Axial Skeleton: Muscle and Joint Interactions

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Chapter 10

Axial Skeleton

Muscle and Joint Interactions

CHAPTER AT A GLANCE

INNERVATION OF THE MUSCLES AND JOINTS WITHIN THE TRUNK AND CRANIOCERVICAL REGIONS

TRUNK AND CRANIOCERVICAL REGIONS

Production of Internal Torque

Special Considerations for the Study of Muscle Actions within the Axial Skeleton

Muscles of the Trunk: Anatomy and Their Individual Actions

Muscles of the Trunk: Functional Interactions among Muscles

Muscles of the Craniocervical Region: Anatomy and Their Individual Actions

Muscles of the Craniocervical Region: Functional Interactions among Muscles That Cross the Craniocervical Region

SELECTED BIOMECHANICAL ISSUES OF LIFTING: A FOCUS ON REDUCING BACK INJURY

SYNOPSIS

ADDITIONAL CLINICAL CONNECTIONS

REFERENCES

STUDY QUESTIONS

Osteologic and arthrologic components of the axial skeleton are presented in Chapter 9. Chapter 10 focuses on the many muscle and joint interactions occurring within the axial skeleton. The muscles control posture and stabilize the axial skeleton, protect the spinal cord and internal organs, produce torques required for movement of the body as a whole, and, lastly, furnish fine mobility to the head and neck for optimal placement of the eyes, ears, and nose. Muscles associated with ventilation and mastication (chewing) are presented in Chapter 11.

The anatomic structure of the muscles within the axial skeleton varies considerably in length, shape, fiber direction, cross-sectional area, and leverage across the underlying joints. Such variability reflects the diverse demands placed on the musculature, from manually lifting and transporting heavy objects to producing subtle motions of the head for accenting a lively conversation.

Muscles within the axial skeleton cross multiple regions of the body. The trapezius muscle, for example, attaches to the clavicle and the scapula within the appendicular skeleton and to the vertebral column and the cranium within the axial skeleton. Protective guarding because of an inflamed upper trapezius can therefore affect the quality of motion throughout the upper extremity and craniocervical region.

The primary aim of this chapter is to elucidate the structure and function of the muscles within the axial skeleton. This information is essential to the evaluation and treatment of a wide range of musculoskeletal impairments, such as postural malalignment, deformity, or instability; muscle injury, spasm, excessive stiffness, or weakness; and generalized neck and back pain.

INNERVATION OF THE MUSCLES AND JOINTS WITHIN THE TRUNK AND CRANIOCERVICAL REGIONS

An understanding of the organization of the innervation of the craniocervical and trunk muscles begins with an appreciation of the formation of a typical spinal nerve root (Figure 10-1). Each spinal nerve root is formed by the union of ventral and dorsal nerve roots: the ventral nerve roots contain primarily “outgoing” (efferent) axons that supply motor commands to muscles and other effector organs associated with the autonomic system. The dorsal nerve roots contain primarily “incoming” (afferent) dendrites, with the cell body of the neuron located in an adjacent dorsal root ganglion. Sensory neurons transmit information to the spinal cord from the muscles, joints, skin, and other organs associated with the autonomic nervous system.

Near or within the intervertebral foramen, the ventral and dorsal nerve roots join to form a spinal nerve root. (Spinal nerve roots are often described as “mixed,” emphasizing the point that they contain both sensory and motor fibers.) The spinal nerve root thickens owing to the merging of the motor and sensory neurons and the presence of the dorsal root ganglion.

The vertebral column contains 31 pairs of spinal nerve roots: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. The abbreviations C, T, L, and S with the appropriate superscript number designate each spinal nerve root—for example, C5 and T6. The cervical region has seven vertebrae but eight cervical nerve roots. The suboccipital nerve (C1) leaves the spinal cord between the occipital bone and posterior arch of the atlas (C1). The C8 spinal nerve root exits the spinal cord between the seventh cervical vertebra and the first thoracic vertebra. Spinal nerve roots T1 and below exit the spinal cord just inferior or caudal to their respective vertebral bodies.

Once a spinal nerve root exits its intervertebral foramen, it immediately divides into a ventral and dorsal ramus (the Latin word ramus means “path”) (see Figure 10-1). Depending on location, the ventral ramus forms nerves that innervate, in general, the muscles, joints, and skin of the anterior-lateral trunk and neck, and the extremities. The dorsal ramus, in contrast, forms nerves that innervate, in general, the muscles, joints, and skin of the posterior trunk and neck. This anatomic organization is depicted generically by the illustration in Figure 10-2.

Ventral Ramus Innervation

Throughout the vertebral column, each ventral ramus of a spinal nerve root either forms a plexus or continues as an individual named nerve.

INDIVIDUAL NAMED NERVES

Many of the ventral rami within the trunk and craniocervical regions do not join a plexus; rather, they remain as individual named nerves. Each of these nerves typically innervates only a part or a segment of a muscle or connective tissue. This is why, for instance, many muscles that extend across a large part of the axial skeleton possess multiple levels of segmental innervation. The two most recognized sets of individual segmental nerves derived from the ventral rami are the intercostal (thoracic) and the recurrent meningeal nerves (see Figure 10-2).

