Mechanism associated with the effect of backrest inclination on biodynamic responses of the human body sitting on a rigid seat exposed to vertical vibration

Abstract Drivers and passengers in vehicles usually sit with their backs supported with a backrest. Sitting with a backrest in vehicles can affect the biodynamic responses of the human body and sitting comfort. Understanding the effect of back support on the biodynamic responses can assist in improving vehicle ride comfort. This study was designed to identify the mechanism associated with the effect of backrest inclination on the apparent masses measured at the seat pan and the backrest during vertical whole-body vibration based on experiment and modelling. The apparent masses of 14 subjects sitting on a rigid seat with various backrest inclination angles of 0o, 10o, 20o and 30o during exposure to vertical vibration at 0.5 ms−2 r.m.s. between 0.5 and 20 Hz were measured at the seat pan and the backrest. A biodynamic model previously developed by the authors was updated to include the interaction between the back of the human body and the seat backrest to represent the measured responses for each subject . The effect of backrest inclination on the biodynamic response and on the model parameters were identified. It was found that as the backrest inclination increased, the resonance frequency in the vertical in-line apparent mass measured at the seat pan increased, and the resonance frequency in the fore-and-aft cross-axis apparent mass measured at the seat pan decreased, while the resonance frequency in the normal apparent mass measured at the backrest was not changed. Statistical analysis found that with increasing backrest inclination, the vertical stiffness of the soft tissues at the ischial tuberosities, middle thighs and front thighs, as well as the stiffness at the lumbar joint tended to decrease, while the fore-and-aft stiffness and the damping of the soft tissues at the ischial tuberosities tended to increase. Analysis with the model of the median apparent masses of the 14 subjects disclosed the vibration modes of the body containing the fore-and-aft sliding motion of the pelvis and thighs, the vertical motion of the body on the soft tissue beneath the ischial tuberosities and thighs, and the bending motion of the spine. With the vertical backrest, the fore-and-aft and vertical motions of the pelvis and thighs were coupled together. As the backrest inclination increased, this coupling decreased and the coupling between the vertical motion of the pelvis and the bending of the spine increased.