Kano ke kihi po‘ohiwi o Honokōhau.

Hard are the shoulder muscles of Honokōhau.

The people of Honokōhau, Maui, were said to be hard workers.

‘Ōlelo No‘eau, compiled by Mary Kawena Pukui, #1503

Cultural Connection

Makahiki — Festival of the New Year. In ancient Hawaiʻi, Makahiki was a roughly four-month time beginning in October or November, in which Hawaiians paused work to gather to pay tithes to chiefs, to feast, and to enjoy dancing competitive games. During the Makahiki season, Hawaiians acknowledge the abundance of the earth and participate in religious festivities and ceremonies to ask the gods, especially the god Lono-i-ka-makahiki, to provide prosperity for the future. The practice of partaking in Makahiki festivities has been revitalized and traditional games such as ʻulu maika (bowling), ʻōʻō ihe (spear/javelin throwing), hukihuki (similar to tug-of-war), pā uma (arm wrestling), haka moa (traditional wrestling), surfing, and canoe races are still practiced today. Training for the Makahiki games includes Hoʻoikaika kino (bodybuilding exercises) that develop spiritual, mental, and physical strength. In Hawaiian culture physical strength is important and these games provide a space for demonstration of such strength while honoring Akua Pa‘ani, god of sports.

Participants of the annual Makahiki festival place gifts on an alter at Puʻuloa.	Participants of the annual Makahiki festival begin the event with the arrival of Lono, akua of agriculture, rain, health and peace.

Muscular System and Homeostasis

The Merrie Monarch Festival is an annual hula competition that celebrates the Hawaiian language and art. When you watch the performances, you will find the integrated action of skeletal muscles, bones, and joints produce body movements from walking to dancing hula, as well as more subtle movements that result in various facial expressions, eye movements, and respiration (Figure 11.1).

Figure 11.1: Merrie Monarch Festival: Hula Hālau O Kamuela under the direction of Kumu Hula Kauʻi Kamanaʻo and Kunewa Mook – 2019 Overall Winners of the Merrie Monarch Festival – Hilo HI – Lokalia Montgomery Perpetual Trophy

In addition, muscle contractions maintain our posture, continually making fine adjustments as we hold the body in stationary positions such as sitting and standing. Understanding skeletal muscle anatomy and function is vital for professionals in the allied health and physical rehabilitation fields who work with patients whose mobility has been disrupted.

11.1 Skeletal Muscle Movements

Most skeletal muscle attaches to bone to produce movement. The tension created by muscle contraction is transferred to tendons, bands of connective tissue that attach muscles to bones.

Origin and Insertion

To produce a movement means to change the angle between two bones at a synovial joint. A skeletal muscle must be attached to a fixed part of the skeleton. The moveable end of the muscle that attaches to the bone being pulled is called the muscle’s insertion, and the end of the muscle attached to a fixed (stabilized) bone is called the origin. For example, when bending the elbow (forearm flexion), forearm bones are brought closer to the humerus by the contraction of biceps brachii muscles that span across the elbow joint. The biceps brachii spans across the elbow joint with its origins on the scapula and insertion on the distal radius (Figure 11.2).

Figure 11.2: Biceps brachii muscle 

Lever Systems

Have you ever used a knife to pry the coconut meat away from the shell?

The rigid knife handle acts as a lever and the tip of the knife pressed into the coconut acts as a fulcrum, the fixed point when a force is applied. The applied force or effort is used to pry out the coconut meat, which is the load or resistance that opposes the movement. The human musculoskeletal system works similarly, with bones acting as stiff levers and joints acting as fulcrums. The load would be an object being lifted (such as bones and associated tissue) or any resistance to a movement (your head is a load when you are lifting it), and the effort is the force applied by the contracting muscle at its insertion. Motion occurs when the effort exceeds the load.

Levers can vary based on the relative position of the load, fulcrum, and effort and are categorized into three types (Figure 11.3).

In a first-class lever, the load and force sit on either side of the fulcrum in the same way as a seesaw. First-class levers are relatively uncommon in the body, but one example is the lever formed by the head resting on the vertebral column which acts as the fulcrum. When the contraction of the posterior neck muscles provides the effort, the anterior portion of the skull (load) is lifted.

In a second-class lever, the load is between the fulcrum and the effort. Second-class levers are also relatively uncommon in the body. One example is raising yourself on your toes. The fulcrum is based at the front of the foot, the load is the weight of the body, and the force is applied through the Achilles tendon in the heel. This creates a mechanical advantage because the load is closer to the fulcrum than the effort as in a wheelbarrow.

In a third-class lever, the force is applied between the load and the fulcrum. The majority of muscles in the body are third-class levers. An example of a third-class lever in the body is the biceps brachii. When flexing the elbow, the elbow is the fulcrum and the force applied by the contracting biceps brachii muscle is the effort. The weight of the forearm and any object a person is carrying in their hand is the load.

Figure 11.3: Lever systems

Fascicle Arrangement

Skeletal muscles are grouped into fascicles, which are bundles of muscle fibers surrounded by a perimysium. Fascicle arrangement, the organization, and orientation of muscle fibers are correlated to the force generated by a muscle; it also affects the range of motion of the muscle. Based on the patterns of fascicle arrangement, skeletal muscles can be classified in several ways (Figure 11.4).

Figure 11.4: Muscle Shapes and Fiber Alignment The skeletal muscles of the body typically come in seven different general shapes.

The fibers of the circular or sphincter muscles are arranged in concentric rings around an opening or recess. When they contract, the size of the openings close. For example, the orbicularis oris muscle surrounds the opening of the mouth. When it contracts, the oral opening becomes smaller. Another example is the orbicularis oculi muscles surrounding the eyes.

When a muscle has a broad origin and is spread over a sizable area, but then the fascicles come to a single, common insertion point, the muscle is called convergent. The convergent muscle is triangular or fan-shaped. The large muscle on the chest, the pectoralis major, is an example of a convergent muscle because it converges on the greater tubercle of the humerus via a tendon.

Parallel muscles have fascicles that are arranged in the same direction as the long axis of the muscle. Most skeletal muscles in the body are parallel muscles. When a parallel muscle is spindle-shaped (tapers at the ends) with an expanded midsection (belly), it is called a fusiform muscle. An example of a fusiform muscle is the biceps brachii. The belly of the biceps brachii muscle is visible when you extend and then flex your forearm.