Intercostal Nerves (T1 to T12): Each of the 12 ventral rami of the thoracic spinal nerve roots forms an intercostal nerve, innervating an intercostal dermatome and the set of intercostal muscles that share the same intercostal space. (Refer to dermatome chart in Appendix III, Part B, Figure III-2.) The T1 ventral ramus forms the first intercostal nerve and part of the lower trunk of the brachial plexus. The ventral rami of T7-T12 also innervate the muscles of the anterior-lateral trunk (i.e., the “abdominal” muscles). The T12 ventral ramus forms the last intercostal (subcostal) nerve and part of the L1 ventral ramus of the lumbar plexus.

Recurrent Meningeal Nerves: A single recurrent meningeal (sinuvertebral) nerve branches off the extreme proximal aspect of each ventral ramus. After its bifurcation, the recurrent meningeal nerve courses back into the intervertebral foramen (hence the name “recurrent” [see Figure 10-2]). As a set, these often very small nerves provide sensory and sympathetic nerve supply to the meninges that surround the spinal cord, and to connective tissues associated with the interbody joints.23 Most notably, the recurrent meningeal nerve supplies sensation to the posterior longitudinal ligament and adjacent areas of the superficial part of the annulus fibrosus. Sensory nerves innervating the anterior longitudinal ligament reach the spinal cord via small branches from nearby ventral rami and adjacent sympathetic connections.23

Dorsal Ramus Innervation

A dorsal ramus branches from every spinal nerve root, innervating structures in the posterior trunk usually in a highly segmental fashion. With the exception of the C1 and C2 dorsal rami, which are discussed separately, all dorsal rami are smaller than their ventral rami counterparts (see Figure 10-2). In general, dorsal rami course a relatively short distance posteriorly (dorsally) before innervating selected adjacent muscles and connective tissues on the back of the trunk (Box 10-1).

The dorsal ramus of C1 (“suboccipital” nerve) is primarily a motor nerve, innervating the suboccipital muscles. The dorsal ramus of C2 is the largest of the cervical dorsal rami, innervating local muscles as well as contributing to the formation of the greater occipital nerve (C2 and C3)—a sensory nerve to the posterior and superior scalp region.

TRUNK AND CRANIOCERVICAL REGIONS

The muscles of the axial skeleton are organized into two broad and partially overlapping areas: the trunk and the craniocervical region (Table 10-1). The muscles within each area are further organized into sets, based more specifically on their location.

The muscles within each area of the body are presented in two sections, the first covering anatomy and individual muscle actions, and the second covering examples of the functional interactions among related muscles. Throughout this chapter, the reader is encouraged to consult Chapter 9 for a review of the pertinent osteology related to the attachments of muscles. Appendix III, Part C should be consulted for a summary of more detailed muscular anatomy and innervation of the muscles of the axial skeleton.

Before beginning the description of the muscles of the trunk, the following fundamental topics will be reviewed, many of which are specifically related to the kinesiology of the axial skeleton.

Production of Internal Torque

By convention, the “strength” of a muscle action within the axial skeleton is expressed as an internal torque, defined for the sagittal, frontal, and horizontal planes. Within each plane, the maximal internal torque potential is equal to the product of (1) the muscle force generated parallel to a given plane, and (2) the length of the internal moment arm available to the muscle (Figure 10-3).

The spatial orientation of a muscle’s line of force determines its effectiveness for producing a torque for a particular action. Consider, for example, the obliquus externus abdominis muscle producing a force across the lateral thorax, with a line of force oriented about 30 degrees from the vertical (Figure 10-4). The muscle’s resultant force vector can be trigonometrically partitioned into unequal vertical and horizontal force components. The vertical force component—about 86% of the muscle’s maximal force—is available for producing lateral flexion or flexion torques. The horizontal force component—about 50% of the muscle’s maximal force—is available for producing an axial rotation torque. (This estimation is based on the cosine and sine of 30 degrees, respectively.) For any muscle of the axial skeleton to contribute all its force potential toward axial rotation, its overall line of force must be directed solely in the horizontal plane. For a muscle to contribute all its force potential toward either lateral flexion or flexion-extension, its overall line of force must be directed vertically. (Realize, though, that a vertically oriented muscle cannot produce any axial rotation because it lacks the moment arm required to produce a torque in the horizontal plane. As described in Chapter 1, a muscle force is incapable of producing a torque within a given plane if it either parallels or pierces the associated axis of rotation.)

The lines of force of muscles that control movement of the axial skeleton have a spatial orientation that varies over a wide spatial spectrum, from nearly vertical to nearly horizontal. This fact is important with regard to a muscle or muscle group’s torque potential for a given action. For instance, because more of the total muscle mass of the trunk is biased vertically than horizontally, maximal efforts usually produce greater frontal and sagittal plane torques than horizontal plane torques.145

Special Considerations for the Study of Muscle Actions within the Axial Skeleton

To understand the actions of muscles located within the axial skeleton, it is necessary to first consider the muscle during both unilateral and bilateral activations. Bilateral activation usually produces pure flexion or extension of the axial skeleton. Any potential for lateral flexion or axial rotation is neutralized by opposing forces in contralateral muscles. Unilateral activation, in contrast, tends to produce flexion or extension of the axial skeleton, with some combination of lateral flexion and contralateral or ipsilateral axial rotation. (The term lateral flexion of the axial skeleton implies “ipsilateral” lateral flexion and therefore is not so specified throughout this chapter.)

