ACL Injury Prevention: Essential Strategies for Athletes

ACL Injury Prevention: Essential Strategies for Athletes

Written By: 

Erik Albarran, PT, DPT, CSCS

Certified Clinical Specialist in Sports Physical Therapy 

SUMMARY:

ACL injuries are common and can have long-lasting effects on athletes. However, understanding the risk factors and implementing prevention strategies can significantly reduce the likelihood of injury. This blog covers:

• Understanding ACL Injuries: Common causes and mechanisms behind ACL tears, with a focus on non-contact injuries.
• Key Risk Factors: Explore both modifiable (e.g., strength, biomechanics, fatigue) and non-modifiable (e.g., anatomy, sex) factors.
• Preventive Measures:
  • Strengthening exercises for quadriceps, hamstrings, and core.
  • Proprioception and neuromuscular control training.
  • Importance of monitoring fatigue and psychological readiness.
• Program Recommendations: Overview of popular ACL injury prevention programs and their common components.
• Takeaway: A comprehensive approach to training can help mitigate ACL injury risk and support long-term athletic health.

Read on to learn more.

ACL Injury Prevention: Essential Strategies for Athletes

Anterior cruciate ligament (ACL) ruptures are the most common ligamentous injuries occurring in the United States. It is estimated that there are anywhere between 100,000 and 200,000 cases yearly (48). Most of these injuries occur through non-contact. This means that the athlete was not tackled or forced into a compromising position. The injury resulted from some action that included components of deceleration, jumping, landing, or cutting. Most often than not, some type of change of direction movement was also involved (48). 

For years the exact mechanism of injury has been debated without a clear consensus. Some common theories that have been suggested to predispose an athlete to injury include intercondylar notch impingement, quadriceps strength, quadriceps to hamstring strength ratio, and axial compressive forces (6). Females are traditionally believed to be at a greater risk to sustain an ACL injury with the injury rate reported between 2-8 times greater than their male counterparts (2, 34). Potential explanations are related to the fact that females have increased knee valgus or abduction moments, generalized joint laxity, genu recurvatum, ACL size (smaller), and potential hormonal effects of estrogen on the ACL (6, 9, 58). 

Conservative treatment following an ACL injury focuses on strengthening the quadriceps and hamstring muscles, restoring proprioception, and improving overall lower extremity neuromuscular control to improve knee alignment during functional activities. Ironically, a lot of the aspects of treatment following an ACL injury work well to prevent the injury to begin with.  

As previously mentioned, there are a variety of potential risk factors that could predispose an individual to suffer an ACL injury. Some of these risk factors are nonmodifiable and there is not much someone can do about it. These nonmodifiable risk factors include sex, anatomy, and prior injury. 

However, the focus of an aspiring preventative program should be correcting some of the more modifiable risk factors such as lower extremity strength and neuromuscular control, proprioception, fatigue, biomechanics, and an athlete’s psychological readiness to return after an injury. However, it must be noted that the aforementioned risk factors are listed because multiple studies have found some association between them and ACL injury. This does not mean that correcting any one of these will guarantee avoidance of injury. With that being said, there are a multitude of studies that support and some that deny the association between injury risk and these skills and both sides will be acknowledged in the following text.

Risk factors: Modifiable

Fatigue:

Fatigue and performance are inversely related by definition and proven by correlation (75). It would be intuitive that as an athlete’s fatigue increases, their performance will decrease. This drop in performance could theoretically cause compensations leading to poor technique and faulty mechanics that result in a movement pattern similar to the mechanism of injury (69). This theory has been anecdotally supported by observing fatigued athletes performance in landing. This has prompted multiple studies that have discovered faulty landing mechanics in fatigued athletes that include larger knee adduction, abduction, and internal rotation compared to their non-fatigued condition. The fatigued athlete also demonstrated increased initial and peak knee abduction and internal rotation motions and peak knee internal rotation, adduction, and abduction moments (32). Particular interest should be placed on females considering the difference was more pronounced in that population. 

