The role of hip extensor strength during cutting

Although most clinicians tend to emphasize hip abductor and external rotator muscle strengthening in injury prevention and rehabilitation, recent research suggests exercises to increase explosive hip extensor strength may be critical for controlling frontal plane motion during cutting tasks.

By Marc Norcross, PhD, ATC, and Sam Johnson, PhD, ATC

Walk into any sports medicine clinic and you are bound to see a patient performing “proximal control” exercises—clamshells, side-lying leg raises, and monster walks, to name just a few. The reason for this ubiquitous clinical practice is likely driven by the fact that excessive frontal plane motion of the hip and knee is thought to be related to many lower extremity injuries, including patellofemoral joint dysfunction,^1-3 iliotibial band friction syndrome,⁴ and ruptures of the anterior cruciate ligament.^5-7

Accordingly, a common clinical paradigm is that if a patient’s hip musculature can be strengthened, their symptoms can be alleviated and/or their injury risk decreased through an improved ability to control frontal plane motion of the hip and knee. However, the results of studies investigating the relationships between hip strength and frontal plane motion during functional tasks have been inconsistent.

The traditional paradigm

The musculature about the knee has limited capacity to control knee valgus motion directly.^8,9 Therefore, the magnitude of knee valgus motion during movement is primarily influenced by joints either distal or proximal to the knee.² Proximally, greater knee valgus angles are associated with greater hip adduction during landing¹⁰ and cutting activities.¹¹ This suggests that increasing the capacity of the hip abductors to control frontal plane hip motion may reduce the magnitude of hip adduction and, in turn, knee valgus during movement. Clinically, the gluteus medius is the most common muscle targeted for this reason because it is the primary producer of hip abduction torque.^12-14

However, greater hip internal rotation is also associated with greater knee valgus angle during movement.¹⁰ As a result, clinicians often also include exercises designed to increase the strength of the hip external rotators to reduce knee valgus by improving control of hip internal rotation.¹⁵ Unfortunately, despite the common acceptance of this clinical treatment paradigm, research evaluating the relationships between hip abductor or external rotator strength and frontal plane motion have reported inconsistent findings.

Frontal plane control benefits associated with explosive gluteus medius strength may be negated when the hip is flexed and/or abducted, which occurs often in sports.

Hollman et al¹⁶ and Norcross et al¹⁷ reported that greater isometric hip abductor strength was associated with greater knee valgus angle or frontal plane projection angle (FPPA) during a single-leg squat—a relationship that is the opposite of what would be expected. Further, these investigators failed to identify any significant associations between isometric external rotator,^16,17 eccentric external rotator,¹⁷ or eccentric hip abductor¹⁷ strength and frontal plane knee motion. Thijs et al¹⁸ also failed to identify a significant relationship between isometric hip abductor strength and FPPA during a forward lunge. Conversely, Claiborne et al¹⁹ reported that greater concentric hip abductor torque measured at 60°/sec was associated with less knee valgus motion during a single-leg squat; and Willson et al²⁰ reported a trend for greater isometric hip abductor (p = .07) or external rotator strength to be associated with a smaller FPPA at the knee during a 45° single-leg squat.

Willy and Davis²¹ reported significantly less hip adduction during a single-leg squat in female runners following the completion of a training program that increased hip abductor and external rotator strength, but these training-related changes in hip adduction were not identified during running. In contrast, Heinert et al²² reported that while frontal plane hip kinematics during treadmill running did not differ significantly between strong and weak groups classified on the basis of isometric hip abductor strength, the weak group had significantly greater knee valgus than the strong group at initial contact and throughout the stance phase.

The equivocal relationship between hip strength and frontal plane motion extends beyond squatting and running tasks. During a single-leg landing task, Jacobs et al²³ observed a trend for less knee valgus and hip adduction to be associated with greater isometric hip abductor strength. This same group also reported a significant relationship between greater peak eccentric hip abductor strength measured at 120°/sec and less peak knee valgus during a single-leg hop.²⁴However, Patrek et al²⁵ failed to identify any meaningful changes in frontal plane hip or knee kinematics during a 40-cm single-leg drop landing following a fatigue protocol that reduced peak isometric hip abductor strength by 43%.