The action of a muscle within the axial skeleton depends, in part, on the relative degree of fixation, or stabilization, of the attachments of the muscle. As an example, consider the effect of a contraction of a member of the erector spinae group—a muscle that attaches to both the thorax and pelvis. With the pelvis stabilized, the muscle can extend the thorax; with the thorax stabilized, the muscle can anteriorly rotate (tilt) the pelvis. (Both of these motions occur in the sagittal plane.) If the thorax and pelvis are both free to move, the muscle can simultaneously extend the thorax and anteriorly tilt the pelvis. Unless otherwise stated, it is assumed that the superior (cranial) end of a muscle is less constrained and therefore freer to move than its inferior or caudal counterpart.

Depending on body position, gravity may assist or resist movements of the axial skeleton. Slowly flexing the head from the anatomic (standing) position, for example, is normally controlled by eccentric activation of the neck extensor muscles. Gravity, in this case, is the prime “flexor” of the head, whereas the extensor muscles control the speed and extent of the action. Rapidly flexing the head, however, requires a burst of concentric activation from the neck flexor muscles, because the desired speed of the motion may be greater than that produced by action of gravity alone. Unless otherwise stated, it is assumed that the action of a muscle is performed via a concentric contraction, rotating a body segment against gravity or against some other form of external resistance.

Muscles of the Trunk: Anatomy and Their Individual Actions

The following section describes the relationships between the anatomy and the actions of the muscles of the trunk. Musculature is divided into three sets: (1) muscles of the posterior trunk, (2) muscles of the anterior-lateral trunk, and (3) additional muscles (see Table 10-1).

SET 1: MUSCLES OF THE POSTERIOR TRUNK (“BACK” MUSCLES)

The muscles of the posterior trunk are organized into three layers: superficial, intermediate, and deep (see Table 10-1).

Muscles in the Superficial and Intermediate Layers of the Back: The muscles in the superficial layer of the back are presented in the study of the shoulder (see Chapter 5). They include the trapezius, latissimus dorsi, rhomboids, levator scapula, and serratus anterior. The trapezius and latissimus dorsi are most superficial, followed by the deeper rhomboids and levator scapula. The serratus anterior muscle is located more laterally on the thorax.

In general, bilateral activation of the muscles of the superficial layer extends the adjacent region of the axial skeleton. Unilateral activation, however, laterally flexes and, in most cases, axially rotates the region. The right middle trapezius, for example, assists with right lateral flexion and left axial rotation of the upper thoracic region.

The muscles included in the intermediate layer of the back are the serratus posterior superior and the serratus posterior inferior. They are located just deep to the rhomboids and latissimus dorsi. The serratus posterior superior and inferior are thin muscles that contribute little to the movement or stability of the trunk. Their function is more likely related to the mechanics of ventilation and therefore is described in Chapter 11.

Muscles within the superficial and intermediate layers of the back are often referred to as “extrinsic” because, from an embryologic perspective, they were originally associated with the front “limb buds” and only later in their development migrated dorsally to their final position on the back. Although muscles such as the levator scapula, rhomboids, and serratus anterior are located within the back, technically they belong with upper limb muscles. All extrinsic muscles of the back are therefore innervated by ventral rami of spinal nerves (i.e., the brachial plexus or intercostal nerves).

SPECIAL FOCUS 10-1   imageMuscles of the Superficial Layer of the Back: an Example of Muscles “Sharing” Actions between the Axial and Appendicular Skeletons

Chapter 5 describes the actions of the muscles of the superficial layer of the back, based on their ability to rotate the appendicular skeleton (i.e., humerus, scapula, or clavicle) toward a fixed axial skeleton (i.e., head, sternum, vertebral column, or ribs). The same muscles, however, are equally capable of performing the “reverse” action (i.e., rotating segments of the axial skeleton toward the fixed appendicular skeleton). This muscular action is demonstrated by highlighting the functions of the trapezius and rhomboids during use of a bow and arrow. As indicated in Figure 10-5, several muscles produce a force needed to stabilize the position of the scapula and abducted arm. Forces produced in the upper trapezius, middle trapezius, and rhomboids simultaneously rotate the cervical and upper thoracic spine to the left, indicated by the bidirectional arrows.23,82 This “contralateral” axial rotation effect is shown for C6 in the inset within Figure 10-5. As the muscle pulls the spinous process of C6 to the right, the anterior side of the vertebra is rotated to the left. The trapezius and rhomboids also stabilize the scapula against the pull of the posterior deltoid, long head of the triceps, and serratus anterior. The shared actions of these muscles demonstrate the inherent efficiency of the musculoskeletal system. In this example, a few muscles accomplish multiple actions across both the axial and the appendicular skeletons.