In pennate (penna = feathers) muscles, the tendon runs through the length of the muscle. The fascicles attach to the central tendon at an angle, somewhat like the quill of a feather with the muscle arranged similar to the feathers. Due to this design, the muscle fibers in a pennate muscle can only pull at an angle, and as a result, contracting pennate muscles do not move their tendons very far. There are three subtypes of pennate muscles. If all the fascicles of a pennate muscle are on the same side of the tendon, the pennate muscle is called unipennate. If the fascicles lie to either side of the tendon, the muscle is called bipennate. If the central tendon branches within a muscle, the muscle is called multipennate. Multipennate muscles look like many feathers with quills inserted into one large tendon. An example is the deltoid muscle of the shoulder, which covers the shoulder but has a single tendon that inserts on the humerus.

Coordination

Although many muscles may be involved in an action, the principal muscle involved is called the prime mover, or agonist. To lift a cup, a muscle called the biceps brachii is the prime mover. A muscle with the opposite action of the prime mover is called an antagonist. The antagonist muscle remains relaxed or stretches when a prime mover contracts to produce a movement. Antagonists also provide resistance to maintain body or limb position, such as holding the arm out or standing erect; and control rapid movement, as in shadow boxing without landing a punch or the ability to check the motion of a limb. Antagonist and agonist are located on opposite sides of the joint. For example, the triceps brachii muscle is an antagonist of the biceps brachii when we flex our arms.

Synergist muscles act around a moveable joint to produce motion similar to or in concert with agonist muscles. For example, in forearm flexion, the brachioradialis and brachialis act as synergists aiding the biceps brachii (Figure 5). Synergists can add extra force to the same movement or reduce the excessive or undesirable force generated by the agonist. A synergist can also act as a fixator that immobilizes a bone or the prime mover’s origin so that the primer can act more efficiently. For example, the scapula in the shoulder is held steady by fixator muscles when the deltoid muscle, which is attached to the scapula, contracts and pulls on the humerus to abduct the arm.

At any given time, muscles can act as agonists, antagonists, or synergists depending on the movement being performed.

Figure 11.5: Prime Movers and Synergists. The biceps brachii flexes the lower arm. The brachioradialis, in the forearm, and brachialis, located deep to the biceps in the upper arm, are both synergists that aid in this motion.

There are also skeletal muscles that do not pull against the skeleton for movements. For example, there are the muscles that produce facial expressions. The insertions and origins of facial muscles are in the skin so that certain individual muscles contract to form a smile or frown, and raise the eyebrows. There also are skeletal muscles in the tongue, and the external urinary and anal sphincters that allow for voluntary regulation of urination and defecation, respectively. In addition, the diaphragm contracts and relaxes to change the volume of the pleural cavities but it does not move the skeleton to do this.

Compartments are the grouping of muscles and associated nerves and blood vessels in your arms and legs that are separated from other tissues by a tough connective tissue membrane called fascia. Imagine plastic food wrap enclosing a piece of meat and that will give you a sense of the fascia around your muscles. Muscles in the same compartments have a common function. For example, flexor compartment muscles in the upper extremities are on the anterior surface.

11.2 Naming of Skeletal Muscles

Names of skeletal muscles may sound similar to a foreign language, but anatomists named the skeletal muscles according to many criteria, each of which describes the muscle in some way. The names of the skeletal muscles use Latin and Greek terminology or roots. Taking the time to learn the root of the words will not only help you learn the names of the skeletal muscles in this chapter, but it is also crucial to understanding the vocabulary of anatomy and physiology. Understanding the names of muscles will help you remember where the muscles are located and what they do (Figure 11.6).

Figure 11.6: Understanding a muscle name from the Latin

Table 11.1: Definitions and Mnemonic Devices for Latin Roots

Figure 11.7: Major Muscles of the Body

The names of most skeletal muscles contain word roots that describe their characteristics. These include naming the muscle after its shape, size, fiber direction, location, number of origins, or its action.

Table 11.2: Muscle Word Roots

11.3 Muscles of the Head

Muscles of Facial Expression

As you watch the hula performances, you will see the facial expressions of the hula dancers as they convey the meanings of the songs. The origins of the muscles of facial expression are on the surface of the skull. The insertions of these muscles have fibers intertwined with connective tissue and the dermis of the skin. Facial expression is a result of the contraction of these muscles. This is because these muscles insert in the skin rather than on bone, and when they contract, the skin moves resulting in facial expression.

Figure 11.8a Facial Expression: Manu Bennett and Jason Momoa perform a Haka, Māori traditional dance

Figure 11.8b: Muscles of Facial Expression. Many of the muscles of facial expression insert into the skin surrounding the eyelids, nose, and mouth, producing facial expressions by moving the skin rather than bones.

The occipitofrontalis muscle moves up the scalp and eyebrows. The muscle has a frontal belly and an occipital belly. In other words, there is a muscle on the forehead (frontalis) and one on the back of the head (occipitalis), but there is no muscle across the top of the head. Instead, the two bellies are connected by a broad tendon called the occipitofrontalis aponeurosis. Recall that tendon connects muscle to bone and an aponeurosis is a sheet of white fibrous tissue that attaches sheet-like muscles through a wide area of attachment. Orbicularis oculi is a circular muscle that closes the eyelids. This is the muscle known as the “blinking muscle”. Orbicularis oris is a circular muscle that moves the lips. This is the muscle known as the “kissing muscle”. Zygomaticus major and minor are the muscles that elevate the mouth laterally when we “smile”. Buccinator is the muscle that allows you to whistle, blow, and suck; and it contributes to the action of chewing. Remember this muscle when you are sucking your favorite guava juice on a straw. Platysma is the muscle that tenses the skin of our neck and depresses the lower lip. Corrugator supercilii is the muscle that lowers the eyebrows, like when we are frowning.

Bell’s Palsy is a paralysis of the nerves of facial expression, specifically facial nerve (CN VII). There is an inflammation of the nerve within the stylomastoid foramen, where the nerves travel through. The muscles on the same side become paralyzed (unilateral paralysis). The main treatment is reducing the symptoms. This condition can be caused by infection with the virus for herpes simplex type 1.

Muscles that Move the Eye

The movement of the eyeball is under the control of the extrinsic eye muscles, which originate outside the eye and insert onto the outer surface of the sclera of the eye (white of the eye). These muscles are found inside the eye orbit. You will study the eye muscles in more detail in the Vision section of the Special Senses chapter and the Cranial Nerves section of the Spinal Cord chapter. In general, we have four rectus muscles and two oblique muscles. The rectus muscles are straightforward, the name tells you what it does. For instance, the superior rectus muscle elevates the eyeballs. On the other hand, the oblique muscles do the opposite, such as the superior oblique depresses the eyeballs.

Muscles that Move the Mandible (Mastication)

Mastication, popularly known as chewing, is an important action in our daily lives. Muscles involved in chewing must be able to exert enough pressure to bite through and then chew food before it is swallowed [Figure with Muscles that Move the Lower Jaw]. The masseter muscle is the main muscle used for chewing because it elevates the mandible (lower jaw) to close the mouth, and it is assisted by the temporalis muscle, which retracts the mandible. While the masseter muscle protracts the mandible, the temporalis muscle retracts the mandible. You can feel the temporalis move by putting your fingers to your temple as you chew.

