Bridging clinical and computational assessments: Standardizing kinematic reporting in modified mallet scale movements

Accurate assessment of shoulder kinematics in Modified Mallet Scale (MMS) movements is essential for clinical evaluation and treatment planning in brachial plexus birth injury (BPBI). However, inconsistencies in reporting methods affect the reliability of kinematic data. This study aims to determine the optimal Euler sequence for reporting shoulder kinematics during MMS movements. Kinematic data from the unaffected arms of ten children with BPBI were collected using inertial measurement units and analyzed retrospectively. Three commonly adopted Euler sequences (ZXY, XZY, YXY) were analyzed. The occurrence of discontinuities, including gimbal lock, phase angle discontinuities, was quantified. Additionally, amplitude coherence was evaluated by comparing reported joint angles with clinically expected values. The ZXY sequence was the only method that exhibited gimbal lock, particularly in global abduction (M1), global external rotation (M2), and hand-to-neck (M3). XZY and YXY showed no gimbal lock; however, YXY showed two instances of phase angle discontinuities. Amplitude coherence analysis identified XZY and YXY as most suitable for M1 and M2, YXY for M3 and hand-to-mouth (M5), XZY for hand-to-back (M4), ZXY for hand-to-belly (M6). The method used to report shoulder kinematics has a major impact on the accuracy of MMS movement analysis. Our findings highlight the need for movement-specific, standardized reporting approaches that reduce discontinuities and maintain amplitude coherence. Establishing such standards is essential for enhancing clinical relevance and ensuring meaningful comparisons across studies, particularly in the evaluation of BPBI.