Few studies have systematically investigated the combined effects of anterior-posterior whole-body vibration, backrest inclination, and seat elasticity on lumbar biomechanics using a full-body FE model. Therefore, to comprehensively analyse the impact of seat elasticity, backrest angle, and vibration frequency on lumbar biomechanics under forward-leaning whole-body vibration, this study employed a validated full-body finite element model to create eight 3D human-seat system models featuring two seat elasticity types and four backrest inclination angles. Sinusoidal whole-body vibration simulations from 1 to 12 Hz were then conducted. The results indicate that the resonance frequency of the human-seat system was 2 Hz higher for rigid seats (6–8 Hz) compared to elastic seats (8–9 Hz). Regarding damping performance, rigid seats performed better under low- and medium-frequency vibrations, while elastic seats were superior at higher frequencies. When the excitation frequency was below resonance, increasing the backrest inclination angle reduced the lumbar response. Conversely, when the excitation frequency exceeded the resonance frequency, a greater backrest angle amplified the lumbar response. Furthermore, the resonance frequency increased with greater inclination for both seat types. Significant increases in stress (approximately increasing by 6.6%–26.9%) or acceleration (approximately increasing by 4.6%–16.8%) were observed when the excitation frequency approached resonance. Although the findings suggest rigid seats may offer advantages for short-term and low-frequency exposure, risk factor analysis based on long-term exposure assessment indicates that for professional vehicle occupant regularly exposed to 6–9 Hz whole-body vibration, a seat configuration using elastic foam with a 30° backrest inclination can reduce the peak Mises stress at the L5–S1 disc by approximately 5%–17% and lower the risk factor by 57%–63%, compared to a traditional rigid upright seat with 0° inclination.
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