Using coupled biomechanical-epidemiological approaches, the authors have studied young athletes in Boone County, Kentucky for nearly a decade.16–18
In that county school system the authors have been tracking all the sixth- through twelfth-grade female soccer, basketball, and volleyball players, by assessing biomechanical performance and following the athletes longitudinally for injury. Load and biomechanical profiles of young athletes can be studied in order to determine how these profiles relate to biomechanical and neuromuscular factors, as well as to determine how they underlie future risk for ACL injury as the athlete's age. These same profiles are also being studied regarding their relationship to patellofemoral pain syndrome, juvenile osteochondritis dissecans, and osteochondral defects. The coupled biomechanical-epidemiological approach has allowed the authors to develop a much greater understanding of the critical ages when females appear to be most vulnerable to injury, as well as the strategies that can be undertaken in order to prevent and treat ACL injuries.16,19–21
When video sequences of actual ACL injury are examined, it appears that there are four common motor performance components that occur, especially in women.16,22
As the at risk female athlete lands, her knee buckles inward. The injured knee is relatively straight. Most if not all of her weight is on a single lower extremity. Finally, her trunk tends to be tilted laterally. In summary, her center of mass is outside of her base of foot support. These same mechanisms also occur in men but the exaggeration of the positioning of the lower extremities and trunk is much greater in women.
Directly related to the aforementioned four common components of the typical mechanism of injury are four neuromuscular imbalances that the authors term ligament dominance, quadriceps dominance, leg dominance, and trunk dominance. Existing evidence suggests that these four neuromuscular imbalances may be associated with the underlying ACL injury mechanisms. Therefore, these imbalances should be screened for and indentified in individual athletes. Identification of faulty movement patterns would allow for implementation of specific interventions, targeted at prevention.
Recall the component of the typical injury mechanism where the knee collapses into a valgus position. The neuromuscular imbalance observed more frequently in women than men responsible for this biomechanical deficit is termed ligament dominance. In the condition termed ligament dominance, muscles do not sufficiently absorb the ground reaction forces, so the joint and the ligaments must absorb high amounts of force over a brief time period. High amounts of force sustained over a short period of time lead to higher impulse forces, which is what likely results in ligament rupture.
Ligament dominance is characterized by use of anatomic (bony configuration and articular cartilage) and static stabilizers (ligaments) to absorb the ground reaction forces encountered during activity, rather than the use of the muscular prime movers of the lower extremity. Especially important for lower extremity muscular control (and avoidance of ligament dominance) is the group of muscles that comprise the posterior kinetic chain: the gluteals (both maximus and medius), the hamstrings, and the gastrocnemius and soleus. The large, powerful posterior chain muscles must be properly recruited in order to absorb the substantial reaction forces imparted by the ground, or they travel to the joint and the ligament. Newton's third law of equal and opposite reaction forces is always obeyed. Therefore, when a female soccer player makes a one-footed cut on the field, or a female basketball player lands from a rebound on a court, she hits the ground or court, and that surface hits her back with an equal and opposite reaction force. The reaction force experienced by the athlete is actually significantly greater than her body weight because her body and body segments have inertia and impart a force to the ground greater than their collective mass. These inertial properties related to velocity of movement lead to forces in multiples of body mass impacting the ground and body. For example, when a female athlete is simply walking across the court, she is impacting the ground and the ground is impacting her back with two to three times her body mass. Forces experienced during the landing, cutting, running and jumping tasks performed during sports activities occur in multiples of body mass much greater than two to three times.
During all activities, the ground reaction force (GRF) is directed toward the center of mass (COM) of the body. The COM, which resides in the trunk segment of the body, is the target of the GRF, and therefore the trunk and control of trunk motion is vitally important for control of GRF's imparted to the body. In addition to the trunk, the positioning of the COM relative to the knee joint and the plantar surface of the foot is crucial with regard to knee protection or injury. The trunk plays a crucial role in the female athlete's ability to control her body in space. This will be further discussed in the upcoming section on trunk dominance. However, in the context of ligament dominance, if the trunk moves, the GRF tracks or follows the movement of the trunk; if an athlete allows her trunk to move laterally, her center of mass moves with it. As the GRF tracks the COM, and if it progresses lateral to the center of the knee joint, the result is movement of the knee joint into a valgus alignment. Thus, the problem with excessive lateral trunk motion or the lack of adequate neuromuscular control of the motion of the trunk in the frontal plane during sports movements: the ground reaction force moves in the direction of the center of mass, lateral to the knee joint, and forces the lower leg into an abducted position (valgus position at the knee). ()
Figure 1. Example of dynamic lower extremity valgus: A combination of motions and rotations at all 3 lower extremity joints, potentially including hip adduction and internal rotation, knee (tibial) abduction, tibial external rotation and anterior translation and (more ...)
