OBJECTIVE To confirm the neuromuscular integrity of the rotator cuff and to investigate the effect of different contraction patterns on shoulder function, we sonographically analyzed dynamic contraction patterns of this muscle group. METHODS Fifty supra- and 50 infraspinatus muscle contraction patterns of patients with different shoulder pathologies examined with a 7.5-MHz transducer were recorded and saved as video files. Both data sets were analyzed by 3 independent observers who then assigned a grade to each contraction pattern: normal, slightly disturbed, severely disturbed, or no contraction. Intra- and inter-observer variations were calculated. In 43 patients with isolated supraspinatus tears, the clinical relation to shoulder function as measured with the Constant score was evaluated. RESULTS In the 100 cases tested, the intraclass correlations for the 3 observers were 0.82, 0.88, and 0.88. The inter-observer reliability was 0.74. No pair of first and second readings for either the supraspinatus or infraspinatus muscle differed by more than 1 grade. In patients with a supraspinatus tear, the contraction type of the supraspinatus correlated significantly with the Constant score (p=0.02). CONCLUSIONS When dynamic ultrasonography is used to assess the contraction patterns of the supra- and infraspinatus muscles, good intra- and inter-observer reliability is attained. Because a better contraction type correlates significantly with better shoulder function, this new diagnostic criterion may improve decision-making in the treatment of shoulder diseases.