Bilateral kinematic and kinetic analysis of the squat exercise after anterior cruciate ligament reconstruction

OBJECTIVES: To characterize the bilateral lower-extremity kinematics and kinetics associated with squatting exercise after anterior cruciate ligament (ACL) reconstruction. DESIGN: We evaluated bilaterally sagittal plane kinematics and kinetics of the ankle, knee, and hip joints during submaximal squatting exercise in rehabilitating patients after ACL reconstruction. Comparisons were performed between involved and noninvolved limbs, and regression models were created to examine the relations between the bilateral kinetic differences and time postsurgery. SETTING: A motion analysis laboratory. PARTICIPANTS: Eight adults (27.9+/-6.8y) with unilateral ACL reconstruction (postsurgical time, 30+/-12wk). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Sagittal plane ankle, knee, and hip peak net moments of force, maximum joint excursion angles, and peak vertical ground reaction forces. RESULTS: Peak vertical ground reaction forces did not differ between limbs. The peak knee extensor moment generated during the submaximal squatting exercise was 25.5% greater in the noninvolved limb than in the involved limb (P=.003). The peak ankle plantarflexor moment did not differ between limbs (P=.85); however, there was a trend toward a greater hip extensor moment in the involved limb (P=.06). The ratio of the peak hip extensor moment to the peak knee extensor moment was 46.5% greater in the involved limb (P=.02). Only the peak dorsiflexion angle differed between limbs (P=.02). None of the linear models examining the relations between differences in the involved limb and noninvolved limb kinetics, and postsurgical time, were statistically significant. CONCLUSIONS: Patients performing the squat exercise, within 1 year of ACL reconstructive surgery, used 2 strategies for generating the joint torques required to perform the movement: (1) in the noninvolved limb, patients used a strategy that equally distributed the muscular effort between the hip and knee extensors, and (2) in the involved limb, patients used a strategy that increased the muscular effort at the hip and reduced the effort at the knee. These intra- and interlimb motor-programming alterations (ie, substitution strategies) could potentially slow or limit rehabilitation, and induce strength and performance deficits.