BACKGROUND: Subacromial pain syndrome is the most common cause of shoulder pain and is associated with altered humeral and scapular kinematics. Symptoms can be improved by rehabilitation. Accurate tools to analyze shoulder kinematic curves are lacking. METHODS: A single-center prospective pilot study using inertial measurement units located on both arms and scapulae to assess bilateral arm elevation in the sagittal, scapular and frontal planes. Reparameterization and signal registration algorithms compared similarity of global shoulder and scapular kinematic curves from participants with subacromial pain syndrome before and after a short rehabilitation program, with a control template combining the curves of asymptomatic participants. A similarity score used curve comparisons
the more closely the curve shapes matched, the closer the score was to zero. We used a paired Wilcoxon test to compare the scores. FINDINGS: We included 9 right-handed symptomatic participants (10 shoulders): 2 males (22 %), mean (SD) age 53.8 (13.7) years, symptom duration 29 (23) months, pain (Numeric Rating Scale) 61.1 (22.4)/100, activity limitation (Quick-Dash): 48.3 (26.6)/100 points, and 10 asymptomatic age-matched right-handed participants (20 shoulders): 4 males (40 %), 54.2 (5.4) years old. Post-rehabilitation similarity scores decreased non-significantly for shoulder elevation (scapular and frontal planes), scapular lateral rotation (sagittal and scapular planes) and anterior-posterior tilt (scapular plane) and significantly for shoulder sagittal elevation (P = 0.004). Participant heterogeneity was high. INTERPRETATION: The similarity methodology, used for the first time in the context of subacromial pain syndrome, offers a new quantitative tool to assess kinematic changes, measure movement-related impairments and monitor patient progress.