Large-reclined seating has emerged as a favored configuration in the luxury transport sector. While the static advantages are evident, the effect of this posture on dynamic comfort is not clear. This study investigated the objective biodynamics and subjective discomfort of the human body sitting in a large-reclined posture (58° from the vertical) under single axis vertical and lateral vibration excitations. The transmissibility of the human–seat system and apparent mass of the human body were measured respectively. The results revealed a critical transition between static and dynamic comfort: while the 58° posture offers superior static relaxation, dynamic discomfort dominates the overall perception when the excitation intensity exceeds a threshold of 0.249 m/s² r.m.s. Objective measurements indicated that dynamic comfort degradation in large-reclined postures is primarily driven by altered inherent biodynamic characteristics. These findings highlight that future luxury vehicle seating must incorporate targeted dynamic isolation to compensate for posture-induced comfort degradation and ensure premium ride quality.

Despite the presence of multi-axis vibration in many vehicles, biodynamic responses of the human body are mostly studied under single-axis conditions. In this study, the fore-aft, lateral, and vertical apparent masses were measured under tri-axis translational excitation with various in-line (“primary-axis”) and cross-axis (“secondary-axes”) magnitudes and backrest inclinations (0°, 10°, and 20°). The softening effect of increasing the primary-axis magnitude was reduced by higher secondary-axes magnitudes, and the effect of secondary-axes magnitudes was similarly attenuated by higher primary-axis magnitudes. With tri-axis vibration, the fore-aft peak modulus at the seat pan first fell, then rose with the increasing backrest inclination as under single-axis conditions, whereas the resonance frequency lost the non-monotonic pattern. Backrest effects on fore-aft and lateral apparent masses were insignificant with single-axis vibration but became significant with tri-axis vibration. These findings suggest that incorporating multi-axis vibration and backrest inclination may improve the accuracy of ride comfort evaluation for realistic driving conditions.