Purpose: Reduced quadriceps activation after anterior cruciate ligament reconstructions (ACL-R) has been reported. Patients often return to daily exercise or sport after ACL-R; however, there is little information regarding quadriceps function after exercise in this population. Therefore, the purpose of this study was to compare quadriceps strength and central activation after repeated bouts of exercise between individuals with and without a history of ACL-R.
Methods: Eighteen recreationally active subjects participated in this study: 8 with unilateral ACL-R at least 6 months earlier and 10 healthy controls. All subjects performed 30 min of continuous exercise on two consecutive days. Exercise included repeated bouts of inclined treadmill walking and jumping exercises. We measured maximal isometric knee extension torque and quadriceps central activation ratio (CAR) using the superimposed burst technique before and after exercise on day 1. Subjects returned 24 h later (day 2) for a repeat testing session.
Results: There was a significant group–time interaction for knee extension torque. ACL-R subjects were weaker at baseline on day 1 of testing (P = 0.01) but exhibited no significant reduction in knee extension torque after exercise (P = 0.13), whereas the healthy control subjects did (P = 0.01). Knee extension torque on day 2 remained significantly lower than baseline for the controls (P = 0.03), but not the ACL-R (P = 0.73). On day 2, both groups experienced significantly reduced knee extension torque after exercise. There was no group–time interaction for CAR, but a group main effect indicated that, on average, ACL-R had significantly lower CAR bilaterally compared with healthy controls.
Conclusions: The quadriceps in ACL-R knees were weaker and showed less reduction of isometric knee extension torque compared with healthy group in day 1. However, there were no group differences of quadriceps strength on day 2.