Muscles in the Deep Layer of the Back: Muscles in the deep layer of the back are the (1) erector spinae group, (2) transversospinal group, and (3) short segmental group (Table 10-2). The anatomic organization of the erector spinae and transversospinal groups is illustrated in Figure 10-6.

In general, from superficial to deep, the fibers of the muscles in the deep layer become progressively shorter and more angulated. A muscle within the more superficial erector spinae group may extend virtually the entire length of the vertebral column. In contrast, each muscle within the deeper, short segmental group crosses only one intervertebral junction.

Although a few exceptions prevail, muscles in the deep layer of the back are innervated segmentally through the dorsal rami of spinal nerves.132 A particularly long muscle within the erector spinae group, for instance, is innervated by multiple dorsal rami throughout the spinal cord. A shorter muscle such as one multifidus, however, is innervated by a single dorsal ramus.100

Embryologically, and unlike the muscles in the extremities and anterior-lateral trunk, the muscles in the deep layer of the back have retained their original location dorsal to the neuraxis. For this reason these muscles have also been called “intrinsic” or “native” muscles of the back. As a general rule, most intrinsic muscles of the back are innervated by dorsal rami of adjacent spinal nerves.

Erector Spinae Group: The erector spinae are an extensive and rather poorly defined group of muscles that run on both sides of the vertebral column, roughly within one hand’s width from the spinous processes (Figure 10-7). Most are located deep to the posterior layer of thoracolumbar fascia (see Chapter 9) and the muscles in the intermediate and superficial layers of the back. The erector spinae consist of the spinalis, longissimus, and iliocostalis muscles. Each muscle is further subdivided topographically into three regions, producing a total of nine named muscles (see Table 10-2). Individual muscles overlap and vary greatly in size and length.23

The bulk of the erector spinae muscles have a common attachment on a broad and thick common tendon, located in the region of the sacrum (see Figure 10-7). This common tendon anchors the erector spinae to many locations (Box 10-2). From this common tendon arise three poorly organized vertical columns of muscle: the spinalis, longissimus, and iliocostalis.132 The general muscle attachments are described in the following sections; more specific attachments can be found in Appendix III, Part C.

Longissimus Muscles: The longissimus muscles include the longissimus thoracis, longissimus cervicis, and longissimus capitis. As a set, these muscles form the largest and most developed column of the erector spinae group. The fibers of the longissimus thoracic muscles fan cranially from the common tendon, attaching primarily to the posterior end of most ribs. In the neck, the longissimus cervicis angles slightly medially before attaching to the posterior tubercle of the transverse processes of the cervical vertebrae (see Figure 10-7). The longissimus capitis, in contrast, courses slightly laterally and attaches to the posterior margin of the mastoid process of the temporal bone. The slightly more oblique angulation of the superior portion of the longissimus capitis and cervicis suggests that these muscles assist with ipsilateral axial rotation of the craniocervical region.

Summary: The erector spinae muscles cross a considerable distance throughout the axial skeleton. This anatomic feature suggests a design more suited for control of gross movements across a large part of axial skeleton (such as extending the trunk while rising from a low chair) rather than finer movements at selected intervertebral junctions.15 Bilateral contraction of the erector spinae as a group extends the trunk, neck, or head (Figure 10-8).47 The muscles’ relatively large cross-sectional areas enable them to generate large extension torque across the axial spine, such as for lifting or carrying heavy objects.37

By attaching to the sacrum and to the pelvis, the erector spinae can anteriorly tilt the pelvis, thereby accentuating the lumbar lordosis. (Pelvic tilt describes a sagittal plane rotation of the pelvis around the hip joints. The direction of the tilt is indicated by the rotation direction of the iliac crests.) As depicted in Figure 10-8, A, the anterior pelvic tilt is accentuated by the increased tension in stretched hip flexor muscles, such as the iliacus.

Contracting unilaterally, the more laterally disposed iliocostalis muscles are the most effective lateral flexors of the erector spinae group. The cranial or cervical components of the longissimus and iliocostalis muscles assist with ipsilateral axial rotation, especially when the head and neck are fully and contralaterally rotated. The iliocostalis lumborum assists slightly with ipsilateral axial rotation.

Transversospinal Muscles: Located immediately deep to the erector spinae muscles is the transversospinal muscle group: the semispinalis, multifidi, and rotatores (Figures 10-9 and 10-10). Semispinalis muscles are located superficially; the multifidi, intermediately; and the rotatores, deeply.

The name transversospinal refers to the general attachments of most of the muscles (i.e., from the transverse processes of one vertebra to the spinous processes of a more superiorly located vertebra). With a few exceptions, these attachments align most muscle fibers in a cranial-and-medial direction. Many of the muscles within the transversospinal group are morphologically similar, varying primarily in length and in the number of intervertebral junctions that each muscle crosses (Figure 10-11). Although somewhat oversimplified, this concept can greatly assist in learning the overall anatomy and actions of these muscles.

