Criterion Validity of Wrist-Worn IMU-Derived Smoothness Metrics During Functional Upper-Limb Tasks

Wearable inertial measurement units (IMUs) may facilitate quantitative assessment of upper-limb motor function outside laboratory settings; however, the validity of IMU-derived movement smoothness metrics compared with optical motion capture remains unclear. This study aimed to investigate the criterion validity of wrist-worn IMU-derived smoothness metrics during functional upper-limb tasks. Twenty-seven healthy adults (14 females, 13 males) performed two functional tasks, Reaching and Hand-to-mouth, with both upper limbs. Movements were simultaneously recorded using a marker-based motion capture system and two wrist-worn IMUs, both sampling at 100 Hz. Acceleration profiles were obtained from MoCap-derived wrist trajectories and IMU signals, and four smoothness metrics were calculated: Spectral Arc Length (SPARC), normalized root mean square (nRMS), Normalized Jerk (NJ), and Log Dimensionless Jerk (LDLJ). Criterion validity was assessed through Pearson correlation, estimation error (RMSE and MAE), Bland–Altman analysis, and proportional bias assessment. During the Reaching task, IMU-derived metrics showed significant correlations with MoCap values for all indices (r = 0.41–0.74), with NJ showing the strongest association. SPARC showed the smallest mean bias between systems. During the Hand-to-mouth task, significant correlations were observed only for SPARC and NJ (r = 0.39–0.47), with increased discrepancies for nRMS and LDLJ. Wrist-worn IMUs provide comparable smoothness estimates to MoCap during specific upper-limb tasks, but agreement depends on both the selected metric and movement characteristics. These findings highlight the need for task- and metric-specific validation before translating laboratory-based movement quality assessment into wearable settings.