Development of Flexible Sensor for Assisting Device at Stand-up and Sit-down Motion

Abstract

The purpose of this study is to develop a simple and flexible sensor to measure the torque and its change with respect to time, acting at human knee during standing-up from and sitting-down on the chair. The outer diameter of the leg increases when people bend their leg due to muscle power. Therefore, the muscle power, torque applied at the knee can be estimated if the diameter is measured. A thin rubber sheet, on which strain gauges were mounted, was equipped in a conventional wearable supporter for human legs. The sensor equipped supporter is flexible and expanded if the muscle becomes thick. The expansion of the rubber sheet is expected to reflect the nearest muscle stretch. However, every portion of the leg/muscles does not show enough change in its diameter/stretch. The location where the sensor equipped supporter was banded onto the leg was altered place to place from the top of the leg to the end in order to find the most appropriate location for estimating the precise torque applied at the knee. We introduced a new index called MAI (muscle activity index) defined by: MAI=1/(t2-t1)intt1 t2|f(t)|dt.Here, f(f) is the strain history of the rubber sheet while t is time. t1 and t2 are the times when the subject starts and finishes the motion, respectively. The relationship between sensor location and MAI effect was revealed. The standing-up and sitting-down tests were conducted under the condition of over-bending, which was considered to simulate the motion of the elderly with pain. The isometric joint torque in the sagittal plane was calculated by multiplying the isometric force defined as the initial force keeping the subject's posture and the length of moment arm at the knee. The tests result showed that the clear responses in diameter change of the muscle and distinctive MAI were found at several locations of the leg. When the subject moved with over-bending, the MAI calculated from the strain of the rubber sheet on the m. rectus femoris was significantly different from those at other locations. The correspondence was also shown between the knee joint torque and the observed strain, when the sensor was put on the m. rectus femoris. This paper concluded that the flexible strain sensor currently developed was applicable to evaluate the standing-up and sitting- down motions as well as to estimate the torque at the knee.