Recall that females tend to land from a jump with less knee flexion than males. The extended knee joint component of the injury mechanism relates to a neuromuscular imbalance that occurs in females that the authors term quadriceps dominance
. Quadriceps dominance refers to the tendency to stabilize the knee joint by primarily using the quadriceps muscles. Women appear to preferentially use the quadriceps more than males in order to stiffen and stabilize the knee joint.5
When females contract their quadriceps it extends their knee, which likely relates to the more extended knee position observed during an ACL injury. In addition to that, the quadriceps serve to stiffen or compress the tibiofemoral joint. The common quadriceps tendon attaches at the superior patella, and then attaches via the infrapatellar tendon to the tibial tubercle on the anterior tibia. When the quadriceps contract they pull the tibia anterior relative to the femur. The resultant biomechanical problem is that the ACL serves to hold the tibia posteriorly (or check anterior translation), and when a female athlete uses her quadriceps to stabilize the joint she induces an anterior shear stress to the tibia and therefore also to the ACL.
Quadriceps dominance relates to ligament dominance. If an athlete preferentially uses the quadriceps instead of the posterior chain muscles to control the limb, she uses a single muscle with a single tendinous insertion for stability and control. This is in contrast to using the group of posterior chain muscles that possess multiple muscles with varied tendon insertions that can be selectively utilized to control the limb during functional tasks. For instance, the hamstring has both medial and lateral tendons that play a vital role in stiffening and stabilizing the knee joint, in the exact opposite direction of the quadriceps at low knee flexion angles. The hamstrings are able to increase flexion at the knee, which provides a better position (mechanical advantage) for using the muscles to absorb force. The hamstrings are considered a synergist with the ACL and are able to pull the tibia posteriorly thereby decreasing the stress on the ACL. Finally, the hamstrings have tendons that insert on either side of the joint that can offer the frontal plane control of motion at the knee that is vital for injury prevention.
The authors' early studies compared high school male and female volleyball players who were on average the same weight and height. Hence, biomechanically, they were quite similar.5
When an athlete blocks a volleyball over a net, two significant force spikes occur right at the point of landing, when the ball of the foot hits and when the heel hits. These forces are in the range of five to seven times body weight. The athlete's center of mass is forward because he/she just blocked the ball over the net, and the landing knee is often near full extension, demonstrating the lack of knee flexion.
If the athlete's COM is forward and the knee is extended, the GRF (when assessing from the side, in the sagittal plane) slams the knee back further into extension. In biomechanical terminology this is referred to as an external force, because the force is coming from the ground. In essence, as the athlete hits the ground, the ground offers an external extension force which extends the hinge of the knee joint. As previously mentioned, according to Newtonian mechanics the mechanisms inside the body must generate an equal and opposite torque in order to prevent the athlete from falling backwards. That torque is called an internal flexion moment or flexion torque, and it is provided mainly by the hamstrings and gastrocnemius at that point of landing. Deficits in recruitment of hamstrings and gastrocnemius during landing may allow for excessive extension torques that increase stress on knee joint passive stabilizers, and provide enough torque to cause injury. The authors of this commentary further determined in the same study of volleyball players that the posterior kinetic chain muscles that flex the knee were activated at three times the level in size-matched males as compared to females when landing from a jump.5
This early study provided documentation of quadriceps dominance
. Other researchers examined the muscular reactions that occurred after pushing the tibia forward with an experimental turnbuckle device. In this study, men activated their hamstrings first, while females first activated their quadriceps.23
In summary, primary activation of the hamstrings is desirable because the hamstrings pull the tibia posteriorly and take stress off the ACL. The quadriceps first muscular control strategy used by women, where the quadriceps pull forward adding to the anterior stress on the ACL, is exactly opposite of the preferred activation pattern used by males.
