The Journal of Bone and Joint Surgery (American). 2009;91:176-185.
doi:10.2106/JBJS.G.01311
© 2009 The Journal of Bone and Joint Surgery, Inc.
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Architectural Analysis and Intraoperative Measurements Demonstrate the Unique Design of the Multifidus Muscle for Lumbar Spine Stability

Samuel R. Ward, PT, PhD1, Choll W. Kim, MD, PhD1, Carolyn M. Eng, BS1, Lionel J. Gottschalk, IV, BS1, Akihito Tomiya, MD, PhD1, Steven R. Garfin, MD1 and Richard L. Lieber, PhD1

1 Departments of Orthopaedic Surgery (S.R.W., C.W.K., C.M.E., L.J.G. IV, A.T., S.R.G., and R.L.L.), Radiology (S.R.W.), and Bioengineering (R.L.L.), V.A. Medical Center and University of California at San Diego, 3350 La Jolla Village Drive, San Diego, CA 92161. E-mail address for R.L. Lieber: rlieber{at}ucsd.edu

Investigation performed at the Departments of Orthopaedic Surgery, Radiology, and Bioengineering, University of California at San Diego and Veterans Administration Medical Centers, San Diego, California

Disclosure: In support of their research for or preparation of this work, one or more of the authors received, in any one year, outside funding or grants in excess of $10,000 from DePuy Spine, the Department of Veterans Affairs Rehabilitation Research and Development, and NIH grants HD048501 and HD050837. Neither they nor a member of their immediate families received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity. No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, division, center, clinical practice, or other charitable or nonprofit organization with which the authors, or a member of their immediate families, are affiliated or associated.

Background: Muscular instability is an important risk factor for lumbar spine injury and chronic low-back pain. Although the lumbar multifidus muscle is considered an important paraspinal muscle, its design features are not completely understood. The purpose of the present study was to determine the architectural properties, in vivo sarcomere length operating range, and passive mechanical properties of the human multifidus muscle. We hypothesized that its architecture would be characterized by short fibers and a large physiological cross-sectional area and that it would operate over a relatively wide range of sarcomere lengths but would have very stiff passive material properties.

Methods: The lumbar spines of eight cadaver specimens were excised en bloc from T12 to the sacrum. Multifidus muscles were isolated from each vertebral level, permitting the architectural measurements of mass, sarcomere length, normalized fiber length, physiological cross-sectional area, and fiber length-to-muscle length ratio. To determine the sarcomere length operating range of the muscle, sarcomere lengths were measured from intraoperative biopsy specimens that were obtained with the spine in the flexed and extended positions. The material properties of single muscle fibers were obtained from passive stress-strain tests of excised biopsy specimens.

Results: The average muscle mass (and standard error) was 146 ± 8.7 g, and the average sarcomere length was 2.27 ± 0.06 µm, yielding an average normalized fiber length of 5.66 ± 0.65 cm, an average physiological cross-sectional area of 23.9 ± 3.0 cm2, and an average fiber length-to-muscle length ratio of 0.21 ± 0.03. Intraoperative sarcomere length measurements revealed that the muscle operates from 1.98 ± 0.15 µm in extension to 2.70 ± 0.11 µm in flexion. Passive mechanical data suggested that the material properties of the muscle are comparable with those of muscles of the arm or leg.

Conclusions: The architectural design (a high cross-sectional area and a low fiber length-to-muscle length ratio) demonstrates that the multifidus muscle is uniquely designed as a stabilizer to produce large forces. Furthermore, multifidus sarcomeres are positioned on the ascending portion of the length-tension curve, allowing the muscle to become stronger as the spine assumes a forward-leaning posture.

Clinical Relevance: These findings demonstrate that the human multifidus muscle is designed to function as a dynamic stabilizer of the lumbar spine.


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