Gym-Based Training Interventions for Anterior Cruciate Ligament Injury Reduction in American Football Players

Abstract

Injuries to the anterior cruciate ligament (ACL) of the knee are one of the most prominent injuries affecting players in American football. One primary aim of training to reduce injury risk is to provide exercises for players on attaining the highest athletic performance with the least orthopedic stress. This review article on ACL injury reduction protocols focuses on the protective and performance-enhancing biomechanical patterns during simple exercises used in a gym-based setting, in the following areas: single-leg balance and trunk stability, single-leg jumping/plyometrics, and reflexive strength training. This supplementary training, as part of a sports performance program, might include training to develop maximum strength, explosive power, acceleration, maximum velocity, bioenergetic endurance qualities, mobility/flexibility, agility, and sport skill acquisition.

Introduction

Injuries to the anterior cruciate ligament (ACL) of the knee are one of the most prominent injury-related issues in American football. In the 2015–2019 seasons of the National Football League (NFL), 314 ACL injuries occurred, at an average of 62 injuries per year, with consistency in the number of injuries reported annually [23]. In a sport in which each team consists of 53 players, this rate of ACL injury is comparable to losing an entire NFL team each year. Much research has been conducted on best practices to reduce these injuries, as they appear to be statistically consistent each year, according to NFL Player Health and Safety data.

A primary aim of training is to reduce injury risk by providing exercises that allow players to attain the highest level of athletic performance with the least amount of orthopedic stress. Efficient biomechanical movement patterns must exist not only in isolated, controlled settings but also in team play, where the very nature of participation is complex and chaotic. This search for stability within chaos leads to the concept of attractors.

Attractors may be understood as stable functional coordination patterns into which the human movement system can settle during activity [6]. These are not joint positions but rather abstract principles associated with the stability of the system that may be used in a variety of circumstances, although not all attractors are helpful. The system will search for stable end-points when forming movement solutions to sport-related problems, but some of these solutions may not be ideal for the reduction of injury.

Three broad categories of attractors are the following [2]:

Passive attractors reduce performance and do not protect against injury. The system finds stability through passive tissues, such as ligaments or tendons, without the benefits of intermuscular and intramuscular coordination. Perhaps no example is better in relation to ACL injury than excessive knee valgus on ground contact when a person is running, landing, or changing direction. This pattern is characteristic of what has been termed “ligament dominance,” a condition of imbalance in which muscles do not sufficiently absorb ground reaction forces (GRF), so the joint and the ligaments must absorb high amounts of force over a brief period [15]. While ligament dominance can provide stability, the cost is a lack of protective or performance-enhancing features and should be avoided in training or competition.

Active, protective, performance-reducing attractors protect against injury but may reduce performance. These are emergency measures taken by the system to achieve stability. In team sports, quadriceps-dominant players may attempt to decelerate their momentum by contacting the ground with the foot far outside of the center of mass (COM), with the knee joint extended. This can result in excessive shear stress to the ACL, especially if the trunk is upright, leaning backward, and/or rotating away from the planted foot. In this scenario, the muscles of the posterior chain are unable to contribute to the load distribution of the GRF, so the quadriceps take on potentially damaging anterior shear forces. One way to mitigate stress to the knee when the foot is planted far out in front of the COM is to add internal rotation at the hip where the trunk rotates toward the stance leg on impact. The eccentric tensioning of muscles around the hip (biceps femoris, gluteus maximus, contralateral latissimus dorsi), plus the ligaments crossing the sacroiliac (SI) joint, can increase friction and stiffness and reduce shear on the knee through activating the posterior chain muscles [2,27]. These activated structures may share in accepting load from the GRF, helping to translate the tibia posteriorly, as the base of support is in front of the COM with an extended knee. Another protective strategy in this circumstance is continuing to run, taking multiple braking steps, each with significantly less GRF, until a stable, powerful plant step can be performed.

Active, protective, performance-enhancing attractors both enhance performance and protect against injury. These are most desirable for enforcing safe movement. For example, maintaining the spine and pelvis in neutral alignment keeps the postural muscles at or near their optimum length (the muscle length at which the maximum muscle force can be generated [4]); this enhances neural communication and force expression through the trunk. Optimum muscle length is also associated with reflexes and preflexes, the latter referring to the immediate muscle responses to disruptive external influences, where perturbations of length and velocity produce changes in contractile force with zero delay [13,18]. A neutral spine has a positive impact on pelvis position, which allows for self-organization of the lower limb muscles as they activate favorable positioning, achieve optimum length, and withstand high forces. This aids in distributing the force of ground contact and the range of motion associated with movement expression across several joints. This may dramatically reduce the risk of imbalance conditions such as quadriceps dominance and trunk dominance, the latter involving poor pelvic control leading to excessive lateral or anterior trunk flexion upon foot plant when braking, another posture associated with ACL rupture [15].

