10 healthy male volunteers (20 shoulders) were randomized to receive a subacromial injection of lidocaine in one shoulder and an injection of 0.9% saline in the contralateral shoulder, to determine if the injection of a local anaesthetic could effect shoulder strength and possibly lead to a falsified clinical assessments of rotator cuff tears. 45 minutes after the injection, there was no significant difference between the two injections for pain. There was a significant decrease in arm elevation strength during the Whipple test for the placebo shoulders but not for the lidocaine shoulders. The two groups also did not differ significantly for abduction strength, external rotation strength, distal latency, the compound motor action potential amplitude, and the motor unit number estimate.