Anatomy and Mechanics of the Abdominal Muscles

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CHAPTER 4 Anatomy and Mechanics of the Abdominal Muscles

Stephen H.M. Brown, PhD, Samuel R. Ward, PT, PhD, Richard L. Lieber, PhD

Relatively little is known about the mechanics of the abdominal muscles. It is clear, however, that abdominal wall muscles are morphologically unique and are responsible for an array of mechanical roles, including the production and control of spine movement, stabilization of the spinal column, generation of intra-abdominal pressure (IAP), and respiration. The four muscles of the abdominal wall consist of three broad, sheetlike muscles that overlay one another (external oblique, internal oblique, transverse abdominis) and the more anterior rectus abdominis, across which the sheetlike muscles are linked (Fig. 4–1). This chapter discusses abdominal muscular anatomy, characteristics and considerations that affect the force-generating and moment-generating capabilities of the muscles, novel data regarding the architectural properties of the muscles, mechanical roles and consequences of the muscles, and finally possible relationships between abdominal muscle function and low back pain and injury.

FIGURE 4–1 A, Cadaveric dissection of abdominal wall from anterior view. Part of inferior external oblique aponeurosis has been cut away to uncover internal oblique deep to it. B, Cadaveric dissection of abdominal wall from anterolateral view. Rectus abdominis was cut and external oblique folded back to uncover deep internal oblique and transverse abdominis. Note dissimilar fiber orientations in different muscles.

Gross Morphologic Anatomy

Rectus Abdominis

The rectus abdominis runs longitudinally down the anterior trunk and is divided into two separate muscles (right and left) by the linea alba. The muscle originates from the lower sternum and costal cartilage of the fifth to seventh ribs,¹ and it inserts into the pubic symphysis (Fig. 4–2A). It is divided transversely along its length by tendinous intersections (normally three) that separate the muscle into four regions (in-series with one another) of muscle fibers. These intersections often do not span the complete mediolateral distance across the muscle, with some fibers therefore extending extra length.^2,³

FIGURE 4–2 A-D, Schematic representations of muscle fiber lines of action for rectus abdominis (A), external oblique (B), internal oblique (C), and transverse abdominis (D).

The mechanical role of the septa of the tendinous intersections is unclear, but two main hypotheses have been proposed. The first purports that these tendons provide bending locations within the muscle, allowing it to fold effectively as the trunk flexes forward, preventing muscle fiber bunching that could occur with extreme shortening of long fibers.⁴ The second hypothesis pertains to the transverse mechanical strength of the rectus abdominis; the oblique and transverse abdominis muscles can apply substantial forces transversely across or, via anchoring on the rectus sheath, through the rectus abdominis.⁵ The tendinous intersections may provide transverse strength to rectus abdominis fibers, preventing them being pulled apart by the forces transmitted by the external oblique, internal oblique, and transverse abdominis.

The external oblique is a large sheetlike muscle, the most superficial of the abdominal wall, with fibers spanning from the rib cage (5th to 12th ribs),¹ running inferomedially, and attaching across two main anatomic regions: the rectus sheath (covering the rectus abdominis) and the iliac crest (Fig. 4–2B). Muscle fiber orientations have been described for the external oblique, internal oblique, and transverse abdominis relative to a line connecting left and right anterior superior iliac spines (ASIS)⁶ and are described here as such. External oblique fibers originating superior to the base of the rib cage are oriented approximately 50 degrees (standard deviation [SD] 7 degrees) inferomedially⁶ and terminate to form the most superficial layers of the rectus sheath. Fibers originating between the base of the rib cage and iliac crest run approximately 59 degrees (SD 11 degrees) inferomedially (slightly more vertically).⁶ The most inferior fibers originating from the rib cage that do not terminate on the iliac crest become aponeurotic superior to the ASIS,⁶ form the lower-most superficial portions of the rectus sheath, and insert into the pubic symphysis. Some authors report that a small proportion of posterior fibers run from the mid-posterior of the iliac crest and terminate as part of the middle⁷ and posterior⁸ layers of the lumbar fascia, although this is not a universal finding.