Figure 11.9: Muscles that Move the Lower Jaw The muscles that move the lower jaw are typically located within the cheek and originate from processes in the skull. This provides the jaw muscles with a large amount of leverage needed for chewing.

Muscles that Move the Tongue

We are aware of the importance of the tongue for mastication and deglutition (swallowing). But the tongue muscles are also important for helping with speech and sound production.

Tongue muscles can be extrinsic or intrinsic. Extrinsic tongue muscles insert into the tongue from outside origins, and the intrinsic tongue muscles insert into the tongue from origins within it. The extrinsic muscles move the whole tongue in different directions, whereas the intrinsic muscles allow the tongue to change its shape (such as, curling the tongue in a loop or flattening it).

The extrinsic muscles all include the word root glossus (glossus = tongue), and the muscle names are derived from where the muscle originates. The genioglossus (genio = chin) originates on the mandible and allows the tongue to move downward and forward. The styloglossus originates on the styloid bone and allows upward and backward motion. The palatoglossus originates on the soft palate to elevate the back of the tongue, and the hyoglossus originates on the hyoid bone to move the tongue downward and flatten it. (Figure 11.10)

Figure 11.10: Muscles that Move the Tongue

Table 11.3: Musculature of the head

11.4 Muscles of the Neck

Muscles for Deglutition

When you are eating a delicious meal of Hawaiian food, the muscles of the neck are working to help you. The muscles of the anterior neck assist in deglutition and speech by controlling the positions of the larynx (voice box), and the hyoid bone, a horseshoe-shaped bone that functions as a solid foundation on which the tongue can move. The muscles of the neck are classified according to their position relative to the hyoid bone [Figure 11.11]. Suprahyoid muscles are superior to it, and the infrahyoid muscles are found inferiorly to the hyoid bone. The suprahyoid muscles elevate the hyoid bone, the floor of the mouth, and the larynx during deglutition. These include the digastric muscle, which has anterior and posterior bellies that work to elevate the hyoid bone and larynx when one swallows; it also depresses the mandible. The stylohyoid muscle moves the hyoid bone posteriorly, elevating the larynx, and the mylohyoid muscle lifts it and helps press the tongue to the top of the mouth, elevating the floor of the mouth and the hyoid bone. The geniohyoid depresses the mandible in addition to elevating and pulling the hyoid bone anteriorly.

The strap-like infrahyoid muscles generally depress the hyoid bone and control the position of the larynx. The omohyoid muscle, which has superior and inferior bellies, depresses the hyoid bone in conjunction with the sternohyoid and thyrohyoid muscles. The thyrohyoid muscle also elevates the larynx’s thyroid cartilage, whereas the sternothyroid depresses it to create different tones of voice.

Figure 11.11: Muscles of the Anterior Neck. The anterior muscles of the neck facilitate swallowing and speech. The suprahyoid muscles originate from above the hyoid bone in the chin region. The infrahyoid muscles originate below the hyoid bone in the lower neck.

Muscles that Move the Head

The head, attached to the top of the vertebral column, is balanced, moved, and rotated by the neck muscles. When these muscles act unilaterally, the head rotates and laterally flexes the neck to its own side, as when you tilt your head to touch your ear to your shoulder. When they contract bilaterally, the head flexes. The major muscle that laterally flexes and rotates the head to the opposite side is the sternocleidomastoid. In addition, both muscles working together are the flexors of the head. Place your fingers on both sides of the neck and turn your head to the left and the right. You will feel the movement originate there. This muscle divides the neck into anterior and posterior triangles when viewed from the side. The scalene muscles work with the sternocleidomastoid to flex the neck [Figure 11.12]

Figure 11.12: (a) Posterior and Lateral Views of the Neck. The superficial and deep muscles of the neck are responsible for moving the head, cervical vertebrae, and scapulas. (b) Muscles of the Neck and Back. The large, complex muscles of the neck and back move the head, shoulders, and vertebral column.

The splenius muscles originate at the midline and run laterally and superiorly to their insertions. From the sides and the back of the neck, the splenius capitis inserts onto the region of the head, and the splenius cervicis extends onto the cervical region. These muscles can extend the head, laterally flex it, and rotate it. So, when you look up for coconuts on a coconut tree next time, think about the splenius!

11.5 Muscles of the Thorax

Muscles that Move the Abdomen and Trunk

It is a complex job to balance the body on two feet and walk upright. The muscles of the vertebral column, thorax, and abdominal wall extend, flex, and stabilize the vertebral column. The deep muscles of the core of the body help maintain posture as well as carry out other functions. The brain sends out action potentials to these various muscle groups to control posture by alternate contraction and relaxation. This is necessary so that no single muscle group becomes fatigued too quickly. If any one group fails to function, body posture will be compromised. Four pairs of abdominal muscles cover the anterior and lateral abdominal region, meeting at the anterior midline and holding the abdominal organs in place. These muscles are: the external obliques, the internal obliques, the transversus abdominis, and the rectus abdominis. [Figure 11.13]

Figure 11.13: Muscles of the Abdomen (a) The anterior abdominal muscles include the medially located rectus abdominis, which is covered by a sheet of connective tissue called the rectus sheath. On the flanks of the body, medial to the rectus abdominis, the abdominal wall is composed of three layers. The external oblique muscles form the superficial layer, while the internal oblique muscles form the middle layer, and the transversus abdominis forms the deepest layer. (b) The muscles of the lower back move the lumbar spine but also assist in femur movements.

There are three flat skeletal muscles in the anterolateral wall of the abdomen. The external oblique, closest to the surface, extends inferiorly and medially, in the direction of sliding one’s four fingers into pants pockets. This muscle forms the inguinal ligament that extends from the iliac spine to the pubic tubercle. Deep to it, is the intermediate internal oblique, extending superiorly and medially, the direction the thumbs usually go when the other fingers are in the pants pocket. The deepest muscle is the transversus abdominis, which is arranged transversely around the abdomen, similar to the front of a belt on a pair of pants. This arrangement of three bands of muscles in different orientations allows various movements and rotations of the trunk. The three layers of muscle also help to protect the internal abdominal organs in a region where there is no bone.

The linea alba is a white, vertical fibrous band that is made of the bilateral rectus sheaths that join at the anterior midline of the body. These enclose the rectus abdominis muscles (a pair of long, linear muscles, commonly called the “sit-up” muscles) that originate at the pubic crest and symphysis, and extend the length of the anteromedial abdominal wall. Each muscle is segmented by three transverse bands of collagen fibers called the tendinous intersections. This results in the look of “six-pack abs,” as each segment hypertrophies on individuals who do many sit-ups and have a toned abdominal wall.

