Light curtain for detecting footfall instants during treadmill walking--an exploratory study.

Footswitches, instrumented insoles, and forceplates are commonly used for detecting footfall instants during treadmill walking. This study aimed to explore the feasibility of using an optoelectronic light curtain for detecting initial foot contact (FC) and toe-off (TO) instants during treadmill walking. A commercial light curtain comprised of an array of parallel infrared light beams was installed 5 mm above the treadmill belt. Ten subjects walked on the treadmill at 0.5, 1.0 and 1.4 m/s and their gait was captured using seven optoelectronic cameras. Footswitches were secured over the heel and big toe of the right foot and the corresponding areas of the shoe. Footfall instants from the light curtain, markers, and foot footswitches were compared with the shoe footswitches serving as a criterion. The respective time differences were computed for each step and used to evaluate the agreement with the criterion and variability across subjects. FC instants from foot footswitches were on average within 10ms of the criterion. Both video and light curtain FC instants were detected at least 30 ms before the criterion. For TO, both the foot footswitches and video instants preceded the criterion. In contrast, the TO from the light curtain occurred >110 ms after the criterion but the variability was the smallest. The results indicated that using a light curtain for detecting footfall instants is feasible. The advantages of light curtain are relatively low cost, no subject preparations, and real-time signals. Greater accuracy is expected with further optimization of the setup.