Hamstring Activity in the ACL Injured Patient: Injury Implications and Comparison With Quadriceps Activity Rachel M. Frank, M.D., Hannah Lundberg, Ph.D., Markus A. Wimmer, Ph.D., Brian Forsythe, M.D., Bernard R. Bach Jr., M.D., Nikhil N. Verma, M.D., and Brian J. Cole, M.D., M.B.A. 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 ACL- deficient 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 ¼ .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 un- derstand muscle function and gait patterns in the acute time period after ACL injury. Level of Evidence: Level III, case control study. I njury to the anterior cruciate ligament (ACL) re- mains one of the most common knee injuries in the United States, with more than 200,000 ACL re- constructions (ACLRs) performed annually. 1 It is well documented that patients with acute ACL injuries often present with a diminished range of motion (ROM), especially in terminal extension. Regaining terminal extension has historically proven to be paramount to successful outcomes in ACLR with respect to postoperative ROM and function. 2,3 With the loss of the primary restraint to anterior tibial translation as a result of an acute ACL injury, the tibia slides anterior until a secondary soft tissue restraint (collateral ligaments, menisci) provides resistance. To compensate, patients may reflexively inhibit their quadriceps activity to avoid unopposed anterior tibial translation. Reflexive quad- riceps inhibition after joint distension, which is almost always present after ACL injury, is thus thought to be one of the main factors responsible for the lack of knee extension incurred after acute ACL injury. 4 Although the lack of terminal knee extension after acute ACL injury has long been thought to be due exclusively to this reflexive inhibition of the quadriceps, it is possible that hamstring spasm may also play a role, because hamstring activation provides an additional restraint to anterior tibial translation. 5-7 Intraoperative stimulation of the ACL in patients undergoing knee arthroscopy has proven to elicit hamstring reflexes. 8 The current literature, however, is From the Department of Orthopaedic Surgery, Rush University Medical Center, Chicago, Illinois, U.S.A. The authors report that they have no conflicts of interest in the authorship and publication of this article. Received May 30, 2015; accepted January 21, 2016. Address correspondence to Rachel M. Frank, M.D., Rush University Med- ical Center, 1611 West Harrison Street, Chicago, IL 60612, U.S.A. E-mail: rmfrank3@gmail.com Ó 2016 by the Arthroscopy Association of North America 0749-8063/15450/$36.00 http://dx.doi.org/10.1016/j.arthro.2016.01.041 Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol -, No - (Month), 2016: pp 1-9 1