A Geometry–Energy–Trend Fusion Framework for Frame-Wise Human Dynamic Stability Assessment

Dynamic stability during human movement reflects interactions among body configuration, support conditions, mechanical state, and short-term state evolution. This study proposes a frame-wise stability score, S(t), integrating three dimensionless instability components: a height-adaptive extrapolated center of mass (XCoM)–base of support (BoS) geometric term, mechanical-energy deviation, and a Lyapunov-type state-trend term. The framework was evaluated using 24 HuMoD walking, running, kicking, and jumping trials from two participants, 26 HuMoD trials for sensitivity analysis, and 73 GAITEX trials. Task means followed the predefined biomechanical ranking (Spearman and Kendall = 1.000), which served as a consistency reference rather than an independent ground truth. Trial-level associations with margin of stability (MoS) and signed CoM–BoS distance were weak, whereas S(t) showed a negative association with center-of-pressure (COP) velocity (r = −0.667); COP velocity showed greater task discrimination (effect size 0.814 vs. 0.350). Ablation analysis showed the largest overall numerical effect for geometry, while trend contributed up to 28.77% of total instability locally. Tested parameter perturbations retained similar qualitative response patterns. These results support S(t) as an interpretable framework for organizing support geometry, mechanical-state deviation, and short-term state evolution on a common frame-wise scale; clinical and predictive validity require independent validation.