Modeling of changes in lumbar joint stiffness by pelvic tightening based on physique and pelvic alignment

Abstract

We have been elucidated the mechanism by which pelvic belt reduces lumbar burden. Pelvic tightening increases the lumbar joint stiffness and inhibits the flexion of the lumbar spine in the forward bending. Flexion of the lumbar joint is substituted by the hip joint, which corrects the posture during forward bending and reduces the lumbar burden (lumbar joint torque). The level of the assisting effect of the tightening force has the individual difference, but at present, it is not clear what factors cause it. In this study, we develop a model that represents the assistance effect based on the physique such as height and weight, and the posture and shape of the pelvis and lumbar spine (pelvic alignment).This study developed a regression model that expresses the amount of change in joint stiffness due to tightening force, using the individual physique and pelvic alignment characteristics as input. The joint stiffness was estimated from motion measurement using optical motion capture and obtained from the equation of motion of the link model representing the human body. Pelvic alignment was measured by X-ray images, and five features were obtained: sacral inclination angle (SS angle), pelvic inclination angle (PT angle), inclination angle of the sacrum relative to the pelvis (PI angle), curvature of the lumbar lordosis, and pelvic aperture angle. Among these, the SS angle, PT angle, and PI angle were measured under two conditions: standing and 45 degrees forward bending and obtained a total of eight pelvic alignment features. In addition, height, weight, BMI, and back muscle strength were added as physique characteristics expressing body size and muscle strength, for a total of 12 variable measured. These features were measured on 15 subjects. Among the 12 features, three to four variables were input to the model by excluding those that were strongly correlated with each other and less capable of explaining changes in the target value, joint stiffness.From the proposed model, changes in joint stiffness could be estimated from pelvic alignment and physique characteristics. At large forward bending angles, we found that to obtain a high assistance effect, tilting of the sacrum anteriorly relative to the pelvis and forward tilting of pelvis is necessary. On the other hand, in the mild forward bending, the person who is higher body weight in addition to the forward tilt of the PT and PI angles tend to obtain higher assistive effect.