Shoulder manual muscle resistance test cannot fully detect muscle weakness

Abstract

Purpose: The shoulder manual resistance test is one of the common clinical assessments for patients with muscle weakness. However, there have been no studies investigating the threshold for muscle weakness. The purpose of this study was to clarify the threshold for muscle weakness in the shoulder manual muscle resistance test. Methods: Fifty-three patients (37.9 ± 20.6 years old) with either rotator cuff tear (21 patients), superior labrum anterior-to-posterior (SLAP) lesion (7 patients), or Bankart lesion (25 patients) of one shoulder were administered three manual muscle resistance tests (abduction strength, external rotation, and belly press tests). Positive results in these tests were defined as a subjective weakness in the involved shoulder compared to the opposite shoulder. Based on this result, the patients were divided into positive and negative groups. Another observer measured isometric strength using a hand-held dynamometer and calculated the side-to-side ratio. Comparing instrument measurement with manual measurement, the cut-off point, at which we can recognise that there is a side-to-side difference, was calculated by receiver operating characteristic analysis. Results: The cases with less than 60% of the muscle strength in the contralateral shoulder were judged as positive in all examinations, whereas among the cases with 60–90% of muscle strength, there was a mixture of negative and positive determinations. The cut-off point was 78.9% in the abduction strength test, 73.8% in the external rotation test, and 84.0% in the belly press test. Conclusion: The side-to-side difference could be manually detected, when muscle strength was less than 75–85% of that on the contralateral side. This finding suggests that it is necessary to understand the limitation of these manual tests in the case of clinical examinations. Therefore, care must be taken for the shoulder manual muscle resistance test as muscle weakness cannot be fully detected by manual measurement. Level of evidence: Case–control study, Level IV.