Tears of the anterior cruciate ligament, or ACL, are a prevalent sports-related injury. 250,000 ACL tears occur on an annual basis, and athletes that participate in sports such as soccer that involve cutting, jumping, and pivoting are at an especially high risk1. The ACL prevents anterior tibial translation (the tibia sliding forward relative to the femur), and prevents the knee from hyperextending. It is also a stabilizer against knee valgus (it prevents the knee from collapsing inwards).
Recently, more and more data has emerged that suggests that performing a dynamic warm-up before practices and games can be effective at reducing the risk of ACL injury11. However, a consensus regarding the most effective components of a prevention program has not been reached. Despite the lack of an evidence-based consensus on the optimal warm-up, there is ample evidence that certain knee movements, muscle activation patterns, and movement strategies predispose the ACL to excessive strain, and therefore potential injury. Therefore, we at MOTUS Specialists Physical Therapy have proposed some foundational concepts of a soccer warm-up that primes the body to activate and move in a way that protects the ACL.
While mobility is an important aspect of ACL injury prevention, it is important to consider how that mobility is gained during warm-up. Static stretching is performed by taking a limb or joint to it’s end-range and staying there for 30-60 seconds. Static stretching has been shown to increase the amount of anterior tibial translation available at the knee4, and such laxity has been shown to be a risk factor for ACL injuries8. Additionally, static stretching actually seems to decrease athletic performance, likely as a result of the inhibitory effect it has on muscle activity2. To the contrary, dynamic stretching, or active movements that take an athlete through his/ her full range of motion, has been shown to increase performance. Furthermore, dynamic stretching is favorable because it allows an athlete to practice actively moving through his/ her full range of motion. Therefore, dynamic stretching should be favored over static stretching for pre-activity warm-ups.
Mobility in certain movements are especially important to reduce ACL injury risk. Specifically, decreased range of motion at the ankle and hip joint can necessitate more motion at the knee, resulting in increased ACL strain. Decreased ankle dorsiflexion (toes/ foot pointing up) and hip external rotation (toes pointing out) can lead to increased knee valgus, a position that directly strains the ACL12. Additionally, athletes with decreased hip internal rotation (toes pointing in) has been found to have an increased risk of ACL tears5. Therefore, active movements into ankle dorsiflexion and hip internal/ external rotation are important components of an effective warm-up.
Movement patterns are another important component of an effective prevention program. While retraining of movement patterns can take extensive training outside of a warm-up routine, it is important to incorporate ideal movement patterns in a warm-up to prime the body to move in a safe and efficient manner. Specifically, training a “hip strategy” with lunging, jumping, and landing is an effective way to reduce strain through the ACL. A hip strategy refers to increased hip flexion/ forward trunk lean during loading and landing tasks. By leaning forward and increasing hip flexion you tension and stretch the hamstring muscles, which prevents anterior tibial translation and reduces load through the ACL14. Additionally, you put the gluteal muscles in a better position to create force to stabilize and support the hip and knee. It has also been shown that increased knee and hip flexion (bending) with landing tasks put an athlete at decreased risk for ACL injury12. It is important that this is controlled hip and knee flexion, as increased quadriceps stiffness during landing has been shown to decrease strain on the ACL9. By increasing the depth of controlled hip and knee movement, you put more demand on the active structures (muscles) and less on the passive structures (ligaments, like the ACL).
Lastly, it is important to perform some light muscle activation exercises that will prime key muscles to activate during activity. It is important to dose this portion of the warm-up appropriately, as you do not want to fatigue the muscles heading into a game or practice. Two important muscles are the hamstrings and the gluteus maximus. The hamstrings work as an ACL agonist (they work together with the ACL to prevent anterior tibial translation), and stronger hamstrings have been correlated with decreased ACL injury risk10. Additionally, it has been demonstrated that the ability of the gluteus maximus to produce force quickly, or generate power, is an important protector against valgus knee movements that predispose the ACL to injury6. Therefore, exercises that strengthen the hamstrings and gluteus maximus outside of warm-up should be coupled with a relatively small volume of activation exercises in the pre-game warm-up to facilitate ideal muscle activation patterns.
As with any warm-up, it is important to ensure that the warm-up increases heart rate/ body temperature and recreates sports-specific skills and movements. However, if the goal of the warm-up is to reduce ACL injury risk, the following components should be included:
Mobility: ankle dorsiflexion, hip internal and external rotation
Movement patterns: controlled knee and hip flexion with landing, hip strategy
Muscular strength: hamstring strength, gluteus maximus power
Acevedo, R. J., Rivera-Vega, A., Miranda, G., & Micheo, W. (2014). Anterior Cruciate Ligament Injury. Current Sports Medicine Reports,13(3), 186-191. doi:10.1249/jsr.0000000000000053
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Cronin, B., Johnson, S. T., Chang, E., Pollard, C. D., & Norcross, M. F. (2016). Greater Hip Extension but Not Hip Abduction Explosive Strength Is Associated With Lesser Hip Adduction and Knee Valgus Motion During a Single-Leg Jump-Cut. Orthopaedic Journal of Sports Medicine,4(4), 232596711663957. doi:10.1177/2325967116639578
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Homan, K. J., Norcross, M. F., Goerger, B. M., Prentice, W. E., & Blackburn, J. (2013). The influence of hip strength on gluteal activity and lower extremity kinematics. Journal of Electromyography and Kinesiology,23(2), 411-415. doi:10.1016/j.jelekin.2012.11.009
Lipps, D. B., Oh, Y. K., Ashton-Miller, J. A., & Wojtys, E. M. (2013). Effect of increased quadriceps tensile stiffness on peak anterior cruciate ligament strain during a simulated pivot landing. Journal of Orthopaedic Research,32(3), 423-430. doi:10.1002/jor.22531
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Michaelidis, M., & Koumantakis, G. A. (2014). Effects of knee injury primary prevention programs on anterior cruciate ligament injury rates in female athletes in different sports: A systematic review. Physical Therapy in Sport,15(3), 200-210. doi:10.1016/j.ptsp.2013.12.002
Pollard, C. D., Sigward, S. M., & Powers, C. M. (2010). Limited hip and knee flexion during landing is associated with increased frontal plane knee motion and moments. Clinical Biomechanics,25(2), 142-146. doi:10.1016/j.clinbiomech.2009.10.005
Sigward, S. M., Ota, S., & Powers, C. M. (2008). Predictors of Frontal Plane Knee Excursion During a Drop Land in Young Female Soccer Players. Journal of Orthopaedic & Sports Physical Therapy,38(11), 661-667. doi:10.2519/jospt.2008.2695
Withrow, T. J., Huston, L. J., Wojtys, E. M., & Ashton-Miller, J. A. (2008). Effect of Varying Hamstring Tension on Anterior Cruciate Ligament Strain During in Vitro Impulsive Knee Flexion and Compression Loading. The Journal of Bone and Joint Surgery-American Volume,90(4), 815-823. doi:10.2106/jbjs.f.01352