2019 年 14 巻 1 号 p. 18-00510
This study focuses on observing and analyzing the occurrence of a fall for humans during normal gait, which perhaps results in severe injury. Previously, the insufficiency of a recovery step, which diminishes the forward rotation of the body after tripping, was suggested as a factor for the failure of fall avoidance motion. However, although it was identified that a slow and/or short recovery step resulted in the failure of fall prevention, the physical process of tripping and falling during the gait was not analyzed sufficiently. In this study, the subject’s reaction motion and fall process against tripping became clearer because the fall motion, which included the phase closer to the ground contact, could be recorded for a longer time than that in most previous studies. Although the subject attempted to mitigate the forward angular momentum and the descent of body induced by tripping by holding on his recovery foot, it was not effective, especially when the recovery step length was short. Among such trials, the larger forward inclination of the body, the excessive forward movement of the center of mass from the support point, and the smaller ground reaction force of the recovery foot in the normal direction was observed. Because the short recovery step resulted in the limitation of the moment arm, which affected the reaction torque of the recovery foot, it became difficult to decelerate the body rotation and prevent the fall. Furthermore, it was also suggested that the forward lean of the body and the decrease of the ground reaction force increased the impact speed and effective mass, which affected the impact force. The observation of such an advanced fall phase also contributes to a more realistic simulation of the human reaction motion for a more precise estimation of the fall injuries.