Muscle Atrophy and Changes in Spinal Morphology

(From the September 2010 “G”Note – By Steven T. Tanaka, DC)

Bevaly DL, Armbrecht G, Richardson CA, et al. Muscle Atrophy and Changes in Spinal Morphology: Is the Lumbar Spine Vulnerable After Prolonged Bed-Rest? Spine 2010; 35:e-pre-publication.

ABSTRACT: Study design: Prospective longitudinal study. Objective: To evaluate the effect of bed-rest on the lumbar musculature and soft-tissues. Summary of Background Data: Earlier work has suggested that the risk of low back injury is higher after overnight bed-rest or spaceflight. Changes in spinal morphology and atrophy in musculature important in stabilizing the spine could be responsible for this, but there are limited data on how the lumbar musculature and vertebral structures are affected during bed-rest. Methods: Nine male subjects underwent 60-days head-down tilt bed-rest as part of the second Berlin Bed-Rest Study. Disc volume, intervertebral spinal length, intervertebral lordosis angle, and disc height were measured on sagittal plane magnetic resonance images. Axial magnetic resonance images were used to measure cross- sectional areas (CSAs) of the multifidus (MF), erector spinae, quadratus lumborum, and psoas from L1 to L5. Subjects completed low back pain (LBP) questionnaires for the first 7-days after bed-rest. Results: Increases in disc volume, spinal length (greatest at lower lumbar spine), loss of the lower lumbar lordosis, and move to a more lordotic position at the upper lumbar spine (P 0.0097) were seen. The CSAs of all muscles changed (P 0.002), with the rate of atrophy greatest at L4 and L5 in MF (P 0.002) and at L1 and L2 in the erector spinae (P 0.0006). Atrophy of the quadratus lumborum was consistent throughout the muscle (P < 0.15), but CSA of psoas muscle increased (P < 0.0001). Subjects who reported LBP after bed-rest showed, before reambulation, greater increases in posterior disc height, and greater losses of MF CSA at L4 and L5 than subjects who did not report pain (all P 0.085). Conclusion: These results provide evidence that changes in the lumbar discs during bed-rest and selective atrophy of the MF muscle may be important factors in the occurrence of LBP after prolonged bed-rest.

COMMENTS: The introduction states that astronauts, after time in space, have a higher incidence of intervertebral disc herniation than the general and the Army aviation population. Bed-rest, even overnight, increases the risk of low back injury upon arising. Bed-rest and space flight increases the size of the disc due to increased disc hydration. There is also lengthening of the spine and loss of the lumbar curve (lordosis). Long-term bed-rest causes atrophy of spinal musculature, particularly extensor muscles. Atrophy is greatest on the multifidus muscles. Since multifidus muscles stabilize the lumbar segments, control the lumbar curve, and withstand compression loading on the spine, these are vital muscles. They provide 60% to 80% of active stiffness of the spine and have vital control of lumbar movement. The motor control of the lumbopelvic musculature is also affected.

This study confirmed many of the previous findings. They found that the multifidus muscles atrophied much more and faster in the lower lumbar spine compared to the other levels of the lumbars. The lumbar erector spinae showed more atrophy in the upper lumbar spine. The quadratus lumborum appears to atrophy symmetrically. The psoas muscles appear to increase in size, particularly at the upper lumbar levels. There was a loss of the lumbar curve (measured in the supine position) and increases in posterior disc height compared to anterior disc height, particularly in the lower lumbar spine.

The increased disc height would put more stress on the anular fibers, reduce stiffness of the disc, and increase intersegmental movement. This on top of atrophied muscles.

Those who had lower back pain following bed-rest had greater posterior disc height and very significant atrophy of the multifidus muscles at the L4 and L5 levels.