The objective of this study is to evaluate occupant posture change during pre‐impact braking and explain the effects of a motorized seatbelt (MSB) on occupant restraint. In order to simulate the pre‐impact condition, low‐speed sled tests on young adult male volunteers were conducted with a vehicle seat, a seatbelt and a foot rest. In this study, two seatbelt systems and two muscle tone states were provided as test conditions. In order to evaluate the potential benefits of the MSB for posture maintenance, the volunteers were restrained by either a current three point seatbelt (SB) or the MSB. In order to examine the effect of muscle exertion on the physical motion, the experiments were conducted in two states: a relaxed state and a tensed state. Compared with the relaxed case, the upper torso motion was constrained by the muscle exertion in the tensed case. In addition, in both relaxed and tensed cases, the MSB worked effectively in restraining the upper body of the volunteers at the accelerating phase. The experiments and the results will provide useful information on occupant kinematics in an emergency situation, which is considered difficult to be measured in the vehicle cabin with high accuracy.