Whole-Body Compensation Supports Early Functional Hygiene Recovery Despite Persistent Internal Rotation Deficits After Reverse Shoulder Arthroplasty

Reverse total shoulder arthroplasty (rTSA) reliably improves pain and forward elevation but often incompletely restores internal rotation, limiting activities of daily living. Functional hygiene tasks require coordinated motion across the shoulder and axial skeleton. The purpose of this study was to longitudinally quantify changes in shoulder kinematics, axial compensation, and neuromuscular activation during a functional internal-rotation task before and after rTSA. Thirty-five patients (mean age 72.3 &#xb1; 6.4 years) undergoing primary rTSA were prospectively evaluated preoperatively and at 3 and 6 months postoperatively using three-dimensional motion capture and dynamic surface electromyography during a standardized hand-behind-back task. Outcomes included wrist-to-sacrum distance, joint kinematics, kinematic compensation, and neuromuscular activation. Linear mixed-effects models with subject-level random intercepts adjusted for age, sex, and body mass index were used. Shoulder internal rotation range of motion decreased at 3 months (19.94&#xb0; vs 29.63&#xb0;, p < 0.001) and remained reduced at 6 months (22.92&#xb0;, p < 0.01). Task efficiency improved, with time to completion decreasing from 2.72 seconds preoperatively to 2.21 seconds at 3 months and 2.20 seconds at 6 months (both p < 0.01). Functional reach initially worsened (wrist-to-sacrum distance: 21.78 vs 17.73 cm, p < 0.001) but returned to baseline by 6 months (18.43 cm, p = 0.69), remaining inferior to the contralateral limb. Compensatory motion increased, including trunk (10.21&#xb0; vs 6.44&#xb0;) and lumbar rotation (7.12&#xb0; vs 5.22&#xb0;) at 6 months (both p < 0.001). Medial deltoid activation amplitude increased at 6 months (p < 0.001), while prolonged triceps activation at 3 months normalized by 6 months. rTSA improves functional task performance by 6 months despite persistent internal rotation deficits. Early functional recovery was accompanied by increased axial compensation and changes in measured neuromuscular recruitment despite incomplete restoration of shoulder rotational motion. These findings provide a mechanistic basis for discordance between objective motion deficits and patient-reported functional improvement.