# Lecture 12: Major Skeletal Muscles

## Anatomy and Physiology I

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the criteria used to name skeletal muscles
2. Explain the functional roles of muscles (agonist, antagonist, synergist, fixator)
3. Describe the lever systems of the body
4. Identify the major muscles of the head, neck, trunk, and limbs, including their origins, insertions, and primary actions

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## Lecture Content

### I. Naming Skeletal Muscles

Muscles are named based on one or more of the following criteria. **Location** is used for muscles such as the temporalis (over the temporal bone) and the intercostals (between the ribs). **Shape** gives rise to names like deltoid (triangular), trapezius (trapezoidal), and rhomboid. **Size** distinguishes muscles like the gluteus maximus (largest) from the gluteus minimus (smallest), and terms like longus (long) and brevis (short) are also size-based. **Direction of fibers** accounts for names like rectus (parallel to the midline), oblique (diagonal), and transversus (perpendicular to the midline). **Number of origins** explains the naming of the biceps (two origins), triceps (three), and quadriceps (four). **Origin and insertion** together produce names such as sternocleidomastoid (sternum and clavicle to the mastoid process). **Action** yields names like flexor, extensor, adductor, abductor, levator (lifts), and depressor (lowers).

### II. Functional Roles of Muscles

The **agonist (prime mover)** is the muscle primarily responsible for producing a specific movement; for example, the biceps brachii is the agonist for elbow flexion. The **antagonist** opposes or reverses the action of the agonist, relaxing or lengthening while the agonist contracts; the triceps brachii is the antagonist during elbow flexion. Agonist-antagonist pairs allow smooth, controlled movements. A **synergist** assists the agonist by stabilizing joints or preventing unwanted movements; the brachialis and brachioradialis assist the biceps during elbow flexion. A **fixator** is a synergist that stabilizes the origin of the agonist so it can act more efficiently; the muscles stabilizing the scapula during arm movements are an example.

### III. Lever Systems

A lever consists of a rigid bar (bone) moving on a fulcrum (joint), with effort (muscle force) applied to move a resistance (load). In a **first-class lever**, the fulcrum lies between the effort and the load, as when the head nods on the atlas (analogous to a seesaw). In a **second-class lever**, the load lies between the fulcrum and the effort, as when standing on tiptoes (analogous to a wheelbarrow). In a **third-class lever**, the effort lies between the fulcrum and the load, as when the biceps flexes the elbow. Third-class levers are the most common type in the body and favor speed and range of motion over force.

### IV. Major Muscles of the Head and Neck

#### Muscles of Facial Expression

The muscles of facial expression originate from skull bones or fascia and insert into skin. They are innervated by cranial nerve VII (the facial nerve). The **frontalis** raises the eyebrows and wrinkles the forehead. The **orbicularis oculi** closes the eyelids in blinking and squinting. The **orbicularis oris** closes and protrudes the lips (the "kissing muscle"). The **buccinator** compresses the cheeks during blowing and chewing, keeping food between the teeth. The **zygomaticus major** draws the corner of the mouth superiorly and laterally, producing a smile. The **platysma** depresses the mandible and tenses the skin of the neck.

#### Muscles of Mastication (Chewing)

The muscles of mastication are innervated by cranial nerve V (the trigeminal nerve, mandibular division). The **masseter** is the strongest muscle for chewing, elevating (closing) the mandible. It originates from the zygomatic arch and inserts on the angle and ramus of the mandible. The **temporalis** elevates and retracts the mandible, originating from the temporal fossa and inserting on the coronoid process of the mandible. The **medial and lateral pterygoids** assist in elevation, protrusion, and lateral (side-to-side) movements of the mandible.

#### Muscles of the Neck

The **sternocleidomastoid (SCM)** is a two-headed muscle on the anterolateral neck, originating from the sternum and clavicle and inserting on the mastoid process of the temporal bone. Unilateral contraction rotates the head to the opposite side and tilts it to the same side, while bilateral contraction flexes the neck. It is innervated by cranial nerve XI (the accessory nerve). The **scalenes (anterior, middle, and posterior)** are lateral neck muscles that elevate the ribs during forced inspiration and flex and laterally bend the neck. The **suprahyoid muscles** (digastric, mylohyoid, geniohyoid, and stylohyoid) elevate the hyoid bone during swallowing. The **infrahyoid muscles** (sternohyoid, omohyoid, sternothyroid, and thyrohyoid) depress the hyoid bone and larynx.

