies and the scheduling of eccentric training sessions.