The authors call the third type of imbalance leg dominance
. In tasks that normally require side-to-side symmetry of the lower extremities, women tend to be more one-leg dominant than their male counterparts. When a female tears her ACL, most if not all her weight is on a single leg.22
When considering the imbalance of leg dominance, it should be acknowledged that most athletes have a preferred plant leg and a preferred kick or drive (in the case of a single leg jump task) leg. However, the difference between limbs in muscle recruitment patterns, muscle strength, and muscle flexibility, tends to be greater, in women than men.10,21,24–26
So, in the context of a pre-season screening process neuromuscular/biomechanical tests should attempt to identify side-to-side differences. Regarding muscular symmetry, if side to side muscle strength or relative recruitment differences are measurable, then an athlete is asymmetric and is considered to be leg dominant. What the authors of this commentary have shown is that those that have greater asymmetry in these force and torque profiles have greater risk of future injury.20
Trunk (Core Dysfunction) Dominance
Those athletes, typically women, who do not adequately sense the position of their trunk in three-dimensional space, or allow greater movement following a perturbation or disturbance of their trunk, have greater risk of future knee, ligament, and ACL injury.22,27
Diminished proprioception of the trunk is one difference that exists between men and women. In women a valgus positioning of the knee is frequently observed during varied motor tasks, and is considered to be high risk for ACL injury. The same positioning is not frequently observed in men, and the trunk is a more common contributor to the at risk position demonstrated by women. For instance, in those studies performed by Zazulak et al27–28
on the association between trunk proprioception and control and future risk of knee ligament injury, trunk motion and trunk proprioception predicted risk of future knee ligament injury in females, but not males.27
Trunk dominance is simply defined as the inability to precisely control the trunk in three dimensional space. Trunk dominance may be related to growth and maturation factors. For example, if you observe a young woman who has just experienced a growth spurt; she has a bigger “machine” to manage with her existing motor programs and neuromuscular strategies. This growth spurt may be likened to putting someone on stilts. So, if an average 10 or 11 year old female grows ten to fifteen centimeters (4-6 inches) in a year she basically functions as if on stilts. As females mature, they also increase body mass and add proportionally more fat body mass than their male counterparts. Her center of mass is higher off the ground, therefore, it is harder for her to control and balance her body. After the adolescent growth spurt, males get what is called a “neuromuscular spurt” where they experience muscular development and get proportionately more powerful. So, although males also get a bigger “machine” after a growth spurt they get a much bigger sized engine in relation to the size of the machine. In contrast, in women and girls, that ratio between the size of the machine and the size and the power output of the engine, basically stays the same and does not adapt to the increased demands.
After maturation, females experience a more massive trunk, a center mass located higher off the ground and fat and lean body mass redistributed in novel ways, but don't get the more powerful engine to control that bigger “machine”. When examining videos of a woman rebounding a basketball or cutting on the soccer field, she will frequently have excess motion of the trunk. Recall, that as discussed in the context of ligament dominance, the uncontrolled side to side movements and GRF's create medial-lateral torques on the knee.
In summary, the authors believe that these four neuromuscular imbalances underlie the mechanisms that relate to ACL injury risk that is greater in females than in males. We have demonstrated these four dysfunctional movement strategies both in the controlled environment of the laboratory and during sport performance and injury analysis on the field or court. Acknowledging that there are differences between males and females in the observed ACL injury mechanism, it is important to understand the important variations observed in prevention strategies.29
When interventions are designed, the observed differences in injury mechanisms should be considered and unique programs that specifically address the neuromuscular deficits observed should be developed for males and females. For example, in females there should be a greater focus on dynamic, ground based core or dynamic trunk training than in a program developed for males.27–28
Authors have demonstrated that single limb balance and wobble board training is more important for men than women, who are on average more proficient than men at single leg balance. Hence, single limb balance training should be included when designing an ACL prevention program specifically for men.30–31
Relationship between Mechanism, Neuromuscular Imbalance and Neuromuscular intervention for ACL injury prevention in Female Athletes.
Interventions to address Quadriceps Dominance
After conducting two decades of research investigations and closely following the emerging evidence published by other researchers, it has become apparent that females are in fact highly quadriceps dominant. The authors believe that the muscles of the posterior chain are present, but are often under-recruited or not effectively “turned on” in many female athletes. For instance, Zazulak et al presented data regarding hip muscular recruitment of women and men, showing that women activate their hamstrings and gluteals less than men.32
The gluteus maximus, the biggest, strongest muscle in the body, is the only triaxial, three plane controller of femoral position. When an athlete primarily contracts her quads, and reduces the contraction of her gluteals and hamstrings, the result is the position of valgus knee collapse. Recall that this is problematic because as an athlete allows the ground to push her knee into the valgus position, her muscle contraction patterns further reinforce that position and the loads experienced at the knee can become high enough to be injurious.
