To measure the effect of scapular protraction on isometric shoulder rotation strength, 20 normal subjects completed 2 maximal isometric internal and external rotation contractions in 2 scapular positions (scapula neutral [SN] and scapula protracted [SP]) from 3 arm positions (90 degrees internal rotation [IR], 45 degrees internal rotation [MR], and 90 degrees external rotation [ER]). Scapular protraction reduced shoulder rotation strength in 5 of 6 test positions (P < .0004), with significant interactions between scapular position and arm position (P < .001) and between scapular position and contraction type (P < .0001). Protraction significantly reduced IR strength by 13% to 24% relative to SN. The effect of SP on ER strength was more position-dependent, increasing strength by 6% in the IR position and decreasing it by 7% in the MR position and 20% in the ER position. In conclusion, acute changes in scapular position affect shoulder isometric IR and ER strength. The potential adverse effects of scapular protraction on shoulder rotation strength should be considered during the evaluation and treatment of shoulder pain.