Documentation:FIB book/ACL Injuries in American Football

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The anterior cruciate ligament (ACL) is a ligament (made of dense connective tissue) in the knee that connects the femur to the tibia.

Anterior Cruciate Ligament Tear[1]

The ACL plays a key role in stabilizing the knee joint through passive restraint[2], as it resists anterior tibial translation and rotational loads[3]. The ACL prevents tibial over rotation in both the medial and lateral directions, prevents varus and valgus stresses, and provides roughly 85% of the restraining force during anterior translation[3]. The knee is further stabilized during functional movement (such as sports) through a combination of active muscular contraction and precise neuromuscular timing[2]. Therefore, changes to the biomechanics or the muscular control of the knee through any sort of altercation will increase the risk of ACL injury[2]. After the ankle, the knee joint is the second most common area of injury of the body. ACL injuries account for 50% or more of those knee injuries. Soccer, football and basketball are a few of the common sports that have an increased risk of ACL injuries. Football in particular was shown to be the sport that athletes sustained the most ACL injuries, with the injury rate during competition being 7 times higher than in practice [3] . ACL injury treatment is also economically costly, averaging around $1 billion annually in the United States for reconstruction surgery alone, and 76.6% of all ACL injuries are surgical[3]. In addition to this, patients need to go to rehabilitation to regain normal function and range of motion post surgery which is an incredibly costly expense and full recovery can take anywhere from six to twelve months depending on injury severity[3].



One epidemiological study looked at the incidence of ACL injuries among high school athletes by sport and sex. The study found that boys' football had the second highest injury rate at 11.1%, only behind girls' soccer at 12.2%. Additionally, the study found that player-to-player contact was the most common mechanism of ACL injuries in high school athletes, at 42.8%[3]. In another study looking at NFL and professional football in the US between 2009-2015, 11.6% were found to have sustained ACL injuries[4]. Moreover, receivers and linebackers were found to have a significantly higher ACL injury risk compared to other NFL players. On the other end of the range, players in the role of defensive end and offensive tackle had significantly lower injury risk relative to other NFL players[5]. A similar epidemiological study on English professional rugby found that the greatest percentage of all days missed due to a knee injuries, 29%, was because of an ACL injury[6].

A football tackle can result in ACL injuries[7]


In terms of biomechanics, ACL injuries occur when a person generates a great force or moment on the knee, which creates excessive tension onto the ACL[5]. Berns et al[8] conducted a study on the effects of combined knee loading on ACL strain on 13 cadaver knees and concluded that the anterior shear force on the proximal end of the tibia and moment around knee valgus was the primary cause of an ACL strain. These forces can be caused by great quadricep muscular force, great posterior ground reaction forces, and small knee flexion angle. Out of these factors, quadricep muscular force is the major contributor to the previously described anterior shear force. It was found that muscular force on the ACL is largest at a knee flexion of 35°[9]. Another study used cadaver knee specimens to demonstrate the likelihood of ACL injuries caused by a 4500 N quadriceps muscle force when the knee is set to 20° flexion[10].

In American Football, this can be caused from contact and non contact collisions and ranges from various of reasons. In terms of contact, high speed collisions increases the risk of ACL tears[5]. This greatly affects offensive players such as wide receivers, tight ends, linebackers, running backs, and fullbacks because they are more involved in tackles. In terms of non-contact situations, change in speed, pivoting and landing awkwardly increases the chances of ACL tears which effect all positions but mainly offensive linesman due to their great mean body mass index relative to any other position[5].

Risk Factors

For certain sports, female athletes can have up to a 3.5 times greater reported ACL injury rate relative to male athletes playing at the same level. Knowledge on contributors to the increased risk for female athletes is limited however, smaller ACL size, decreased ACL stiffness, and different knee bone geometry due to sex-differences have been identified as potential factors[11]. It has been demonstrated that female athletes typically exibit greater knee valgus, hip internal rotation, and hip adduction moment than male athletes[12]. Additionally, some studies have noted the possible effect of female sex hormones on the structure and composition of the ACL[13].

The level of an athlete's neurocognitive function may also play a role in increasing risk of ACL injury. Through tests designed to measure memory and reaction time, athletes who experienced ACL injuries demonstrated lower scores compared to the control group. The authors suggest that these differences may be related to reduced neuromuscular control and coordination, increasing the risk of ACL injuries for affected athletes[14].

Previous ACL injuries put athletes at a higher risk of a repeat ACL injury, especially within 12 months of knee reconstruction surgery[15]. One possible explanation is the tendency for ACL reconstruction patients to demonstrate higher internal tibial rotation compared to control groups, especially during activities that place high rotational stress on the knee[12].

