Anterior cruciate ligament (ACL) injuries are one of the most common and potentially debilitating sports related injuries. Approximately 80,000 ACL injuries occur annually within the United States with roughly 50,000 of these injuries requiring surgical reconstruction. A conservative estimation of the annual cost to treat ACL injuries is nearly one billion dollars. 70% of all ACL injuries occur without contact and injured individuals frequently report that the injury occurred while performing a movement they had performed numerous times before without incidence. Characteristics such as decreased femoral notch width, increased Q-angle and higher blood estrogen concentrations have been associated with greater risk for sustaining an ACL injury (Hewett et al., 2006). However, anthropometric and hormonal characteristics are examples of factors associated with ACL injuries that are not under the functional control of an athlete and therefore are not easily modified to reduce injury risk. The focus of this investigation was on factors associated with ACL injury that fall under the immediate control of the athlete. Previous studies suggest that quadriceps to hamstrings activation, quadriceps to hamstrings strength ratio, ground reaction force characteristics and the knee angle associated with an athletic maneuver affect ACL loading, are under the functional control of the athlete, and can be modified through training. However, it is not known how these four characteristics interact with each other and ACL loading to either protect the knee or contribute to ACL injury.

The objectives of this study were:​ (1) to establish a method of assessing the effects quadriceps to hamstrings activation, quadriceps to hamstrings strength ratio, ground reaction force characteristics (magnitude, direction, and location on the foot), and knee angle have on one another and on tibial shear force (TSF), a measure of ACL loading, during a common movement known to be associated with ACL injuries, and (2) to determine if TSF can be reduced by altering movement mechanics to modify those factors that have the greatest impact on TSF.

These objectives were achieved through an integration of theoretical, observational, and experimental methods. The theoretical component used data from previous studies and SIMM (Software for Interactive Musculoskeletal Modeling, Motion Analysis Corp, Santa Rosa, CA) to develop a mathematical model of the internal structures of the knee that could assess each one of the factors under investigation during a jump stop, an athletic movement closely associated with the typical ACL injury mechanism. The jump stop movement was then performed using various different techniques and kinematic, kinetic and anthropometric data were used in an inverse dynamics analysis to obtain the resultant force and moment at the knee joint. The resultant force and moment at the knee were then distributed among the internal structures of the knee to obtain TSF. The observational component of the study was performed to better understand the jump stop movement and accurately identify the conditions under which the jump stop is performed during the training of our subject population. Fourteen subjects were then tested in the laboratory performing the jump stop under conditions representative of those that were observed inthe-field. Using four Falcon high speed digital video cameras, Motion Analysis software, a Kistler force platform and a Noraxon telemetered electromyography (EMG) system, data were collected to assess the subjects’ baseline quadriceps to hamstrings activation, quadriceps to hamstrings strength ratio, magnitude, location and direction of the resultant ground reaction force, knee angle and to calculate the baseline TSF associated with the jump stop. Using the findings from the theoretical phase, the athletes were instructed to modify their jump stop technique in ways that would reduce TSFs without compromising jump stop performance in the intervention condition. Modifications to movement mechanics included;​ increasing knee flexion, increasing the height of the jump amplitude approach, and contacting the ground with the front portion of the foot first. Peak TSF (PTSF) was compared from the baseline condition to the intervention condition to assess whether the modifications to movement technique had reduced TSF.

100% of the subjects tested were able to make drastic reductions in PTSF from the baseline to the intervention condition. The average reduction in PTSF was 56.4% (0.38 ± 0.28 BW vs. 0.81 ± 0.42;​ P<.001). Reduction was the result of slight modifications to their movement mechanics that did not significantly decrease performance and even showed a significant 2.5 cm increase in takeoff vertical jump height (23.4 ± 7.0 vs. 25.9 ± 6.2;​ P = 0.015). The largest factors affecting ACL loading were identified to be knee angle, quadriceps activation and the direction of the peak resultant ground reaction force (PRGRF). An 8.7 degree increase in the knee angle at contact, resulted in a 26.1% reduction in PTSF. The factors that had the largest interactions with the other factors significantly related to PTSF were;​ GRF magnitude, knee angle, quad activation, and PRGRF direction.

The findings from this study identify crucial elements that should be included in any noncontact ACL injury prevention program, such as the Sportsmetrics program and the Prevent injury Enhance Performance (PEP) program. A program that results in an athlete committing these types of movement mechanics to habit may have the best chance of reducing non-contact ACL injury risk.