Evaluation of Sternocleidomastoid Muscle Activity of a Passenger in Response to a Car's Lateral Acceleration While Slalom Driving

Human factors are becoming one of the most important factors that are considered for automobile design and test. However, the ride comfort of a passenger or driver is mainly dependent on a subjective assessment by a test driver or questionnaire investigation, and, therefore, a quantitative evaluation of the ride comfort is being pursued as one of the research goals in the automobile industry. In this paper, actual-vehicle and driving simulator (DS) experiments were carried out to evaluate the sternocleidomastoid (SCM) muscle activity of a passenger in response to a car's lateral acceleration while slalom driving. Interestingly, the SCM muscle of the passenger on the side opposite the direction of the car's lateral acceleration contracts to keep the head stable against the body shaking. The electromyography (EMG) signal of the SCM muscle in a modified car was significantly lower than in a normal car, because the 1-10 Hz low-frequency vibrations of the body frame of the modified car during the slalom driving were decreased through improvements of the rigidity of car's body frame. A passenger feels more discomfort when the EMG signal of the SCM muscle increases, and less as the signal decreases. The DS experiment, with the addition of more experimental conditions, arrived at the same conclusion, which testified that the DS is a powerful tool with which to evaluate passenger discomfort. In conclusion, the EMG of the SCM muscle can be considered as an objective and effective method with which to quantify the effect of vehicle properties on human discomfort in both actual-vehicle and DS experiments for slalom driving.