Hamstring and Quadriceps Strength Balance in Normal and Hamstring Anterior Cruciate Ligament-Reconstructed Subjects

OBJECTIVE: The purpose of this study was to evaluate and characterize the agonist-antagonist strength balance (hamstring/quadriceps [H/Q] ratio and dynamic control ratio [DCR]) about the knee specific to velocity, range of motion, and contraction type. We hypothesized that there would be systematic variation in the H/Q ratio and DCR based on knee joint angle, angular velocity, and contraction type. We also hypothesized that these ratios would be altered in the anterior cruciate ligament (ACL)-reconstructed group in favor of protecting the ACL graft (relative knee flexor strength when strain on the ACL is the greatest). DESIGN: Cross-sectional design. SETTING: A tertiary care sport medicine clinic. PATIENTS OR PARTICIPANTS: Sixteen subjects more than 1 year after hamstring tendon ACL reconstruction were compared with 30 active uninjured control subjects. INTERVENTIONS: Isokinetic strength testing was performed over 5 degrees to 95 degrees knee joint range of motion, 5 angular velocities (50, 100, 150, 200, 250 degrees/s), for concentric and eccentric contractions. MAIN OUTCOME MEASUREMENTS: Angle and velocity-matched H/Q ratio maps and DCR maps were produced for each group. Difference maps allowed quantification of the differences between the groups. RESULTS: Angle and velocity-matched H/Q ratio maps demonstrated systematic variation based on joint angle, velocity, and contraction type for both the control (H/Q, approximately 0-1.42; DCR, approximately 0-1.57) and the ACL-reconstructed group (H/Q, approximately 0-1.33; DCR, approximately 0-1.35). Difference maps demonstrate regional (angle and velocity-specific) alteration in the ratio between the ACL-reconstructed and control groups. CONCLUSIONS: Specific imbalances were demonstrated in the ACL-reconstructed group compared with control. In high knee flexion angles, the low H/Q ratio may represent a compromised ability of the hamstrings to stabilize the knee joint throughout the full range of motion. Near full knee extension shifts in favor of the knee flexors may represent an attempt to stabilize the knee at the angle of greatest ACL strain. These finding have implications for graft donor site selection and postoperative rehabilitation as well as provide insight into the neuromuscular control of the knee.