Homan et al²⁶ did not identify differences in frontal plane knee kinematics during a double-leg jump landing between groups of women classified as having high versus low isometric hip abductor or external rotator strength. However, they did find that participants in the low hip abductor and external rotator strength groups had significantly greater gluteus medius and gluteus maximus activation, respectively, as measured using surface electromyography.²⁶ Although the authors concluded that strength did not influence frontal plane knee kinematics directly, weaker individuals did utilize greater neural drive than stronger individuals to achieve a similar kinematic profile.²⁶

Despite its general acceptance, the published evidence related to the traditional clinical paradigm between hip abductor and external rotator strength and frontal plane motion is inconclusive. Although the reasons for this inconsistency are likely multifaceted, two potentially relevant factors are important to consider with respect to the relationship between hip strength and frontal plane motion: 1) the functional divisions of the gluteus maximus, and 2) the discrepancy between the way hip strength is most commonly assessed during scientific studies and the way the hip muscles actually function during a dynamic task.

Gluteus maximus functional divisions

The gluteus maximus is most commonly thought of as a single muscle that functions primarily as a hip extensor and external rotator.¹³ However, there is evidence suggesting that it has two distinct functional divisions. Stern²⁷ described the anatomy of the gluteus maximus and concluded that while the lower portion is oriented to produce hip extension, the upper portion via its insertion on the tensor fascia latae is most effective at producing hip abduction. Grimaldi et al²⁸ reported different changes in the upper and lower portions of the gluteus maximus in response to advanced hip joint pathology and concluded the upper portion should primarily be considered a hip abductor. Lyons et al²⁹ highlighted the synergy between the activation of the gluteus medius and the upper portion of the gluteus maximus during the single-leg support phases of walking and stair climbing—further supporting the hip abductor role of the upper gluteus maximus.

Recently, Selkowitz et al³⁰ reported that during therapeutic exercises incorporating hip abduction, external rotation, or both, the upper portion of the gluteus maximus had significantly greater activation amplitude than the lower portion. However, no significant differences in activation amplitude were identified between the upper and lower portions during exercises that focused primarily on hip extension.³⁰ This finding is especially interesting, as it suggests the role of the upper gluteus maximus is not limited to hip abduction and that it can contribute to the external rotation function of the gluteus maximus.^13,31 Further, the results indicate the upper gluteus maximus also functions synergistically with the lower portion to produce hip extension.³⁰ Therefore, while it may not be possible clinically to isolate and test the strength of the upper portion of the gluteus maximus, greater overall hip extension strength might be indicative of greater strength of this functional division.

Despite this functional role of the upper gluteus maximus, relatively few studies have directly investigated the influence of hip extensor strength on frontal plane motion during dynamic activities. Thijs et al¹⁸ reported no significant association between isometric hip extension strength and FPPA at the knee during a forward lunge. Stearns and Powers³² reported that, following a hip-focused training program that increased isometric hip extension strength, women exhibited a 1.2° smaller peak knee valgus angle during a double-leg drop jump (p = .07). However, because the training program also facilitated an increase in isometric hip abduction strength, the individual influence of hip extension strength on this kinematic change is not clear.

Hollman et al³³ observed no change in frontal plane knee or hip motion during double-leg vertical jumps following a fatigue protocol despite a 25% reduction in isometric hip extension strength. They suggested that fatigue-related kinematic changes may have been effectively mitigated by a 33% increase in gluteus maximus activation during postfatigue vertical jumps. In a subsequent article, Hollman et al³⁴ reported that, after controlling for hip motion, isometric hip extensor strength and gluteus maximus activation were predictive of knee valgus angle at peak knee flexion during double-leg vertical jumps.