Semispinalis Muscles: The semispinalis muscles consist of the semispinalis thoracis, semispinalis cervicis, and semispinalis capitis (see Figure 10-9). In general, each muscle, or main set of fibers within each muscle, crosses six to eight intervertebral junctions. The semispinalis thoracis consists of many thin muscle fasciculi, interconnected by long tendons. Muscle fibers attach from transverse processes of T6 to T10 to spinous processes of C6 to T4. The semispinalis cervicis, much thicker and more developed than the semispinalis thoracis, attaches from upper thoracic transverse processes to spinous processes of C2 to C5. Muscle fibers that attach to the prominent spinous process of the axis (C2) are particularly well developed, serving as important stabilizers for the suboccipital muscles (described ahead).

The semispinalis capitis lies deep to the splenius and trapezius muscles. The muscle arises primarily from upper thoracic transverse processes. The muscle thickens superiorly as it attaches to a relatively large region on the occipital bone, filling much of the area between the superior and inferior nuchal lines (see Figure 9-3).

The semispinalis cervicis and capitis are the largest muscles that cross the posterior side of the neck. Their large size and near-vertical fiber direction account for the fact that these muscles provide 35% to 40% of the total extension torque of the craniocervical region.145 Right and left semispinalis capitis muscles are readily palpable as thick and round cords on either side of the midline of the upper neck, especially evident in infants and in thin, muscular adults (Figure 10-12).

Multifidi: Multifidi are situated just deep to the semispinalis muscles. The plural “multifidi” indicates a collection of multiple fibers, rather than a set of individual muscles. All multifidi share a similar fiber direction and length, extending between the posterior sacrum and the axis (C2).6,23,132 In general, the multifidi originate from the transverse process of one vertebra and insert on the spinous process of a vertebra located two to four intervertebral junctions above (see Figure 10-10, A).

Multifidi are thickest and most developed in the lumbosacral region (see multiple attachments listed in Box 10-3).100 The overlapping fibers of the multifidi fill much of the concave space formed between the spinous and transverse processes. The multifidi provide an excellent source of extension torque and associated stability to the base of the spine. Excessive force in the lumbar multifidi—from either active contraction or protective spasm—may be expressed clinically as an exaggerated lordosis.

Rotatores: The rotatores are the deepest of the transversospinal group of muscles. Like the multifidi, the rotatores consist of a large set of individual muscle fibers. Although the rotatores exist throughout the entire vertebral column, they are best developed in the thoracic region (see Figure 10-10, B).132 Each fiber attaches between the transverse process of one vertebra and the lamina and base of the spinous process of a vertebra located one or two intervertebral junctions above. By definition, the rotator brevis spans one intervertebral junction, and the rotator longus spans two intervertebral junctions.

Summary: On average, the transversospinal muscles cross fewer intervertebral junctions than the erector spinae group. This feature suggests that, in general, the transversospinal muscles favor a design for producing relatively fine controlled movements and stabilizing forces across the axial skeleton.15,22

Contracting bilaterally, the transversospinal muscles extend the axial skeleton (see Figure 10-8, B). Increased extension torque exaggerates the cervical and lumbar lordosis and decreases the thoracic kyphosis. The size and thickness of the transversospinal muscles are greatest at either end of the axial skeleton. Cranially, the semispinalis cervicis and capitis are very well-developed extensors of the craniocervical region; caudally, the multifidi are very well-developed extensors of the lower lumbar region, accounting for two thirds of the muscular-based stability in this region.154

Contracting unilaterally, the transversospinal muscles laterally flex the spine; however, their leverage for this action is limited because of their close proximity to the vertebral column. The more obliquely oriented transversospinal muscles assist with contralateral axial rotation. From a relatively fixed transverse process, contraction of a single left multifidus or rotator longus, for example, can rotate a superiorly located spinous process toward the left and, as a result, rotate the anterior side of the vertebra to the right. Compared with all the trunk muscles, however, the transversospinal muscles are secondary axial rotators. The leverage for this rotation is relatively poor because of the muscle’s proximity to the vertebral column. (Compare the multifidi with the obliquus abdominis externus, for example, in Figure 10-3, C). Furthermore, the prevailing line of force of most transversospinal muscle fibers is directed more vertically than horizontally, thereby providing a greater force potential for extension than for axial rotation.

Short Segmental Group of Muscles: The short segmental group of muscles consists of the interspinalis and the intertransversarius muscles (see Figure 10-10). (The plural “interspinales and intertransversarii” is often used to describe all the members within the entire set of these muscles.) They lie deep to the transversospinal group of muscles. The name “short segmental” refers to the extremely short length and highly segmented organization of the muscles. Each individual interspinalis or intertransversarius muscle crosses just one intervertebral junction. These muscles are most developed in the cervical region, where fine control of the head and neck is so critical.132

Each pair of interspinalis muscles is located on both sides of, and often blends with, the corresponding interspinous ligament. The interspinales have a relatively favorable leverage and optimal fiber direction for producing extension torque. The magnitude of this torque, however, is relatively small considering the muscles’ small size and therefore low force potential.

