Difficulty: Introductory | Prerequisites: Basic anatomical terminology (anterior, posterior, superior, inferior, medial, lateral), familiarity with skeletal landmarks from Chapter 7.
This is the first of three sets of notes covering the major superficial muscles of the human body, drawn from the Chapter 8 muscles worksheet. Here we focus on the head, neck, and trunk, which form the axial musculature. These muscles control facial expression, head movement, breathing, spinal stability, and trunk rotation. You need to know their names, their locations relative to one another, and what they do. If you can place each muscle on an anterior or posterior diagram and recall its primary action, you are in good shape for the practical exam.
The head and neck muscles covered here handle facial expression (frontalis, zygomaticus, orbicularis oris, occipitalis), neck rotation and flexion (sternocleidomastoid), and head/shoulder positioning (trapezius, semispinalis capitis). The trunk muscles manage breathing and rib stabilisation (serratus anterior, pectoralis major), trunk flexion (rectus abdominis), trunk rotation and lateral flexion (external oblique), and spinal extension (erector spinae).
Frontalis
The muscle of the forehead. It raises the eyebrows and wrinkles the forehead. Think of it as the muscle you use when you look surprised.
Occipitalis
The muscle at the back of the skull. It works with the frontalis via a shared tendon (the epicranial aponeurosis) to pull the scalp backward. In simple terms, frontalis and occipitalis are two ends of the same unit that moves your scalp.
Zygomaticus
Runs from the zygomatic bone (cheekbone) to the corner of the mouth. It pulls the mouth upward and laterally. This is your smiling muscle.
Orbicularis oris
The circular muscle surrounding the mouth. It closes and purses the lips. Think of it as the muscle you use to whistle or kiss.
Sternocleidomastoid (SCM)
A large, paired muscle on each side of the neck, running from the sternum and clavicle to the mastoid process of the temporal bone. Unilateral contraction rotates the head to the opposite side; bilateral contraction flexes the neck. In simple terms, this is the muscle that lets you turn your head and tilt your chin down.
Semispinalis capitis
A deep posterior neck muscle that extends the head (tilts it backward) and helps rotate it. It sits beneath the trapezius, running from the upper thoracic and lower cervical vertebrae to the occipital bone. Think of it as one of the muscles that lets you look up at the ceiling.
Trapezius
A large, diamond-shaped muscle covering the upper back and posterior neck. Its upper fibres elevate the scapula (shrugging), its middle fibres retract the scapula, and its lower fibres depress it. It is visible from both the anterior and posterior views.
Pectoralis major
The large, fan-shaped chest muscle. It flexes, adducts, and medially rotates the arm at the shoulder. In simple terms, this is the muscle that lets you push things away from your chest or bring your arms together in front of you.
Serratus anterior
A saw-toothed muscle along the lateral rib cage, deep to the scapula. It protracts the scapula (pulls it forward around the rib cage) and stabilises it during pushing movements. Think of it as the muscle that anchors your shoulder blade when you throw a punch, which is why it is sometimes called the "boxer's muscle."
External oblique
The most superficial of the lateral abdominal muscles. It compresses the abdomen and assists in trunk rotation and lateral flexion. The fibres run inferomedially, like putting your hands in your front pockets.
Rectus abdominis
The paired, strap-like muscle running vertically on the anterior abdomen, separated by the linea alba. It flexes the lumbar spine (curling the trunk forward) and compresses the abdominal contents. This is the muscle that produces the visible "six-pack" when well developed and body fat is low.
Erector spinae
A group of three parallel muscle columns (iliocostalis, longissimus, spinalis) running along the vertebral column on the posterior side. Together they extend the spine and maintain upright posture. In simple terms, these are the muscles that keep you from slumping forward.
