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Research Papers

Role of Trunk Muscles in Generating Follower Load in the Lumbar Spine of Neutral Standing Posture

[+] Author and Article Information
Kyungsoo Kim

Institute of Natural Sciences, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Koreakyungsoo@khu.ac.kr

Yoon Hyuk Kim1

School of Advanced Technology, Kyung Hee University, Yongin-si, Gyeonggi-do 446-701, Koreayoonhkim@khu.ac.kr

1

Corresponding author.

J Biomech Eng 130(4), 041005 (May 27, 2008) (7 pages) doi:10.1115/1.2907739 History: Received March 06, 2007; Revised December 17, 2007; Published May 27, 2008

Recently, experimental results have demonstrated that the load carrying capacity of the human spine substantially increases under the follower load condition. Thus, it is essential to prove that a follower load can be generated in vivo by activating the appropriate muscles in order to demonstrate the possibility that the stability of the spinal column could be maintained through a follower load mechanism. The aim of this study was to analyze the coordination of the trunk muscles in order to understand the role of the muscles in generating the follower load. A three-dimensional finite element model of the lumbar spine was developed from T12 to S1 and 117 pairs of trunk muscles (58 pairs of superficial muscles and 59 pairs of deep muscles) were considered. The follower load concept was mathematically represented as an optimization problem. The muscle forces required to generate the follower load were predicted by solving the optimization problem. The corresponding displacements and rotations at all nodes were estimated along with the follower forces, shear forces, and joint moments acting on those nodes. In addition, the muscle forces and the corresponding responses were investigated when the activations of the deep muscles or the superficial muscles were restricted to 75% of the maximum activation, respectively. Significantly larger numbers of deep muscles were involved in the generation of the follower load than the number of superficial muscles, regardless of the restriction on muscle activation. The shear force and the resultant joint moment are more influenced by the change in muscle activation in the superficial muscles. A larger number of deep trunk muscles were activated in order to maintain the spinal posture in the lumbar spine. In addition, the deep muscles have a larger capability to reduce the shear force and the resultant joint moment with respect to the perturbation of the external load or muscle fatigue compared to the superficial muscles.

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Copyright © 2008 by American Society of Mechanical Engineers
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Figures

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Figure 1

The three-dimensional finite element model of a lumbar spine developed from T12 to S1 in a neutral standing posture with 117 pairs of trunk muscles: 59 pairs of deep muscles (12 thoracic multifidus, 20 lumbar multifidus, 6 interspinales, 10 intertransversarii, and 11 rotatores) and 58 pairs of superficial muscles (5 longissimus pars lumborum, 4 iliocostalis pars lumborum, 12 longissimus pars thoracis, 8 iliocostalis pars thoracis, 11 psoas, 5 quadratus lumborum, 6 external oblique, 6 internal oblique, and 1 rectus abdominus). (a) thoracic multifidus; (b) lumbar multifidus; (c) interspinales; (d) intertransversarii; (e) rotators; (f) longissimus pars lumborum and iliocostalis pars lumborum; (g) longissimus pars thoracis; (h) iliocostalis pars thoracis; (i) psoas; (j) quadratus lumborum, external oblique, internal oblique, and rectus abdominus.

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Figure 4

A diagram explaining the modified follower load concept that the compressive load aims for the area near the body center with a half-radius of the body

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Figure 3

The corresponding responses of the lumbar spine model. There was no big difference in the follower forces among all three cases. The changes of the shear forces between the superficial 75% case and the normal case were greater than those between the deep 75% case and the Normal case. The resultant joint moment around the y-axis was biggest in the superficial 75% case. (a) Follower force; (b) shear force; (c) joint moment around y-axis.

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Figure 2

The deformed posture of the lumbar spine model in the sagittal plane when 300N of upper body weight and 3Nm of the resulting flexion moment were applied to T12, and 10N of weight was added to each lumbar vertebra L1–L5. The posture of the spine model was well preserved regardless of the restriction on muscle activation.

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