Internal Oblique

The internal oblique is a large sheetlike muscle that lies deep to the external oblique and superficial to the transverse abdominis. Within different regions of the muscle, its fibers run at angles highly oblique to one another, creating a fanlike appearance (Fig. 4–2C). Fibers originating from the base of the rib cage run from the costal margins of the 10th to 12th ribs¹ to the iliac crest at an angle of approximately 48 degrees (SD 13 degrees) superomedially.⁶ These fibers terminate across the rectus abdominis to create deep and superficial layers of the rectus sheath. Fibers spanning the region between the base of the rib cage and the iliac crest run at an angle of approximately 35 degrees (SD 10 degrees) superomedially.⁶

All of the fibers in this region attach to the iliac crest but can terminate at one of three anatomic locations: (1) medial fibers terminating as deep and superficial layers of the rectus sheath; (2) more lateral fibers terminating on the costal margin of the base of the rib cage; (3) and the most posterior fibers terminating to form a fascial layer deep to the erector spinae (middle layer of lumbar fascia), which ends at the transverse processes,⁷^–⁹ with some fibers adjoining a superficial fascial layer overlying the erector spinae (posterior layer of lumbar fascia), which attaches to the spinous processes (Fig. 4–3).^8,⁹ In the final functional region of the abdominal wall, fibers originate from the anterior of the iliac crest, terminate as deep and superficial layers of the rectus sheath, and are oriented at completely different angles to the more superior fibers. The fibers originating between the iliac crest and the ASIS run approximately horizontally and become continually more inferomedially oriented below the ASIS (up to an angle of approximately 16 degrees [SD 10 degrees] inferomedially).⁶

FIGURE 4–3 Transverse cross section of abdomen at mid–lumbar spine level. Internal oblique, external oblique, and transverse abdominis come together at anterior of abdominal wall to form a common aponeurosis, which continues to form the rectus sheath surrounding rectus abdominis. At posterior of abdominal wall, internal oblique and transverse abdominis terminate into middle (MLF) and posterior (PLF) layers of lumbar fascia.

Transverse Abdominis

The transverse abdominis is the deepest of the sheetlike abdominal wall muscles. Its fibers above the base of the rib cage originate from the costal margins of the ribs and terminate as the deepest layers of the rectus sheath (Fig. 4–2D). These fibers run almost horizontally, with only a slight inferomedial orientation of 3 degrees (SD 9 degrees).⁶ The fibers between the base of the rib cage and iliac crest run from the posterior fascia (terminating as the same fascial layers as the internal oblique [discussed earlier; see Fig. 4–3]) to the rectus sheath and are oriented at approximately 13 degrees inferomedially. Fibers originating from the anterior iliac crest and ASIS run approximately 21 degrees (SD 11 degrees) inferomedially and terminate again as the deepest slips of the rectus sheath.⁶

Rectus Sheath

Each of the abdominal wall muscles attaches at the anterior margin of the torso through an aponeurosis that ultimately leads to the formation of the rectus sheath (Fig. 4–4). Detailed investigations of the morphology of the abdominal wall aponeuroses reveal a bilayered arrangement stemming from each muscle. Specifically, the aponeurosis of each of the external oblique, internal oblique, and transverse abdominis can be anatomically separated into two layers, one arising from the superficial fascial layer of the muscle and the other arising from the deep fascial layer of the muscle.^10,¹¹ The superficial and deep layers of the rectus sheath comprise three fascial layers (superficial—two from external and one from internal oblique, and deep—one from internal oblique and two from transverse abdominis). It is thought that the functional or mechanical purpose of this structural arrangement is to enable transfer of forces generated by the abdominal wall muscles (external oblique, internal oblique, transverse abdominis) around the torso, creating a pressurized abdominal cavity that assists in stiffening the spinal column.^4,¹² Although highly variable and inferior to the umbilicus, most individuals display a gradual movement of deep internal oblique and transverse abdominis aponeurotic fibers from the posterior to the anterior of the rectus sheath; this may be due to the need for increased resistance to bulging of the anterior wall in the lower abdomen.

FIGURE 4–4 A, Schematic transverse cross section of anterior abdominal aponeurosis and rectus sheath. Note trilaminar nature of superficial and deep layers of rectus sheath. Fascial sheets arise from superficial and deep layers of each of external oblique, internal oblique, and transverse abdominis to form aponeurosis and rectus sheath and terminate across midline of the body through the linea alba. B, Ultrasound image of transverse cross section of anterior abdominal wall.

The connective tissue networks overlying each of the three abdominal wall muscles, giving way to the formation of the rectus sheath, also provide a strong mechanical shear linkage between the muscle layers. This linkage has been shown to transmit forces mechanically among the muscle layers in a rat preparation ¹³ and has been hypothesized to afford composite laminate structural properties that assist in strengthening the abdominal wall and stiffening the spinal column. Additional study of mechanical interactions between the muscle layers is needed to further understanding of abdominal muscle function, in particular related to deformation during contraction or movement and force generation and transmission around the abdomen.