Each right and left pair of intertransversarius muscles is located between adjacent transverse processes. The anatomy of the intertransversarii as a group is more complex than that of the interspinales.132 In the cervical region, for example, each intertransversarius muscle is divided into small anterior and posterior muscles, between which pass the ventral rami of spinal nerves.

Unilateral contraction of the intertransversarii as a group laterally flexes the vertebral column. Although the magnitude of the lateral flexion torque is relatively small compared with that of other muscle groups, the torque likely provides an important source of intervertebral stability.

SET 2: MUSCLES OF THE ANTERIOR-LATERAL TRUNK (“ABDOMINAL” MUSCLES)

The muscles of the anterior-lateral trunk include the rectus abdominis, obliquus externus abdominis, obliquus internus abdominis, and transversus abdominis (Figure 10-13). As a group, these muscles are often collectively referred to as the “abdominal” muscles. The rectus abdominis is a long straplike muscle located on both sides of the midline of the body. The obliquus externus abdominis, obliquus internus abdominis, and transversus abdominis—the lateral abdominals—are wide and flat, layered superficial to deep, across the anterior-lateral aspects of the abdomen.

The abdominal muscles have several important physiologic functions, including supporting and protecting abdominal viscera and increasing intrathoracic and intra-abdominal pressures. As will be further described in Chapter 11, increasing the pressures in these cavities assists with functions such as forced expiration of air from the lungs, coughing, defecation, and child birth. This chapter focuses more on the kinesiologic functions of the abdominal muscles.

Formation of the Rectus Sheaths and Linea Alba: The obliquus externus abdominis, obliquus internus abdominis, and transversus abdominis muscles from the right and left sides of the body fuse at the midline of the abdomen through a blending of connective tissues. Each muscle contributes a thin bilaminar sheet of connective tissue that ultimately forms the anterior and posterior rectus sheaths. As depicted in Figure 10-14, the anterior rectus sheath is formed from connective tissues from the obliquus externus abdominis and the obliquus internus abdominis muscles. The posterior rectus sheath is formed from connective tissues from the obliquus internus abdominis and transversus abdominis. Both sheaths surround the vertically oriented rectus abdominis muscle and continue medially to fuse with identical connective tissues from the other side of the abdomen. The connective tissues thicken and crisscross as they traverse the midline, forming the linea alba (the Latin word linea means “line,” and albus, “white”). The linea alba runs longitudinally between the xiphoid process and pubic symphysis and pubic crest.

The crisscross arrangement of the fibers within the linea alba adds strength to the abdominal wall, much like the laminated structure of plywood. The linea alba also mechanically links the right and left lateral abdominal muscles, providing an effective way to transfer muscular force across the midline of the body.

Anatomy of the Abdominal Muscles: The rectus abdominis muscle consists of right and left halves, separated by the linea alba. Each half of the muscle runs longitudinally, widening as it ascends within an open sleeve formed between the anterior and posterior rectus sheaths. The muscle is intersected and reinforced by three fibrous bands, known as tendinous intersections. These bands blend with the anterior rectus sheath. The rectus abdominis arises from the region on and surrounding the crest of the pubis, and it attaches superiorly on the xiphoid process and cartilages of the fifth through seventh ribs.

The anatomic organization of the obliquus externus abdominis, obliquus internus abdominis, and transversus abdominis muscles is different from that of the rectus abdominis. As a group, the more laterally placed muscles originate laterally or posterior-laterally on the trunk and run in a different direction toward the midline, eventually blending with the linea alba and contralateral rectus sheaths (Table 10-3).

The obliquus externus abdominis (informally referred to as the “external oblique”) is the largest and most superficial of the lateral abdominal muscles. The external oblique muscle travels in an inferior-and-medial direction, similar to the direction of the hands placed diagonally in front pockets of pants. The obliquus internus abdominis (or less formally the “internal oblique”) is located immediately deep to the external oblique muscle, forming the second layer of the lateral abdominals. Fibers originate from the iliac crest and, to a varying degree, blend with the adjacent thoracolumbar fascia. From this lateral attachment point, the fibers course in a cranial-and-medial direction toward the linea alba and lower ribs. As evident in Figure 10-13, C, the inferior attachments of the internal oblique muscle extend to the inguinal ligament. The average fiber direction of the internal oblique muscle is nearly perpendicular to the average fiber direction of the overlying external oblique muscle.

The transversus abdominis is the deepest of the abdominal muscles. The muscle is also known as the “corset muscle,” reflecting its role in compressing the abdomen as well as stabilizing the lower back through attachments into the thoracolumbar fascia.132 Of all the abdominal muscles, the transversus abdominis has the most extensive and consistent attachments into the thoracolumbar fascia,136 followed closely by the internal oblique muscle.

Actions of the Abdominal Muscles: Bilateral action of the rectus abdominis and oblique abdominal muscles reduces the distance between the xiphoid process and the pubic symphysis. Depending on which body segment is the most stable, bilateral contraction of these abdominal muscles flexes the thorax and upper lumbar spine, posteriorly tilts the pelvis, or both. Figure 10-15 depicts a diagonally performed sit-up maneuver that places a relatively large demand on the oblique abdominal muscles. During a standard sagittal plane sit-up, however, the opposing axial rotation and lateral flexion tendencies of the various abdominal muscles are neutralized by opposing right and left muscles.

