A Frontal Plane Model of the Lumbar Spine Subjected to a Follower Load: Implications for the Role of Muscles

Abstract

Compression on the lumbar spine is 1000 N for standing and walking and is higher during lifting. Ex vivo experiments show it buckles under a vertical load of 80-100 N. Conversely, the whole lumbar spine can support physiologic compressive loads without large displacements when the load is applied along a follower path that approximates the tangent to the curve of the lumbar spine. This study utilized a two-dimensional beam-column model of the lumbar spine in the frontal plane under gravitational and active muscle loads to address the following question: Can trunk muscle activation cause the path of the internal force resultant to approximate the tangent to the spinal curve and allow the lumbar spine to support compressive loads of physiologic magnitudes? The study identified muscle activation patterns that maintained the lumbar spine model under compressive follower load, resulting in the minimization of internal shear forces and bending moments simultaneously at all lumbar levels. The internal force resultant was compressive, and the lumbar spine model, loaded in compression along the follower load path, supported compressive loads of physiologic magnitudes with minimal change in curvature in the frontal plane. Trunk muscles may coactivate to generate a follower load path and allow the ligamentous lumbar spine to support physiologic compressive loads.

Numerical Analysis on Quantitative Role of Trunk Muscles in Spinal Stabilization

“…The validity of the model developed could be verified by comparing the response of the present model to results obtained in previous studies (18) . In addition, the changes of the response of model and the role of trunk muscles were also investigated when 10% deviation of the directions of joint force from the follower load path was allowed.…”

“…subjected to a follower load using the differential equation theory by Patwardhan et al (18) However, the model in Ref. (18) was restricted in that it assumed that only one muscle was active in each muscle pair at all levels.…”

“…(18) was restricted in that it assumed that only one muscle was active in each muscle pair at all levels. In order to analyze the model with many muscles, such as the lumbar spine model with its complex configurations and large number of muscles, the stiffness analysis method is appropriate.…”

“…Crisco and Panjabi (17) formulated a set of secondorder differential equations and boundary conditions that describe the lumbar spine model, including the trunk muscle activations with the classical beam-column theory, and analyzed the lateral stabilizing potential of multisegmental muscles. Patwardhan et al (18) used an analytic model in Ref. (17) to evaluate the muscle force activation subjected to a follower load in the frontal plane.…”

Effect of prosthesis endplate lordosis angles on L5-S1 kinematics after disc arthroplasty

“…In addition, ex vivo studies are unable to fully replicate the physiologic loads. The follower preload, used to simulate normal loads, was not applied during axial rotation and lateral bending due to limitations of the technique used to apply the follower preload [15,16]. The use of bilateral cables to apply compressive follower preload during lateral bending and axial rotation tests is inappropriate for spine testing.…”