Evolving Base of Support Associated Multicontact Stability Analysis Reveals Distinct Sit-to-Stand Motor Control Pattern in Stroke Survivors

Sit-to-stand (STS) transition is a prerequisite for independent mobility and a primary benchmark for post-stroke functional recovery. However, conventional event-based measures fail to continuously resolve weight transfer, and the classical Extrapolated Center of Mass (XcoM) and Margin of Stability (MoS) framework cannot represent evolving multicontact support, vertical acceleration dynamics, or whole-body angular momentum, thereby obscuring phase-specific motor control strategies. To address this problem, we proposed a continuous biomechanical framework for analyzing multicontact STS dynamics by integrating a time-evolving effective base of support (BoS) with a Vertical-Dynamics-Informed Extrapolated Center of Mass (VDI-XcoM) that incorporates vertical dynamics and an angular-momentum-related correction. Combining these formulations yields the Multicontact Margin of Stability (MMoS). The framework was evaluated in a method agreement study (MAS) and a clinical comparison study (CCS). The MAS showed agreement between the rapidly deployable Kinect-based method and the laboratory reference for VDI-XcoM and effective-BoS trajectories, while MMoS metrics exhibited high within-session repeatability in the CCS. Critically, whereas conventional discrete descriptors and the pre-seat-off minimum MoS failed to differentiate stroke survivors from healthy controls, MMoS successfully isolated a low-stability-margin phase before seat-off in stroke survivors, showing a shallower primary valley and smaller negative area than controls. Effective-BoS analysis demonstrated that the increase in plantar AP support in stroke survivors was insufficient to offset seat support reduction, while VDI-XcoM decomposition identified smaller velocity- and angular-momentum-related changes in the margin between the extrapolated body state and effective BoS before seat-off. Together, these findings characterize incomplete seat-to-foot transfer and reduced pre-seat-off forward-momentum generation and transfer after stroke. By unifying evolving support boundary with body-state dynamics, the framework provides phase-specific biomechanical information for objective post-stroke STS analysis and may support the motor control assessment in rehabilitation treatment.