This review article focuses on protocols for reducing ACL injury involving active, protective, and performance-enhancing attractors with simple movements used in a gym-based setting: single-leg balance and trunk stability, single-leg jumping/plyometrics, and reflexive strength training. A holistic program would strike a balance between isolated exercises in a closed, controlled setting and sport-specific skills in the context of a football game by also including neuromuscular strength (particularly in the eccentric and isometric regimens) [8,9,12,19,22], visuospatial attention [5,20], and neurocognitive performance, including perception-action coupling [1,10,26].

Developing Stable Positions in Training for ACL Injury Reduction

High-velocity braking, where the foot is planted far outside the COM, is potentially one of the most injurious movements for the ACL. It has been reported that peak GRF can exceed 5 times body mass within 50 ms during a penultimate foot contact of a severe 135° change of direction [14]. When the body stops forward momentum in the sagittal plane, a hinging motion at the hip with neutral spine can activate the posterior chain through eccentric lengthening. This co-contraction with the quadriceps may reduce the risk of anterior tibial shift [11]. With a rounded spine, the posterior chain may not be properly tensed and the anterior muscles may take too much of the load, leading to shearing forces that may rupture the ACL. Therefore, proper hinge patterns may be built into sports performance training, from slow, high-force movements (such as single-leg Romanian deadlift) to faster, more-explosive movements (such as landing from a single-leg depth drop).

In American football, movements are multiplanar, with a player’s motion inevitably occurring in the transverse plane—for example, turning the head and upper body to locate the ball in the air or keeping an eye on an opponent’s cues. Such movements may increase the risk of dynamic lower extremity valgus resulting from a combination of rotation at the hip, knee, and ankle, including hip adduction with internal rotation, tibial abduction, tibial external rotation with anterior translation, and ankle eversion—all associated with increased ACL injury risk [17]. A “chest out” spinal posture when rotating may reduce the risk of these dangerous positions [2]. This may also ensure that the postural muscles are kept close to their optimum length, with appropriate isometric and eccentric tension distributed over several joints. Exercises may be built into a training program to enforce this protective attractor, such as a forward lunge with internal rotation toward the lead leg while holding a weighted plate and rapidly extending the arms during the rotation. The player must resist the temptation to go with the momentum of the plate punch, thereby avoiding a flexed spinal position, and instead maintain an upright, chest out position as the arms are extended.

To avoid quadriceps dominance when changing direction on the football field, a player should activate the musculature around the hip and posterior chain. During sprint acceleration, the gluteal and hamstring muscles provide linear propulsion. Applying horizontal force requires the hamstring muscle group during the swing phase to push into the ground with each step [21]. Greater transmission of force into the ground is accomplished by initiating the effort at the hip, through retraction of the swing leg, leading to a “whip from the hip” that results in a coordinated sequencing among the muscles of the hip, knee, and lower leg [7]. When changing direction, these same attractors apply, even if the movement plane differs. Effective change of direction occurs with the pelvis in a neutral position, the swing leg retraction initiated at the hip, the foot planted beneath the COM, and force transmitted through the ground.

Thus, an ACL injury risk reduction program can guide players to align their motor control strategies with active, protective, performance-enhancing attractors. These attractors ideally exist during complex activities, such as representative learning where dynamic training interventions are designed in consideration of the interacting constraints on movement behaviors, adequately sampled informational variables from specific performance environments, and functional coupling between perception and action processes [25].

Training Interventions for Reducing ACL Risk in Football Players

Implementing ACL injury risk reduction programs aimed at making small improvements in biomechanical movement patterns can have a dramatic effect. For example, neuromuscular training interventions to alleviate imbalances such as ligament dominance, quadriceps dominance, trunk dominance, and leg asymmetries have been shown to reduce ACL injury risk by 30% to 80% (average 50% reduction in relative risk) [15,16]. Training in using protective attractors as a roadmap and developing global neuromuscular strength may address most of the neuromuscular imbalance associated with ACL injury risk.

Balance/trunk stability exercises, single-leg jump training, and reflexive strength training may be carefully constructed to challenge the system in activities of greater balance, speed, and complexity.

