The objective of the current study is to analyze the influence of changes of the physical motion on human body under low-level impact accelerations. Five human volunteers in good health participated in the experiment under the supervision of an ethics committee. Each subject sat on a seat mounted on the sled that glided forward on the rails and simulated actual car motion when the passenger applied the emergency brake. Applied acceleration was around 1.0 g. During the experiments, the physical motion (measured by three-dimensional motion capturing system), acceleration, and EMG signals were recorded. In the relaxed case, muscle responses were observed to start activating at around 100ms after the impact, when the pelvis acceleration was at its peak value due to the interaction between the lumbar spine and the seat. Depending on the location within the body, the reflex time of each muscle is different. Furthermore, the head-neck-torso accelerations were strongly influenced by the muscle activities after impact. These results can contribute in clarifying the relationship between the physical motion and muscle forces with respect to the difference in activation level of muscles. This study was also design to establish an injury prediction approach to verify the influence of human body posture changes just before impact on the occupant injuries in a traffic accident.