Influence of seat backrest and headrest on head-neck vibration responses based on modal analysis: Uniaxial and biaxial excitations

As the carrier of the human perceptual system, the head is directly affected by vibrations transmitted from the seat. However, the biodynamic behaviour of the head-neck under different vibration conditions remains unclear. To systematically investigate the influence of backrest/headrest on the biodynamics of different head-neck regions under uniaxial ( X and Z ) and biaxial ( X + Z ) vibrations using modal and random response analyses, this study developed five 3D biodynamic models of the human-seat system, including rigid/elastic seats with/without a backrest and headrest. The results revealed three primary resonance modes in the X and Z axes for the human-seat system. Compared with the seat stiffness, the backrest/headrest lead to a greater increase in the peak frequency of the head-neck response. The backrest/headrest induced two peak values in the X -direct-axis seat-to-head transmissibility (STHT). Under uniaxial excitation, the backrest significantly reduced the X -direct-axis STHT but amplified other direct- and cross-axis STHTs. The backrest and biaxial excitation jointly affected cross-axis coupling and multimodal collaboration, significantly increasing STHT. Across all five backrest angles and three excitation conditions, peak intervertebral disc (IVD) stress consistently occurred in the C5–C6 segment. Moreover, the IVD stress response was more sensitive to X -axis excitation than to Z -axis excitation. Notably, backrest/headrest supports significantly mitigated this localized stress. In addition, spatial distribution revealed that the STHT amplitude varied significantly among different positions on the head, and was much larger than that on the neck. This study provided new research methods and a theoretical basis for the evaluation of vehicle ride comfort and the optimization of seat under complex vibrations.