Movement smoothness matters: The missing piece in the functional assessment of rotator cuff patients. Insights from a stereophotogrammetry experimental study

Purpose: This study evaluated whether movement smoothness metrics can effectively distinguish between pathological and healthy upper limbs in patients with rotator cuff tears (RCT), and aimed to identify the most reliable metrics across time and frequency domains. It also investigated the task dependence of movement smoothness across different planes of arm elevation. Methods: Thirty-three patients with unilateral RCT performed bilateral synchronous arm elevation tasks in sagittal, scapular, and frontal planes. Movement smoothness was quantified using metrics such as log dimensionless squared jerk (LDSJ), mean arrest period ratio (MAPR) and spectral arc length (SPARC, SPAL), obtained using a 3D Optoelectronic motion capture system. The coefficient of variation (CoV) was used to assess the inter-subject variability. Differences between limbs were tested with the Mann-Whitney U test; task dependency was evaluated with the Wilcoxon signed-rank test. Results: Pathological limbs consistently showed lower smoothness than healthy ones across all planes, with SPAL, SPARC, DSJ and LDSJ significantly differentiating the two sides (p < 0.05). SPARC and MAPR exhibited the lowest inter-subject variability, supporting their validity. Smoothness was also task-dependent, with frontal plane movements being significantly smoother than those in the sagittal or scapular planes (p < 0.001). Conclusion: Smoothness metrics can effectively differentiate pathological from healthy limb movements in patients with RCT, with variability across movement planes. Among the tested metrics, SPARC emerged as the most robust and consistent measure, supporting its use as a biomechanical biomarker for functional assessment and clinical decision-making. Level of evidence: Level II.