Anatomy of the Erector Spinae: Review

and O. Gille²

(1)

Orthopedic Medicine, University Hospital, Bordeaux, France

(2)

Spinal Unit, University Hospital, Bordeaux, France

Keywords

AnatomySpineMuscleFunctionLow back painCamptocormia

Introduction

The final decades of the twentieth century have seen a sharp rise in the prevalence of back pain in the western world [1]. From a purely osteoarticular perspective, this may seem paradoxical insofar as, at the same time, working and living conditions seem to have improved [1]. Recent animal and human studies have repeatedly advanced the importance of paraspinal muscle dysfunction in the pathogenesis of low back pain [2–4]. Similarly, the increase in disorders of sagittal balance of the spine, related to idiopathic paravertebral atrophy with increasing age underscores the functional importance of these muscles [5]. These muscles, because of their insertions, are extensors of the spine, that is to say that their concentric contraction tends to induce an extension of the spinal segments with respect to each other. This conception of their action is, not sufficient to account for their protective role of the spine. It also seems a little limited to account for their postural and dynamic actions. Is their action unambiguous? What synergistic action do they have with each other and with other muscles? What mechanisms can induce their dysfunction? What are the postural and dynamic consequences of their dysfunction? Are their lesions irreversible or can we resist the durability of these induced disorders? Technological advances have recently allowed us to conduct work aimed to address all of these issues. They are based on a fine analysis of the trunk movements and the simultaneous recording of the activity of many muscles, as well as on the magnetic resonance (MRI) of contractile components of paravertebral masses.

This is a synthesis of current knowledge based on a literature review. It aims to expose the recent functional anatomy data of the paraspinal muscles and emphasize the clinical implications that may arise.

In the first part, we will study the topographic and functional organization of the paravertebral muscles, including a recall of the descriptive anatomy of the paravertebral muscles and the different functional anatomy models that have been proposed to explain the postural role of lumbar and abdominal muscles and therapeutic prospects they offer. Then we will summarize the functional implication of the erector spinae during walking.

Secondly, we will recall the theories that tend to explain the muscular role in the perpetuation of chronic low back pain.

Descriptive Anatomy of the Paravertebral Gutter

It should be noted that, compared with the joints of the limbs which, when they are in one plane, have a relatively simple muscular control, the organization of the axial muscular system is more complex because of the high degree of freedom associated with stacked vertebrae. Most spinal muscles are housed in the paravertebral gutter, which is between the spinous and transverse processes of the vertebrae. The muscles in the paravertebral gutter are organized in four superimposed planes [6]. To emphasize this organization, we chose to report it, as a whole, at the cervical, thoracic, and lumbosacral level, starting with the deepest plane. They are covered by the muscles of the posterolateral wall of the trunk whose spinal insertions form the thoracolumbar fascia [7].

Plane of the Transverse—Spinous Process Muscles (Deepest)

It extends from the second cervical vertebra to the sacrum. It includes three muscles. Intertransverse muscles (intertransversarii) are small muscles from transverse process to transverse process. They are even, symmetrical, and located laterally all along the spine. Their unilateral concentric contractions provoke homolateral inclination of the spine. The 11 rotatores muscles are small symmetrical muscles that fill the base of the paravertebral gutters inserting on the transverse process to the lamina of the superjacent vertebra. Their concentric contraction induces an extension and a contralateral rotation of the superjacent vertebra. The multifidus muscle is composed of muscle bundles that medially fill the paravertebral gutter, inserting on a transverse process and which send muscular extensions to each of the superjacent lumbar spinous processes. Its contraction, concentric, causes an extension and a contralateral rotation of the vertebrae of termination with respect to the vertebra of insertion.

Plane of the Spinalis and Semispinalis Muscles

It covers the multifidus muscle. It consists of two groups of muscles called spinalis (spinalis dorsi, spinalis cervicis, and spinalis capitis) and semispinalis (semispinalis capitis, semispinalis cervicis, and semispinalis dorsi/thoracis), which span from top to bottom. The spinalis muscles are located nearest to the midline, spanning from top to bottom in the paravertebral gutter and have an action of extension of the spine.

