There are several pathologies and clinical presentations that may indicate targeting the hip abductors, but are you selecting the most effective interventions?
Lets start off with some anatomy and biomechanics…
The hip abductor musculature consists of the gluteus medius, gluteus minimus, and tensor fasciae latae as determined by Clark et al. Additionally, the piriformis and sartorius muscles work as secondary hip abductors. In addition to being the largest hip abductor muscle, based on its origin and insertion (external surface of the ilium above the anterior gluteal line to the lateral aspect of the greater trochanter), the gluteus medius is also provided with the greatest abductor moment arm. These factors contribute to its place as the dominant abductor muscle and the primary focus of most rehabilitation programs.
Outside of this muscle’s obvious role (hip abduction), it has the additional responsibility of controlling frontal plane stability of the pelvis during walking and other functional activities. During a large portion of stance phase, the hip abductors stabilize the pelvis over a relatively fixed femur (Hurwitz et al). Additionally during the single limb support phase of gait, without adequate hip abductor torque on the stance leg, the pelvis and trunk may drop toward the side of the swinging limb. This hip abductor activation also contributes most of the compressive forces generated between the acetabulum and femoral head. These functions are of utmost importance in normalizing and improving movement quality during functional activities and/or gait. In summary of this muscle’s actions, the gluteus medius concentrically abducts the hip, isometrically stabilizes the pelvis, and eccentrically controls hip adduction and internal rotation.
There are a bounty of different exercises that are said to ‘target’ the hip abductors or primarily the gluteus medius, but where’s the evidence?
In 2004, Fry et al proposed that 40-60% of maximal voluntary isometric contraction (MVIC) should be considered the threshold for producing adaptive muscular strengthening. Bolgla et al examined the muscular activation of the gluteus medius during 3 weight-bearing (WB) and 3 non-weight-bearing (NWB) exercises in healthy, young subjects via electromyography. This study found that the average percentage of MVIC was increased during WB exercises due to the additional external torque required to maintain pelvic stability. This study showed that the WB pelvic drop exercise demonstrated the greatest gluteus medius activation at 52% MVIC. While all, but 2 of the exercises are within the proposed threshold for muscular strengthening, the total number of exercises was minimal and none of the exercises would be considered ‘functional’ in nature. However, Ayotte et al evaluated the electromyographical activity of some of the major hip muscles during 5 unilateral WB exercises. Gluteus medius activity ranged from 52-36% MVIC with the unilateral wall squat garnering the highest muscular activity. As there was not a mean difference amongst the 5 activities, it is suggested that similar gluteus medius activity is required for all activities measured. To build upon these findings, both Boren et al and Philippon et al evaluated the activity of the gluteus medius during more advanced exercises. Boren et al found the most demanding exercises to be 3 different plank variations along with the single-leg squat task. This increased muscular activity may have been secondary to the increased stabilization necessary to properly perform the task and gives credence to the use of these exercises for more advanced patients or those who have progressed near the end of their treatment.
In clinical practice, unstable surfaces are often included to further stress balance and theoretically impose greater demands on the musculature stabilizing the pelvis. Krause et al investigated the alteration in muscular activation when comparing stable to unstable surfaces. While values of percent MVIC did increase with the addition of an unstable surface, these increases did not reach statistical significance and cannot be considered superior to exercises performed on stable surfaces. It should be noted that this study only used one ‘unstable’ surface (Cor-Tex Balance Trainer) and other surfaces may have yielded more beneficial results.
Additionally, you must consider the interaction of other muscles acting with or against the gluteus medius. It has been proposed that individuals who demonstrate excess femoral internal rotation during functional tasks may be relying too heavily on the tensor fasciae latae (TFL) to control their pelvis in the presence of weak or inhibited gluteus medius musculature. The TFL is generally considered a hip internal rotator in addition to its role as an abductor, thus its overutilization has been theorized to cause abnormal pelvic control. Powers et al conducted a study comparing the %MVIC of the gluteus medius versus that of the TFL in 11 different therapeutic exercises utilizing indwelling fine-wire electrodes. They ranked the exercises based on their calculation of the Gluteal-TFL Activation Ratio. The authors concluded, exercises that produced significant activation of the gluteus medius while minimizing TFL activity included the clam, side-step, unilateral bridge, and hip extension in quadruped on elbows with knee extended or flexed. It must be understood that in general, these comparisons of MVIC have their limitations. Not all the testing procedures were consistent between studies and all subjects measured were young and asymptomatic, which does not necessarily correlate with patients seen in a rehabilitation setting. Also, several studies utilized surface EMG electrodes, which are notorious for crosstalk being recorded from adjacent muscles.
While measurement of MVIC and its application to exercise selection is very beneficial, it should not be your only means of determining the proper prescription. You must take into consideration your patient’s current stage in the rehabilitation process, potential contraindications, and where your patient’s deficits actually lay. Your patient may have a 5/5 grade on his side-lying hip abductor manual muscle test (MMT), but he demonstrates an inability to control hip adduction and internal rotation during your running and/or gait analysis. The evidence says gluteus medius activity during side-lying hip abduction is 62% when averaged across three studies, pretty good right? Would this be an appropriate exercise for this patient? Probably not. He has no problem with NWB concentric activation of his gluteus medius, but loses isometric and eccentric pelvic control during demanding functional activities. You must select exercises appropriate for your patient, do not get blinded by the statistics, they do not tell the whole story. Use MVIC data to determine which exercises among those appropriate for your patient are most effective. This approach will give you a foundation to stand on when determining your patient’s exercise prescription.
Below is a summary chart of the Gluteus Medius %MVIC data compiled from all the studies referenced in this article.