Injury Mechanisms

Contact Injuries

Specific to American football, contact events cause the majority of ACL injuries within the sport. Partially due to increased player contact incidence, football players are four times as likely to sustain ACL injuries than players of other sports[16]. Contact injuries of the ACL are commonly caused by external forces on the knee resulting in valgus collapse[17]. This most commonly occurs in the case where an offensive lineman is locked up with and blocking an opponent and another player falls onto the lineman's legs. Cadaveric and computer simulation studies of contact related ACL injuries are not an area researchers are investigating. This is likely due to the large number of ways an individual can be tackled, and the huge complexity of making a representative experiment. A study where a player falls onto an offensive lineman’s legs is an area that future research could look into.

Due to the nature of direct loading of the knee through contact, ACL contact injuries are often seen alongside other injuries of the knee. In 15-17% of ACL reconstructions have an association between ACL tears and ramp lesions. These ramp lesions are difficult to detect with an MRI, and if not diagnosed ramp lesions may result in abnormal knee laxity and meniscus lesions. A paper by Seil et al. conducted a survey among patients who underwent ACL reconstructive surgery. Patients with contact induced ACL injuries were 2.98 times more likely to suffer from ramp lesions when compared to patients with ACL injuries caused through noncontact mechanisms.[18]

ACL injuries are associated with bone bruising, which can be a precursor to later degenerative changes. A study by Bisson et al. examined the factors associated with bone bruising in patients undergoing ACL reconstruction. Bruises of the lateral tibial plateau are typically seen with higher severity in contact ACL injury cases in relation to noncontact injuries.[19]

Non-contact Injuries

Non-contact ACL injuries cause many players to spend months away from games in recovery. Video analysis has been used to asses ACL injury mechanisms in football players. Video analysis type studies are useful in determining the mechanisms of ACL injuries, but are unable to isolate and test specific scenarios. Often ACL non-contact injuries occur due to high speed or high acceleration steps where the runner pivots or changes direction. An examination of Australian football players found 37% of these occurred during sidestepping maneuvers, 16% land and step, 10% stopping, and 5% during a crossover cut.[15] Most injuries were also observed to have occurred at extended knee angles which place extra stress on the ACL and reduce the protective role of hamstrings.[20] Knee valgus collapse was also frequently seen in non-contact situations.[21]

Dynamic computer models have been used to try and estimate an ACL injury threshold, which in one paper was defined to be the maximum axial force sustained by the knee before the joint opened enough to cause injury which was 8 degrees medially or laterally. Similarly to what was seen on camera this model showed that valgus alignment reduced the injury threshold compared to neutral alignment.[22] One limitation of this study is that only varus/valgus opening as an injury mechanism where other mechanisms or combinations can occur in an ACL injury. This model also ignores the motion of the leg in the sagittal plane, but motion occurs here during normal activity such as a cutting maneuver or landing from a jump. This paper considers a scenario where this motion doesn’t apply, but would be a more rare scenario for an athlete.

A biomechanics study by Levine et al. tested seventeen cadaveric legs with a drop stand to try and understand the mechanism of noncontact ACL injuries. The quadriceps and hamstring tendons were clamped in metal tendon grips to allow for simulated muscle loads (1200 N to the quadriceps and 800N to the hamstrings) but other skin musculature and soft tissue was not disrupted. An impactor hit the floor plate with either half or full body weight (350 N or 700 N) from 30 or 60 cm above the foot to simulate a vertical landing. Multiple combinations of knee abduction, tibial rotation moments, and anterior tibial shear force under axial impact were done to examine 2 loading groups. The first focused on the effects of knee abduction moments and the other on the effects of internal tibial rotation moments on ACL strain. Biomechanical and video analysis studies demonstrate that an anterior tibial shear force, knee abduction, and internal tibial rotation moments are the most likely loading conditions responsible for noncontact ACL injuries. These loading conditions result from the same kinds of high intensity maneuvers identified in Orchard et al. This experiment showed that this combination were the most critical dynamic loading conditions that lead to ACL failure. This research does agree with Chaudhari et al. that an increased risk of ACL injury occurs with valgus rotation of the tibia. This cadaveric study does have some limitations. The researchers observed from the injury severities that higher magnitudes of loading may have been applied. This may have been due to the weaker cadaveric tissue increasing the severity of tissue damage.[23]

Injury Prevention Programs

Currently, there is strong evidence that ACL injury prevention programs help reduce the risk of non contact ACL tears in American football by 85%[24]. Table 1 shows a few common injury prevention programs articles.

