Conflicting time-dependent effects of treadmill exercise on joint contracture after anterior cruciate ligament reconstruction in rats

We examined the time-dependent differences in the effects of treadmill exercise on joint contracture after anterior cruciate ligament (ACL) reconstruction. Rats received ACL reconstructive surgery. After surgery, rats were reared with or without treadmill exercise (60 min/day) starting at 3 days (when inflammatory reactions are active) or 14 days (when inflammatory reactions have subsided) post-surgery. Untreated rats were used as controls. Data were collected at two or four weeks post-surgery. The range of motion (ROM) decreased following surgery at both time points and treadmill exercise during the first two weeks post-surgery further decreased the ROM. Moreover, treadmill exercise during first two weeks post-surgery upregulated the pro-inflammatory cytokine interleukin-1?? gene in the joint capsule and aggravated joint capsule fibrosis. For exercise started at 14 days post-surgery, the ROM recovered to control levels at four weeks post-surgery. In addition, delayed ex-ercise contributed to resolving inflammatory and fibrotic reactions. Treadmill exercise initiated soon after sur-gery aggravates joint contracture via enhanced inflammatory and fibrotic reactions in the joint capsule. Conversely, exercise initiated after active inflammation has subsided facilitates joint capsule reorganization and increases joint ROM. Therefore, aggressive exercise should be started only after active inflammation is resolved to improve joint contracture following ACL reconstruction.