Table 11.4: Muscles of the thoracic cage

Muscles for Breathing

The muscles of the chest serve to facilitate breathing by changing the size of the thoracic cavity. When you inhale, your thoracic cavity increases because the cavity expands, allowing air to get into the lungs. Alternately, when you exhale, your chest falls because the thoracic cavity decreases in size, forcing air out of the lungs.

The most important muscle associated with breathing is the diaphragm. The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation of the diaphragm [Figure 11.14]. This muscle separates the thoracic and abdominal cavities and is dome-shaped at rest. The superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity. The inferior surface is concave, creating the curved roof of the abdominal cavity. The muscle fibers of this muscle converge to form the central tendon. When you inhale, the diaphragm contracts and the central tendon is pulled towards the abdominal region, increasing the dimensions of the thoracic cavity.

Figure 11.14: Muscles of the Diaphragm. The diaphragm separates the thoracic and abdominal cavities.

Defecating, urination, and even childbirth involve cooperation between the diaphragm and abdominal muscles (this cooperation is referred to as the “Valsalva maneuver”). You hold your breath by a steady contraction of the diaphragm; this stabilizes the volume and pressure of the peritoneal cavity. When the abdominal muscles contract, the pressure cannot push the diaphragm up, so it increases pressure on the intestinal tract (defecation), urinary tract (urination), or reproductive tract (childbirth). The movements of the diaphragm also help with the venous blood returning to the heart from the inferior regions of the body.

As a group, the intercostal muscles assist with breathing by changing the dimensions of the rib cage. [Figure 11.15] The external intercostal muscles aid in the inspiration of air during breathing because when they contract, they expand the thoracic cavity, by elevating the ribs. The internal intercostal muscles, just under the externals, are used for expiration because they depress and draw the ribs together to constrict the thoracic cage. The innermost intercostal muscles are the deepest, and they act as synergists for the action of the internal intercostals.

Figure 11.15: Intercostal Muscles. The external intercostals are located laterally on the sides of the body. The internal intercostals are located medially near the sternum. The innermost intercostals are located deep to both the internal and external intercostals.

Muscles that Move the Pelvic Floor and Perineum

The pelvic floor is a muscular sheet that defines the inferior portion of the pelvic cavity. The muscles in this area support the pelvic viscera. The pelvic diaphragm, spanning anteriorly to posteriorly from the pubis to the coccyx, comprises the levator ani and the ischiococcygeus. Its openings include the anal canal and urethra, and vagina in women.

The large levator ani consists of two skeletal muscles, the pubococcygeus, and the iliococcygeus. The levator ani is considered the most important muscle of the pelvic floor because it supports the pelvic viscera. It resists the pressure produced by contraction of the abdominal muscles so that the pressure is applied to the colon to aid in defecation and to the uterus to aid in childbirth (assisted by the ischiococcygeus, which pulls the coccyx anteriorly). This muscle also creates skeletal muscle sphincters at the urethra and anus. [Figure 11.16]

Figure 11.16: Muscles of the Pelvic Floor. The pelvic floor muscles support the pelvic organs, resist intra-abdominal pressure, and work as sphincters for the urethra, rectum, and vagina.

The perineum is the diamond-shaped space between the pubic symphysis (anteriorly), the coccyx (posteriorly), and the ischial tuberosities (laterally), lying just inferior to the pelvic diaphragm (levator ani and coccygeus). Divided transversely into triangles, the anterior is the urogenital triangle, which contains the external genitalia and urethra. The bulbospongiosus is found in men and women. In men, this muscle ejects urine or semen and stiffens the penis; while in women, this muscle narrows the opening of the vagina and stiffens the clitoris. The ischiocavernosus muscle helps with the erection of the penis or clitoris.

The posterior is the anal triangle, which contains the anus. The external anal sphincter constricts the anal opening and to defecate, this muscle must voluntarily relax. The perineum is also divided into superficial and deep layers with some of the muscles common to men and women. Women also have the compressor urethrae and the sphincter urethrovaginalis, which function to close the vagina. In men, there is the deep transverse perineal muscle that plays a role in ejaculation. [Figure 11.17]

Figure 11.17: Muscles of the Perineum. The perineum muscles play roles in urination in both sexes, ejaculation in men, and vaginal contraction in women.

Table 11.5: Muscles of the Pelvic Floor and Perineum

Muscles that Move the Vertebral Column

The back muscles stabilize and move the vertebral column, and are grouped according to the lengths and direction of the fascicles.

The erector spinae group composes most of the muscle mass of the back and it is the primary extensor of the vertebral column. It controls flexion, lateral flexion, and rotation of the vertebral column, and maintains posture while helping a person to stand erect. The erector spinae comprises the iliocostalis (laterally found) group, the longissimus (intermediately localized) group, and the spinalis (medially placed) group.

The transversospinalis muscles are localized deep to the erector spinae muscle and run from the transverse processes to the spinous processes of the vertebrae. These muscles help with the stabilization of the vertebrae. [Figure 11.18]

The COVID-19 pandemic has forced all of us to spend more time on laptop computers and mobile phones at kitchen tables or sofas while we study or take classes remotely. Sitting long hours in an ergonomically incorrect position leads to strain on the neck, lower back, and hip causing repetitive trauma. When we use social media, we tend to bend our necks to look down at our phones or lean over the laptop for hours. Proper body position is important, but taking frequent breaks from the screen and moving can also reduce the risk of developing these ergonomically related problems.

Figure 11.18: Posterior and Lateral Views of the Neck. The superficial and deep muscles of the neck are responsible for moving the head, cervical vertebrae, and scapulas.

11.6 Upper Extremity

Muscles of the shoulder and upper extremity serve to stabilize and position the pectoral girdle and move the arm, forearm, and wrists, hands, and fingers. The upper extremity can move through a large range of motion and the muscles here are capable of accomplishing intricate movements such as playing the ukulele.

Figure 11.19: ʻUkulele Player: A Kohala Seminary student poses with her ukulele in this 1912 photo.

Muscles that Move the Scapula and Humerus

Your pectoral (shoulder) girdle consists of the clavicle (collarbone) and scapula (shoulder blade). The muscles that cross the pectoral girdle act as stabilizers for the scapula which serves as the origin for many muscles that move the shoulder (glenohumeral) joint. The full range of motion at the shoulder is highly dependent upon scapular movement. For example, if your scapula did not move, you would not be able to reach your arm over your head.

The pectoralis minor and serratus anterior are important stabilizers of the scapula that originate from the anterior thorax. The trapezius, rhomboid major, and rhomboid minor position the scapula posteriorly. These muscles help to position the scapula against the posterior thoracic wall.

