Research Review By Dr. Demetry Assimakopoulos©

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Date Posted:

November 2016

Study Title:

Prevalence of gluteus medius weakness in people with chronic low back pain compared to healthy controls

Authors:

Cooper NA, Scavo KM & Strickland KJ et al.

Author's Affiliations:

Department of Physical Therapy and Rehabilitation Science, Carver College of Medicine, University of Iowa, Iowa City, Iowa; Department of Rehabilitation Therapies, University of Iowa Hospitals and Clinics, Iowa City, Iowa.

Publication Information:

European Spine Journal 2016; 25(4): 1258-1265.

Background Information:

Low back pain (LBP) is now the greatest cause of disability worldwide (1)! It is generally accepted that abdominal and low back rehabilitation is an important part of managing low back pain. However, few guidelines and strategies to date have focused specifically on hip abductor weakness (2-4). With this in mind, the authors sought to quantify the prevalence of hip girdle weakness and tenderness in people with chronic non-specific LBP (CLBP).

Pertinent Results:

The authors recruited 150 subjects with CLBP and 75 age matched controls. Sixty-six subjects had bilateral symptoms, and 84 had unilateral symptoms. There was perfect inter-rater reliability for the Trendelenberg sign and muscle tenderness. The muscle strength testing (MST) inter-class coefficient value was 0.597 (fair-good agreement). Those with LBP tended to have greater body mass index (BMI).

Gluteus medius (Gmed) strength was significantly decreased on the affected, painful side, compared to the unaffected side and control groups. TFL strength was significantly greater on the unaffected side compared to controls, but not the affected side. Gluteus maximus (Gmax) MST was equivocal side-to-side and compared to controls. There were significant between group differences for the Trendelenberg sign, as it was more frequently present on the affected side (52.2%) compared to the unaffected side (7.1%) and controls (9.7%). There was a greater prevalence of gluteal (68.1%) and lumbar paraspinal (53.2%) tenderness on the affected side compared to the unaffected side (4.8% gluteal; 23.8% paraspinal) and controls (11.2% gluteal; 0.7% paraspinals). There was no difference between sides or groups in piriformis tenderness.

The ROC (receiver operating characteristic) curve analysis (used to determine an appropriate cut-off point for, in this case, a clinical measure) showed that an MST score of ≤ 3/5 was the optimal cut off point for using Gmed weakness to predict the presence of LBP from the total sample. LBP was correlated with higher BMI, Gmed weakness, low back tenderness and a positive Trendelenberg sign, with the strongest contributor being Gmed weakness.

Clinical Application & Conclusions:

The authors of this controlled trial endeavoured to find differences in hip abductor tenderness and weakness in individuals with CLBP compared to controls. They successfully identified a subpopulation of individuals with CLBP with signs of Gmed weakness, gluteal tenderness and a positive Trendelenburg sign. Only Gmed weakness was a statistically significant predictor of LBP when compared to the control group, which suggests that Gmed muscle weakness contributes to the presentation of CLBP. However, it is unclear if Gmed weakness is a cause or consequence of LBP. The authors believe that there is a separate subgroup of LBP suffers who present with Gmed weakness and tenderness, which might benefit from targeted Gmed strengthening. This reflects what many of us see in practice, further promoting a comprehensive approach to the assessment and rehabilitative management of CLBP. Clinicians should evaluate patients’ hip mobility and functional strength, allowing these findings to assist in the prescription of appropriate rehabilitation exercise (ex. clamshells, band-walk variations, lunge patterns etc.), daily mobility drills, and so on.

Study Methods:

A total of 150 subjects were recruited. Control subjects without LBP were age and gender matched. Subjects in the experimental group had non-specific LBP lasting ≥ 3 months. Non-specific LBP was defined as pain anywhere from the inferior rib margin to the inferior gluteal fold. Patients were excluded if they had a defined etiology, such as radiculopathy, neurogenic claudication, fracture, spinal tumour, and other specific pathology. Each patient underwent a standardized history to rule out the presence of paresthesia, weakness, bowel and bladder dysfunction, lower extremity pain, history of trauma, systemic illness, weight loss and night pain.

The physical examination included deep tendon reflexes and sensory testing to rule out neurological involvement. The investigators additionally performed the straight leg raise (SLR) and Trendelenberg tests, and screened for the presence of groin pain with internal hip rotation. Muscle strength testing (MST) of the gluteus medius (Gmed), TFL and gluteus maximus (Gmax) was also performed (5). Gmed strength testing was performed by placing the patient side-lying with their hip abducted and slightly extended. The patient was instructed to keep the pelvis slightly rotated forward. Resistance was applied to the ankle. TFL and Gmax strength testing were performed in a standard fashion. Strength was scored out of 5.

The examiners palpated the bilateral gluteal musculature, greater trochanters, lumbar paraspinals and piriformis for tenderness. Tenderness was defined as reproduction of the subject’s pain complaint when using enough pressure to blanch the examiner’s finger nail.

Demographic information, weight, height and BMI were compared between both groups using t-tests. Inter-rater reliability of MST, Trendelenberg sign and palpation tenderness were assessed by way of two-way random effects model. MST results between groups and side were assessed using Friedman’s test with post-hoc comparisons. Positive Trendelenberg sign and palpation tenderness were assessed between groups and sides using Cochran’s Q. An receiver operating characteristic (ROC) curve was used to determine the appropriate cut off for muscle strength weakness. A hierarchal linear regression was performed to identify predictors of the presence of LBP in a total sample population. Demographic differences were treated as co-variables.

Study Strengths / Weaknesses:

Strengths:
  • The researchers examined the hip, specifically for the presence of groin pain with end range internal hip rotation. Often, the hip is not assessed in studies pertaining to LBP. It would have been amazing if they took this a step further, and calculated what percentage if LBP patients with gluteal tenderness also suffered from a lack of ipsilateral hip ROM.
  • The statistical analysis was robust.
  • The authors took into account the composite abduction and extension that is performed by Gmed (keep in mind that Cambridge et al. [2015] took extension angle into account when performing an EMG analysis of hip abduction, and proved that extension angle might not matter. This was reviewed on RRS previously).
Weakness:
  • Gmax, Gmed and piriformis are located at different tissue depths. Simply applying enough pressure to blanch the examiner’s finger nail might not allow the examiner to reach sufficient depth for deeper muscles. Also, the examiners did not palpate for tissue tension, which might have improved palpation specificity.
  • The results of this study do not allow the reader to determine whether Gmed weakness is a cause or effect of LBP. From the perspective of LBP treatment, the evidence is uncertain regarding whether simply isolating Gmed and improving its strength and endurance is enough to decrease LBP.
  • The muscle strength testing was performed clinically, and not with a more sensitive instrument, such as dynamometry.

Additional References:

  1. Hoy D, March L, Brooks P et al. The global burden of low back pain: estimates from the Global Burden of Disease 2010 study. Ann Rheum Dis 2014; 73: 968–974.
  2. Nourbakhsh MR &Arab AM. Relationship between mechanical factors and incidence of low back pain. J Orthop Sports Phys Ther 2002; 32: 447–460.
  3. Arab A & Nourbakhsh M. The relationship between hip abductor muscle strength and iliotibial band tightness in individuals with low back pain. Chiropr Osteopat 2010; 18:1.
  4. Kendall KD, Schmidt C & Ferber R. The relationship between hip-abductor strength and the magnitude of pelvic drop in patients with low back pain. J Sport Rehabil 2010; 19: 422–435.
  5. Hislop HJ & Montgomery J. Daniels and Worthingham’s muscle testing: techniques of manual examination. Saunders, Philadelphia. 2002.