As described in Chapter 9, the axes of rotation for all motions of the vertebral column are located in the region of the interbody joints. The relative posterior placement of the axes relative to the trunk equips the abdominal muscles, most notably the rectus abdominis, with very favorable leverage for generating trunk flexion torque (Figure 10-16). Note in Figure 10-16 that, with the exception of the psoas major, all muscles have a moment arm to produce torques in both sagittal and frontal planes.

Contracting unilaterally, the abdominal muscles laterally flex the trunk. The external and internal obliques are particularly effective in this action owing to their relatively favorable leverage (i.e., long moment arms) (see Figure 10-16) and, as a pair, relatively large cross-sectional area. The combined cross-sectional area of the external and internal obliques at the level of the L4-L5 junction is almost twice that of the rectus abdominis muscle.106

Lateral flexion of the trunk often involves activation of both trunk flexor and extensor muscles. For example, lateral flexion against resistance to the right demands a contraction from the right external and internal oblique, right erector spinae, and right transversospinal muscles. Coactivation amplifies the total frontal torque while simultaneously stabilizing the trunk within the sagittal plane.12

By far, the internal and external oblique muscles are the most effective axial rotators of the trunk.7,12,73,141 The external oblique muscle is a contralateral rotator, and the internal oblique muscle is an ipsilateral rotator. The strong axial rotation potential of these muscles reflects their relatively large cross-sectional area and favorable leverage (see Figure 10-3, C for long moment arm length of the obliquus externus abdominis). During active axial rotation in a particular direction, the external oblique muscle on one side functions synergistically with the internal oblique on the other side.141 This functional synergy produces a diagonal line of force that crosses the midline through the muscles’ mutual attachment into the linea alba (see Figure 10-15). Contraction of the two muscles therefore reduces the distance between one shoulder and the contralateral iliac crest.

Several electromyographic (EMG) studies using intramuscular (fine-wire) electrodes demonstrate some degree of bilateral activation of the transversus abdominis during axial rotation.38,82,141 It has been shown that during axial rotation the middle and lower fibers of the transversus abdominis coactivate at slightly different times than the upper fibers.141 Although the exact role of the transversus abdominis during axial rotation is uncertain, the muscle appears to function more as a stabilizer for the oblique abdominal muscles than a torque generator of axial rotation. Bilateral activation of the transversus abdominis can stabilize the ribs, linea alba, and thoracolumbar fascia—areas that serve as attachments for the internal or external oblique muscles.

The torque demands placed on the axial rotators of the trunk vary considerably based on the nature of an activity and position of the body.12 Torque demands are relatively large during high-power axial rotations, such as sprinting, wrestling, and throwing a discus or javelin. The demands may be very low, however, during activities that involve slow twisting of the trunk while in an upright position, such as during walking over level surfaces.

Axial rotation performed primarily within the horizontal plane places little to no gravity-induced external torque on the rotator muscles. The muscles’ primary resistance, in this case, is caused by the inertia of the trunk and the passive tension created by stretching antagonist muscles.

SPECIAL FOCUS 10-2   imageRole of Trunk Extensors as “Rotational Synergists” to the Oblique Abdominal Muscles

The external and internal oblique muscles are the primary axial rotators of the trunk. Secondary axial rotators include the ipsilateral latissimus dorsi, the more oblique components of the ipsilateral iliocostalis lumborum, and the contralateral transversospinal muscles. These secondary axial rotators are also effective extensors of the trunk. During a strong axial rotation movement, these extensor muscles are able to offset or neutralize the potent trunk flexion potential of the oblique abdominal muscles.98,154 Without this neutralizing action, a strenuous action of axial rotation would automatically be combined with flexion of the trunk. The aforementioned extensor muscles resist the flexion tendency of the oblique abdominal muscles, but also contribute slightly to the axial rotation torque.

The multifidi muscles provide a particularly important element of extension stability to the lumbar region during axial rotation.128,156 Pathology involving the apophyseal joints or discs in the lumbar region may be associated with weakness, fatigue, or reflexive inhibition of these muscles. Without adequate activation from the multifidi during axial rotation, the partially unopposed oblique muscles would, in theory, create a subtle and undesirable flexion bias to the base of the spine.

Comparing Trunk Flexor versus Trunk Extensor Peak Torque: In the healthy adult, on average, the magnitude of a maximal-effort trunk flexion torque is typically less than maximal-effort trunk extension torque. Although data vary based on gender, age, history of back pain, and angular velocity of the testing device, the flexor-to-extensor torque ratios determined isometrically for the trunk and craniocervical regions are between 0.45 and 0.77.20,80,114,145 Although the trunk flexor muscles normally possess greater leverage for sagittal plane torque (see Figure 10-16), the trunk extensor muscles possess greater mass and, equally important, greater overall vertical orientation of muscle fibers.95,106 The typically greater torque potential of the trunk extensor muscles reflects the muscles’ predominant role in counteracting gravity, either for the maintenance of upright posture or for carrying loads in front of the body.