The muscles that cross the shoulder joint and move the humerus include both muscles that arise from the axial skeleton (axial) and the scapula. The two axial muscles are the pectoralis major and the latissimus dorsi. The pectoralis major is a fan-shaped muscle that covers most of the superior portion of the anterior thorax. The broad and triangular latissimus dorsi (“latissimus” = broadest, “dorsum” = back) is located on the inferior part of the back. It produces many of the movements required for swimming which is why many swimmers have very well-developed “lats”.

The other sets of shoulder muscles originate on the scapula. The deltoid is a thick and powerful muscle that covers the shoulder joint and is responsible for the abduction of the arm. When you receive a vaccine shot, this is the site of intramuscular injections. The subscapularis originates on the anterior scapula, and it fills the subscapular fossa where it medially rotates the arm. Named for their locations, the supraspinatus (superior to the spine of the scapula) abducts the arm and the infraspinatus (inferior to the spine of the scapula) laterally rotates the arm. The long teres minor laterally rotates and extends the arm. The teres minor lies inferior to the infraspinatus and superior to the teres major. The thick and flattened teres major is responsible for extending the arm and assisting in adduction and medial rotation.

When you hear about friends or family who suffer from rotator cuff injuries, it is related to the four deep muscles of the shoulder. The tendons of the subscapularis, supraspinatus, infraspinatus, and teres minor connect the scapula to the humerus, forming the rotator cuff (musculotendinous cuff), the circle of tendons around the shoulder joint. These muscles are important stabilizers of the shoulder joint. The supraspinatus muscle is subject to wear and tear more than the other three muscles due to its location when abducting the arm. When baseball pitchers undergo shoulder surgery it is usually on the rotator cuff, which becomes pinched and inflamed, and may tear away from the bone due to the repetitive motion of bringing the arm overhead to throw a fastpitch.

Figure 11.20 Muscles that Position the Pectoral Girdle The muscles that stabilize the pectoral girdle make it a steady base on which other muscles can move the arm. Note that the pectoralis major and deltoid, which move the humerus, are cut here to show the deeper positioning muscles. (OpenStax)

Figure 11.21 Muscles that Move the Humerus (a, c) The muscles that move the humerus anteriorly are generally located on the anterior side of the body and originate from the sternum (e.g., pectoralis major) or the anterior side of the scapula (e.g., subscapularis). (b) The muscles that move the humerus superiorly generally originate from the superior surfaces of the scapula and/or the clavicle (e.g., deltoids). The muscles that move the humerus inferiorly generally originate from the middle or lower back (e.g., latissimus dorsi). (d) The muscles that move the humerus posteriorly are generally located on the posterior side of the body and insert into the scapula (e.g., infraspinatus). (OpenStax)

Table 11.7: Muscles That Move the Scapula

Muscles that Move the Radius and Ulna

Our muscles that move the forearm produce flexion, extension, pronation, and supination at the elbow (a hinge joint). When you are doing biceps curls at home or the beach, have you wondered about the muscles involved? The forearm flexors include the biceps brachii, brachialis, and brachioradialis. The two-headed biceps brachii crosses the shoulder and elbow joints to flex the forearm. It also takes part in supinating the forearm at the radioulnar joints and flexing the arm at the shoulder joint. The brachialis, which sits deep to the biceps brachii, and the brachioradialis also contribute to forearm flexion.

The primary extensor of the forearm is the triceps brachii and it is a large muscle located on the posterior side of the arm. As the name implies, it has three heads of origin where the long head is from the scapula where the lateral and medial heads are from the humerus.

Found on the anterior aspect of the forearm, the pronator teres is responsible for pronation at the elbow or turning your palm down. The supinator muscle, found on the posterior aspect of the forearm, performs supination, turning your palm up, along with the biceps brachii.

Figure 11.22 Muscles that Move the Forearm The muscles originating in the upper arm flex, extend, pronate, and supinate the forearm. The muscles originating in the forearm move the wrists, hands, and fingers. (OpenStax)

Table 11.9: Muscles That Move the Radius and Ula

Muscles that Move the Wrist, Hand, and Fingers

The wrist, hand, and finger movements are facilitated by two groups of muscles and these muscles help us perform a lot of daily functions, such as playing the ukulele. The extrinsic muscles originate outside of the hand and insert within it (“ex”=outside) whereas the intrinsic muscles originate and insert within the hand. You will find that the muscle names give hints to their origin, insertion, or action.

Based on the location and function, these muscles are categorized into two groups, anterior and posterior compartments. The muscles in the anterior compartment of the forearm are primarily flexors and pronators and they originate on the medial humerus and insert into the wrist and hand. Muscles of the anterior compartment include; flexor carpi radialis, palmaris longus, flexor carpi ulnaris, flexor digitorum superficialis, flexor pollicis longus, and flexor digitorum profundus.

The tendons of the forearm muscles attach to the wrist and hand. The flexor retinaculum extends over the palmar surface of the carpal bones where you can find a narrow space called the carpal tunnel. The carpal tunnel is a small space that houses 9 flexor tendons and the median nerve. Overuse of the finger and wrist flexors can cause inflammation in this space resulting in Carpal Tunnel Syndrome. In this condition, the median nerve is compressed which can result in painful or numb sensations in the hand, loss of grip and strength in the hand, or a combination of those symptoms.

Figure 11.23: Carpal Tunnel Horizontal/cross-section of the carpal tunnel showing the carpal bones, transverse carpal ligament, and median nerve. The carpal tunnel is the passageway by which nine muscle tendons and a major nerve enter the hand from the anterior forearm. The walls and floor of the carpal tunnel are formed by the U-shaped grouping of the carpal bones, and the roof is formed by the flexor retinaculum, a strong ligament that anteriorly unites the bones.

Golfer’s elbow is a condition that is caused by repetitive strain of the flexor muscles, especially the flexor carpi radialis, from swinging a golf club. It can also be seen in other actions commonly observed when playing the piano and moving weights.

The muscles in the posterior compartment of the forearm are extensors and supinators that originate on the lateral humerus, radius, and ulna. Posterior compartment muscles include; extensor carpi radialis longus, extensor carpi radialis brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis.

Unlike the extrinsic muscles that we mentioned earlier, the intrinsic muscles of the hand both originate and insert within the hand. These muscles allow your fingers to also make more precise movements for actions, such as sewing, typing, or writing. These muscles are divided into three groups. The thenar muscles are on the radial aspect of the palm (thumb side). The hypothenar muscles are on the medial aspect of the palm (pinky side), and the intermediate muscles are midpalmar.

