Modeling of the response of a seated passenger to vibrations and impulsive forces

Lower back disorders are the main chronic condition experienced by professional vehicle operators and occupants. The current international standard for the evaluation of human exposure to mechanical vibrations underestimates high accelerations typically experienced in off-road vehicle rides. Although the existing epidemiological data lack evidence of morbidity patterns, there is strong evidence that impulsive forces increase the risk of back problem and that the approach suggested by the current standard does not adequately account for high acceleration events. Considering the ethical concerns associated with in-vivo experimentation and the complex logistics of long-term epidemiological investigations, modeling remains a primary option.; A biodynamic model of a seated passenger was built and simulated with the aid of ADAMS computer code. Its purpose is to explain the mechanism of maintenance of stable trunk posture during rides over rough terrain and to estimate the loading on the lumbar spine due to the mechanical shocks transmitted through the vehicle seat. The model is anatomically realistic and includes the inertial properties of the upper torso, the compliance of the lumbar spine and can accept linear and rotational acceleration inputs. The muscular response mimics an alert passenger contracting the spinal musculature to avoid collision with adjacent objects and maintain a stable posture. Kinematic profiles from vehicle rides on rough terrain are used to subject the model to realistic conditions. Electromyographic data from such rides serve to validate certain features of the model.; A stable posture can be achieved with a minimum amount of muscular activity. High impulsive rides require a higher level of muscle intervention to counteract the upperbody sway. The more elevated muscular activity increases the compressive load on the lumbar spine. Although it does not reach its mean compressive strength, we are well within the range of material fatigue. Lateral acceleration demands a substantially higher level of co-contraction of antagonist muscles than vertical acceleration. The upper-body appears to be more stable in the transversal than in the longitudinal plane.