Another potential fatigue based theory is that sudden increases in training load results in a general fatigue state that may cause an overall increase in injury risk (26). This is due to the fact that fatigued muscles are not able to absorb the same amount of energy that a non-fatigued muscle would (27). This could lead to excessive strain on the soft tissue which, if not corrected, could lead to injury. For this reason, it is important to monitor fatigue levels and loading in athletes to gauge if modifications need to be made for an individual athlete, session, or entire program. 

There has not been a specific monitoring method identified, however commonly used measures include objective tests and subjective self-reported measures. Some objective tests include counter movement jump, heart rate monitoring, and blood lactate concentration (56, 69). Self-reported measures include rate of perceived exertion and questionnaires with relation to fatigue and physical and mental wellbeing (56, 69). However, as mentioned previously, injury risk is multifactorial and fatigue as a standalone descriptor is not a good indicator of injury risk (11). 

The main take away from this section is to assess fatigue in some way and take it into consideration with future training or potential modifications.

Environmental:

There has been a heavy criticism of artificial turf playing surfaces in recent years and their perceived contribution to increased rates of injuries. However, this has not necessarily caused any immediate change from the governing organizations or facility management. The cost of maintenance for artificial turf is significantly lower than natural grass so financially speaking there is no competition. Especially considering that there is no proof that artificial turf is a direct cause of the injuries. 

However, recent studies have shown that there is some merit to the claim (20, 37, 65). Even so, there are some discrepancies that these studies came across such as differences in sex, the specific sport that was played, and whether the field was used for training or an official game. Due to this, no specific claim could be made to support or deny the use of natural grass over artificial turf. 

Aside from the constitution of the field, another potential factor to consider is the weather and the condition of the field. There have been multiple studies that have found that the moisture of the field is a big factor on injury rates (43, 47). These conclusions suggest that the actual risk factor is the traction between the foot and the playing surface. A humid and wet surface decreases the traction which allows for more give when planting a foot and potentially decreasing the shear and torsional forces experienced by the knee with cutting and pivoting movements. This theory is further supported by another study that looked at the type of natural grass where the thickness of the thatch layer could increase the traction and thus increase injury rate as well (42). This would explain why artificial turf has received bad publicity due to the fact that it is generally associated with higher traction than natural grass (41). This does shed some light as to why most professional teams will water the field prior to practice and games.

The main takeaway from this section is that there is still inconclusive evidence and no significant argument that can be made against artificial turf. 

Biomechanics:

As mentioned previously, the most common mechanism of injury for ACL tears includes a change of direction or cutting movement with some component of deceleration. For many years, the knocked knee position was vilified and pointed to as the cause of ACL injuries. It was termed the “position of no return” (21). This position was described as the lower extremity falling into hip internal rotation and adduction, knee valgus, and tibial external rotation on a pronated, externally rotated foot (5, 21). However, this assumption was formulated from video recordings of ACL injuries and not by biomechanical assessments. Naturally, after an athlete suffered an ACL injury, the game footage would be reviewed, and a very clear observation would be made during the play in question. The injured lower extremity would be in the “position of no return” and thus the misconception was born. 

With the advancement of technology, more sophisticated assessments have identified specific biomechanics and postures that lead to increased forces and strain on the ACL. For example, there is an increased risk of injury due to greater peak forces as a result of decreased hip and knee flexion angles during landing (19). However, increased hip and knee flexion would inevitably lead to a forward leaning posture which would lower the center of mass and move it closer to the base of support. It would come to no surprise then that there has also been evidence to suggest that the center of mass position relative to base of support at point of contact influences potential ACL injury as well (8). Landing too far posterior of the base of support increases the potential for injury while leaning forward when landing brings the center of mass closer to the base of support and decreases the potential for injury (8). This is supported by multiple studies that found that a less erect posture was associated with decreased risk of ACL injury compared to landing with a more upright posture (14, 16, 24). These are basically two ways of saying the same thing. 

