Background: This study aimed to clarify the muscle activation patterns of the deltoid and periscapular muscles in the active elevation of the upper extremity in massive rotator cuff tear (MRCT) patients with nonpseudoparalysis and to help prescribe specific physical therapy for patients with pseudoparalysis for whom surgery is not indicated. Methods: The MRCT group included 31 shoulders of 27 patients (10 men and 17 women: age 74.9 ± 5.0 years) with active shoulder flexion ≥90°. The control group included 21 men and 8 women, aged 77.1 ± 5.4 years, without evidence of rotator cuff injuries on ultrasonography. The muscle activation ratio (R-muscle value) was calculated from the surface electromyography signal after root mean square processing for signal smoothing. The R-muscle value was calculated for the following muscles (anterior, middle, and posterior deltoid; upper, middle, and lower trapezius; and lower serratus anterior [SA]) over 3 shoulder flexion intervals (0°-30°; 30°-60°; and 60°-90°). Between-group differences were evaluated using analysis of variance, with Bonferroni adjustment for multiple comparisons. Results: R-muscle values were greater in the MRCT group than in the control group for all 3 components of the deltoid muscle for the 0°-30° interval, as well as for the posterior deltoid, lower SA, and upper and middle trapezius for the 30°-60° interval. Conclusion: The findings support rehabilitation targeting activation of the deltoid, lower SA, and upper and middle trapezius muscles at different angles through the range of shoulder flexion to facilitate the transition of patients with MRCT from pseudoparalysis to nonpseudoparalysis status.