Objective: This study examined the influence of chronic anterior cruciate ligament deficiency on muscle activation patterns displayed during abrupt deceleration, relative to timing of tibiofemoral shear forces (Fs) generated during the task.
Design: Experimental data were collected for both limbs of 11 chronic functional anterior cruciate ligament deficient subjects and 11 matched controls.
Methods: Electromyographic, high speed film and ground reaction force data were sampled as subjects landed in single-limb stance on a force platform after receiving a chest level pass and decelerating abruptly. Temporal characteristics of each muscle burst relative to initial foot-ground contact were derived and Fs were calculated from net joint reaction forces and the patellar tendon force occasioned by the net moments and inertial forces predicted about the knee.
Results: Compared to controls, anterior cruciate ligament deficient subjects displayed a delay in hamstring activation so that peak hamstring activity was more synchronous with initial contact and with the high Fs which occurred after initial contact.
Conclusions: It was concluded the delayed hamstring activation was an adaptation developed to enable peak muscle activity to better coincide with the high anterior Fs, thereby stabilizing the injured limb against a giving-way episode via increased joint compression and posterior tibial drawer.