Background <p>Two key terms used to describe the resultant shoulder dysfunction are "pseudoparesis" and "pseudoparalysis," which differentiate the degree of motion loss—pseudoparesis involves limited active shoulder elevation to 45°–90°, while pseudoparalysis refers to a more severe restriction, with active shoulder elevation limited to less than 45 degrees.</p> Methods <p>Between 2010 and 2020, 406 patients who underwent shoulder arthroscopy for massive rotator cuff tear were divided into pseudoparesis, pseudoparalysis, and normal-motion groups according to shoulder range of motion. In the pre-operative period, lateral acromion angle, critical shoulder angle, acromiohumeral distance, acromion index, shoulder abduction moment index, and fatty degeneration were then compared between groups on conventional radiographs and magnetic resonance imaging.</p> Results <p>77 patients with pseudoparalysis, 132 patients with pseudoparesis, and 197 patients with normal range of motion were evaluated. Shoulder abduction moment index &lt; 0.77 predicted the presence of pseudoparesis or pseudoparalysis with 89.5% optimism-corrected sensitivity and 92.8% specificity. Bootstrap validation confirmed minimal optimism bias. The absence of pseudoparesis or pseudoparalysis in patients with stage 1 fatty degeneration was statistically significant (<i>p</i> = 0.026).</p> Conclusions <p>The shoulder abduction moment index may serve as a pre-operative screening tool for identifying patients at risk of pseudoparesis or pseudoparalysis in massive rotator cuff tears. In addition, the presence of stage 1 fatty degeneration in the supraspinatus may predict the possibility that active shoulder range of motion is within normal limits.</p> Level of Evidence <p>Level 4, Retrospective Cohort Study.</p>