Another aspect of the hip joint that may increase injury risk is the abduction moments in landing which is also heightened in females due to an increased Q angle (32). An abduction moment is the external force that makes the knee want to fall in. This is countered by the muscles of the hip that move the lower leg out, most notably the gluteus medius.

When it comes to the knee, the positioning associated with injury is most often described as landing with the knee in near full extension, pivoting in near full extension on a planted foot, or a knee hyperextension or hyperflexion mechanism (12, 15, 54). The specific forces acting on the knee in these positions include knee valgus, varus, internal rotation, and external rotation moments, and anterior translation force (29, 30, 39, 68). To continue to debunk the “position of no return”, it has since been shown that it was in fact internal rotation and not external that causes increased tension on the ACL (15, 29, 30). It has also been shown that neither individual force is responsible for ACL injuries and it is a combination of forces that leads to the ligamentous rupture (4, 68). The combination credited as the main factor in ACL tears is specifically an anterior directed force on the tibia with a concomitant knee valgus and/or tibial internal rotation at around 20-30 degrees of knee flexion (4, 10, 29, 30, 33, 68). Unlike other topics in this text, there are not many studies objecting to the biomechanics of the knee having a significant influence in ACL injury.

The last joint of the lower extremity, the ankle, has also been subject to controversy for its potential involvement in ACL injury risks. There have been studies that found an association between subtalar hyper pronation and ACL injury risk (1, 3, 64). Meaning that the more uncontrolled flat foot during weight bearing, the higher chance of ACL injury. However, other studies have found the opposite in that foot mechanics had no significant correlation with ACL injury risk (22, 49). A potential explanation for this is in the ankle’s ability to dorsiflex. In other words, how far the knee can get past the toes without the heel coming off the ground. The most common compensation for decreased dorsiflexion is pronation of the foot. The lack of dorsiflexion could also limit the amount of flexion available at the knee and thus contribute to the relatively extended knee that has been identified as a risk factor. It would come to no surprise then that a literature review of motion analysis studies found an association with increased dorsiflexion range of motion and proper single leg squat and landing (61). However, it cannot be said with complete certainty that this is the main contributor to injury risk and for that reason there is no clear consensus on the involvement of the positioning of the ankle joint as a risk factor. 

Another potential factor in ACL injury that has received a lot of attention is the hamstring’s ability to protect the ACL. The nature of the hamstring’s pull on the lower leg causes knee flexion but also tibial posterior translation. This makes the hamstrings a great support to the ACL to restrict tibial anterior translation. It has been shown that weak hamstrings result in greater ground reaction forces which could potentially increase the risk of ACL injury (18). This could be due to the fact that hamstrings are direct antagonists to the quadriceps. Isolated quadricep contraction results in knee extension and anterior tibial translation which would stress the ACL. Isolated hamstring contraction does the opposite and thus reduces the stress on the ACL. However, when there is a concomitant quadriceps contraction the hamstring’s protective influence on the ACL decreases if the knee is flexed less than 30 degrees or near full extension (25, 46, 62). This further supports the previously mentioned studies that found an increased risk of injury with decreased knee flexion.

The main take away from this section is that an increase in hip and knee flexion and greater hip and hamstring strength are associated with decreased risk of ACL injury.

Psychological readiness:

The mind is a powerful thing. An athlete’s mental status heavily influences the athlete’s self-efficacy, confidence, and, eventually, success on the field. It is easy to see how the psychological readiness of an athlete plays a big role in their success on the field, in rehabilitation, and their potential to return to their prior level of activity and competition. 

