Female athletes are currently reported to be 4–6 times more likely to sustain a sport-related non-contact anterior cruciate ligament (ACL) injury than are male athletes.1^–3 Altered neuromuscular trunk and hip control strategies during the execution of sports movements may result in lower limb joint mechanics (motions and loads) that increase the risk of ACL injury in female athletes. Hip abduction strength and control may be the critical modulator between altered trunk control and ultimate lower limb knee loads responsible for ACL rupture.

During peak growth velocity of both height and mass, the tibia and femur grow at rapid rates in pubertal athletes of both sexes.4 Rapid growth of the two longest levers in the human body initiates height increases and increased height of the centre of mass, making muscular control of trunk more challenging. In addition, increased body mass and longer joint levers leads to the development of greater joint forces and moments that are more difficult to balance and dampen at the lower limb joints during high-velocity manoeuvres.5^–7 Brent et al used a longitudinal study approach to determine that in a population of 11–22-year-old athletes, male athletes increased their relative hip abduction strength at a significantly greater rate than did female athletes.8 Female athletes also showed decreased power and increased valgus knee motion during jumping and landing as they matured.5 6 Mature female athletes who showed decreased hip abduction control were also more likely to show increased knee valgus angle during a dynamic cutting task.9 Based on these data, it can be hypothesised that after the onset of puberty and the initiation of peak height velocity, the increased tibia and femur lever length, with increased body mass and height of the centre of mass, lead to decreased “core stability” or control of trunk motion during dynamic tasks in the absence of increases in strength and recruitment of the musculature at the hip.7 10 As female athletes reach maturity, decreased “core stability” may underlie their tendency to increased dynamic lower limb valgus load (hip adduction and knee abduction) during dynamic tasks.5^–7 11^–13

The aim of the current study was to determine the effect of trunk and hip focused neuromuscular training (TNMT) on lower limb hip and knee strength. The hypothesis tested was that TNMT would change standing isokinetic hip abduction strength with no similar changes in knee flexion/extension or in the strength measures for control subjects.

METHODS

Dynamic strength

Isokinetic knee extension/flexion strength was collected with subjects seated on a dynamometer with the trunk perpendicular to the floor, and with hip and knee flexed to 90°. Before each data-collection set, a warm-up set consisting of five knee flexion/extensions for each leg at 300°/second was performed (fig 1). The test session consisted of 10 knee flexion/extension repetitions for each leg at 300°/second. Peak flexion and extension torques were recorded. Hip abduction strength was measured with the subjects standing erect, fully supported with a stabilisation strap around the pelvis and their hands gripping a stable hand rest. The test leg was positioned lateral to the opposite leg at zero degrees of hip and knee flexion. The axis of abduction/adduction of the hip was aligned with the axis of rotation of the dynamometer. The resistance pad was affixed to the subject’s leg, just proximal to the knee joint. Before data collection, subjects were familiarised with how to perform the test and were allowed to perform 5–10 submaximum repetitions until they became comfortable with the testing technique and the tester confirmed appropriate technique. The athletes were then asked to perform five repetitions with maximum exertion at 120°/second (fig 2). The initial test leg was alternated between subjects to control for a side-learning effect. Peak hip abduction torques were recorded and analysed by a single investigator. The intrarater reliability (ICC_3,1) for the presented hip abduction test methods showed good to excellent reliability on the left (ICC = 0.857; 95% CI 0.577 to 0.971) and right (ICC = .916; 95% CI 0.727 to 0.984) sides.8

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Figure 1 Example of the isokinetic knee flexion/extension testing.

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Figure 2 Example of the isokinetic standing hip-abduction testing.

Training protocol

Table 1 shows the trunk and hip focused neuromuscular training protocol that was used with female athletes to target deficits in trunk control and improve hip strength and power. Five phases were used for each exercise to facilitate progression in difficulty. These exercises were designed to improve the athletes’ ability control the trunk and improve “core stability” during dynamic activities. Each exercise within the individual phases progressively increased in intensity to challenge the performance of the exercise techniques continually. End-stage progressions incorporated lateral trunk perturbations that forced the athlete to decelerate and control the trunk in the coronal plane in order to execute the prescribed technique successfully (⇓fig 3, 4). All of the exercises selected for the initial phase, before progression, were adapted from previous epidemiological studies that reported reductions in ACL injury risk (table 1).14^–17 The protocol progressions were developed from previous biomechanical investigations that reported reductions in knee abduction load in female athletes after their training protocols.18^–21 The novelty of this training approach is that the protocol incorporated exercises that perturb the trunk to improve control of trunk musculature, in attempt to improve hip abduction strength and “core stability.” Our hypothesis was that improved hip and trunk control would decrease the magnitude of peak kinematics and kinetics that induce high knee abduction loading in female athletes.

