June 2015

Hip strength, balance, and risk of ACL injury

6ACL-shutterstock_92975599-copyBy Jatin P. Ambegaonkar, PhD, ATC, OT, CSCS; Lindsey M. Mettinger, MS, ATC; Shane V. Caswell, PhD, ATC; Andrea Burtt, MS, ATC; Shruti J. Ambegaonkar, PT, PhD; and Nelson Cortes, PhD

Hip strength is associated with performance on the Star Excursion Balance Test in female collegiate athletes, a finding that adds to the evidence that hip strengthening programs can help reduce the risk of anterior cruciate ligament injuries in that patient population.

More than 250,000 anterior cruciate ligament (ACL) knee injuries occur annually in the US alone, where ACL injury direct surgical costs are near $850 million per year,1 with an additional $2 billion of indirect costs for postsurgical care and rehabilitation. Many long-term sequelae (eg, early-onset knee osteoarthritis) are possible.2 Despite clinicians implementing prevention programs,3 ACL injury incidence rates remain high.4 Overall, the ACL injury problem continues to be of great concern, particularly to female athletes.5,6

The risk of ACL injury in female athletes is three to eight times greater than in similarly trained male athletes.7 Female athletes are at greater risk for ACL injury than their male counterparts for many reasons,7 including nonmodifiable (eg, anatomical, hormonal) and modifiable (eg, neuromuscular) risk factors,6,8 and the risk tends to be higher in sports that require cutting and landing motions (eg, lacrosse, basketball).5,9,10 Neuromuscular control of the core8,11 and hip musculature12-14 plays an important role in lower body mechanics and may influence ACL and lower body injury risk.

Clinicians can use SEBT balance testing as a measure of neuromuscular control to track athletes’ improvement after a hip muscle strengthening intervention.

Core musculature and ACL injury risk

The core is an important stabilizer of the knee and lower body movement during activity,15,16 which underscores its potential to influence injury risk. The core musculature includes the rectus abdominis, transversus abdominis/internal obliques, external obliques, and erector spinae muscles.15-19 During reaction-based tasks (eg, running, landing), the transversus abdominis/internal obliques are key dynamic stabilizers of the spine, lumbopelvic region, and the whole trunk-pelvis segment, collectively referred to as the “core.”17,20-22 The rectus abdominis, external obliques, and erector spinae muscles control trunk position relative to its base of support.16 The transversus abdominis is the first muscle activated during lower body movements.16 In a series of prospective studies published in 2007 examining the effects of core stability on lower body injury risk,18,22 Zazulak et al reported that a logistic regression model (which included core-specific factors, including trunk displacement, low back pain history, and proprioception) predicted knee, ligament, and ACL injury risk in female athletes with 84%, 89%, and 91% accuracy, respectively.

Figure 1. Star Excursion Balance Test (SEBT) directions: anterior reach (1A), posterolateral reach (1B), posteromedial reach (1C).

Figure 1. Star Excursion Balance Test (SEBT) directions: anterior reach (1A), posterolateral reach (1B), posteromedial reach (1C).

Hip musculature and ACL injury risk

In a 2005 study, Zazulak et al noted less hip muscle (specifically gluteus maximus) activation in female athletes than male athletes during landing and suggested these muscle activation patterns may be an important factor in the increased susceptibility of female athletes to ACL injuries.23 Other researchers have also found associations between strength and biomechanical risk factors for ACL injury. Individuals with greater hip abduction strength have less knee valgus motion during single-leg squats,12 and individuals with greater hip external rotation strength have lower vertical ground reaction forces and external knee adduction and flexor moments during landing.14

Stearns and Powers found that a hip-focused training program was associated with benefits for recreationally active women; specifically, their knee/hip extensor moment ratios and knee adductor moments were positively affected, and their lower body mechanics during a drop-jump changed in a manner consistent with reduced risk of ACL injury.24

Balance and ACL injury risk

Deficits in neuromuscular postural stability or balance have also been suggested to increase lower body injury risk.25,26 The terms “postural stability” and “balance” are often used interchangeably, but can describe different ideas. For example, as operationalized in the Star Excursion Balance Test (SEBT), balance is the ability to maintain postural stability (standing on one leg) while reaching as far as possible with the other leg in a specified direction without losing support.27,28

Tekscan-Injury-Risk-200x300_v3Poor SEBT performance has been found to predict increased lower body injury risk in multiple sports.26,29,30 Plisky et al30 noted that female athletes with lower SEBT reach distances (less than 94% of leg length) are 6.5 times more likely to have a lower body injury than those with higher reach distances. Other researchers have noted that SEBT scores are lower in individuals who have ACL deficiency31 and women who have undergone ACL reconstruction surgery.32

Overall, increased core endurance, greater hip muscle strength, and better SEBT performance are associated with reduced ACL and lower body injury risk.8,15,16,18,22,33 In our 2014 study published in the International Journal of Sports Physical Therapy34 we investigated relationships among core endurance, hip strength, and balance performance on the SEBT.