One of the most common ACL specific psychological readiness questionnaires is the Anterior Cruciate Ligament Return to Sport after Injury Scale (ACL-RSI). It is easy to administer and easy to understand. Multiple studies have looked at the association and predictive ability of questionnaires and surveys to identify an athlete’s perception and mental status when it comes to their physical performance. It has been shown that greater psychological readiness is associated with return to sport (59). Associations have also been found between an athlete’s psychological readiness and physical characteristics. For example, a significant negative correlation has been seen with the ACL-RSI and kinematic limb symmetry. Athletes that scored lower on the questionnaires demonstrated relatively weaker quadricep strength and increased asymmetries and vice versa (72, 74).

However, a recent study looking at the correlation between psychological readiness and likelihood of sustaining a second ACL injury found that the athletes that suffered a second injury had higher questionnaire scores (71). This could be explained by having athletes that are too confident and potentially borderline reckless. This false sense of security could lead these athletes to return too soon or to perform activities that they are not prepared for. However, it must be noted that this study had a small sample size and that all secondary injuries were also hamstring tendon autografts which have a higher retear rate compared to other autografts (60). This would make sense considering the hamstring’s involvement in assisting the ACL. Harvesting a graft from the hamstrings would intuitively weaken the hamstring complex which could lead to increased risk of injury as mentioned previously. It is no surprise that when looking at the biomechanics of athletes following a hamstring autograft, they found increased knee valgus moments (57). 

Even though psychological readiness has been shown to be an indicator on both sides of the spectrum, as far as injury risk is concerned, it is not often assessed. A recent study that surveyed hundreds of surgeons indicated that only 21% were assessing psychological readiness when considering clearing an athlete for return to sport (66). As previously mentioned, no risk factor on their own is a great indicator of injury risk. Multiple factors must be considered. For example, if an athlete scores well on their physical exam but poor on their psychological readiness, they are most likely not ready to return to their sport. If an athlete scores poorly on their physical exam but great on their psychological readiness, they also most likely are not ready to return to their sport. 

The main takeaway from this section is to consider psychological readiness when making important participation and return to sport considerations.

What now?

After reviewing the modifiable risk factors, it is obvious that some of these are more actionable than others. Depending on an individual’s situation, some of these may have to be advocated for. For instance, fatigue assessment and monitoring could be performed on an individual basis but if an athlete is part of a team or some institution, it may be a bit more difficult. The importance of monitoring will need to be made apparent to the coaching or training staff and they will need to be open to potential modifications based on the results of the assessments. This could be met with some resistance from the staff. It could be seen as adding another component to an already busy schedule. It could be seen as an athlete trying to get out of their workouts. For these reasons it is imperative to address the importance and long-term benefit that fatigue monitoring can provide to the team. Some of the benefits include maximizing performance by avoiding fatigue leading up to important games or matches as well as avoiding potential injury to keep athletes available for selection. Similarly, the playing environment and field will also have to be advocated for in a team setting but as noted previously, there is little support for natural grass over artificial turf. 

Psychological readiness can also be monitored relatively easily, however the interpretation of these questionnaires should be left in the hands of professionals such as sports psychologists, physicians, physical therapists, and athletic trainers. This does not mean that an athlete requires a medical professional to improve in this aspect. An athlete should continue to train and expose themselves to different scenarios to improve their confidence in their physical capabilities. A study looking at participation in neuromuscular training and a second injury prevention program demonstrated a significant improvement in both the athlete’s psychological response and self-reported function (70). 

This leads us to the most actionable risk factor on this list, biomechanics. Multiple injury prevention programs have been developed such as Perform+, Prevent Injury and Enhance Performance (PEP), and the Knee Control Program. Another helpful document with preventative recommendations is the American Physical Therapy Association Clinical Practice Guidelines: Exercise-Based Knee and Anterior Cruciate Ligament Injury Prevention. These programs are all a bit different and include different exercises, but the general focus and common components are very similar. These components include lower extremity and core strengthening, balance and proprioceptive work, plyometrics, and agility training. 

