Measurement of step length and foot clearance during human walking using an inertial sensor attached to the foot

To establish a supportive technology for reducing the risk of falling in older people, it is essential to clarify gait characteristics in elderly individuals that are possibly linked to risk of falling during actual daily activities. In this study, we developed a system to monitor human gait in an outdoor environment using a wearable sensor unit consisting of a tri-axial accelerometer and tri-axial gyroscope, and a global positioning system (GPS). Step-by-step stride length and foot clearance were estimated from the sensor unit attached to the dorsum of the foot. Specifically, stride length and foot clearance were calculated by integrating the translational acceleration over time during the swing phase. The measured acceleration vector however contains not only the translational acceleration but also the gravitational acceleration, and the latter must be separated and removed for correct calculation of displacement. For this, the change in the spatial orientation of the sensor unit with respect to the global coordinate system was estimated based on the data from the tri-axial gyroscope (angular velocity vector). The gravitational acceleration vector was then subtracted for integration. The GPS data were used to trace how the gait parameters changed by location. Waking of three healthy participants was simultaneously measured using the proposed system and a conventional motion capture system to evaluate the accuracy of the estimated foot trajectory. The results demonstrated that the swing phase trajectory of the sensor unit attached to the foot was successfully reconstructed using the proposed system. This system will serve as a useful tool to monitor human walking during daily activities.