Figure 11.24: Hand, including hand muscles

Table 11.10: Muscles That Move the Wrist/Hand

11.7 Lower Extremity

The lower extremity is attached to the thorax via the pelvic girdle. There is much less motion at the pelvic girdle as compared to the pectoral girdle, however, this articulation is very stable and strong, forming a solid foundation for the lower extremity. As compared to the upper extremity muscles, leg muscles produce far less range of motion but are much more powerful as they are important in stabilizing and moving the body.

Muscles that Move the Femur

Many of the muscles that act on the femur originate from the pelvic girdle. The iliopsoas (psoas major and iliacus combined) connects the thorax to the femur and is a strong hip flexor. On the opposite side of the body, the large and powerful gluteus maximus produces hip extension. The gluteus medius is a fan-shaped muscle that stabilizes the pelvis and produces hip abduction. It also serves as a common site for intramuscular injection due to the thickness of the muscle, rich blood supply which promotes faster absorption, and lack of major nerves and blood vessels within the belly of the muscle. The tensor fascia latae works with the gluteus medius and iliopsoas in producing flexion and abduction at the hip. This muscle is unique in that it inserts into the iliotibial tract, a thick band of connective tissue that runs the length of the lateral thigh and attaches at the knee. Deep within the gluteal area are the six external rotators of the hip which includes the piriformis muscle. The piriformis is clinically relevant in that the large sciatic nerve exits directly below it, and in some cases pierces this muscle. If the piriformis is tight this can cause symptoms of sciatica.

Figure 11.25 Hip and Thigh Muscles The large and powerful muscles of the hip that move the femur generally originate on the pelvic girdle and insert into the femur. The muscles that move the lower leg typically originate on the femur and insert into the bones of the knee joint. The anterior muscles of the femur extend the lower leg but also aid in flexing the thigh. The posterior muscles of the femur flex the lower leg but also aid in extending the thigh. A combination of gluteal and thigh muscles also adduct, abduct, and rotate the thigh and lower leg.

Table 11.11: Muscles of the Anterior Compartment

Muscles that Move the Femur, Tibia, and Fibula

The deep fascia separates the thigh into anterior, posterior, and medial compartments. Muscles within these compartments typically share a common action and innervation.

Figure 11.26: Compartments of thigh

Anterior Compartment

The anterior compartment of the thigh is composed of the quadriceps femoris, four separate muscles which share a common insertion. The rectus femoris is the only muscle of the four that crosses the hip and knee joint. It is sometimes referred to as the “kicking” muscle due to its ability to produce hip flexion and knee extension. The vastus lateralis is found on the lateral aspect of the thigh while the vastus medialis is medial. The vastus intermedius lays deep to the rectus femoris, between the vastus medialis and vastus lateralis. All four muscles insert into the quadriceps tendon which continues as the patellar ligament inserting at the tibial tuberosity. As a result, all four muscles contribute to knee extension and stabilization of the knee joint.

Figure 11.27: Femoral triangle

Also found in the anterior compartment is the sartorius, a very long muscle that extends from the anterior superior iliac spine to the medial side of the proximal tibia. This muscle allows you to cross one leg on top of the other as it produces hip flexion, abduction, and lateral rotation at the hip in addition to flexion at the knee.

Compartment syndrome is a condition that results from excess external or internal pressure, such as from a direct blow or overuse of the muscles within the compartment. Symptoms include pain, burning, weakness, and paralysis. Because the deep fascia that encloses the compartments is very strong, pressure may continue to build and cut off the blood supply to muscles and vessels within the compartment. In severe cases, a fasciotomy may be necessary. In this surgical procedure, the fascia is cut to relieve the pressure.

Table 11.12. Muscles of the Anterior Compartment

Posterior Compartment

The hamstrings, a group of three muscles including the biceps femoris, semitendinosus, and semimembranosus, collectively comprise the posterior compartment of the thigh. These muscles are responsible for hip extension and knee flexion. Tendons of these muscles are found at the back of the knee and frame the popliteal fossa, the diamond-shaped space at the back of the knee. These muscles share a common origin at the ischial tuberosity and attach at three different locations across the knee joint.

Hamstring strains are common amongst athletes that produce explosive movements or change direction quickly. A strain is stretching or tearing of muscle tissue (recall that a sprain refers to tearing of a ligament). Someone with a hamstring strain will have pain with hip extension and knee flexion and in some cases present with a contusion and/or palpable deformity of the muscle.

Table 11.13. Muscles of the Posterior Compartment

Medial or Adductor Compartment

The medial compartment is also referred to as the adductor compartment since the muscles found within it are primarily responsible for adducting the femur. Many of the muscles found within this compartment are named for their action and size including; adductor longus, adductor brevis, and adductor magnus. The pectineus and gracilis muscles are also found within the medial compartment and contribute to the adduction of the femur.

Table 11.14 Muscles of the Medial Compartment

Muscles that Move the Foot and Toes

Similar to the thigh muscles, the muscles of the leg are divided by deep fascia into anterior, lateral, and posterior compartments.

Cross-section through middle of leg. Fascial compartments of leg are colored.	Fascial compartments of leg. Anterior compartment of leg. Lateral compartment of leg. Deep posterior compartment of leg. Superficial posterior compartment of leg.

Figure 11.28: Cross-section showing compartments of leg

The anterior compartment muscles of the lower leg are responsible for dorsiflexing the ankle and extending the toes. The tibialis anterior is found on the lateral surface of the tibia and produces dorsiflexion in addition to inversion. The extensor digitorum longus and extensor hallucis longus are responsible for extending the five toes and assisting with dorsiflexion. The superior and inferior extensor retinaculum hold these tendons in place at the ankle.

The lateral compartment of the leg is composed of two muscles; the fibularis longus (peroneus longus) and the fibularis brevis (peroneus brevis). These muscles produce plantarflexion and evert the ankle. A lateral ankle sprain is a common injury that results from excessive inversion of the foot and ankle. In addition to torn ligaments, the fibularis longus and brevis may also be strained. In some cases, the fibularis brevis tendon can avulse (tear away) from its attachment on the base of the fifth metatarsal.

The posterior compartment of the lower leg can be further subdivided into superficial and deep layers. The gastrocnemius and soleus are found in the superficial compartment and share a common insertion, the calcaneal or Achilles tendon. These muscles are very powerful plantar flexors and help to keep us upright. The gastrocnemius is the most superficial of the two and is easily observable (balls of muscle just underneath the knee joint) while the flat soleus is found just underneath the gastrocnemius.

The flexor digitorum longus, flexor hallucis longus, and tibialis posterior are found in the deep posterior compartment of the lower leg and also contribute to plantar flexion in addition to flexion of the toes.

Similar to the hand, the foot also possesses intrinsic muscles that originate and insert directly within it. These muscles are important stabilizers and help to support the arches of the foot.

