Multibody modelling of the human body for vibration induced direct and cross-axis seat to head transmissibility

During vertical or fore-and-aft vibrations of a seated human body, the motion of the head is influenced by backrest forces transmitted to the lumbar region of the spine. Thus, it is essential to consider backrest support while developing a human body model to capture direct and cross-axis seat to head transmissibility. For this purpose, twelve degrees of freedom seated human body model of the appropriate level of complexity with inclined backrest support has been developed in this work. The interconnection of masses is modelled using rotational and translational springs-dampers and the contact with an inclined backrest gives a better simulation of forces transmitted to the lower torso in the x-z direction. The model parameters were identified and optimized using a multi-objective genetic algorithm by minimizing the least square difference between experimental head motion and analytical results. Subsequently, a sensitivity study was carried out to find the effect of model parameters on the peak transmissibility modulus and resonant frequency. This will help in refining the design of the seat and backrest for improving comfort. Modal analysis was carried out for an improved understanding of the relative motion of different sections. Power absorbed at different locations of the human body was studied to identify regions susceptible to long term damage. Internal forces in the neck region were also determined and these can be used to characterize the neck pain due to prolonged exposure to such vibrations. The research work reported in this work offers an in-depth understanding of biodynamic responses of backrest supported human body subjected to vertical/horizontal vibrations. We expect it to offer vehicle designers some insights to control human body parameters (acceleration/forces /moments/power absorbed/modal displacements/lower back pain) and design appropriate restraints-supports for improved comfort.