Anatomy and Biomechanics of the Back Muscles in the Lumbar Spine With Reference to Biomechanical Modeling

Lone Hansen, PhD; Mark de Zee, PhD; John Rasmussen, PhD; Thomas B. Andersen, PhD; Christian Wong, PhD; Erik B. Simonsen, PhD


Spine. 2006;31(17):1888-1899. 

In This Article

Psoas Major

The psoas major muscle covers the anterolateral aspects of the lumbar spine and inserts on the femur. According to anatomic textbooks,[53,54] the psoas major is a long fusiform muscle that occurs from the transverse processes of the lumbar vertebrae, the bodies of 2 vertebrae, and their intervertebral disc (from T12 to L5). In addition, it also occurs from a series of tendinous arches extending across the bodies of the lumbar vertebrae, allowing passage of vessels and nerves. It descends along the pelvic brim, passes beneath the ligamentum inguinale, and ends in a tendon, which is attached to the trochanter minor of the femur.

Psoas major is a primary flexor of the hip joint, and when acting from below (both sides), it bends the trunk forward.[51] Its secondary rotational action is controversial. Hooper[55] suggested that the iliopsoas plays no significant part in the rotation of the femur. However, Skyrme et al[56] found lateral rotation of the femur with the hip in abducted position, which was shown by traction of the muscle in cadavers and supported by the findings of Le Floch-Prigent.[57] Because the psoas occurs from the lumbar spine, it implies that it also might have an action on the lumbar spine. Electromyography has shown that the psoas major is active during upright standing,[58] and during bending and lifting,[59,60] which was supported by Yoshio et al,[61] who concluded that the psoas major has an antigravity function.

However, Andersson et al[62] found no postural activity in psoas during standing but, rather, in sitting with a straight back. Bogduk et al[63] studied the effect of psoas major on each segment of the lumbar spine. It was discovered that the psoas had no substantial role as a flexor or extensor of the lumbar spine but, rather, that the psoas major exerted large compression and shear loading on the lumbar joints. They also concluded that the psoas major would not be suited as a lateral flexor of the lumbar spine because of the small moments produced.

In contrast, Farfan,[64] supported by Andersson et al,[62] concluded that the psoas major is ideally placed for lateral flexion of the spine. Gracovetsky[65] suggested that the psoas acts as a controller of the spine during walking and balances the moment of bodyweight in upright relaxed standing.[66] Penning[67] supported the latter and suggested that the psoas major probably functions as a stabilizer of the lumbar lordosis in upright posture. Based on 6 young subjects, the fiber type of the iliopsoas is given by Johnson et al[68]: type I = mean 49.2% (95% confidence interval 39.5-58.5) and type II = mean 50.8% (95% confidence interval 41.2-60.5); Pierrynowski,[69] supported these and reported a 50% distribution.

To define the muscle's lines of action, a further description is necessary. According to Dumas et al,[70,71] the psoas major muscle consists of 10 lines of action based on dissection of one 39-year-old male: 5 going from the intervertebral discs (T12-L1-L4-L5) and 5 from the transverse processes (L1-L5). Bogduk et al[63] presented a very precise and detailed description of the morphology (fascicular and segmental anatomy) based on dissection of 3 male cadavers. They defined the muscle as a series of overlapping segmental fascicles, including 1 more irregular fascicle from the body of L5 (Figure 1, Table 3 ).

M. psoas major. The sites of attachment (shaded areas) and the lines of action of the fascicles of psoas major as seen in anteroposterior projection. Reprinted with permission from Spine 1992;17:897—913.[1]

Stokes and Gardner-Morse[72] also used these 11 fascicles for static analyses of muscles as well as spinal forces. In contrast to anatomic textbooks, Bogduk et al[63] found that the tendinous arches do not constitute a separate site of attachment. The deepest fibers of psoas major could be traced to distinct sites of attachment to either the intervertebral disc or adjacent margin of the vertebral body. To establish the lines of actions for each of the 11 fascicles, Bogduk et al,[63] and Stokes and Gardner-Morse[7] as well as Dumas et al[71] drew lines from the vertebral attachments to a common point immediately anterior to the iliopubic eminence, where the tendon passes over the ilium and curves posteriorly toward the lesser trochanter. In all 3 articles, figures with lines of actions were presented. Data for physiologic cross-sectional area/cross-sectional area, length, moment arms, and maximal moments are presented in Table 3 and Table 4 .

The muscle masses filling the space between the transverse and spinous processes are known as the transversospinal system. It consists of semispinalis (thoracis, cervicis, and capitis), multifidi, and rotatores (thoracis, cervicis, and lumborum). The fascicles all run obliquely upwards and medially from the transverse processes to the spinous processes of the vertebrae. As a collective function, they extend the vertebral column and rotate the trunk to the opposite side.[53] Donisch and Basmajian[78] suggested that the transversospinal muscles adjust small movements between individual vertebrae as a kind of dynamic ligament. The semispinalis muscles are not represented in the lumbar region and, therefore, are not further described here.


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