Forty collegiate female athletes performed the SEBT bilaterally in anterior, posterolateral, and posteromedial directions (assessed as percentage of leg length), McGill’s anterior, posterior, and left and right plank core endurance tests (seconds), and hip abductor, flexor, extensor, and external rotator isometric strength tests bilaterally (measured in Newtons) using handheld dynamometry.

Star Excursion Balance Test. Lower quarter balance was measured using the SEBT in the anterior (Figure 1A), posterolateral (Figure 1B), and posteromedial (Figure 1C) directions.30,35,36 Reach distances were averaged and normalized to limb length (%LL, cm), which was measured from the anterior superior iliac spine to the medial malleolus bilaterally.28 Six different SEBT scores were calculated: three directional scores on the right leg and three directional scores on the left leg. Then all six scores were averaged to result in a single composite SEBT score per participant.

McGill’s core endurance tests. Core endurance was measured using McGill’s tests.37 These tests consist of four positions: the trunk anterior flexor test, the right and left lateral plank, and trunk posterior extensor test as described previously,37 in which the maximum time (in seconds) participants can hold a static position is measured (Figures 2A-E).

Isometric hip strength. Isometric hip strength (hip abductors, flexors, external rotators, extensors) was measured bilaterally using a handheld dynamometer (Figures 3A-D).

Primary findings

All scores (means and standard deviations) are presented in Table 1. The strength of the relationships between variables was described as detailed by Portney and Watkins, ie, 0 to .25 = little or no relationship; .26 to .50 = fair degree of relationship; .51 to .75 = moderate to good relationship, and .76 to 1 = good to excellent relationship.38

Our primary findings were that significant positive existed relationships between:

  1. Anterior balance and hip flexor and extensor strength;
  2. Posterolateral balance and hip abductor, extensor, and flexor strength, and
  3. Posterior core endurance and hip extensor strength bilaterally.
The Star Excursion Balance Test assesses an athlete’s ability to maintain postural stability on one leg while reaching as far as possible with the other leg in a specified direction without losing support.

Balance and hip strength

SEBT combined scores showed a fair degree of positive correlation with hip abductor, flexor, and extensor strength. Right anterior SEBT scores showed a fair positive correlation bilaterally with hip flexor and extensor strength, while left anterior SEBT scores had a fair positive correlation bilaterally with hip flexor strength. Right posterolateral SEBT scores were fairly positively correlated with right hip abductor and extensor strength, and left posterolateral SEBT reach scores were fairly positively correlated with right hip abductor and left hip flexor strength. Posteromedial SEBT reach scores were not correlated with any hip strength scores.

We found that women with greater hip strength scores also had overall better SEBT scores. We examined hip abductor, flexor, external rotator, and extensor muscle strength, as previous research has shown these muscles are activated during the SEBT33 (while muscle activation does not indicate muscle strength directly, the two measures are associated).29

These findings are interesting in the context of previous research on SEBT scores and injury risk, as well as research on SEBT outcomes after hip strengthening interventions. As mentioned earlier, female basketball players with worse SEBT composite scores (less than 94% of limb length) reportedly are 6.5 more times more likely to have a lower body injury than those with higher SEBT scores.30 Rasool and George found that SEBT scores can improve up to 36% after two to four weeks of neuromuscular balance training.39 Hip strengthening exercises have been found to be more effective than traditional rehab­ilitation for improving sagittal plane dynamic balance (anterior reach on the Y-balance test) three months post-ACL recon­struc­tion.40 Similarly, Filipa et al found that a lower body strength­ening and core stability program increased SEBT scores in soccer players.36

Figure 2. McGill’s core endurance tests: anterior trunk flexor test, starting position (2A) and testing position (2B); left (2C) and right (2D) lateral musculature plank test; and posterior trunk extensor test (2E).

Figure 2. McGill’s core endurance tests: anterior trunk flexor test, starting position (2A) and testing position (2B); left (2C) and right (2D) lateral musculature plank test; and posterior trunk extensor test (2E).

When combined with prior work, our finding that greater hip strength is associated with better SEBT performance is important for clinicians, who can use this information to encourage female athletes to participate in hip strengthening and balance training programs.

Anterior SEBT scores were positively correlated bilaterally with hip flexion strength. This finding suggests that when hip flexors (eg, quadriceps muscles) are stronger, an individual can reach farther in the anterior
direction. This is consistent with the results of a study by Earl et al that found that both vastus medialis and vastus medialis obliquus activations (both com­­po­nents of the quadriceps muscles) were greater in the anterior SEBT directions than in the other directions.33 Although these quadriceps muscles do not cross the hip joint themselves, we believe these muscle activations reflect overall quadriceps muscle activity during SEBT anterior reaches.

hip-table1SEBT anterior scores were also fairly positively correlated bilaterally with hip extension strength in our study. This finding suggests that hip extensors control the pelvis and trunk, helping to maintain balance, during anterior reaches. SEBT right anterior scores were also fairly positively correlated with left hip external rotator muscle strength but not with right hip external rotator muscle strength. The fair degree of the relationships suggests that, as anterior reach motion is primarily in the sagittal plane, extension and external rotator strength may not be as influential in this direction. Overall, several SEBT scores were positively correlated with multiple hip muscle strength measures, suggesting that examining anterior reach may provide a good general measure of hip muscle strength and vice versa.