Interventions to encourage female athletes to become less quadriceps dominant emphasize “turning on” or emphasizing the recruitment of the posterior kinetic chain muscles. This can be achieved in multiple ways. Plyometric exercises that utilize the position of 90/90 degrees flexion at the knee and hip are highly effective. () Russian hamstring curls using a band across the chest facilitate control utilizing not only the eccentric (lowering) part of a closed kinetic chain hamstring curl, but also the concentric (raising) part. () Athletes benefit by training all muscles of the posterior kinetic chain in both eccentric and concentric modes.
Squat jump sequence. Note, in a tuck jump the knees are flexed toward the trunk while in the air. See Appendix 1 for more detail on the tuck jump.
“Russian” hamstring exercise with elastic resistance around the trunk to emphasize concentric and eccentric hamstring contractions.
The authors also use exercises that enhance recruitment of the posterior chain while requiring core activation of the abdominals and hip stabilizers. Instruct the athlete to put their heels on a medium size swiss ball, and assume a plank position, maintaing a neutral pelvis (a flat body position) and then pull the knees into flexion, by rolling their heels on the ball. The athlete pulls the ball toward their rear-end by flexing the knees, achieving a dynamic hamstring curl while the pelvis remains stable. () To increase the difficulty, the athlete can do the same movement using a single leg which incorporates both balance and strengthening work.
Dynamic core stabilization and hamstring curl on swiss ball. Note: the athlete must stabilize the trunk/core as the legs flex and extend.
Interventions to address Leg Dominance
Single leg balance and single leg hopping techniques are useful for addressing leg dominance. () Although it may seem counterintuitive, the more single leg activities an athlete performs, the more side to side symmetry is restored. The human body possesses complex neurologic mechanisms that attempt to achieve balance, one side versus the other. This is especially true when performing single leg tasks; the body makes use of neuromuscular feedback loops and bilateral neurological systems to influence symmetry during dynamic control of such tasks. Note that maximum cross-over effects are achieved when both lower extremities are utilized in single limb activities, alternately. Additionally, single leg hop activities may influence synergistic recruitment of the posterior chain musculature, which facilitates not only the muscular control to be successful with unilateral tasks, but may also decrease quadriceps dominance during dynamic tasks.
Example of three components of dynamic neuromuscular training: plyometrics, single limb balance, and perturbations. Note: Single limb balance and perturbations are especially helpful in decreasing leg dominance.
Interventions to address Trunk Dominance
Addressing trunk dominance is achieved by core training, but not necessarily the core training related to the increase in performance of the prime movers (such as the rectus abdominis). Instead, contemporary focus regarding training for the core relates to the specific instruction in activities that provide stability offered by the local musculature (such as transversus abdominis and multifidus) and pelvic/hip stabilizers. Trunk dominance is difficult to measure in real time, especially during dynamic function. Many of the training techniques described within this commentary are testing procedures at the same time. The swiss ball exercise described previously (see ) can be used as a measure of an athlete's ability to control their core, by assessing the stability of the pelvis in the transverse and frontal planes. When an athlete lacks core stability the pelvis deviates into rotated or anterior/posterior tilted positions.
Another important consideration of core stability is the ability of the patient to control the pelvis on the hips and lower extremity in space via the stabilization functions of the abductors and rotators of the hip. Therapists who work with athletes after an ACL tear and subsequent reconstruction should be aware that one of the best predictors of future risk is hip external rotation strength. The hip external rotators serve as a proximal stabilizing component of the core and lower extremity. In a recent article published by Paterno et al, subjects whose hip external rotation strength was less than optimal after an ACL reconstruction had an eight times greater chance of sustaining another ACL injury.21
An exercise to address the hip rotation facet of core control is closed chain internal and external rotation of the hip using elastic resistance. () In this exercise the patient assumes a single limb stance position, wraps elastic tubing around their pelvis and concentrically and eccentrically controls the transverse plane rotation/derotation of the pelvis on the hip, while keeping the trunk erect. Feedback about maintaining upright posture and neutral pelvic position both the sagittal and frontal planes is essential.
Figure 6. Closed-kinetic chain hip internal and external rotation using elastic resistance. Elastic resistance is secured lateral to the pelvis at the level of the hip/pelvis, and is wrapped around the pelvis in order to exert a pull opposite the desired rotation. (more ...)
Finally, those athletes that went on to sustain a second ACL injury had increased frontal plane excursion of the trunk and knee, and decreased flexion through the knee and hip during sport competition.21
Trunk deviations seem to be closely related to lower quarter deviations as previously described in the discussion about the COM and base of support relationship to valgus positioning of the lower extremity. If athletes allow significant medial collapse at the knees they had approximately a four and a half fold greater risk of injury. If athletes showed asymmetry during landing, one side versus the other, they had about three times greater risk of a second ACL injury.21