SET 3: ADDITIONAL MUSCLES (ILIOPSOAS AND QUADRATUS LUMBORUM)

Although the iliopsoas and quadratus lumborum are not anatomically considered muscles of the trunk, they are strongly associated with the kinesiology of the region.

Iliopsoas: The iliopsoas is a large muscle consisting of two parts: the iliacus and the psoas major (see Figure 12-27). As are most hip flexors, the iliopsoas is innervated by the femoral nerve, a large branch from the lumbar plexus. The iliacus has a proximal attachment on the iliac fossa and lateral sacrum, just anterior and superior to the sacroiliac joint. The psoas major attaches proximally to the transverse processes of T12 to L5, including the intervertebral discs. The two muscles fuse distal to the inguinal ligament and typically attach as a single tendon to the lesser trochanter of the femur.

The iliopsoas is a long muscle, exerting a potent kinetic influence across the trunk, lumbar spine, lumbosacral junction, and hip joints. Crossing anterior to the hip, it is a dominant flexor, drawing the femur toward the pelvis or the pelvis toward the femur. In the latter movement, the iliopsoas can anteriorly tilt the pelvis, a motion that increases the lordosis of the lumbar region (review in Figure 9-63, A). With muscular assistance from the abdominal muscles, a strong bilateral contraction of the iliopsoas can also rotate the pelvis and superimposed trunk over fixed femurs. Based on this ability, the iliopsoas is as much a respected trunk flexor as a hip flexor. This discussion resumes later in the chapter.

Function of the Psoas Major at the Lumbosacral Region: In the anatomic position the psoas major demonstrates leverage for lateral flexion of the lumbar spine (see Figure 10-16).95 Little, if any, leverage exists for axial rotation.

The flexor and extensor capacity of the psoas major differs throughout the lumbosacral region. Across the L5-S1 junction, the psoas major has an approximate 2-cm moment arm for flexion (Figure 10-17).107 The psoas major is therefore an effective flexor of the lower end of the lumbar spine relative to the sacrum. Progressing superiorly toward L1, however, the line of force of the psoas major gradually shifts slightly posterior, falling either through or just posterior to the multiple medial-lateral axes of rotation (see cross-section at L3 in Figure 10-16). The muscle’s location reduces or eliminates its flexor or extensor capacity. The psoas major therefore is neither a dominant flexor nor extensor of the lumbar region, but rather a dominant vertical stabilizer of the region.57,127 (The term “vertical stabilizer” describes a muscular function of stabilizing a region of the axial skeleton in a near-vertical position while maintaining its natural physiologic curve.) Because of the lack of effective leverage in the lumbar region, the psoas major has a minimal role in directly influencing the degree of lordosis.127 The iliopsoas, however (as all hip flexor muscles), can indirectly increase the lordotic posture of the lumbar spine by tilting the pelvis anteriorly across the hip joints.

Quadratus Lumborum: Anatomically, the quadratus lumborum is considered a muscle of the posterior abdominal wall. The muscle attaches inferiorly to the iliolumbar ligament and iliac crest and superiorly to the twelfth rib and the tips of the stout transverse processes of L1 to L4 (Figure 10-18). The relative thickness of the muscle is evident by viewing Figure 10-16. The quadratus lumborum is innervated by the ventral rami of spinal nerves T12-L3.

Contracting bilaterally, the quadratus lumborum is an extensor of the lumbar region. Its action is based on the line of force passing about 3.5 cm posterior to the medial-lateral axis of rotation at L3.107

Contracting unilaterally, the quadratus lumborum has very favorable leverage as a lateral flexor of the lumbar region.57 The axial rotation potential of the quadratus lumborum, however, is minimal.

Clinically, the quadratus lumborum is often called a “hip hiker” when its role in walking is being described, especially for persons with paraplegia at or below the L1 neurologic level. By elevating (hiking) one side of the pelvis, the quadratus lumborum raises the lower limb to clear the foot from the ground during the swing phase of brace-assisted ambulation.

The psoas major and the quadratus lumborum run nearly vertically on both sides of the lumbar vertebrae (see Figure 10-16). A strong bilateral contraction of these muscles affords excellent vertical stability throughout the lumbar spine, including the L5-S1 junction. Theoretically, exercises that increase the volitional control and conditioning of these muscles may benefit a person with pain related to instability of the lumbar region.

Muscles of the Trunk: Functional Interactions among Muscles

Thus far in this chapter, the discussion of the muscles of the trunk has focused primarily on their anatomy and, for the most part, individual actions (Table 10-4). The upcoming discussion pays more attention to the functional interactions among the muscles or muscle groups. Two themes are explored: (1) muscular-based stability of the trunk, and (2) muscular kinesiology of performing a standard sit-up movement. The second interaction exemplifies a classic kinesiologic relationship between the trunk and hip muscles.

TABLE 10-4.

Actions of Most Muscles of the Trunk

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