Characterization of hamstring reflexes during anterior cruciate ligament disruption: In vivo results from a goat model

The existence of an anterior cruciate ligament-hamstring reflex are: the ex tent to which these reflexes can protect the knee, and the extent to which they are affected by rupture of the anterior cruciate ligament remain contr oversial. We evaluated the temporal components of the anterior cruciate lig ament-hamstring synergy by simulating an injury to the ligament in a goat m odel. Reflexive hamstring activation in anesthetized goats was evaluated wh en the anterior cruciate ligament was loaded with static subfailure, dynami c subfailure, and dynamic failure loads. Reflexive hamstring activation was not found in response to static subfailure loading but was observed in res ponse to dynamic subfailure and failure loading. The latency of the reflex evoked by dynamic failure loading was shorter than that evoked by dynamic s ubfailure loading. The findings suggest that the extent to which the hamstr ing reflexes can protect the knee may be bounded by the ability of these mu scles to generate force rapidly and the amplitude and time-course of the lo ads applied to the knee joint. The present data present a framework for fur ther investigation of the contribution of anterior cruciate ligament-hamstr ing reflexes to the stability of the knee joint under high loads and loadin g rates.