Collectively, these investigations provide some support for a relationship between greater hip extension strength and lesser frontal plane hip and knee motion. However, relatively few studies have been performed, and they are limited by the use of fairly low-level (forward lunge)¹⁸ or double-leg (drop³² and vertical jump^33,34) tasks rather than a single-leg dynamic movement such as cutting. Further, all of these investigators chose to measure peak hip extension strength during an isometric contraction rather than quantifying the capacity to produce explosive strength, which is how muscles function during athletic movements.

Why measure explosive strength?

The majority of studies that have evaluated the influence of hip strength on frontal plane hip and knee motion have quantified peak strength during a maximal voluntary isometric contraction (MVIC).^{16,18,20-23,25,26,32-34} Although this may be clinically feasible, the time to peak torque during an MVIC is appreciably longer than the time available during landing to control peak frontal plane motion. Peak knee valgus and hip adduction occur within 150 ms of initial contact during landing.^35,36 However, peak torque during MVIC is generally not achieved until at least 250 ms after torque onset.^37,38 As a result, the explosive strength characteristics of a muscle group are typically assessed by quantifying the rate of torque development (RTD), or the slope of the isometric torque-time curve.^39,40

Further, despite being related,^40,41 peak strength and explosive strength have been shown to increase differentially in response to training.⁴² Consequently, explosive hip strength is not only modifiable, it also may be more relevant than absolute isometric hip strength for controlling frontal plane motion during the 150 ms following ground contact.

Our recent research

Given this background, we sought to address some of the limitations of previous investigations by conducting a study that examined the relationships between explosive hip extensor and abductor strength and frontal plane hip and knee kinematics during a single-leg jump-cut task.⁴³

Forty healthy female recreational athletes performed isometric hip extensor and abductor contractions as hard and as fast as possible in response to a light stimulus on a dynamometer. Participants were positioned during the two conditions in an effort to isolate the gluteus medius (abduction) and gluteus maximus (extension). RTD of the hip extensors and abductors was then calculated during the 200 ms after torque onset. Frontal plane knee and hip kinematics were also recorded during a jump-cut task and compared between participants in the highest and lowest RTD tertiles for each hip strength condition.⁴³ Although no differences in frontal plane kinematics were identified between the high and low hip abductor RTD groups, women in the high hip extensor RTD group exhibited less knee valgus and hip adduction motion during the jump-cut task than those in the low hip extensor RTD group.⁴³

Conclusions and clinical implications

The lack of a relationship between explosive hip abductor strength and hip adduction and knee valgus was extremely surprising given the traditional hip strength and frontal plane motion paradigm. However, we concluded the lack of relationship may have been driven by the manner in which the jump-cut task was performed. On average, participants landed with about 30° of hip flexion and slightly more than 10° of hip abduction.⁴³ Unfortunately, the capacity of the gluteus medius muscle to produce hip abduction torque decreases as the hip is flexed¹² and abducted.^12-14 It is possible that any frontal plane control benefits that might be associated with greater explosive gluteus medius strength are negated during movements that are performed with the hip in significant flexion and abduction.⁴³

Conversely, the effectiveness of the gluteus maximus to function as a hip abductor seems to increase as the hip is flexed.^44,45 Therefore, the greater frontal plane control by participants with more explosive hip extensor strength may have been because the upper portion of the gluteus maximus was able to effectively control femoral motion.⁴³

Considering that many athletic tasks are performed with the hip in a flexed and/or abducted position, we proposed that increasing explosive gluteus maximus strength by performing exercises with a specific intention to move quickly—which has been shown to be the primary stimulus for improving explosive strength⁴⁶—may be important in lower extremity rehabilitation and injury prevention programs.⁴³ However, the effectiveness of this novel addition to the traditional hip strength paradigm in controlling frontal plane motion needs further evaluation.

Marc Norcross, PhD, ATC, is an assistant professor and graduate coordinator of the Kinesiology Program, and Sam Johnson, PhD, ATC, is a clinical assistant professor and clinical education coordinator for the Athletic Training Program, both in the College of Public Health and Human Sciences at Oregon State University in Corvallis.

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