A finite element model of trunk with muscles by both active and passive contractions

Prevalence of LBP has led researchers to find out biomechanics of lumbar spine in conjunction with the surrounding muscles and the role of intra-abdominal pressure in different activities. Despite substantial number of experimental studies in this field, limitations of in-vivo experiments have guided researchers to utilize mathematical modeling to obtain more information regarding the spine biomechanics. An important aspect of such spinal modeling is to replicate the muscle behavior which includes nonlinear mechanical properties. In this study, we developed a finite element model of the spine with the surrounding muscles consisting both active and passive behavior of the muscles at the same time. In order to have more realistic muscles, we used the combination of a 1-D nonlinear element and a 2-D linear elastic shell. The shell element can take initial stress, while the 1-D element can take force-displacement relationship. For each muscle, 1-D elements were laid on shell elements in the direction of muscle fibers. In a specific activity level, initial stress and stiffness of shell elements and force-displacement relation of 1-D elements were changed based on mathematical model of muscles published in the literature. The effects of two muscles of the model were investigated on lumbar spine and IAP. These muscles are rectus abdominis and transversus abdominis. Results show that TA has an important effect on IAP generation and this muscle can produce extensor moment due to increasing IAP. RA has flexor role and has no important effect on IAP. Also analyses have predicted a reduction in lumbar spine compression force with increasing IAP.