Study investigating the correlation between downward movement conditions of the seat and lap belt movement from the iliac crests of a human occupant in the front passenger seat during frontal impact. Simulations were conducted using an AM50 THUMS human model under two seat conditions: vertical downward movement of 150 mm, and 150 mm vertical downward movement combined with forward rotation. Results show the lap belt detached from both iliac crests during lean forward seat movement, whereas it detached from only one iliac crest during vertical movement. Abdominal injuries can be caused by downward seat movement together with forward rotation during frontal impacts.