This is an excerpt from Preventing Noncontact ACL Injuries eBook by Human Kinetics.
Questions regarding why an individual would adopt a strategy that includes potentially injurious mechanics have largely focused on factors such as muscle strength, motor patterns, and sensory systems.
Available strength is one of the fundamental ingredients for skilled movement. Appropriate strength is needed to generate movements that transport and decelerate the body's center of mass during athletic tasks. It is particularly important for tasks performed during sports that are associated with noncontact ACL injuries (landing and rapid change-of-direction tasks). Executing such tasks in the absence of adequate strength results in poor athletic performance and is thought to result in lower-extremity biomechanics that place an athlete at risk for injury. Studies have traditionally focused on quadriceps and hamstring strength at the knee joint, while more recent investigations have begun to direct their attention to proximal muscles of the hip and trunk.
Quadriceps and Hamstrings Early investigation of muscular control of the knee focused on the quadriceps and hamstring muscles because they have a direct influence on the knee joint. As previously discussed, in vitro and in vivo studies found that an isolated quadriceps contraction is capable of exerting an anterior shear force on the tibia at small knee flexion angles and is therefore considered to be an ACL antagonist (Markolf et al. 1990; Durselen, Claes, and Kiefer 1995; Beynnon et al. 1995; Fleming et al. 2001). Conversely, isolated hamstring contractions are capable of exerting a posterior shear force on the tibia at all knee flexion angles (Yasuda and Sasaki 1987; Beynnon et al. 1995; Fleming et al. 2001) and are considered ACL protective. Cocontraction of the hamstrings and quadriceps has been found to reduce, not eliminate, strain on the ACL that is generated by the quadriceps alone (Beynnon et al. 1995; Renstrom et al. 1986). Moreover, the quadriceps and hamstring muscle cocontraction works to stabilize the knee in the frontal plane (Lloyd and Buchanan 1996; Lloyd and Buchanan 2001). Based on function of the quadriceps and hamstring muscles, a preferential use of the quadriceps over the hamstring is thought to place an individual at risk for injury.
Several studies have reported that female athletes have both decreased quadriceps and hamstring strength when compared to males (Huston and Wojtys 1996; Barber-Westin, Noyes, and Galloway 2006; Hewett et al. 1996; Uhorchak et al. 2003; Lephart et al. 2002). Barber-Westin and colleagues (2006) assessed isokinetic quadriceps and hamstring strength in more than 1,000 athletes, aged 9 to 17 years old. After controlling for body mass, females aged 14 to 17 were found to have significantly weaker hamstrings and quadriceps than age-matched males. These results support the work of Uhorchak and colleagues (2003), who found similar differences in 859 young adults aged 17 to 23. Despite the apparent sex difference in quadriceps and hamstring strength, available strength was not found to be a predictor for ACL injury in a prospective study (Uhorchak et al. 2003).
An imbalance of hamstring to quadriceps strength, or a smaller H/Q strength ratio, is thought to be indicative of a quadriceps-dominant pattern. Athletes with smaller H/Q strength ratios are thought to preferentially use their quadriceps muscles to stabilize the knee during dynamic activities (Hewett 2000). Hewett et al. (1996) noted females had an increased H/Q strength ratio following participation in a jump training program. Because this program was found to be successful in reducing serious knee injuries (Hewett et al. 1996), these authors suggested that the reduction in this strength ratio was the mechanism by which the training program was successful. However, previous studies evaluating hamstring and quadriceps strength in males and females found sex differences in strength but no differences in H/Q strength ratio between sexes. In addition, to date there is no evidence to suggest that there are differences in strength ratios between males and females. Stronger evidence linking hamstring and quadriceps strength ratios is needed to determine if such factors are important risk factors for ACL injury.
There is some evidence to suggest that deficits in available quadriceps and hamstring strength influence lower-extremity movement patterns. A relationship between strength and function is supported by studies evaluating single-leg support tasks. When compared to males, female athletes who demonstrate decreased quadriceps strength performed single-leg landing and forward hop tasks with less knee flexion and greater hip internal rotation. The combination of these factors suggests that the females may have compensated for decreased quadriceps strength by stiffening the knee and using hip internal rotation range of motion to lower the center of mass (Lephart et al. 2002). Additionally, Claiborne and colleagues (2006) found a correlation between isokinetic knee flexor and extensor strength and knee valgus motion during a single-leg squat. Individuals with less strength demonstrated greater knee valgus at the initiation of the task. This is not surprising because the quadriceps and hamstrings contribute to frontal-plane control of the knee (Lloyd and Buchanan 1996; Lloyd and Buchanan 2001). Together, these studies suggest that inadequate quadriceps or hamstring strength can result in compensatory mechanisms that may place an individual at risk for ACL injury by contributing to excessive transverse- and frontal-plane motion at the hip and knee.