Identifying rotator cuff disorders through shoulder movement quality: An exploratory study using a single inertial measurement unit.

Abstract

To investigate differences in shoulder movement quality between rotator cuff disorders and other shoulder disorders (e.g., frozen shoulder and long head of the biceps tendinitis) by quantifying movement smoothness employing Spectral Arc Length (SPARC) derived from a single wrist-mounted Inertial Measurement Unit (IMU), exploring its potential for objective disorder identification. In this exploratory cross-sectional study, thirty older adults with shoulder disorders were classified by physical therapists into rotator cuff disorder or other shoulder disorder groups. Angular velocity during active shoulder flexion and abduction was recorded via a single wrist-mounted IMU. Movement smoothness was quantified using SPARC and compared between groups alongside pain intensity (Numerical Rating Scale; NRS). The rotator cuff disorder group demonstrated significantly lower SPARC values during abduction; numerically lower values were observed during flexion, although the difference was not statistically significant. For both movements, the Areas Under the Curve (AUCs) were 0.70-0.74, with specificities of 0.78-0.89 and Kappa coefficients of 0.32-0.38. Conversely, no significant intergroup differences in NRS scores were detected within the statistical power of this study, suggesting that movement smoothness captures movement quality associated with shoulder function impairments not reflected in subjective pain measures. Movement smoothness quantified by SPARC from a single IMU demonstrated exploratory potential for distinguishing differences in movement smoothness as a biomechanical indicator of functional quality between rotator cuff and other shoulder disorders. This simple, low-cost, objective approach may provide a complementary tool for assessing shoulder function in clinical practice, home-based evaluation, and telerehabilitation settings.