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Assessment of Hip Abductor Muscle Strength. A Validity and Reliability Study
Katharina S. Widler, MSc1; Julia F. Glatthorn, MSc1; Mario Bizzini, MSc1; Franco M. Impellizzeri, MSc1; Urs Munzinger, MD1; Michael Leunig, MD1; Nicola A. Maffiuletti, PhD1
1 Neuromuscular Research Laboratory, Schulthess Clinic, Lengghalde 2, CH-8008 Zurich, Switzerland. E-mail address for N.A. Maffiuletti: nicola.maffiuletti@kws.ch
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Disclosure: The authors did not receive any outside funding or grants in support of their research for or preparation of this work. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.
Investigation performed at the Neuromuscular Research Laboratory, Schulthess Clinic, Zurich, Switzerland

The Journal of Bone and Joint Surgery, Inc.
J Bone Joint Surg Am, 2009 Nov 01;91(11):2666-2672. doi: 10.2106/JBJS.H.01119
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Background: Hip abductors are the most important muscles around the hip joint. It is therefore essential to assess their function in a valid and reliable way. Since the optimal body posture for the assessment of hip abductor strength is unknown, we tested the validity and reliability of unilateral hip abductor strength assessment in three different body positions. We hypothesized that the validity would be better in the side-lying position because of the consistent stabilization of the contralateral (untested) hip.

Methods: Sixteen healthy subjects participated in two identical testing sessions. Unilateral isometric hip abductor muscle strength was measured, with use of a stabilized commercial dynamometer, with the subject in the side-lying, supine, and standing positions. Construct validity was based on concomitant recordings of gluteus medius electromyographic activity from the tested and contralateral hips. The body position permitting greater muscle activation and abductor strength on the tested hip, while minimizing muscle activation in the contralateral hip (that is, lower contralateral-to-tested electromyographic ratio), was considered the most valid. Coefficients of variation, the Bland and Altman limits of agreement, and intraclass correlation coefficients were calculated to determine test-retest reliability of hip abductor strength.

Results: Maximal hip abductor strength was significantly higher in the side-lying position compared with the standing and supine positions (p < 0.05). The contralateral-to-tested electromyographic ratio for the side-lying position was significantly lower than that for the supine and the standing position (p < 0.01). Test-retest reliability of strength measurements in terms of coefficients of variation (3.7% for side-lying, 6.1% for supine, and 4.2% for standing) and limits of agreement (±6.9% for side-lying, ±8.4% for supine, and ±7.5% for standing) was better in the side-lying position. All intraclass correlation coefficients were high to moderate (0.90 for side-lying, 0.83 for supine, and 0.88 for standing).

Conclusions: The side-lying body position offers the most valid and reliable assessment of unilateral hip abductor strength.

Clinical Relevance: We recommend the use of the side-lying body position whenever hip abductor function is assessed.

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    These activities have been planned and implemented in accordance with the Essential Areas and policies of the Accreditation Council for Continuing Medical Education (ACCME) through the joint sponsorship of the American Academy of Orthopaedic Surgeons and The Journal of Bone and Joint Surgery, Inc. The American Academy of Orthopaedic Surgeons is accredited by the ACCME to provide continuing medical education for physicians.
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