Architectural analysis and intraoperative measurements demonstrate the unique design of the multifidus muscle for lumbar spine stability

Abstract

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 microm, yielding an average normalized fiber length of 5.66 +/- 0.65 cm, an average physiological cross-sectional area of 23.9 +/- 3.0 cm(2), 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 microm in extension to 2.70 +/- 0.11 microm 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.

Fig. 1

Posterior schematic of the lumbosacral region. In the repeated images (left to right), the shaded areas depict the regions sampled for each segmental level (from T12 to L5).

Fig. 2

Representative preoperative and intraoperative lateral radiographs of the spine, made in the various conditions measured. Note that these conditions were used to determine the physiological conditions under which biopsy specimens were obtained for the in vivo data plotted in Fig. 5. A: Preoperative standing flexion lateral radiograph. B: Intraoperative lateral radiograph. C: Preoperative standing neutral lateral radiograph. D: Preoperative standing extension lateral radiograph.

Fig. 3

Scatterplot showing the relationship between physiological cross-sectional area and fiber length. As physiological cross-sectional area is proportional to muscle force and fiber length is proportional to muscle excursion, this type of plot illustrates the functional design of a muscle. These data illustrate that the multifidus has the largest force-generating capacity in the lumbar spine and is designed for stability. (The data on muscles other than the multifidus were adapted from the article by Delp et al..)

Fig. 4

Bar graphs showing muscle characteristics as a function of segmental level of origin. A: Sarcomere length. B: Fiber length. The dotted line in A indicates optimal sarcomere length in human skeletal muscle. Sarcomere length did not vary among segmental levels, but muscle fibers originating from T12, L4, and L5 were significantly shorter compared with those arising from L1, L2, and L3. †p < 0.05.

Fig. 5

Sarcomere length operating range of the multifidus plotted on the human skeletal muscle sarcomere length-tension curve (black line). Blue circles represent average sarcomere length obtained by means of biopsy with the patient in the prone (n = 8) or lumbar flexion (n = 5) position. These data demonstrate that the multifidus muscle operates on the ascending limb of the length-tension curve and becomes intrinsically stronger as the spine is flexed (arrow). Schematic sarcomeres are shown on the ascending and descending limb to scale, based on the quantification of actin and myosin filament lengths reported previously.