Criterion Validity of Manual Muscle Testing in Detecting Shoulder External Rotator Muscle Strength

Background: Manual muscle testing (MMT) is a key component of the physical examination of patients with shoulder disorders. Previous literature suggests the presence of shoulder external rotator muscle weakness on MMT is highly indicative of infraspinatus tendon pathology, however the absence of such weakness cannot confidently rule this out. Although the diagnostic value of MMT in determining infraspinatus tendon pathology seems directly related to its accuracy in detecting true strength deficits, the latter has yet to be established.

Methods: Forty-three consecutive patients (12 females) referred to an outpatient shoulder clinic with a unilateral shoulder complaint were recruited for this study. An orthopaedic surgeon performed bilateral and simultaneous assessment of shoulder external rotator muscle strength. Strength was rated as normal when no side-to-side difference was noted between the involved and uninvolved side, or diminished, when the involved side was felt to be weaker than the uninvolved side. A physical therapist performed bilateral testing of shoulder external rotator muscle strength with the use of a hand-held dynamometer (HHD). Strength was considered deficient when the involved side displayed a deficit of ≥10% compared with the uninvolved side.

Results: Based on MMT, 18 patients were rated as weak, while 25 patients were rated with normal strength. Based on testing with a HHD, 25 patients displayed a strength deficit of ≥10% on the involved side. The sensitivity and specificity of MMT in detecting shoulder external rotator muscle weakness was 60.0% and 83.3%, respectively.

Conclusions: Manual muscle testing seems more accurate in detecting shoulder external rotator muscle weakness than in ascertaining its absence. Our findings help explain the greater accuracy of MMT in ruling in infraspinatus pathology than in ruling it out. Finally, in the absence of weakness on MMT of the shoulder external rotators, testing may need to be augmented by a HHD.