Right posterolateral SEBT reach scores were positively correlated with right leg hip abductor and extensor strength, and left posterolateral SEBT reach scores were positively correlated with right hip abductor and left hip flexor strength. We did not find these results consistently in all muscles, which may be a reflection of the moderate correlations between posterolateral reaches and hip strength. Still, our findings suggest that individuals with stronger hip abductors, flexors, and extensors may be able to reach farther backward and laterally without losing balance. Such movements are frequently performed in sports that require the individual to backpedal (eg, a basketball, lacrosse, or soccer player on defense). Thus, posterolateral SEBT reach is a good measure of hip muscle strength and can be used by clinicians to examine athletes’ functional performance and progress during rehabilitation.

Interestingly, posteromedial SEBT reach scores were not correlated with hip strength scores in either limb. Norris et al41 found that hip muscle activation in the SEBT posteromedial direction was lower than in the anterior and medial directions. This finding, combined with our findings of no correlations in the posteromedial direction with hip strength suggest that this motion (crossing over backward) may not be as clinically important as the other directions as a measure of hip and lower body function.

Balance and core endurance

Figure 3. Hip muscle strength tests: abductors (3A), flexors (3B), external rotators (3C) and extensors (3D).

Figure 3. Hip muscle strength tests: abductors (3A), flexors (3B), external rotators (3C) and extensors (3D).

We noted fair positive correlations between left lateral core endurance and right posteromedial SEBT scores, but not consistently for the other directions. We are unsure of the reason for this finding, as the core should be active bilaterally during dynamic lower body movements. Still, the limited correlations we observed suggest lateral core training programs could potentially improve posteromedial balance. No other correlations existed between core endurance and balance.

Similarly, Gordon et al did not find any relationships among core endurance and balance (as measured by the SEBT).28 However, Shirey et al42 examined core muscle activation during single-leg squats and reported that voluntary activation of the core musculature had a more positive effect on frontal plane hip and knee kinematics (less mediolateral hip displacement, more knee flexion) than when the same individuals did not activate the core.42

The discrepancy between our findings and those of Shirey et al may be due to the differing tasks. Overall, further studies are needed to determine if core endurance and SEBT scores are related.

Core endurance and hip strength

Core extensor endurance was positively correlated with hip extensor strength. Most of the prime mover muscles of the lower body (eg, hamstrings, quadriceps, iliopsoas muscles) attach in similar anatomical areas (eg, ilium, ischium, pubic bones) as the core musculature, leading clinicians to believe that hip extensors and posterior hip muscles influence the lower back. However, we found inconsistent relationships between hip external rotation and SEBT scores. Gordon et al did not find significant relationships between hip external rotation strength and SEBT scores, and attributed this finding to muscles other than the external rotators being involved in balance.28

In our examination of additional hip muscles, while we noted positive relationships among the hip flexors, extensors, and abductors and SEBT scores, we measured hip muscles only isometrically rather than during a dynamic activity. Future researchers may use dynamic hip strength tests (eg, isotonic or isokinetic testing) to measure hip musculature and its relationships with balance.

Our findings add to the literature that suggests hip muscle strengthening programs may help reduce the risk of anterior cruciate ligament injury and other lower body injuries in female athletes.

Clinical implications

Overall, the primary clinical implications of our findings are that (1) hip strength is associated with SEBT performance, but core endurance is not, and (2) greater hip muscle strength is related to better balance performance on the SEBT in multiple directions. These findings add to the literature that suggests hip muscle strengthening programs may help reduce the risk of ACL injury and other lower body injuries in female athletes. The findings further suggest that clinicians can use SEBT balance testing as a measure of neuromuscular control to track these improvements.

Jatin P. Ambegaonkar, PhD, ATC, OT, CSCS, is the operations director of the Sports Medicine Assessment, Research, and Testing (SMART) Laboratory at George Mason University in Fairfax, VA. Lindsey M. Mettinger, MS, ATC, is a former master’s of science student in the Exercise, Fitness, and Health Promotion (EFHP) Program; Shane V. Caswell, PhD, ATC, is the executive director of the SMART Laboratory; Andrea Burtt, MS, ATC, is a former master’s of science student in the EFHP Program; Shruti J. Ambegaonkar, PT, PhD, is an assistant professor in the Athletic Training Education Program; and Nelson Cortes, PhD, is an associate professor and faculty member in the SMART Laboratory, all at George Mason.

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