In addition to the intrinsic muscles, the plantar fascia is also important to support the foot, specifically the longitudinal arch. It runs along the bottom of the foot from the calcaneus to the toes. Plantar fasciitis (inflammation of the plantar fascia) is a common injury that produces pain in the heel at the origin of the plantar fascia. This condition can occur in anyone who spends a lot of time on their feet, particularly if they are wearing flat shoes without arch support (slippahs). They will often describe intense pain at the heel first thing in the morning when stepping out of bed.

Figure 11.29 Muscles of the Lower Leg The muscles of the anterior compartment of the lower leg are generally responsible for dorsiflexion, and the muscles of the posterior compartment of the lower leg are generally responsible for plantar flexion. The lateral and medial muscles in both compartments invert, evert, and rotate the foot (OpenStax).

Table 11.15: Muscles That Move the Foot and Toes

Chapter Summary

Key Terms

abduct

move away from midline in the sagittal plane

abductor pollicis longus

muscle that inserts into the first metacarpal

adductor

moves the bone toward the midline

adductor brevis

muscle that adducts and medially rotates the thigh

adductor longus

muscle that adducts, medially rotates, and flexes the thigh

adductor magnus

muscle with an anterior fascicle that adducts, medially rotates and flexes the thigh, and a posterior fascicle that assists in thigh extension

agonist

(also, prime mover) muscle whose contraction is responsible for producing a particular motion

anal triangle

posterior triangle of the perineum that includes the anus

antagonist

muscle that opposes the action of an agonist

anterior compartment of the forearm

(anterior flexor compartment of the forearm) deep and superficial muscles that originate on the humerus and insert into the hand

anterior compartment of the thigh

region that includes muscles that flex the thigh and extend the leg

appendicular

of the arms and legs

axial

of the trunk and head

belly

bulky central body of a muscle

bi

two

biceps brachii

two-headed muscle that crosses the shoulder and elbow joints to flex the forearm while assisting in supinating it and flexing the arm at the shoulder

biceps femoris

hamstring muscle

bipennate

pennate muscle that has fascicles that are located on both sides of the tendon

brachialis

muscle deep to the biceps brachii that provides power in flexing the forearm.

brachioradialis

muscle that can flex the forearm quickly or help lift a load slowly

brevis

short

buccinator

muscle that compresses the cheek

circular

(also, sphincter) fascicles that are concentrically arranged around an opening

compressor urethrae

deep perineal muscle in women

convergent

fascicles that extend over a broad area and converge on a common attachment site

corrugator supercilii

prime mover of the eyebrows

deep transverse perineal

deep perineal muscle in men

deglutition

swallowing

deltoid

shoulder muscle that abducts the arm as well as flexes and medially rotates it, and extends and laterally rotates it

diaphragm

skeletal muscle that separates the thoracic and abdominal cavities and is dome-shaped at rest

digastric

muscle that has anterior and posterior bellies and elevates the hyoid bone and larynx when one swallows; it also depresses the mandible

erector spinae group

large muscle mass of the back; primary extensor of the vertebral column

extensor

muscle that increases the angle at the joint

extensor carpi ulnaris

muscle that extends and adducts the hand

extensor digiti minimi

muscle that extends the little finger

extensor digitorum

muscle that extends the hand at the wrist and the phalanges

extensor digitorum longus

muscle that is lateral to the tibialis anterior

extensor hallucis longus

muscle that is partly deep to the tibialis anterior and extensor digitorum longus

extensor indicis

muscle that inserts onto the tendon of the extensor digitorum of the index finger

extensor pollicis brevis

muscle that inserts onto the base of the proximal phalanx of the thumb

extensor pollicis longus

muscle that inserts onto the base of the distal phalanx of the thumb

extensor radialis longus

muscle that extends and abducts the hand at the wrist

extensor retinaculum

band of connective tissue that extends over the dorsal surface of the hand

external intercostal

superficial intercostal muscles that raise the rib cage

external oblique

superficial abdominal muscle with fascicles that extend inferiorly and medially

extrinsic eye muscles

originate outside the eye and insert onto the outer surface of the white of the eye, and create eyeball movement

extrinsic muscles of the hand

muscles that move the wrists, hands, and fingers and originate on the arm

fascicle

muscle fibers bundled by perimysium into a unit

femoral triangle

region formed at the junction between the hip and the leg and includes the pectineus, femoral nerve, femoral artery, femoral vein, and deep inguinal lymph nodes

fibularis brevis

(also, peroneus brevis) muscle that plantar flexes the foot at the ankle and everts it at the intertarsal joints

fibularis longus

(also, peroneus longus) muscle that plantar flexes the foot at the ankle and everts it at the intertarsal joints

fixator

synergist that assists an agonist by preventing or reducing movement at another joint, thereby stabilizing the origin of the agonist

flexion

movement that decreases the angle of a joint

flexor

muscle that decreases the angle at the joint

flexor carpi radialis

muscle that flexes and abducts the hand at the wrist

flexor carpi ulnaris

muscle that flexes and adducts the hand at the wrist

flexor digitorum longus

muscle that flexes the four small toes

flexor digitorum profundus

muscle that flexes the phalanges of the fingers and the hand at the wrist

flexor digitorum superficialis

muscle that flexes the hand and the digits

flexor hallucis longus

muscle that flexes the big toe

flexor pollicis longus

muscle that flexes the distal phalanx of the thumb

flexor retinaculum

band of connective tissue that extends over the palmar surface of the hand

frontalis

front part of the occipitofrontalis muscle

fusiform

muscle that has fascicles that are spindle-shaped to create large bellies

gastrocnemius

most superficial muscle of the calf

genioglossus

muscle that originates on the mandible and allows the tongue to move downward and forward

geniohyoid

muscle that depresses the mandible, and raises and pulls the hyoid bone anteriorly

gluteus maximus

largest of the gluteus muscles that extends the femur

gluteus medius

muscle deep to the gluteus maximus that abducts the femur at the hip

gracilis

muscle that adducts the thigh and flexes the leg at the knee

hyoglossus

muscle that originates on the hyoid bone to move the tongue downward and flatten it

hypothenar

group of muscles on the medial aspect of the palm

iliacus

muscle that, along with the psoas major, makes up the iliopsoas

iliococcygeus

muscle that makes up the levator ani along with the pubococcygeus

iliocostalis group

laterally placed muscles of the erector spinae

iliotibial tract

muscle that inserts onto the tibia; made up of the gluteus maximus and connective tissues of the tensor fasciae latae

infrahyoid muscles

anterior neck muscles that are attached to, and inferior to the hyoid bone

infraspinatus

muscle that laterally rotates the arm

innermost intercostal

the deepest intercostal muscles that draw the ribs together

insertion

end of a skeletal muscle that is attached to the structure (usually a bone) that is moved when the muscle contracts

