A distributed force method for studying muscle activity in human-seat interaction

Musculoskeletal (MSK) models are widely used in seat-related studies to assess muscle activity, yet external loading conditions are often simplified, limiting the representation of realistic human–seat interactions. This study introduces a distributed force method for integrating realistic human–backrest contact loading into MSK simulations to improve the evaluation of muscle responses during sitting. The proposed approach derives a force field from pressure-sensing mat measurements and enables customizable force application based on selected vertebral levels and force point distributions. The method was validated using a full-body MSK model in OpenSim through a case study of seated isometric contraction across three postures (mid, left, and right) under varying backrest inclination angles and cushion geometries. Three distributed force configurations were applied to assess erector spinae (ES) muscle activation. Simulation results show moderate-to-strong agreement with EMG measurements, and the distributed loading approach reduces the mean absolute error (MAE) by up to 1.27%, with greater improvements observed for more upright postures and backrest cushions with geometric features. Overall, this work demonstrates the feasibility and benefits of incorporating distributed forces into MSK modeling and supports its use for more accurate and comprehensive seat ergonomic assessments.