Increasing pre-activation of the quadriceps muscle protects the anterior cruciate ligament during the landing phase of a jump: An in vitro simulation Javad Hashemi a,d, ⁎, Ryan Breighner a , Taek-Hyun Jang a , Naveen Chandrashekar b , Stephen Ekwaro-Osire a , James R. Slauterbeck c a Texas Tech University, Lubbock, TX, United States b University of Waterloo, Ontario, Canada c University of Vermont, Burlington, VT, United States d Texas Tech University Health Sciences Center, United States abstract article info Article history: Received 10 April 2009 Received in revised form 1 September 2009 Accepted 26 September 2009 Keywords: Anterior cruciate ligament ACL Quadriceps ACL strain In vitro simulation ACL injury mechanism We hypothesize that application of an unopposed quadriceps force coupled with an impulsive ground reaction force may induce anterior cruciate ligament (ACL) injury. This situation is similar to landing from a jump if only the quadriceps muscle is active; an unlikely but presumably dangerous circumstance. The purpose of this study was to test our hypothesis using in vitro simulation of jump landing. A jump-landing simulator was utilized. Nine cadaveric knees were tested at an initial flexion angle of 20°. Each ACL was instrumented with a differential variable reluctance transducer (DVRT). Quadriceps pre-activation forces (QPFs) ranging from 25 N to 700 N were applied to each knee, followed by an impulsive ground reaction force produced by a carriage-mounted drop weight (7 kg) that impulsively drove the ankle upward. ACL strain was monitored before landing due to application of QPF (pre-activation strain) and at landing due to application of the ground reaction force (landing strain). No ACLs were injured. Pre-activation strains exhibited a positive correlation with QPF (r = 0.674, p b 0.001) while landing strains showed a negative correlation (r =-0.235, p = 0.032). Total ACL strain (pre-activation + landing strain) showed no correlation with QPF (r = 0.023, p = 0.428). Our findings indicate that elevated QPF increases pre-activation strain but reduces the landing strain and is therefore protective post-landing. Overall, there is a complete lack of correlation between “total” ACL strain and QPF suggesting that the total strain in the ACL is independent of the QPF under the simulated conditions. © 2009 Elsevier B.V. All rights reserved. 1. Introduction The human anterior cruciate ligament (ACL) primarily serves as a restraint against anterior tibial translation at low flexion angles. There are an estimated 80,000 to 250,000 cases annually where the stability of the knee is compromised and the ACL fails [1,2]. Additionally, about 70% of these failures are classified as non-contact [3]. Activities in which non-contact ACL injuries occur include pivoting or side step cutting, decelerating while the knee is in an extended position, or landing from a jump with the latter being the most often cited [4]. No consensus as to the root cause of these injuries exists among researchers in the field [2]. Despite a lack of agreement as to the cause, many risk factors for non- contact ACL injury have been proposed. Among these, sex is the most widely cited with females exhibiting a 4–6 fold greater incidence of ACL injury [1]. In addition to sex, environmental, anatomical, hormonal, and neuromuscular risk factors have also been identified [2]. A frequently cited mechanism of ACL injury is a process in which the quadriceps muscle force is applied at an exceptionally aggressive level to cause severe anterior tibial translation and subsequent ACL injury. An abundance of evidence in the literature supports this plausible theory [5–13]. The proponents of the theory suggest that a combination of low knee flexion, strong quadriceps muscle contrac- tion, and a posteriorly directed ground reaction force can increase ACL loading and cause injury [6,8,10,12]. A posteriorly directed GRF tends to increase the flexion of the knee post-landing and, for the body to resist excessive flexion of the knee, the quadriceps load has to increase; this additional increase in the quadriceps force is believed to increase the ACL strain and potentially cause injury [12,24]. Some argue that hamstring co-contraction will resist anterior tibial translation induced by the aggressive quadriceps pull. However, proponents of the quadriceps pull mechanism contend that at low flexion angles (less than 15°), hamstring co-contraction does not significantly reduce anterior tibial translation and is therefore not protective of the ACL [9,14]. The Knee 17 (2010) 235–241 ⁎ Corresponding author. Department of Mechanical Engineering, Texas Tech University, MS1021, Lubbock, TX 79409, United States. Tel.: +1 806 7423563; fax: +1 806 742 3540. E-mail address: javad.hashemi@ttu.edu (J. Hashemi). 0968-0160/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.knee.2009.09.010 Contents lists available at ScienceDirect The Knee