Frontalis sits on the forehead, superficial, easy to identify on anterior diagrams
Zygomaticus runs diagonally from cheekbone to mouth corner
Orbicularis oris encircles the mouth, no bony attachment on the lip side (it blends with other facial muscles)
Sternocleidomastoid is a key landmark on the anterior neck
Visible as a prominent diagonal band when the head is turned
Named for its attachments: sternum + clavicle to mastoid process
Unilateral contraction: rotates head to the opposite side (contracting the right SCM turns your head left)
Bilateral contraction: flexes the neck
Occipitalis is at the rear of the skull, often tested as the posterior counterpart to the frontalis
Semispinalis capitis lies deep to the trapezius on the posterior neck
Extends and rotates the head
Part of the transversospinalis muscle group
Trapezius is visible from the back as a large diamond shape from the skull to the mid-thoracic spine and out to the scapular spine
Upper, middle, and lower fibres have distinct actions
Pectoralis major covers most of the upper anterior chest
Origin: clavicle, sternum, upper costal cartilages
Insertion: intertubercular groove of the humerus
Actions: flexion, adduction, medial rotation of the arm
Serratus anterior wraps around the lateral rib cage
Often visible as finger-like projections along the ribs in lean individuals
Anchors the scapula to the thoracic wall
Rectus abdominis runs from the pubic crest to the xiphoid process and costal cartilages 5 to 7
Intersected by tendinous inscriptions, giving the segmented appearance
External oblique is the outermost abdominal wall layer
Fibre direction: inferomedial ("hands in pockets")
Works with the internal oblique on the opposite side during rotation
Trapezius dominates the upper posterior view (see above)
Erector spinae runs as three columns along the vertebral column
Iliocostalis (most lateral), longissimus (intermediate), spinalis (most medial)
Primary action: extension of the vertebral column
Also assists with lateral flexion when contracting unilaterally
External oblique is visible from the posterior view as well, wrapping around the lateral trunk
The sternocleidomastoid is one of the first muscles physiotherapists assess in whiplash injuries, because forced hyperextension and rebound flexion strain it directly. The erector spinae group is central to lower-back pain: weakness or imbalance in these muscles is a common contributor to chronic lumbar discomfort, which is why core-stability exercises feature so heavily in rehabilitation programmes.
Students often confuse the frontalis and occipitalis as separate, unrelated muscles. They are the two bellies of the occipitofrontalis (or epicranius), connected by the epicranial aponeurosis.
The sternocleidomastoid rotates the head to the opposite side, not the same side. Contracting the right SCM turns the face to the left. This trips people up regularly.
The "six-pack" is the rectus abdominis. External oblique is lateral, not anterior-centre. They are different muscles with different fibre directions and different actions.
Erector spinae is a group of three muscle columns, not a single muscle. Exam questions may ask you to name the individual columns.
⚠️ Be able to place every muscle on both the anterior and posterior diagrams. Several muscles (trapezius, sternocleidomastoid, external oblique) appear on both views.
⚠️ Know the action of each muscle, not just the name and location. Practical exams often pair "identify" with "what does it do?"
⚠️ The sternocleidomastoid's contralateral rotation is a classic exam question.
⚠️ Be ready to distinguish serratus anterior from external oblique on the lateral trunk. Their positions overlap visually but their actions differ.
True or false: The frontalis and occipitalis are connected by the epicranial aponeurosis. ___
Contracting the left sternocleidomastoid rotates the head to the _____ side.
True or false: The external oblique is the deepest layer of the abdominal wall. ___
The erector spinae group consists of three columns: iliocostalis, __________, and spinalis.
Fill in the blank: The serratus anterior is sometimes called the "__________ muscle" because of its role in stabilising the scapula during punching.
Answers: 1. True. 2. Right. 3. False (it is the most superficial). 4. Longissimus. 5. Boxer's.
Q: Name the muscle that raises the eyebrows and wrinkles the forehead.
A: Frontalis.
Q: What is the primary action of the sternocleidomastoid when both sides contract simultaneously?
A: Flexion of the neck (bringing the chin toward the chest).
Q: Which muscle protracts the scapula and is sometimes called the "boxer's muscle"?
A: Serratus anterior.
Q: A student identifies a muscle on the posterior trunk running in three parallel columns along the spine. What muscle group is this?
A: Erector spinae (iliocostalis, longissimus, spinalis).
Q: Which abdominal muscle has fibres running inferomedially and is the most superficial layer of the lateral abdominal wall?
A: External oblique.
Q: Name the circular muscle around the mouth responsible for pursing the lips.
A: Orbicularis oris.
This material connects directly to the skeletal system (Chapter 7), because muscle origins and insertions reference bony landmarks such as the mastoid process, zygomatic bone, sternum, clavicle, and vertebral column. It also links to the nervous system, since each of these muscles is innervated by a specific nerve (the facial nerve for muscles of facial expression, the spinal accessory nerve for the trapezius and SCM). Understanding trunk musculature is foundational for the respiratory system unit, where the role of the diaphragm, intercostals, and abdominal muscles in breathing mechanics is covered in detail.
Frontalis, occipitalis, occipitofrontalis, epicranius, zygomaticus, orbicularis oris, sternocleidomastoid, SCM, semispinalis capitis, trapezius, pectoralis major, pec major, serratus anterior, boxer's muscle, external oblique, obliques, rectus abdominis, six-pack muscle, erector spinae, iliocostalis, longissimus, spinalis, axial musculature, muscles of facial expression, trunk muscles, neck muscles, anterior view, posterior view, Chapter 8, anatomy and physiology, UF, muscle identification