Background Current conservative management of subacromial shoulder impingement (SSI) includes generic strengthening exercises, especially for internal (IR) and external (ER) shoulder rotators. However, there is no evidence that the strength or the ratio of strength between these muscle groups is different between those with SSI (cases) and an asymptomatic population (controls). Objective To identify if isokinetic rotator cuff strength or the ratio of strength is significantly different between cases and controls. Study design Case Control Study. Method Fifty one cases with SSI and 51 asymptomatic controls matched for age, gender, hand dominance and physical activity level completed isokinetic peak torque glenohumeral IR and ER testing. Within the SSI group, 31 dominant limbs were symptomatic and 20 non-dominant limbs were symptomatic. IR and ER were measured separately using continuous reciprocal concentric (con) and eccentric (ecc) contraction cycles at a speed of 60 degrees per second and again at 120 degrees per second. Values of peak torque (PT), relative peak torque (RPT) and ratios were compared using independent t-tests between the SSI and asymptomatic groups. Results Significant strength differences between the two groups were present only when the symptomatic SSI shoulder was the dominant shoulder (con ER PT at 60°/second, ecc ER PT at 120°/second, ecc ER RPT at 120°/second and ecc IR PT at 60°/second and 120°/second). Conclusions Changes in rotator cuff strength in SSI may be related to limb dominance, which may have implications for strengthening regimes. Level of evidence Level 3a.