Selective Load Control of Lumbar Muscles in Robot-assisted Isometric Lumbar Stabilization Exercise

Abstract Background: Lumbar stabilization exercise was developed as a rehabilitation therapy to improve the condition of patients with low back pain. However, many patients discontinue performing exercises for several reasons. Basically, the main reason is difficult to provide an appropriate intensity of exercise load to the patient. First, there is no way to provide the desired strength of the load on the target lumbar muscle. Also, it is difficult to precisely control the intensity of exercise for a gradual increase in load. Lastly, especially in calisthenics, there are cases where a large load acts on unexpected parts, such as the upper and lower limbs. Therefore, in order to allow the patient to continuously exercise and recover gradually, there is a need for a method capable of exercising with a desired load on the target lumbar muscles. Methods: We propose a rehabilitation assistive device to help the patient quantitatively control the lumbar spine load. In isometric lumbar stabilization exercise, our proposed method is to provide compensating gravity precisely. The proposed device applies assist force in the direction opposite to gravity, with a series elastic actuator placed below the patient in a lying posture, enabling precise load control on the lumbar and reducing vertical load on the spine. To validate the effectiveness, we experimented with 20 healthy subjects on three exercises and analyzed electromyography data using non-parametric statistical methods to determine whether the load could be controlled gradually for the target lumbar muscles. Results: From the statistical analysis with the three exercises, it was verified that for the two exercises with a single degree of freedom, there were significant differences even with a small load change on the target lumbar muscle in the exercise using the device. However, in one exercise with multi degrees of freedom, a load change were different than expected. Conclusions: It was found that the proposed device can fine-tune the load on the target lumbar muscle for exercise with a single degree of freedom. To control the lumbar muscles that were not controlled in this study, dynamic analysis of various exercise postures and additional experiments are needed.