Research Review By Demetry Assimakopoulos ©

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Date Posted:

September 2011

Study Title:

Rationale and implementation of anterior cruciate ligament injury prevention warm-up programs in female athletes

Authors:

Bien DP

Author's Affiliations:

Physical Therapy Department, University Orthopedics, Providence, Rhode Island

Publication Information:

Journal of Strength and Conditioning Research 2011; 25(1): 271-285.

Background Information:

Female athletes competing at the high school and collegiate levels experience a 3-4 times greater likelihood of anterior cruciate ligament (ACL) strains when compared to male athletes. As a result of the tremendous number of subsequent surgeries, osteoarthritic comorbidities, financial implications and psychosocial costs associated with these injuries, numerous preventive programs have been proposed. Many of these interventions have been introduced in a warm up format and have had some success in decreasing the incidence of non-contact ACL injury. These warm up programs have a number of potential advantages, which include decreased time requirements in training, improved practicality, cost effectiveness and convenience because they are performed on-field. These qualities foster an increase in compliance with therapy and avoidance of fatigue (which is a risk factor for non-contact ACL injury and for lack of success in intervention programs).

On the other hand, one problem with ACL injury prevention programs in general is a lack of high intensity overload, which decreases the possibility of sport performance enhancement (i.e. increase speed and vertical jump). Additionally, if these programs are not performed efficiently and effectively, they have the potential to increase fatigue and time demand.

A few of the many problems with much of the research in this field is poor methodological quality, due to a limited number of subjects, subject drop out and a lack of delineation between contact, non-contact ACL injury and poor compliance. Because of the lack of formal evidence with superior quality, a review of the current literature is necessary to discover the best way to implement warm up programs to decrease the risk of non-traumatic ACL injuries in female athletes, while at the same time fostering physical performance gains.

Summary:

Injury Mechanism:

Research shows that an approximate 70% of ACL injuries are non-contact in nature (3). Investigations have shown that knee loading in multiple planes (frontal, saggital and transverse) is the primary mechanism of injury. The highest knee loads occur during a dynamic knee valgus in combination with internal rotation and quadriceps activation with insufficient activation of the hamstrings while performing knee extension (i.e. a poor cutting motion). Video analyses of ACL injuries have shown that anterior tibial shear (particularly around 20-30° of knee extension) and movements involving a combination of knee valgus, hip internal rotation and tibial external rotation near full knee extension while the foot is firmly planted are the 2 predominant loading patterns causing non-traumatic ACL injury. However, some authors have stated that the uniplanar anterior tibial shear mechanism does not produce enough force to cause ACL tears during sporting activities.

Many of these injuries occur during sporting tasks which involve deceleration, directional change and landing from a jump. It is theorized that neuromuscular and biomechanical differences in the lower extremity and trunk are the factors which predispose female athletes to these injuries.

Hip and Knee Pathomechanics and Interventions:

Postural and neuromuscular control of the proximal lower extremity are important factors in ACL injury in females. The fact that females experience greater hip adduction and internal rotation and decreased knee flexion while performing athletic activities predisposes them to an increased knee valgus stress. Greater knee valgus, while in dynamic postures, is the most common mechanism of non-traumatic ACL injury (1). These postures are even more pronounced during unanticipated single leg landings, in comparison to anticipated landings. This suggests that fatigue and unexpected perturbations are significant elements which increase ACL injury risk, due to an increased knee valgus leg posture.

There is some evidence showing that neuromuscular control of the hip is of particular importance in the prevention of ACL injury (4). The activation of gluteus maximus and medius may have an important role in protecting the ACL from tearing. Weakness of the gluteus medius may contribute to dynamic valgus collapse of the knee because of an inability to maintain hip abduction during high intensity single-leg weight-bearing activities such as landing, cutting or changing direction. In fact, studies have shown that contraction of both the abductors and adductors of the hip can substantially increase knee joint stiffness, which in theory can protect the ACL.

In addition to abduction, hip external rotation is important in preventing non-traumatic ACL injury. The gluteus maximus is a powerful external rotator of the hip. Various studies have shown that decreased gluteus maximus activity in female athletes during single-leg landing can lead to increased femoral internal rotation and knee valgus postures (6). Thus, it is important to target both gluteus maximus and medius during off-season and in-season resistance training programs to increase neuromuscular control of the knee and decrease the inherent risk of non-traumatic ACL injury.