<image>An anterior view of the head and neck showing the major muscles of facial expression and mastication. On the left side, the superficial muscles are intact: frontalis, orbicularis oculi, zygomaticus major, orbicularis oris, buccinator, and platysma are labeled. On the right side, deeper structures are exposed showing the masseter, temporalis (seen through a window in the temporal fascia), and the sternocleidomastoid on the neck. The hyoid bone is visible with suprahyoid and infrahyoid muscle groups indicated. Each muscle is labeled with a line pointing to its location.</image>

### V. Muscles of the Trunk

#### Muscles of the Anterior Trunk (Abdominal Wall)

The **rectus abdominis** is the "six-pack" muscle, running vertically from the pubic crest to the xiphoid process and costal cartilages 5 through 7. It flexes the vertebral column (trunk flexion) and compresses the abdomen. Tendinous intersections create its segmented appearance. The **external oblique** is the superficial lateral abdominal muscle, with fibers running inferomedially in the "hands in pockets" direction. It compresses the abdomen and rotates and laterally flexes the trunk. The **internal oblique** lies deep to the external oblique, with fibers running superiorly and medially (perpendicular to the external oblique), performing the same actions. The **transversus abdominis** is the deepest abdominal muscle, with horizontally running fibers that compress the abdomen and is the most important muscle for core stability. The **linea alba** is the midline tendinous seam from the xiphoid to the pubic symphysis, formed by the fusion of the abdominal muscle aponeuroses.

#### Muscles of Breathing

The **diaphragm** is the dome-shaped muscle separating the thoracic and abdominal cavities and is the primary muscle of inspiration. It originates from the inferior border of the rib cage, the xiphoid process, and the lumbar vertebrae, and inserts into the central tendon. When it contracts, the dome flattens, increasing thoracic volume and decreasing intrathoracic pressure so that air flows in. It is innervated by the phrenic nerve (C3, C4, C5 — "C3, 4, 5 keeps the diaphragm alive"). The **external intercostals** lie between the ribs with fibers running inferiorly and anteriorly; they elevate the ribs during inspiration, expanding the rib cage. The **internal intercostals** also lie between the ribs but with fibers running inferiorly and posteriorly (perpendicular to the external intercostals); they depress the ribs during forced expiration.

#### Muscles of the Back

The **erector spinae group** consists of three columns running vertically along the vertebral column: the iliocostalis (lateral), longissimus (intermediate), and spinalis (medial). They extend and laterally flex the vertebral column and are the chief extensors of the spine, maintaining upright posture. The **trapezius** is a large, flat, diamond- or trapezoid-shaped muscle covering the upper back and posterior neck. It originates from the occipital bone, the ligamentum nuchae, and the spinous processes of C7 through T12, and inserts on the clavicle, acromion, and spine of the scapula. Depending on which fibers contract, it elevates, retracts, depresses, or rotates the scapula. The **latissimus dorsi** is a large, flat muscle of the lower back (the "lats"), originating from the spinous processes of T7 through L5, the thoracolumbar fascia, the iliac crest, and ribs 9 through 12, and inserting into the intertubercular groove of the humerus. It extends, adducts, and medially rotates the arm and is the primary "pull-up" muscle.

### VI. Muscles of the Upper Limb

#### Muscles Acting on the Shoulder / Moving the Arm

The **deltoid** is a thick, triangular muscle capping the shoulder, originating from the clavicle, acromion, and spine of the scapula and inserting on the deltoid tuberosity of the humerus. Its middle fibers abduct the arm (making it the primary abductor), its anterior fibers flex and medially rotate the arm, and its posterior fibers extend and laterally rotate the arm. The **pectoralis major** is a large, fan-shaped chest muscle originating from the clavicle, sternum, and costal cartilages 1 through 6 and inserting into the intertubercular groove of the humerus. It flexes, adducts, and medially rotates the arm.

The **rotator cuff muscles (SITS)** are four muscles that stabilize the glenohumeral joint. The **supraspinatus** initiates abduction (the first 15 degrees) and is the most commonly injured rotator cuff muscle. The **infraspinatus** laterally rotates the arm. The **teres minor** also laterally rotates the arm. The **subscapularis** medially rotates the arm. The **teres major** assists the latissimus dorsi in extending, adducting, and medially rotating the arm.

#### Muscles Acting on the Forearm (Elbow)

The **biceps brachii** is a two-headed muscle on the anterior arm, originating from the coracoid process (short head) and the supraglenoid tubercle of the scapula (long head) and inserting on the radial tuberosity. It flexes the elbow and is a powerful supinator of the forearm. The **brachialis**, deep to the biceps, is the strongest elbow flexor, originating from the anterior distal humerus and inserting on the coronoid process of the ulna. The **brachioradialis** is a forearm muscle that flexes the elbow, especially in the neutral position (thumb up). The **triceps brachii** is a three-headed muscle on the posterior arm, with the long head originating from the infraglenoid tubercle, the lateral head from the posterior humerus, and the medial head also from the posterior humerus. It inserts on the olecranon of the ulna and is the prime extensor of the elbow.

