Estimating Lumbar Load During Motion with an Unknown External Load Based on Back Muscle Activity Measured with a Muscle Stiffness Sensor

A forward bending motion is essential in everyday tasks, such as carrying objects, shoveling snow, and performing farm work. However, many people suffer from lumbar pain resulting from forward bending motion, which causes a lumbar disc load owing to the changing of the lumbar shape. We have developed a wearable lumbar load estimation system, which measures the skin shape on the back using a curvature sensor. Because the lumbar load varies with the external load, the lumbar load should be estimated based on the external load. Therefore, we have developed a method for estimating an unknown external force using a muscle stiffness sensor. Muscle strength can be estimated by measuring the muscle hardness from the surface, and the relationship between the external force and the muscle force can be modeled. Using this method, we estimate the dependence of the lumbar load on external forces in real time. In addition, we simplify the calculation by converting the external load into a load resulting from a person’s own weight. We incorporate the proposed method into our wearable sensor system, estimate the lumbar load, and compare this with the results of a musculoskeletal dynamics simulation.