It is well known that female soccer players are at far greater risk of non-contact ACL injury than are their male counterparts. Estimates place women anywhere from 3-6 times at greater risk than men. Over the years, debate has centered on the specific reasons for this increased risk. Of all the potential factors that have been identified, the most important appears to be the neuromuscular control and biomechanical technique used when lading, stopping or cutting. Because of this, many researchers have focused on designing training routines that will correct these deficits and lower the risk of injury. A recent study published in the American Journal of Sports Medicine confirms that neuromuscular training, as part of a routine soccer training session greatly reduces the incidence of ACL injuries.
Non-contact ACL injuries occur when an athlete lands from a jump, stops suddenly or performs a cutting maneuver. How these movements are performed, particularly the biomechanics of the hip and knee flexion can greatly place the ACL at risk of injury. When young boys and girls, land, stop or cut, the knee undergoes a valgus or abduction movement (see figure at right). That is, the knees tend move inward and take on a knock-kneed position. This also causes the knee to rotate inward and opens up the medial (inside) joint space. To counter the valgus and rotational torque, the ACL is placed under tremendous stress. As boys mature through puberty, they experience a neuromuscular growth spurt. They begin to utilize preferred positions where the knee undergoes more flexion and moves slightly outward (varus). In addition, the hip undergoes more flexion. This position relieves much of the stress on the ACL and lowers risk of injury. For some reason, as girls mature, they don’t experience same sort of growth spurt and continue to land with knee valgus little hip flexion. In effect, the impact of landing or cutting is absorbed by the knees, through knee valgus as opposed to hip flexion.
A second factor predisposing women to ACL injuries is reduced core proprioception or neuromuscular control of the trunk. A lack of neuromuscular control in the trunk region makes it difficult to positioning the body’s center of gravity over the base of support. This is especially true when there are rapid changes in trunk position. The inability to maintain proper center of gravity position exaggerates valgus torque and increases the stress placed on the ACL during landing, stopping and cutting maneuvers.
Researchers and clinicians have begun to advocate a program of “neuromuscular training” in an effort to correct biomechanical and neuromuscular differences between men and women. The goal is to improve technique by strengthening the supporting muscles and improving proprioception. Several programs have been developed. For the most part, they incorporate four key components: instruction on proper technique, core stability training (balance training), plyometrics and strength training.
In a recent study published, Dr. Julie Gilchrist of the Centers for Disease Control’s Division of Unintentional Injury Prevention enrolled 26 Division I soccer teams in an injury prevention training program. An additional 35 teams served as the control group (a total of 1435 athletes). The specific training program is shown in the figure on the right (a similar program is available from the Santa Monica ACL Prevention Program, linked below). The program was included as part of the warm-up that preceded each training session and was performed three times per week over the course of the fall season.
The researchers found that the rate of non-contact ACL injures was reduced by an impressive 70% in the training group compared to the control group. For athletes with prior ACL injury, the rate of re-injury was cut by more than half. Based on this, the authors conclude that focused training on neuromuscular control, strength and technique can reduce the risk of ACL injuries and may be of particular benefit for athletes with prior injury. The authors are quick to point out that the program used can be easily incorporated as part of regular training and does not require the use of special equipment. It is also important to point out that the program was used for only a single season and was not implemented during the off-season or pre-season. It is possible that use of year-round neuromuscular training or a more intensive off-season program could pay off with even larger dividends.
The results of the Gilchrist et al. study are not entirely surprising given the results of previous laboratory studies. Neuromuscular training improves landing technique, neuromuscular recruitment patterns, biomechanics and core proprioception. In short, athletes who have undergone neuromuscular training perform landing and cutting exercises with noticeably less knee valgus and more hip flexion. That is, the athletes tended to land in the preferred position rather than the knock-kneed position. Laboratory studies also show increased muscular activity of the semitendenosus muscle. This is important given that the semitendenosus muscle is one of two medial hamstrings. Along with the semimembranosus, this muscle attaches near the inside of the knee. Increased force by the medial hamstrings should “close down” the medial joint space and prevent valgus movement when landing or cutting. Increased hamstring force should also prevent the tibia from sliding forward. Both effects would have the effect of reducing ACL stress and lowering the risk of injury. Finally, core proprioception, stability and balance are all improved following neuromuscular training. Players increase their ability to stabilize their center of gravity over their base of support. All of these changes should lead to reduced stress placed on the ACL when performing lading, stopping and cutting movements.
The benefits found by Dr. Gilchrist confirm the laboratory findings of improved knee control. They also confirm what many are seeing as an emerging trend. That is the use of a neuromuscular training program that incorporates technique training, plyometrics, core stability / balance and strengthening exercises can reduce the risk of ACL injury. At this point, it is clear that female soccer teams should incorporate some form of neuromuscular training into their regular practice routine.
A final note. There are several excellent website that discuss the causes of ACL injuries in women and describe neuromuscular training programs. Dr. Timothy Hewett on the University of Cincinnati and the Cincinnati Children’s Hospital is one of the leading experts on female ACL injury causes and prevention. Two excellent interviews with Dr. Hewett explain the predisposition of women to ACL injuries and describe “Dynamic Neuromuscular Analysis” can lower the risk of injury. They are linked below:
Article 1 and Article 2
Also, a detailed neuromuscular training program developed by Santa Monica ACL Prevention Program (used in the Gilchrist study) can be found here. Be sure to click on the “AAF Logo for ACL Classroom Session” for video explanations of ACL injuries, knee anatomy as well as specific exercises.
Chappell JD, Limpisvasti O. (2008) Effect of a neuromuscular training program on the kinetics and kinematics of jumping tasks. American Journal of Sports Medicine, 36:1081-1086.
Gilchrist J, Mandelbaum BR, Melancon H, Ryan GW, Silvers HJ, Griffin LY, Watanabe DS, Dick RW, Dvorak J. (2008) A randomized controlled trial to prevent noncontact anterior cruciate ligament injury in female collegiate soccer players. American Journal of Sports Medicine, 36:1476-1483.
Zazulak BT, Hewett TE, Reeves NP, Goldberg B, Cholewicki J. (2007) Deficits in neuromuscular control of the trunk predict knee injury risk: a prospective biomechanical-epidemiologic study. American Journal of Sports Medicine, 35:1123-1130.
Zebis MK, Bencke J, Andersen LL, Døssing S, Alkjaer T, Magnusson SP, Kjaer M, Aagaard P. (2008) The effects of neuromuscular training on knee joint motor control during sidecutting in female elite soccer and handball players. Clinical Journal of Sports Medicine, 18:329-337.