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The Anterior Cruciate Ligament (ACL) is a ligament of the knee, running from the anterior tibia to the posterior femur. (6) When the ACL is injured it may also cause problems in the meniscus because the two structures are biomechanically interdependent. An illustration of the ACL is shown below:
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Injury to the ACL can often cause injury to the tibial plateau as well. (6) In layman’s terms, common mechanisms of injury to the ACL are turning, landing, and sudden deceleration. (6) Senter and Hame(1) give a broader description of common mechanisms, “Hyperflexion and hyperextension with the application of tibial torque.” In alpine skiers a mechanism known as “phantom foot,” is commonly seen when a skier falls backward, hyperflexing the knee with weight on the inside of the ski, internally rotating the tibia. (1) Deceleration, particularly with massive quadriceps contraction and limited hamstrings contraction, with the knee extended, appears to be the most common of mechanisms for ACL injury. (1,2,5) However, the root of this mechanism is the tibial torsion it causes, which puts increased stress upon the ACL. (1) The role of equipment in this injury is significant as shown in the case of “phantom foot.” Improved ski bindings prevent ankle and tibia fractures but not ligamentous knee injuries. (1) The interaction between cleated shoes and surfaces has also been shown to have relation to the risk of ACL injury. (1) This interaction is known as “the ‘release coefficient’ (the rotational force on a foot divided by the axial load on the foot), which represents the torque needed to release an engaged shoe-surface interface.” (1) Simply put, release coefficient is a measure of how firmly in place a foot is, and the resultant risk of ACL injury. Release coefficient is affected by many factors including size and number of cleats, as well as the type of surface and temperature. (1) Greater torsional resistance, and therefore higher rate of ACL injury, was seen in shoes with small pointed cleats in the center and long, irregular cleats along the edge.
In this paper I will discuss the prevention of injury to the anterior cruciate ligament. It is vital that an effort is made to prevent this injury. In 2008 it was estimated that 80,000 ACL injuries occurred in the United States. (2) The program I suggest makes use of flexibility and strengthening programs, equipment, and education about proper technique.
Any flexibility and strength training program for preventing ACL injury should be carried out over the long term. Short term programs, 4-7 weeks in duration, have been shown to be insufficient in reducing ACL injury risk for females. (3) Programs should especially be aimed at athletes who are considered high-risk for ACL injury, because they are more responsive to neuromuscular training. (3) A combination of plyometric and basic strength training exercises is effective in reducing risk factors, while use of only one method is ineffective. (3)
In a program of strength training and plyometric exercises, the hamstrings should be emphasized. Strength of this muscle group is needed to limit loading of the ACL when the quadriceps contract vigorously and the knee is extended. If the hamstrings cannot produce enough force the ACL may be injured. (2) The glute-ham raise is a valuable exercise for ACL protection, designed to engage the hamstring muscles from knee extension. The glute-ham raise is shown below:
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Proprioceptive Neuromuscular Facilitation (PNF) stretching is also effective in developing the hamstrings to prevent ACL injury. PNF hamstring stretching can be done in the position shown below:
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PNF stretching serves a dual role. During the resistance, strengthening is enhanced again from an extended knee position. During the passive stretch a greater range of motion is achieved, as compared to unassisted stretching. (7)
A sample week of preventative strength and flexibility training could be done as follows:
| | Monday | Wednesday | Friday |
| glute/ham | 3sets10 | | 3sets10 |
| knee ext. | 3sets10 | | 3sets10 |
| box jumps | | 3sets8 | |
| | | | |
| PNF hamstring | 2x1min. | 2x1min. | 2x1min. |
Much consideration should be given to the risk to the ACL caused by equipment selection. As discussed earlier, the release coefficient of shoes should be considered. For example, shoes with large irregular spikes around the edge and pointy spikes in the middle should be avoided, as well as warm surfaces, because they increase release coefficient. (1) Ski boot release systems should be considered, although they have not yet been developed to protect from knee ligament injuries. Since the 1970s the focus in boot release has been preventing ankle and tibia fractures. (1)
To prevent ACL injury, running technique should focus on three things (a) keeping the heel landing under the body, (b) gradual acceleration and deceleration, and (c) bending at the knees. Making sure the heel lands under the body is important simply because it has been observed that ACL injured legs are often placed infront of the body (2), potentially causing a braking that loads the ACL. A frequently recognized mechanism of injury is rapid acceleration or deceleration and the tremendous subsequent quadriceps contraction. (2) The solution is to practice gradual acceleration and deceleration. This decreases the demand for quadriceps contraction and limits the jarring forces of stopping a person’s momentum in only a few steps. Finally, bending the knees is helpful because it has been observed that twisting injuries, such as ACL tears, may have been avoided if the knee were more flexed. (1) This technique should be implemented into a prevention program by making a conscious effort, when decelerating, to “drop the hips.” This imagery conveys the proper idea of knee bend when already in the vulnerable position of decelerating.
Injury to the ACL is extremely common in the United States (2), and can cause lasting damage, altering or ending athletic careers because “ACL-injured people often suffer long-term complications, such as meniscal lesions, impairment of normal knee function, and arthrofibrosis.” (2 pg396) For this reason it is important to give active persons the best chance to participate without injuring themselves. Research suggests that with the use of the injury prevention strategy previously outlined, some ACL injuries can be avoided. The basic mechanism of tibial torsion can generally be avoided with proper mechanics and equipment. If such torsion does occur, the improvements in strength may limit its severity. This prevention through strength, equipment, and mechanics can save many athletic careers.
References
1. Carlin Senter and Sharon L. Hame. “Biomechanical Analysis of Tibial Torque and Knee Flexion Angle.” SportsMed Volume 36. Issue 8 (2006): 635-641
2. Yohei Shimokochi and Sandra J. Shultz. “Mechanisms of Noncontact Anterior Cruciate Ligament Injury.” Journal of Athletic Training Volume 43. Issue 4 (2008): 396-408
3. Myer, Ford, Brent, and Hewett. “Differential Neuromuscular Training Affects on ACL Injury Risk Factors in ‘High-Risk’ versus ‘Low-Risk’ Athletes.” BioMed Central Musculoskeletal Disorders Volume 8, Issue 39 (2007)
4. Shea, Apel, Pfeiffer, and Traughber. “The Anatomy of the Proximal Tibia in Pediatric and Adolescent Patients: Implications for ACL Reconstruction and Prevention of Physeal Arrest.” Knee Surgery, Sports Traumatology, Arthroscopy Issue 15 (2007): 320-327
5. Jones, Appleyard, Mahajan, and Murrell. “Meniscal and Chondral Loss in the Anterior Cruciate Ligament Injured Knee.” SportsMed Volume 33, Issue 14 (2003): 1075-1089
6. Anderson, Parr, Hall. Foundations of Athletic Training. Baltimore, MD: Lippincott Williams & Wilkins; 2009
7. KPE253, Flexibility, Walter Abbott and David Cusano, March 2007


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