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Figure 3 Example progression of training exercises targeting hip control during each phase (see table 2 for explanation).

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Figure 4 Example progression of training exercises targeting trunk control during each phase (see table 2 for explanation).

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Table 1 Example trunk and hip focused neuromuscular training protocol

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Table 2 Explanation of exercises targeting hip and trunk control.

RESULTS

A significant interaction of group and time was observed with statistical analysis. The TNMT group increased bilateral isokinetic hip abduction strength approximately 15% (13.5% in the dominant limb, mean (SD) 46.6 (10.1) to 52.9 (11.4) foot-pounds and 17.1% in the non-dominant limb 46.1 (10.4) to 54.0 (10.7) foot-pounds; p<0.01). However, there were no observed effects of TNMT on isokinetic knee extension (p = 0.57) or knee flexion (p = 0.57) strength. There was no change in the control group with time between pre-test and post-test measures.

DISCUSSION

The aim of this study was to determine the effect of a training program on lower limb hip and knee strength. Hip abduction strength was increased in a group of female athletes after TNMT. Recent investigations indicate an association of hip abduction strength with dynamic valgus alignments that increase ACL injury risk.9 22^–25 Padua et al examined the relationship between hip strength quantified with a hand-held dynamometer and joint kinematics during a drop-jump task in males and female athletes.24 They reported that decreased hip strength (gluteus medius and gluteus maximus) was related to greater knee valgus at initial contact and greater peak knee valgus.24 In addition, hip abduction strength (gluteus medius) was the only significant predictor of initial and peak valgus angles during the drop vertical jump.24 Claiborne et al reported a significant relationship between increased concentric hip abduction strength and reduced frontal plane knee abduction motion during a single-leg squatting task.25 Similarly, female athletes who showed increased eccentric isokinetic hip strength showed decreased knee valgus angles when landing from a jump.26 In a follow-up investigation, Jacobs et al evaluated isometric hip abduction peak torque and found that female athletes showed a relationship between decreased hip strength and increased knee valgus displacement and hip adduction during landing.27 Interestingly, these relationships were not evident in the male athletes in this population study, who also showed increased hip abduction strength and decreased knee valgus displacement relative to the female athletes in the study.27

The theory that hip strength and control may dictate knee motion and loads underlines the importance of the improved hip abduction strength that was obtained from TNMT in the current project. As female athletes reach maturity, there appears to be an absence of adaptation in hip and “core” strength, concomitant with increased risk of high-risk biomechanics and increased risk of ACL injury.5 6 8 28^–30 Hip abduction strength measurement may be advantageous to identify and target high-risk female athletes with neuromuscular training.18 Secondly, if this association between maturation and decreased adaptive changes in hip abduction strength and recruitment is present in this population, TNMT may be effective intervention in pubertal and pre-pubertal female athletes to induce a “neuromuscular spurt” (defined as the natural adaptation of increased power, strength and coordination that occur with increasing chronological age and maturational stage in adolescent boys) in female athletes.8 31 Cumulatively these techniques may increase the effectiveness of ACL injury prevention programmes in young female athletes.

CONCLUSION

Hip abduction strength and control may be the critical modulator between altered trunk control and ultimate knee loads responsible for ACL rupture. Schmitz et al reported increased gluteus medius activation in response to trunk displacement.32 The results of the current study indicate that 10 weeks of TNMT increases standing hip abduction strength in female athletes. Hip abduction strength and recruitment may improve the ability of female athletes to increase control of lower limb alignment and decrease motion and loads resulting from increased trunk displacement during sports activities.10 25 Further investigations are needed to determine if improved hip strength after TNMT translates into reduced knee abduction load in female athletes at high risk of ACL injury. If this association is observed, then parallel investigations should be undertaken to determine if TNMT is effective in pubertal and pre-pubertal athletes to determine its effectiveness to increase relative hip strength and control, which is often reduced as young female athletes mature.8 31