Mechanical Properties of the Abdominal Muscles

Muscle force–generating capacities depend on architectural characteristics.^14,¹⁵ Specifically, the number, length, and orientation of fibers acting in parallel with the axis of force generation, together with the moment arms around the joints the muscle crosses, determine the functional capabilities of a muscle.

Most information regarding the physiologic cross-sectional area (PCSA) of the abdominal muscles comes from various imaging modalities, including computed tomography (CT), magnetic resonance imaging (MRI), and ultrasonography. More recent work in the authors’ laboratory has further documented the PCSA of the abdominal muscles from cadaveric dissections of 11 donors ranging in age from 52 to 94 years (mean age 77.7 years [SD 16.3 years]).¹⁶ McGill and colleagues¹⁷ and Marras and colleagues,¹⁸ measuring young healthy men using CT and MRI, measured a rectus abdominis PCSA of approximately 8 cm² at the level of the L4-L5 disc. This value is much larger than the mean 3.3 cm² measured from the elderly cadavers.¹⁶ Similarly, the overall force-generating capacities of the internal and external oblique muscles, documented in McGill¹⁹ measuring healthy young men, were much larger than in the cadaveric dissections of Brown and colleagues¹⁶ (approximately 16 cm² and 19 cm² vs. 6.6 cm² and 8.6 cm² for the external oblique and internal oblique). This discrepancy between the data from cadavers and imaging data from young men likely points to an aging-related atrophy of the muscles. Similar measures of transverse abdominis PCSA have been reported, however, between the two modalities (approximately 5 cm² in the studies by McGill¹⁹ and Brown and colleagues¹⁶). In addition, Marras and colleagues¹⁸ reported PCSA gender differences (males greater than females) for external oblique and internal oblique in young individuals and reported an increasing rectus abdominis PCSA toward lower vertebral levels; these findings were not apparent in the cadaveric analyses.¹⁶

Although lines of action in the external oblique, the transverse abdominis, and in particular the internal oblique run at various angles in different regions of the muscles, all fibers act directly in line with connective tissue attachments through which they apply force. It is inappropriate to consider these muscles as pennate to a force line of action but more appropriately as applying force to the body across a wide span of fiber angles and attachments.

The ability of a muscle to generate force also depends on the instantaneous length and velocity of muscle fibers or, more specifically, of the sarcomeres that make up the muscle fibers.²⁰ The more recent study by Brown and colleagues¹⁶ reported fixed sarcomere lengths of the abdominal muscles in the approximate neutral spine posture. Sarcomere lengths of the rectus abdominis and external oblique (mean 3.29 µm [SD 0.22 µm] and 3.18 µm [SD 0.37 µm] for the rectus abdominis and external oblique) in this position were well above optimal (optimal approximately 2.70 µm in human muscle²¹), whereas the lengths of the internal oblique and transverse abdominis (mean 2.61 µm [SD 0.21 µm] and 2.58 µm [SD 0.16 µm] for the internal oblique and transverse abdominis) were slightly below optimal. Because anteriorly acting fibers of the internal oblique shorten during flexion, whereas more laterally acting fibers lengthen, biomechanical modeling predicts the muscle, as a whole, to produce maximum force near the neutral spine posture. The rectus abdominis and external oblique (which shorten during spine flexion) and the transverse abdominis (which primarily lengthens during spine flexion) act together at optimal force-generating length in the mid-range of lumbar flexion, where the internal oblique can still generate in the range of 90% or greater of its maximum force.

Normalized fiber lengths were also calculated for the muscles to provide an indication of their excursion capabilities (Fig. 4–5).¹⁶ A muscle with longer fibers can produce force over a greater range of lengths because a greater number of sarcomeres act to produce this overall length change effectively. This also has direct implications for the velocities at which a muscle can produce force because in a longer muscle each sarcomere experiences a lower relative velocity compared with a shorter muscle-changing length at the same rate. The data in Figure 4–5 imply that the rectus abdominis and external oblique have the potential to undergo greater length changes and produce force at higher absolute velocities compared with the internal oblique and transverse abdominis. How these muscles adapt to changes in body shape (e.g., to chronic visceral weight gain or loss) has yet to be explored, and any potential adaptations (or lack thereof) can have important implications for abdominal muscle function related to obesity.