intercostal muscles

muscles that span the spaces between the ribs

intermediate

group of midpalmar muscles

internal intercostal

muscles the intermediate intercostal muscles that draw the ribs together

internal oblique

flat, intermediate abdominal muscle with fascicles that run perpendicular to those of the external oblique

intrinsic muscles of the hand

muscles that move the wrists, hands, and fingers and originate in the palm

ischiococcygeus

muscle that assists the levator ani and pulls the coccyx anteriorly

lateral compartment of the leg

region that includes the fibularis (peroneus) longus and the fibularis (peroneus) brevis and their associated blood vessels and nerves

lateralis

to the outside

latissimus dorsi

broad, triangular axial muscle located on the inferior part of the back

levator ani

pelvic muscle that resists intra-abdominal pressure and supports the pelvic viscera

linea alba

white, fibrous band that runs along the midline of the trunk

longissimus group

intermediately placed muscles of the erector spinae

longus

long

masseter

main muscle for chewing that elevates the mandible to close the mouth

mastication

chewing

maximus

largest

medialis

to the inside

medius

medium

minimus

smallest

multipennate

pennate muscle that has a tendon branching within it

mylohyoid

muscle that lifts the hyoid bone and helps press the tongue to the top of the mouth

oblique

at an angle

occipitalis

posterior part of the occipitofrontalis muscle

occipitofrontalis

muscle that makes up the scalp with a frontal belly and an occipital belly

omohyoid

muscle that has superior and inferior bellies and depresses the hyoid bone

orbicularis oculi

circular muscle that closes the eye

orbicularis oris

circular muscle that moves the lips

origin

end of a skeletal muscle that is attached to another structure (usually a bone) in a fixed position

palatoglossus

muscle that originates on the soft palate to elevate the back of the tongue

palmaris longus

muscle that provides weak flexion of the hand at the wrist

parallel

fascicles that extend in the same direction as the long axis of the muscle

patellar ligament

extension of the quadriceps tendon below the patella

pectineus

muscle that abducts and flexes the femur at the hip

pectoral girdle

shoulder girdle, made up of the clavicle and scapula

pectoralis major

thick, fan-shaped axial muscle that covers much of the superior thorax

pectoralis minor

muscle that moves the scapula and assists in inhalation

pelvic diaphragm

muscular sheet that comprises the levator ani and the ischiococcygeus

pelvic girdle

hips, a foundation for the lower limb

pennate

fascicles that are arranged differently based on their angles to the tendon

perineum

diamond-shaped region between the pubic symphysis, coccyx, and ischial tuberosities

piriformis

muscle deep to the gluteus maximus on the lateral surface of the thigh that laterally rotates the femur at the hip

popliteal fossa

diamond-shaped space at the back of the knee

posterior compartment of the thigh

region that includes muscles that flex the leg and extend the thigh

prime mover

(also, agonist) principle muscle involved in an action

pronator teres

pronator that originates on the humerus and inserts on the radius

psoas major

muscle that, along with the iliacus, makes up the iliopsoas

pubococcygeus

muscle that makes up the levator ani along with the iliococcygeus

quadriceps tendon

(also, patellar tendon) tendon common to all four quadriceps muscles, inserts into the patella

rectus

straight

rectus abdominis

long, linear muscle that extends along the middle of the trunk

rectus femoris

quadricep muscle on the anterior aspect of the thigh

rectus sheaths

tissue that makes up the linea alba

rhomboid major

muscle that attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae

rhomboid minor

muscle that attaches the vertebral border of the scapula to the spinous process of the thoracic vertebrae

rotator cuff

(also, musculotendinous cuff) the circle of tendons around the shoulder joint

sartorius

band-like muscle that flexes, abducts, and laterally rotates the leg at the hip

scalene muscles

flex, laterally flex, and rotate the head; contribute to deep inhalation

semimembranosus

hamstring muscle

semitendinosus

hamstring muscle

serratus anterior

large and flat muscle that originates on the ribs and inserts onto the scapula

soleus

wide, flat muscle deep to the gastrocnemius

sphincter urethrovaginalis

deep perineal muscle in women

spinalis group

medially placed muscles of the erector spinae

splenius

posterior neck muscles; includes the splenius capitis and splenius cervicis

splenius capitis

neck muscle that inserts into the head region

splenius cervicis

neck muscle that inserts into the cervical region

sternocleidomastoid

major muscle that laterally flexes and rotates the head

sternohyoid

muscle that depresses the hyoid bone

sternothyroid

muscle that depresses the larynx’s thyroid cartilage

styloglossus

muscle that originates on the styloid bone, and allows upward and backward motion of the tongue

stylohyoid

muscle that elevates the hyoid bone posteriorly

subscapularis

muscle that originates on the anterior scapula and medially rotates the arm

supinator

muscle that moves the palm and forearm anteriorly

suprahyoid muscles

neck muscles that are superior to the hyoid bone

supraspinatus

muscle that abducts the arm

synergist

muscle whose contraction helps a prime mover in an action

temporalis

muscle that retracts the mandible

tendinous intersections

three transverse bands of collagen fibers that divide the rectus abdominis into segments

tensor fascia lata

muscle that flexes and abducts the thigh

teres major

muscle that extends the arm and assists in adduction and medial rotation of it

teres minor

muscle that laterally rotates and extends the arm

thenar

group of muscles on the lateral aspect of the palm

thyrohyoid

muscle that depresses the hyoid bone and elevates the larynx’s thyroid cartilage

tibialis anterior

muscle located on the lateral surface of the tibia

tibialis posterior

muscle that plantar flexes and inverts the foot

transversus abdominis

deep layer of the abdomen that has fascicles arranged transversely around the abdomen

trapezius

muscle that stabilizes the upper part of the back

tri

three

triceps brachii

three-headed muscle that extends the forearm

unipennate

pennate muscle that has fascicles located on one side of the tendon

urogenital triangle

anterior triangle of the perineum that includes the external genitals

vastus intermedius

quadricep muscle that is between the vastus lateralis and vastus medialis and is deep to the rectus femoris

vastus lateralis

quadricep muscle on the lateral aspect of the thigh

vastus medialis

quadricep muscle on the medial aspect of the thigh

Sources

• OpenStax Anatomy and Physiology (http://openstax.org/details/books/anatomy-and-physiology)
• Principles of Anatomy and Physiology, 15th edition, Gerard J. Tortora; Brayn H. Derrickson
• https://www.assp.org/docs/default-source/psj-articles/cpjunkin_1220.pdf?sfvrsn=412a8c47_2
• https://www.nytimes.com/2020/09/04/well/live/ergonomics-work-from-home-injuries.html
• http://www.hawaiihistory.org/