The spinalis cervicis is stretched between the lateral surfaces of the C2–C7 spinous processes. The spinalis dorsi extends between the lateral surfaces of the spinous processes of T1 to L3. The semispinalis muscles are more lateral and extend laterally and obliquely downward. The semispinalis capitis extends from the superior nuchal line of the occiput to the transverse processes of T1–T6. It covers the spinalis cervicis. The semispinalis thoracis is stretched from spinous processes of C2 to T4 to transverse processes from T2 to T11. It is covered at the top by the semispinalis capitis and at the bottom by the spinalis dorsi.

Plane of Longissimus and Iliocostal Muscles

The muscles of this plane (longissimus capitis, longissimus cervicis, longissimus thoracis, and iliocostalis) overlap partially and laterally, over the spinalis and semispinalis muscles. They are extensors of the spine, extending downwards towards the midline. The fibers of the longissimus muscles are the most medial and intertwine from top to bottom. The longissimus capitis muscle extends laterally to the semispinalis cervicis. It inserts itself on the mastoid process and ends on the transverse processes from C3 to T1. The longissimus muscle of the thorax inserts on the transverse processes of the 12 thoracic vertebrae, on the upper edge of the posterior arches of the last eight ribs and unites inferiorly with the iliocostalis muscle to form the lumbosacral mass, largely encompassing the spinous processes of the last 4 lumbar vertebrae, the posterior third of the iliac crests, and posterosuperior iliac spines. The iliocostalis muscle, more lateral than the longissimus muscle, inserts on the transverse processes of the last four cervical vertebrae, arising as flattened tendons from the posterior arch of the last ten ribs. It is then further reconstituted by fascicles laterally inserting on the posterior arches of the last six ribs, culminating in a larger volume. It ends by providing the lateral fibers of the lumbosacral mass.

Plane of the Splenius Muscles

It partially covers the semispinalis muscles . It is limited to the upper cervical and thoracic regions and includes two muscles: splenius capitis and cervicis. The splenius capitis inserts on the mastoid and superior nuchal line and ends on the cervical spinous processes. It is an extensor of the head and ipsilateral rotator. The splenius cervicis wraps laterally and caudally around the splenius capitis. It inserts on the transverse processes from C1 to C3 and ends on the spinous processes of the first five thoracic vertebrae. It is an extensor of the neck.

The thoracolumbar fascia that covers the paravertebral muscles seems particularly interesting to study. The dimensions and nature of this electrically inactive fascia allow electromyographic recording of the activity of the erector spinae muscles (longissimus and iliocostalis) by surface electrodes over almost the entire height of the vertebral column [8]. Moreover, it is perforated by vessels from the paravertebral muscles in relation to a fascial zone that accompanies their neurovascular bundle and separates the multifidus and longissimus muscles [9] and seems to be a natural pathway for the surgical approaches of the lumbar spine (Wiltse) [10].

Anatomical Models (Figs. 1 and 2)

The oldest model of functional anatomy of the spinal muscles is based on the observation of the effect of the concentric contraction of the muscles. The paravertebral muscles (dorsal to the spinal axis) oppose the abdominal (ventral) muscles. Thus, truncal extension movements are related to a contraction of the paravertebral muscles, while the flexion movements are related to the contraction of the abdominal muscles [11]. The vertebral column is then considered as a guyed mast (with radiating cable stays) by the anterior and posterior muscles of the pelvis and whose inclinations respond to alternating shortening of antagonistic muscles (Fig. 1). However, this model did not respond to the alternation of activity of the paravertebral muscles observed during anterior flexion movements of the trunk [12]. Thus, some authors have explored the activity of all the muscles surrounding the spine during flexion-extension of the trunk [12]. The observation of simultaneous contractions of the paravertebral and psoas muscles led these authors to propose the notion of a composite beam that is actively constituted by contractions of the deep muscles that stabilize the lumbar spine by adhering to the vertebrae during trunk movements. The latest model based on the different conditions of stabilization and mobilization of the trunk tends to oppose the action of the central qualified muscles: multifidus (M), internal oblique (IO), transversus abdominis (TA), psoas (P), and rectus abdominis (RA); to the action of so-called peripheral muscles : external oblique (OE), erector spinae (ES) including longissimus (L) and iliocostalis (IC) but also divided into ES pars lumborum (LES) and ES pars thoracis (TES), gluteus maximus (GM) and rectus femoris (RF). In the latter model, the core muscles are considered protective muscles of the spine, while the peripheral muscles are seen as stabilizing and mobilizing muscles (Fig. 2).

