BACKGROUND: Subacromial injection of a local anesthetic is used to eliminate pain as a confounding factor in clinical assessment of abduction strength in shoulders with a suspected rotator cuff tear. If strength remains diminished despite pain relief, a rotator cuff tear is likely. The effect of injecting local anesthetic into the subacromial space on the strength of a normal shoulder is unknown, although it could affect strength by impairing suprascapular or axillary nerve function. We hypothesized that subacromial injection of a local anesthetic could decrease shoulder abduction and/or external rotation strength, resulting in physical examination findings that could mislead the clinician. METHODS: A double-blinded, randomized, placebo-controlled design was used to evaluate the effect of subacromial injection of lidocaine on shoulder strength in ten healthy male volunteers. The contralateral shoulder served as the placebo control for each treated shoulder. Abduction and external rotation strength measurements and electromyographic assessment were performed before and after the subacromial injection. Ultrasonography was used to verify the integrity of the rotator cuff and to document the distribution pattern of the injected local anesthetic. RESULTS: The injection was subacromial in eighteen (90%) of twenty shoulders. There was no significant difference in pain or electromyographic parameters between shoulders injected with lidocaine and those injected with 0.9% saline solution (p > 0.05). In the Whipple position, placebo injection into the subacromial space decreased strength significantly compared with the pre-injection state (95 ± 17 to 84 ± 20 N, p = 0.012), whereas a similar decrease observed in the lidocaine group did not reach significance (97 ± 15 to 87 ± 14 N, p = 0.092). In 90° of abduction in the scapular plane (supraspinatus test position), there was no significant decrease in strength in either group. CONCLUSIONS: Subacromial injection reached the subacromial bursa in most cases (90%) without radiographic guidance. The injection of a local anesthetic into the subacromial bursa had no relevant effect on shoulder strength and did not falsify the clinical assessment of strength. LEVEL OF EVIDENCE: Diagnostic Level I. See Instructions for Authors for a complete description of levels of evidence.