#### Muscles of the Forearm and Hand

The **forearm flexors (anterior compartment)** generally flex the wrist and fingers and pronate the forearm. They include the flexor carpi radialis, flexor carpi ulnaris, palmaris longus, flexor digitorum superficialis, flexor digitorum profundus, pronator teres, and pronator quadratus. The **forearm extensors (posterior compartment)** generally extend the wrist and fingers and supinate the forearm. They include the extensor carpi radialis longus and brevis, extensor carpi ulnaris, extensor digitorum, extensor digiti minimi, and supinator. The intrinsic hand muscles include the thenar (thumb) and hypothenar (little finger) groups, along with the lumbricals and interossei for fine finger movements.

### VII. Muscles of the Lower Limb

#### Muscles of the Hip and Thigh

The **iliopsoas** is the primary and strongest hip flexor, composed of the iliacus and psoas major, inserting on the lesser trochanter of the femur. The **gluteus maximus** is the largest and most superficial gluteal muscle and the most powerful hip extensor, originating from the ilium, sacrum, and coccyx and inserting on the gluteal tuberosity of the femur and the iliotibial tract. It extends and laterally rotates the thigh and is important in climbing stairs and rising from sitting. The **gluteus medius**, deep to the gluteus maximus, is the primary hip abductor and is important for stabilizing the pelvis during walking, preventing the contralateral hip from dropping (a positive Trendelenburg sign indicates weakness). The **gluteus minimus** is the smallest and deepest gluteal muscle, assisting in abduction and medial rotation.

The **quadriceps femoris group** comprises four muscles of the anterior thigh that are the primary knee extensors. The **rectus femoris** crosses both the hip and knee, flexing the hip and extending the knee. The **vastus lateralis, vastus medialis, and vastus intermedius** extend the knee only. All four muscles insert via the patellar tendon onto the tibial tuberosity through the patella.

The **hamstrings** are three muscles of the posterior thigh that are the primary knee flexors and hip extensors. The **biceps femoris** (with long and short heads) is the lateral hamstring. The **semitendinosus** and **semimembranosus** are the medial hamstrings. The long heads originate from the ischial tuberosity, and the group inserts on the tibia and fibula.

The **adductors** are the medial thigh muscles that adduct the thigh, including the adductor longus, adductor brevis, adductor magnus, gracilis, and pectineus. The **sartorius** is the longest muscle in the body, running obliquely across the anterior thigh. It flexes, abducts, and laterally rotates the thigh and flexes the knee — the "tailor's muscle" for cross-legged sitting.

#### Muscles of the Leg (Acting on the Ankle and Foot)

The **anterior compartment** contains the dorsiflexors and toe extensors. The **tibialis anterior** is the primary dorsiflexor and also inverts the foot. The extensor digitorum longus extends toes 2 through 5 and assists dorsiflexion. The extensor hallucis longus extends the great toe.

The **lateral compartment** contains the evertors. The **fibularis (peroneus) longus and brevis** evert the foot and assist plantar flexion.

The **posterior compartment** contains the plantar flexors. The **gastrocnemius** is the superficial two-headed calf muscle and the primary plantar flexor, originating from the medial and lateral femoral condyles and inserting on the calcaneus via the Achilles (calcaneal) tendon. It also assists knee flexion. The **soleus**, deep to the gastrocnemius, is a powerful plantar flexor important in standing, originating from the posterior tibia and fibula and also inserting on the calcaneus via the Achilles tendon. Together, the gastrocnemius and soleus form the **triceps surae**, sharing the Achilles tendon, which is the strongest tendon in the body. The **tibialis posterior** is a deep posterior muscle that inverts the foot and assists plantar flexion.

<image>Two panels showing major muscles of the body. Panel A (Anterior view): A full-body muscular figure from the front showing labeled muscles including frontalis, orbicularis oculi, orbicularis oris, sternocleidomastoid, deltoid, pectoralis major, biceps brachii, rectus abdominis, external oblique, iliopsoas, sartorius, quadriceps femoris (rectus femoris, vastus lateralis, vastus medialis), tibialis anterior, and gastrocnemius. Panel B (Posterior view): A full-body muscular figure from the back showing labeled muscles including trapezius, deltoid (posterior fibers), infraspinatus, teres minor, teres major, latissimus dorsi, triceps brachii, erector spinae, gluteus medius, gluteus maximus, hamstrings (biceps femoris, semitendinosus, semimembranosus), gastrocnemius, soleus, and Achilles tendon.</image>