Various exercises have been proven to substantially increase maximal voluntary isometric contraction (MVIC) of the gluteus maximus and medius. Exercises such as single-leg deadlifts, single leg squats, side lying hip abduction and side planks can all facilitate strength gains. It is important to emphasize that many of the exercises which target the hip be performed while in single leg stance in order to more optimally recruit these muscles and to more closely mimic a sporting environment (A-la Mike Boyle, for those trainers out there in the RRS world). Also, integrating these exercises into warm-up routines can enhance dynamic neuromuscular control of the hip prior to an athletic event.

Hamstring Activation and Strength Deficits and Effective Interventions:

Aberrant activation of the hamstring muscles has been implicated in non-traumatic ACL injury in female athletes. Activation of this muscle group may increase dynamic knee stability by countering anterior and lateral tibial shear, and tibial rotation. Studies have shown that females with decreased pre-activation of semitendinosis during a dynamic cutting motion have an increased risk of future ACL rupture (7). It has also been shown that activation of the medial hamstring muscles in particular may be important in limiting excessive knee valgus and tibial external rotation. The author also states that feedforward activation of the medial hamstring during athletic tasks is of particular importance in the protection of the ACL. Thus both targeted training and neuromuscular training of the medial hamstring muscles to resist going into dangerous postures are required for the greatest protection.

In addition to the medial/lateral hamstring muscular and feedforward imbalance, female athletes also display quadriceps dominance while performing sport tasks. This imbalance may also increase ACL injury risk. A hamstring:quadriceps relative strength ratio of less than < 0.55 have been identified as the threshold for ACL injury. This change to quadriceps dominance has been shown to occur in female athletes in adolescence, which confirms the importance of interventions in early puberty to prevent ACL injury later in life.

Fortunately, research shows that it is possible to augment this ratio though neuromuscular intervention programs, such as perturbation training during a disturbed walking task and eccentric hamstring loading (partnered Russian eccentric hamstring curl, single-leg deadlifts with trunk rotation). It is important to note that these tasks can be utilized in a warm up format or during a regular training regimen.

Additionally, movement speed and resistance can be changed to encourage regular strength improvements and increased hamstring activation. Interestingly, foot position can also be altered to selectively activate the medial hamstring musculature by internally rotating the foot during lower extremity exercises. Greater internal rotation of the foot can be utilized while performing exercises such as the single-leg deadlift and hamstring bridging exercises. Both of these exercises can be easily performed during a pre-game warm-up.

Leg Dominance:

A difference in strength/power of greater than or equal to 20% between legs may signify a neuromuscular imbalance between limbs that can predispose to significant ACL injury. This pattern has even been seen to produce greater knee valgus angles during drop vertical jumps, due to the incredible increase in vertical ground reaction force during the activity in the non-dominant leg. The authors suggest that it is inherently important to integrate neuromuscular and plyometric training into a regular conditioning program, focusing on both single-leg and double-leg stances.

Core Training Component:

Weakness of the core stabilizers has been shown predispose female athletes to ACL injury because greater lateral trunk flexion during athletic tasks can move the ground reaction force vector laterally. This change increases the lever arm relative to the centre of the knee joint which could increase knee valgus loading. Because of this biomechanical alteration, it is important to ensure that female athletes have tremendous core stability in all planes and along the kinetic chains which incorporate the abdominal musculature.

In addition to core strength and endurance, it is important to ensure that female athletes have adequate core proprioception. Active proprioceptive repositioning of the trunk has the potential to alter dynamic knee stability and has adequately predicted knee injury risk in females but not males. Suffice it to say, incorporating core stability training which focus on enhancing proprioception through perturbation and correction of body sway on top of traditional core strengthening techniques is important in injury prevention. While the research on this topic is emergent, it is important for therapists to understand that training the core is imperative to ensuring developments in strength and power in athletic populations, so performing tasks targeted to the core will not hurt the athlete in any way. The Superman exercise, side planks and various pieces of commercial equipment which target the abdominal musculature can be effective in engaging the core. (Clinical note: while training your patients/athletes, ensure that they engage the core in a neutral spinal posture, to decrease the chances of injury).

