Effects of chronic anterior cruciate ligament deficiency on muscle activation patterns during an abrupt deceleration task

Objective. This study examined the influence of chronic anterior cruciate l igament deficiency on muscle activation patterns displayed during abrupt de celeration, relative to timing of tibiofemoral shear forces (F-S) generated during the task. Design. Experimental data were collected for both limbs of 11 chronic funct ional 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 a fter receiving a chest level pass and decelerating abruptly. Temporal chara cteristics of each muscle burst relative to initial foot-ground contact wer e derived and F-S were calculated from net joint reaction forces and the pa tellar tendon force occasioned by the net moments and inertial forces predi cted about the knee. Results. Compared to controls, anterior cruciate ligament deficient subject s displayed a delay in hamstring activation so that peak hamstring activity was more synchronous with initial contact and with the high F-S which occu rred after initial contact. Conclusions. It was concluded the delayed hamstring activation was an adapt ation developed to enable peak muscle activity to better coincide with the high anterior F-S, thereby stabilizing the injured limb against a giving-wa y episode via increased joint compression and posterior tibial drawer.