The lower extremity strengthening aspects of these programs will encompass the quadriceps, hamstrings, and glutes. It has been shown that increased quadriceps strength is associated with improved general physical function and greater psychological readiness to return to play (53). Increased hip abductors, extensors, and external rotator strength has also been associated with improved mechanics in single leg activities such as landing and squatting (61). As mentioned before, hamstring strength is also a significant contributing factor to injury recovery and prevention whether they are working with the quadriceps to create an isometric contraction and thus a more stable knee or if it is supporting the ACL to resist the anterior tibial translation. A study looking at the relationship between quadricep and hamstring strength in injured and non-injured individuals found that the non-injured individuals had a significantly greater relative strength in their hamstrings compared to their quadriceps (35). Lastly, core strength was not specifically mentioned in this text but it is important to acknowledge. A study looking at trunk displacement, proprioception, and low back pain as predictors of knee ligament injury found that only low back pain was a predictor for males but all three were significant predictors for female athletes (73). 

Proprioception (kinesthesia) is the body's ability to sense its position and movement relative to space and itself. Neuromuscular control is the unconscious muscular response to stimuli in reference to joint stability. Both skills have been identified as potential areas of improvement to decrease the likelihood of injury (50, 23). A study looking at the quadriceps and the hamstring activation patterns noticed an inhibition into the quadriceps and an excitation in the hamstrings that was elicited by strain on the ACL (50). The ACL’s mechanoreceptors (sensory nerves that respond to deformation) responded to the strain by initiating a reflex arc that decreased the muscle activation of the quads and increased the muscle activation of the hamstrings. This proprioceptive response is an attempt to decrease the strain in the ACL by modifying, either increasing or decreasing, the muscle’s ability to fire. In a more direct comparison study, individuals rehabilitating from an ACL reconstruction were randomized into a group with a focus on strength training and one focused on neuromuscular training. The neuromuscular protocol included several plyometric, balance, and single leg exercises. The neuromuscular group demonstrated significantly higher scores in multiple return to sport assessments (23). This does not mean that neuromuscular training should be the sole focus of the rehabilitation protocol, but it heavily supports its inclusion. 

Plyometrics training can be described as having quick explosive movements such as landing and jumping. A more specific description would be movements reliant on a tendon’s ability to stretch and recoil. This elastic property is called the stretch shortening cycle. Agility is an individual’s ability to perform a change of direction or a change in velocity in response to a stimulus. This does not include ladder drills or well-known pre-planned movements such as pro-agility, T-drill, and Illinois agility drill. These drills are pre-planned change of direction activities and require no stimulus to react to, therefore they are not considered agility drills. There is some debate that agility training is superior to change of direction training and has the potential to differentiate elite level athletes (13, 31, 38, 44, 45). However, both skills are important and require consideration when developing a program. Plyometrics and change of direction drills have been a staple in many return to sport protocols and for good reason. Multiple studies and systematic reviews that include plyometric and change of direction training as part of the protocol demonstrate improvement in return to sport testing and certain biomechanical measures (19, 23, 55, 67).

Complete avoidance of injury is impossible. However, incorporating lower extremity strength training and drills focusing on proprioception, neuromuscular control, plyometrics, change of direction, and agility to an athlete‘s regular regimen is highly recommended. The addition of these components may prepare athletes to encounter the challenges faced when returning to their sport or as a preventative measure (7, 17, 36, 40, 51, 52, 63, 67). As mentioned prior, there have been multiple injury prevention programs developed, however none have been shown to be necessarily superior to others. For this reason, the consensus is a recommendation to include exercises that address the common components utilized by these programs.

Common Exercises

• General warm up: typical dynamic exercises
• Strengthening:
  • Plank variations
  • Bilateral, staggered, unilateral
  • Sagittal, frontal
• Balance: double, staggered, unilateral
• Plyometrics:
  • Multidirectional
  • Bilateral, staggered, unilateral
  • Sagittal, frontal, transverse

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