Feedforward Conditioning Mechanism:

The current literature shows that ACL injuries occur in less than 100 msec. This time frame is much faster than typical reflexive muscular activation. This fact suggests that non-traumatic ACL injuries occur in intervals that are much too fast to allow the body to muscularly react to prevent the injury. Successful injury prevention programs should address the efficiency of these feedforward mechanisms to enhance dynamic stability of the knee. This can be done with perturbation training and neuromuscular training designed to simulate sport-specific tasks, while the whole time emphasizing safer kinematics.

Plyometrics:

This is yet another mechanism to facilitate muscle activation and feedforward neuromuscular adaptations. Among the various positive whole body effects of this type of training, plyometrics have been specifically shown to decrease ground reaction forces, decrease hip abduction/adduction moments during landing activities (allowing for a more neutral knee posture while landing), increased lower extremity power and to decrease the incidence of serious knee injury. In addition, plyometric training has the potential to increase recruitment and timing of gluteus medius, which minimizes joint displacement in the sagittal, coronal and transverse planes. Other reports have shown that plyometric exercises which employ greater knee and hip flexion during landing allows for greater control and improvement of sagittal and frontal plane kinematics and an increase in hamstring activation to limit anterior tibial shear. The authors specifically recommend isolated single-leg weight-bearing activities to minimize asymmetrical loading during double leg stances. Otherwise, there is a potential to create leg dominance. It is also important to note that therapists meticulously observe their athlete’s plyometric technique and sport-specific tasks, as verbal cueing can create positive changes in technique and decreases in vertical ground reaction forces. They should also ensure that plyometric exercises are performed in multiple planes to more closely mimic a sporting event.

Balance:

Balance and proprioceptive training have been proven to significantly reduce ACL injury risk factors. This can be done by increasing the athlete’s single-leg total dynamic stability and anterior-posterior balance through using time trials and perturbation training. These exercises can increase the antagonist coactivation of the knee flexor muscles thus stabilizing the knee.

Agility Training:

Manoeuvres involving a quick change in direction, such as a cutting technique, have frequently been implicated as the cause of non-traumatic injury in female athletes. The supposed key for performing these tasks efficiently and without injury is to perform neuromuscular movement training at various angles and planes, combined with hip, hamstring and core strengthening to limit ACL vulnerability during these movements.

Agility training which emphasizes safe biomechanics and good posture is important, especially while performing exercises that employ an unanticipated change in direction. Exercises using unanticipated movements are ideal because they more closely simulate sporting conditions. It is suggested that particular attention be made to lateral movements, because they produce little knee flexion and the greatest valgus angles.

Program Duration:

The current literature states that interventions greater than 8 weeks in duration are needed for substantial neuromuscular changes and performance training effects, especially for female athletes at high risk for non-traumatic ACL injury.

Screening:

Despite the importance of implementing programs for negating the risk of ACL injury, proper screening tools should be used to identify female athletes who posses injury risk factors. Decreased hand-held dynamometry strength is a reliable tool to identify young, healthy female athletes who have weakness of the gluteus medius and maximus in hip abduction, extension and external rotation. Dynamometry may also be helpful in identifying hamstring weakness in comparison to quadriceps strength. However, identification of hamstrings:quadriceps ratio and timing of activation of the hamstrings may be more important in determining ACL injury risk than static measures.

Video analysis used to identify knee collapse into genu valgum is an incredibly reliable screening tool. Video analysis during jump landing has been able to identify poor biomechanical postures such as excessive hip adduction, femoral internal rotation, tibial external rotation and foot pronation. This tool has the added benefit of providing the client with biofeedback. Video recording combined with verbal cueing can create a powerful tool for nullifying poor neuromuscular activation.

The Landing Error Scoring System (LESS) has been developed as a clinical tool to assess jump landing biomechanics. It has been shown to identify female athletes who are at higher risk of ACL injury with great validity and reliability (5). However, the creators of this assessment protocol acknowledge that it may be insufficient in detecting injury risk in athletes who perform a side or cross cutting manoeuvre. Another assessment strategy developed by Myer et al. is a series of successive jumps over a 10-second time period. Unfortunately, validity and reliability have not been established in a clinical trial.