Balance and Trunk Stability Exercises

Incorporating variations of balancing on 1 limb, especially with added perturbations to disrupt stability, has been shown to aid in decreasing asymmetry between legs. The exercises achieve this by activating neuromuscular feedback loops and bilateral neurologic systems to influence symmetry when balancing on 1 limb [15]. Perturbations may involve closing the eyes to remove the benefit of visual focus or taking a weight plate and circling it around the head swiftly to challenge the trunk to remain stable against varying lever arms of resistance (see Fig. 1). In addition, moving the head from left to right while balancing on 1 leg can provide natural perturbation and challenge the core musculature, especially the pelvic and hip stabilizers. Strength and stability in the hip external rotators are predictors of future ACL injury risk, with an 8-fold greater chance of sustaining repeated ACL injury in those with reduced hip external rotation strength following ACL reconstruction [24]. Keeping a slight bend in the knee while performing these exercises is encouraged for hip and knee muscle activation.

Fig. 1.

Single-leg balance with plate halo.

An isometric split squat hold with perturbation simulates a braking step with the lead leg and is less stable than a traditional bilateral squat position; added perturbations such as lateral band resistance around the lead knee, pulling the knee into valgus requires the player to resist and maintain neutral knee position. Other perturbations may include manipulation of the trunk through a lateral band resistance pulling the body along the transverse plane away from the lead leg or a band pulling the body along the frontal plane in lateral flexion toward the lead leg, requiring the player to resist these forces to maintain neutral posture (see Fig. 2).

Fig. 2.

Split squat variations.

These balance and stability exercises may be performed for 1 to 2 sets of 30 to 60 seconds of balancing on each leg.

Single-Leg Jump Training

Performing jumping and plyometric exercises on 1 limb has been described as useful in addressing asymmetry and quadriceps dominance by influencing synergistic recruitment of the posterior chain [15]. Accentuated single-leg landings, such as a depth drop from a high box (eg 24 in.), may expose players who are quadriceps-dominant and ligament-dominant to medial collapse. Constant exposure to this lower extremity valgus can dramatically increase the risk of ACL injury for athletes, and so it is important to assess for and mitigate this risk.

Depth drops performed to the side may challenge the athlete to stabilize a lateral force when jumping in the same direction as the working leg or a medial force when jumping in the opposite direction. This will challenge trunk and pelvic control in the frontal plane under time pressure. To challenge the transverse plane, the player may perform a 90° turn in midair after jumping from the box and attempt to stick the landing on impact (see Fig. 3). To add an element of unpredictability, a player may perform a bilateral vertical jump on a flat surface and have a partner provide a push as the player leaves the ground so that perturbation is applied in the air, after which the player must find stability and stick the landing on 1 limb (see Fig. 4).

Fig. 3.

Single-leg depth drop variations.

Fig. 4.

Jump with partner perturbations.

These single-leg jumping exercises may be performed for 2 to 4 total sets of 2 to 4 repetitions on each leg.

Reflexive Strength Training

Reflexive training, recently popularized by Dutch professor and coach Frans Bosch, is used for coordination and stability by performing more complicated variations of basic strength training patterns [3]. An example is performing an explosive forward-lunge pattern while holding a weight plate and, as soon as the front foot hits the ground, rotating the trunk toward the lead leg and explosively extending the arms in a “punch” motion (see Fig. 5). This develops multiple protective attractors simultaneously, such as upright trunk position, extended force closure around the pelvis, and chest out when rotating, all in a single swift motion. Variations may include a lateral lunge pattern, where a partner provides an unpredictable lateral push, causing the player to reflexively activate the proper structures to stabilize into the lateral lunge movement (see Fig. 6). The goal for reflexive strength training is to develop coordinative structures with speed and perturbation, not necessarily to progressively increase the overload used.

Fig. 5.

Lunge with plate punch.

Fig. 6.

Lunge with partner perturbations.

Reflexive strength training exercises may be performed for 2 to 4 sets of 3 to 6 repetitions on each leg.

Conclusion

These exercises and training interventions for American football players have the primary goal of working toward active, protective, performance-enhancing attractors of movement. Exercises in balance/trunk stability, single-leg jumping, and reflexive strength may supplement an existing sports performance program. Such a regimen might involve developing maximum strength, explosive power, acceleration, maximum velocity, bioenergetic endurance qualities, mobility/flexibility, agility, and sport skill acquisition. These interventions are far from all-inclusive and the reader is encouraged to search the literature on ACL injury mechanisms, rehabilitation, and risk reduction.