Hamstring Activity in the ACL Injured Patient: Injury Implications and ComparisonWith Quadriceps Activity

Purpose: To investigate the potential causes of diminished knee extension after acute anterior cruciate ligament (ACL) injury using both surface electromyography (sEMG) analysis of the quadriceps and hamstrings, and gait analysis to assess muscle action and tone. Methods: Consecutive patients with an acute ACL tear underwent sEMG and gait analysis within 2 weeks of injury, before ACL reconstruction. Standard motion analysis techniques were used and sEMG data were collected simultaneously with gait data. T-tests were used to determine differences between the ACLdeficient and control subjects in knee flexion angles, peak external knee joint moments, and total time that a muscle was activated (“on”) during gait. External knee moments were expressed as a percentage of body weight times height. Results: Ten patients (mean age 24 4 years) were included at a mean 10.2 days between injury and analysis; 10 uninjured, matched control subjects were included for comparison. There were significant increases in minimum flexion angle at heel strike (5.92 3.39 v 3.49 4.55, P < .001) and midstance (14.1 6.23 v 1.20 4.21, P < .001) in the injured limb compared with controls. There were significantly lower maximum external extension moments at heel strike ( 0.99 0.46 v 2.94 0.60, P < .001) and during the second half of stance in the injured limb compared with controls ( 0.56 1.14 v 3.54 1.31, P < .001). The rectus femoris was “on” significantly less during gait in the injured leg compared with controls (49.1 7.76% v 61.0 14.8%, P 1⁄4 .044). There were no significant differences in hamstring activity “on” time during gait (P > .05). Conclusions: In patients with acute ACL injury, the ACL-deficient limb does not reach as much extension as controls. Although the rectus femoris is “on” for shorter periods during the gait cycle, there is no difference in hamstring time on during gait. This information may help clinicians better understand muscle function and gait patterns in the acute time period after ACL injury. Level of Evidence: Level III, case control study.