Unique skeletal muscle design contributes to spine stability

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Unique skeletal muscle design contributes to spine stability

http://www.eurekalert.org/pub_releases/2009-01/uoc--usm010709.php

The novel design of a deep muscle along the spinal column called the

multifidus muscle may in fact be key to spinal support and a healthy

back, according to researchers at the University of California, San

Diego School of Medicine. Their findings about the potentially

important " scaffolding " role of this poorly understood muscle has

been published on line in advance of the January issue of the Journal

of Bone and Joint Surgery.

" The multifidus muscle was formerly thought to be relatively

unimportant based on its fairly small size, " said L. Lieber,

Ph.D., Professor and Vice Chair of UC San Diego's Department of

Orthopedic Surgery and Director of the National Center for Skeletal

Muscle Rehabilitation Research, based at UC San Diego. Lieber is also

Senior Research Career Scientist at the Veterans Affairs San Diego

Health System. " Our research shows that it's actually the strongest

muscle in the back because of its unique design. It's like a long,

skinny pencil packed with millions of tiny fibers. "

The researchers discovered that the multifidus has a unique packing

design consisting of short fibers arranged within rods, and that

these fibers are stiffer than any other in the body. Using laser

diffraction methods that they developed to measure muscle internal

properties during back surgery, they demonstrated that the

multifidus' unique design serves a critical function as a stabilizer

of the lumbar spine. These findings could have implications for

surgery, according to R. Garfin, M.D., Professor and Chair of

UCSD's Department of Orthopaedic Surgery.

" It is important to identify what each individual muscle does, and

this is just a start, showing that the multifidus contributes

significantly to spinal stabilization, " said Garfin. " The more we

know about what muscles do, the better we can devise therapeutic

interventions such as physical therapy to target specific muscles. "

Garfin explained that currently surgery to treat spinal disorders

could actually disrupt the multifidus muscle, which could lead to

decreased stabilization and lower back pain. Minimally invasive spine

surgery techniques strive to minimize surgical trauma to these

muscles in order to best preserve their function.

The lower back, or lumbar spine, can be vulnerable to many pain-

causing injuries or disorders because the lumbar vertebrae carry the

most body weight and are subject to the most force and stress along

the spine. Muscular instability is a risk factor in many injuries and

consequent chronic lower back pain, according to Lieber.

" The multifidus back muscle keeps us vertical and takes pressure off

the discs, " said Lieber. " When muscle function is poor due to back

problems, support is lost. "

He explained that many muscles get weaker as they are extended. But

the researchers discovered that, unlike all other muscles, the

multifidus actually becomes stronger as it lengthens, when the spine

flexes.

" The length of the sarcomere—the structure within the muscle cell

where filaments overlap to produce the movements required for muscle

contraction—is shorter in the multifidus than in any other muscle

cell, " explained study's first author R. Ward, P.T., Ph.D.,

Assistant Professor of Radiology at UC San Diego School of

Medicine. " But as it gets longer, for instance as a person leans

forward, the multifidus actually strengthens. "

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Contributing authors to the study include UCSD researchers Choll W.

Kim, M.D., Ph.D., Carolyn M. Eng, B.S., Lionel J. Gottschalk, B.S.;

and Akhito Tomiya, M.D., Ph.D. Tohoku University School of Medicine,

Sendai, Japan. Research was supported by the Department of Veterans

Affairs Rehabilitation, Research and Development; the National

Institutes of Health and DePuy Spine of Raynham, MA.