../images/451372_1_En_21_Chapter/451372_1_En_21_Fig1_HTML.png — Fig. 1

Traditional anatomical model (modified from Granata and Wilson [11])

../images/451372_1_En_21_Chapter/451372_1_En_21_Fig2_HTML.png — Fig. 2

Conventional anatomical model

In addition to histological arguments clearly differentiating paraspinal deep muscles from the superficial muscles, the construction of the latter model is based primarily on information provided by the analysis of dynamic multi-channel EMG tracings from trunk muscles recorded synchronously with data kinematics and dynamics. Three categories of muscle behavior can be studied by these electromyographic techniques: the study of the muscular synchronization during voluntary movement, the analysis of the reaction times of the spinal muscles reacting to an environmental stimulus, and the measurement of the strength and endurance for each muscle based on the analysis of the frequency of the action potentials and the surface markers. All of these behaviors have been studied in healthy subjects and compared to those observed in patients with low back pain during standardized flexion-extension movements with the pelvis held stationary, free movement, or in response to disturbances of balance.

The study of the synchronization of the spinal muscles during movement and their reaction times in response to a disruption of sagittal balance reveals, in healthy subjects, a stereotyped muscular organization of the abdominal and lumbar muscles. Thus, in subjects without low back pain we find:

during standardized movements, a symmetrical co-contraction of P and MF or RA and MF [11, 13], to protect the lumbar spine during movement,
during free movements, a symmetrical initial and protective co-contraction of MF, OI, and RF, then a mobilizing and independent contractions of ICT, OE, GM [14],
during postural reactions induced by a sudden disturbance of sagittal balance, a co-contraction of MF and TA [15], followed by alternating anti-phase contractions of the agonist and antagonist muscles accompanying stabilization oscillations around the new sagittal balance position [3].

On the other hand, in chronic low back pain , we find a certain desynchronization resulting in:

during standardized movements, by a loss of symmetric co-contraction of MF and RA [16],
during free movements, by a loss of the specificity of peripheral movement-inducing muscles associated with LES hypoactivity compensated by hyperactivity of TES [17],
during the reactions to a disturbance, by a delay of the stabilizer reaction of central muscle [15, 18] and a loss of secondary phase opposition of the agonist and antagonist muscles [3] in favor of a co-contraction of disseminated muscles [19].

The clinical consequence of the desynchronization of abdominal and lumbar muscles can be advanced to explain the clinical lumbar instability defined as the presence of lumbalgic (low back pain) recurrences starting abruptly following an inappropriate movement, which reflects a decrease of muscular vigilance as well as the dysfunction of the corrective programs [20].

The estimation of muscular strength and endurance also reveals changes occurring in low back pain relative to a subject free from spinal pathology. Thus, in those without low back pain we find:

a symmetry of force and EMG activity of RA and MF [17, 21, 22],
EMG activity of extensors proportional to the applied force [22],
a decrease in EMG activity of RA and OE during heavy lifting,
and during endurance exercises, the effort is initially provided by the axial muscles then relayed by the peripheral muscles [23]. Fatigability predominates at the MF at the level of L5 [24].

In contrast, we find in low back pain :

an asymmetry of force and activity for EMG and MF [17, 21, 22],
an initial overuse of OE and underuse of axial muscles [19, 23],
EMG over-activity for extensors for the applied force [22],
an absence of a decrease in EMG activity of RA and OE [19] during heavy lifting,
in endurance, effort is initially provided by the OE muscles then relayed by the TES muscles and fatigue prevails at the thoracolumbar level [17, 23].