Table 1: Commonly cited injury prevention program articles[24]

Study Journal Year Subjects No. of subjects Sport Program type Duration Frequency Incidence of ACL injury per 1000 occurrences
Ghilchrist et al.[25] AJSM 2008 Female 583 trained

852 control

Soccer M-F approacha 20 min/session 3 days/week, entire season 0.057 trained group

0.189 control group

Caraffa et al.[26] KSSTA 1996 Male 300 trained

300 control

Soccer N-M approacha 20 min/session Entire season Not reported
Heidt et al.[27] AJSM 2000 Female 42 trained

258 control

Soccer M-F approacha 2 days/week

7 weeks

0.25 trained group

0.33 control group

Hewett et al. [28] AJSM 1999 Female/male 366 trained

463 control

Soccer Volleyball Basketball M-F approacha 60–90 min/session 3 days/week, 6 weeks 0.12 trained group

0.22 control group

Mandelbaum et al. [29] AJSM 2005 Female 1885 trained

3818 control

Soccer M-F approacha 20 min/session 3 days/week, 12 weeks 0.09 trained group

0.49 control group

Peterson et al. [30] AOTS 2005 Female 134 trained

142 control

Handball N-M approachb 3 days/week, 8 weeks 0.08 trained group

0.49 control group

Pfeiffer et al.[31] JBJS 2006 Female 577 trained

862 control

Soccer Volleyball


Plyo approachc 20 min/session 2 days/week, entire season 0.078 trained group

0.167 control group

Kiani et al. [32] AIM 2010 Female 1506 Soccer M-F approacha 2 days/week pre-season, 1 day/week in-season 0.04 trained troup

0.20 control group

Soligard et al. [33] BJSM 2008 Female 1055 trained

837 control

Soccer M-F approacha 20 min/session entire season Not reported

AJSM American Journal Sports Medicine, KSSTA Knee Surgery, Sports Traumatology, Arthroscopy, AOTS Archives of Orthopedic and Trauma Surgery, JBJS Journal of Bone and Joint Surgery, AIM Archives of Internal Medicine, BJSM British Journal of Sports Medicine

aM-F approach: multi-faceted approach included a warm-up, strengthening, plyometrics, and agility activities

bN-M approach: neuromuscular approach included proprioceptive, balance, and stability activities

cPlyo approach: plyometric-based approach included jumping and cutting activities

The Six Principles of Injury Prevention Programs

These ACL injury prevention programs are focused on six key principles[24].

1) Age: Fewer ACL Injuries were reported in younger athletes who performed neuromuscular training programs than older athletes.

2) Biomechanics: Correct biomechanics decreases the probability of ACL strains with different movement patterns and sporting activities.

3) Compliance: Compliance of performance of an ACL injury prevention program that is greater than 66% resulted in a reduction rate of ACL injuries to 82%. However when compliance was below 66%, the reduction rate of ACL injuries dropped to 44%.

4) Dosage: The recommended length of the session is 20-30 minutes several times a week and continued throughout the season. However this can vary from one program to another. A higher frequency in session attendance resulted in a reduction in ACL injury.

5) Feedback: Verbal cueing from a coach, training partner or video can help correct any  faulty automation or movements such as landing techniques. This will help prevent any specific movements that can aggravate an ACL strain. Bracing techniques are an example that can be aided from verbal cues to avoid ACL tears from vulnerable landing positions[34]. This is a common problem for offensive linesman, where locking up with another athlete is common.  

The difference between a proper landing technique (left) and an improper landing technique (right)

6) Exercise Variety: Prevention programs include many types of exercises (plyometrics, balance, strengthening) decreases injury risk.

The overall objective of these programs are to increase flexibility, strength and balance simultaneously through plyometric. Thus, the programs focus on neuromuscular training such as muscle strengthening, muscle recruitment patterns, landing and deceleration patterns, proprioception, and plyometrics[24]. These sort of exercises are adopted in injury prevention programs throughout all sports from soccer to American Football, which could yield low incidences of ACL injuries as shown in Table 1. Cochrane et al[34] conducted a video analysis 34 ACL injuries among Australian Football Players and deduced that NFL training programs should focus on specific muscle activation strategies to stabilise the knee motion and rotation. A specific muscle group that should be targeted for activation is hamstring in order to oppose the quadricep that strains the ACL upon knee extension.

Limitations and Future Work

The research on ACL injury and its prevention on American Football players is a dynamic and evolving field that has many pending questions unanswered. There were not many biomechanics-related research articles dedicated solely to American Football as they were mainly focused on volleyball and soccer. Consequently, many of the biomechanics related information were adopted from research articles dedicated to other sports. Furthermore, the body of literature on direct contact induced ACL injuries were limited as most existing research had mainly focused on non-contact mechanisms. This is probably due to the complexity and unpredictability of direct contact ACL Injuries.

A future biomechanical research that would contribute to the research of ACL injuries on American Football players is to investigate the effects of knee position, impact location, and impact magnitude on ACL injury likelihood. Studies to measure actual forces on the ACL and other ligaments in the knees for different sports and levels of play have been suggested since 1991[12].


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