The single-leg hop for distance, triple-hop for distance, the 6-meter timed hop and the cross over hop tests are 4 lower extremity functional tests which have been proven reliable in visually identifying poor neuromuscular mechanics that predispose to ACL injury. Additionally, these tests can assist in identifying leg dominance. Also, the Star Excursion Balance Test has been found to be a reliable clinical tool to assess dynamic lower extremity balance and stability, while also predicting the risk for lower extremity injury (2).
Star Excursion Balance Test While a multitude of assessment protocols exist for assessing core stability, a universally accepted, valid and reliable way of assessing this muscle group remains elusive. Additionally, identification of which muscles actually create the core and whether strength or endurance is more crucial in stabilizing the spine is also up for debate. Front and side medicine ball tosses have been shown to be a reliable measurement of core musculature power. Similarly, a single-leg squat test has also been shown to identify poor hip and knee mechanics while at the same time, screening for core stability. Suffice it to say, more research is needed to find a valid and reliable means of assessing core stability.

Put simply, multiple assessment strategies are needed to identify weaknesses and inadequacies in technique which lead to ACL injury. All of these techniques can be utilized both in a clinical setting and on the field while training athletes. Using the assessment techniques stated prior in addition to any other assessment strategies that are available, allows the therapist to create objective baseline measures for each person whose care they are responsible for.

Clinical Application & Conclusions:

A thorough knowledge of the various kinetic chains within the body, combined with an ability to detect biomechanical and neuromuscular risk factors in female athletes are important in identifying and auditing those at risk for ACL injury. Multifaceted neuromuscular training programs for the hip, hamstrings and core plus plyometrics, proprioception, balance and agility training are inherently necessary in limiting the possibility of ACL injury. This training may be introduced in a warm-up, on field setting, in combination with visual and verbal feedback to cue proper motor patterns. Specifically, clinical interventions should be longer than 8 weeks in duration to allow sufficient time for neuromuscular change. Additionally, a multifaceted prescreening audit process is required to identify females who are at risk for non-traumatic ACL injury and to establish baseline values to measure progress.

Study Methods:

This paper was a narrative literature review.

Study Strengths / Weaknesses:

Weaknesses:
  • The author did not speak enough about the relationship between pes planus (flat feet) and ACL injury.
  • The author seemed to ignore Dr. Stu McGill’s research on core stability assessment, which is a significant body of literature that can guide clinical practice.
Strengths:
  • Very comprehensive review overall.
  • Included details on the kinetic chain and the influence of muscles and both proximal and distal to the knee joint.
  • The clinical assessment was touched upon, to make it applicable to every day practice.

Additional References:

  1. Boden, BP, Torg, JS, Knowles, SB, and Hewett, TE. Video analysis of anterior cruciate ligament injury: Abnormalities in hip and ankle kinematics. Am J Sports Med 37: 252–259, 2009.
  2. Kinzey, SJ and Armstrong, CW. The reliability of the star-excursion test in assessing dynamic balance. J Orthop Sports Phys Ther 27: 356–360, 1998.
  3. McNair, PJ, Marshall, RN, and Matheson, JA. Important features associated with acute anterior cruciate ligament injury. N Z Med J 103: 537–539, 1990.
  4. Myer, GD, Ford, KR, Brent, JL, and Hewett, TE. Differential neuromuscular training effects on ACL injury risk factors in ‘‘high risk’’ versus ‘‘low risk’’ athletes. BMC Musculoskelet Disord 8: 39, 2007.
  5. Padua, DA, Marshall, SW, Boling, MC, Thigpen, CA, Garrett, WE Jr, and Beutler AI. The Landing Error Scoring System (LESS) is a valid and reliable clinical assessment tool of jumplanding biomechanics: The JUMP-ACL study. Am J Sports Med 37: 1996–2002, 2009
  6. Zazulak, BT, Ponce, PL, Straub, SJ, Medvecky, MJ, Avedisian, L, and Hewett, TE. Gender comparison of hip muscle activity during single-leg landing. J Orthop Sports Phys Ther 35: 292–299, 2005.
  7. Zebis, MK, Andersen, LL, Bencke, J, Kjaer, M, and Aagaard, P.Identification of athletes at future risk of anterior cruciate ligament ruptures by neuromuscular screening. Am J Sports Med 37: 1967– 1973, 2009.