The Muscular System
How muscles work together, how they're named, and the major muscles of the body
By the end of this unit you can…
- ✓Describe prime movers, antagonists, synergists and fixators
- ✓Use the seven naming criteria to decode a muscle's name
- ✓Explain lever systems and fascicle arrangements
- ✓Name the location and main action of the major muscles of the head, neck, trunk and limbs
Key terms
Practice →1How muscles work together
Muscles can only pull, never push. Whatever one muscle (or group) does, another can reverse. Muscles are classified by their role in a movement:
- Prime mover (agonist)
- Major responsibility for producing a specific movement — e.g. the biceps brachii in elbow flexion.
- Antagonist
- Opposes or reverses a movement; it relaxes and stretches while the prime mover acts — e.g. triceps brachii opposes elbow flexion. Antagonists can become prime movers for the opposite action.
- Synergist
- Helps the prime mover by adding force or reducing unwanted movements — e.g. brachioradialis assists elbow flexion.
- Fixator
- A synergist that immobilizes a bone (the origin of the prime mover) so the prime mover has a stable base — e.g. muscles stabilizing the scapula during arm movement.
2Naming skeletal muscles
| Criterion | Examples |
|---|---|
| Location | Temporalis (over the temporal bone), intercostals (between ribs), tibialis anterior |
| Shape | Deltoid (triangle), trapezius (trapezoid), serratus (saw-toothed), rhomboid (diamond) |
| Relative size | Maximus (largest), minimus (smallest), longus (long), brevis (short) |
| Direction of fibers | Rectus (straight, parallel to midline), transversus (across), oblique (at an angle) |
| Number of origins | Biceps (2), triceps (3), quadriceps (4) |
| Attachments (origin → insertion) | Sternocleidomastoid — sternum & clavicle → mastoid process |
| Action | Flexor, extensor, adductor (magnus), levator (scapulae), supinator |
3Levers & fascicle arrangement
Bones act as levers, joints as fulcrums, and muscle contraction provides the effort to move a load. If the effort is farther from the fulcrum than the load → mechanical advantage (power lever: move a large load a short distance with less effort). If the effort is closer → mechanical disadvantage (speed lever: fast, wide movements but more force needed). Most skeletal muscles operate at a mechanical disadvantage — trading force for speed and range.
| Lever class | Arrangement | Body example |
|---|---|---|
| First class | Fulcrum between effort and load (E-F-L) | Lifting your head off your chest (neck extensors) |
| Second class | Load between fulcrum and effort (F-L-E) | Standing on tiptoe (calf muscles) — always mechanical advantage |
| Third class | Effort between load and fulcrum (F-E-L) | Most common — biceps flexing the elbow; always mechanical disadvantage |
Fascicle arrangement affects range of motion and power: parallel (sartorius — long, wide range), fusiform (biceps), circular (sphincters — orbicularis oris), convergent (pectoralis major), and pennate — unipennate, bipennate (rectus femoris), multipennate (deltoid). Pennate muscles pack in the most fibers, so they are the most powerful but shorten less.
4Head & neck
| Muscle | Action |
|---|---|
| Occipitofrontalis (frontal belly) | Raises eyebrows, wrinkles forehead |
| Orbicularis oculi | Closes the eye (blinking, squinting) |
| Orbicularis oris | Closes and protrudes lips (“kissing muscle”) |
| Zygomaticus major & minor | Raise the corners of the mouth — smiling |
| Buccinator | Compresses the cheek (whistling, sucking); holds food between teeth |
| Platysma | Tenses neck skin; depresses mandible |
| Masseter & temporalis | Prime movers of jaw closure (chewing) — the masseter is the strongest jaw muscle |
| Sternocleidomastoid | Both: flex the neck. One alone: rotates head to the opposite side |
| Suprahyoid & infrahyoid groups | Move the hyoid and larynx during swallowing and speech |
5Trunk: breathing, abdomen & back
Breathing: the diaphragm is the prime mover of inspiration — contracting flattens it and enlarges the thoracic cavity. External intercostals elevate the ribs (inspiration); internal intercostals depress them (forced expiration).
| Abdominal wall muscle | Fibers | Action |
|---|---|---|
| Rectus abdominis | Vertical, either side of the linea alba; tendinous intersections (“six-pack”) | Flexes and rotates the lumbar spine; stabilizes pelvis |
| External oblique | Down and medially (hands in pockets) | Flex and laterally rotate the trunk; compress abdomen |
| Internal oblique | At right angles to the external oblique | Same as external oblique |
| Transversus abdominis | Horizontal — deepest | Compresses abdominal contents |
Back: the erector spinae group (iliocostalis, longissimus, spinalis) is the prime mover of back extension and maintains posture; quadratus lumborum flexes the spine laterally. Superficial back muscles like the trapezius and latissimus dorsi move the shoulder and arm.
6Shoulder & upper limb
| Muscle | Main action |
|---|---|
| Pectoralis major | Prime mover of arm flexion; adducts and medially rotates |
| Latissimus dorsi | Prime mover of arm extension; adducts and medially rotates (“swimmer's muscle”) |
| Deltoid | Prime mover of arm abduction; common IM injection site |
| Trapezius | Elevates, retracts, depresses and rotates the scapula; extends head |
| Serratus anterior | Protracts the scapula (“boxer's muscle”) — punching, pushing |
| Rotator cuff (SITS) | Supraspinatus, Infraspinatus, Teres minor, Subscapularis — stabilize the shoulder joint |
| Biceps brachii | Flexes the elbow and supinates the forearm |
| Brachialis | Prime mover of elbow flexion |
| Triceps brachii | Prime mover of elbow extension |
| Brachioradialis | Synergist in elbow flexion |
| Flexor / extensor carpi groups | Flex / extend the wrist (anterior / posterior forearm) |
7Hip & lower limb
| Muscle / group | Main action |
|---|---|
| Iliopsoas (iliacus + psoas major) | Prime mover of hip flexion |
| Gluteus maximus | Prime mover of hip extension (climbing stairs, rising from sitting) |
| Gluteus medius & minimus | Abduct and medially rotate the thigh; keep the pelvis level when walking; IM injection site (gluteus medius) |
| Adductor group (magnus, longus, brevis, gracilis, pectineus) | Adduct the thigh (medial compartment) |
| Sartorius | Flexes, abducts and laterally rotates the thigh — longest muscle (“tailor's muscle”, crossing legs) |
| Quadriceps femoris (rectus femoris + vastus lateralis, medialis, intermedius) | Extend the knee (rectus femoris also flexes the hip); vastus lateralis is a common IM injection site in infants |
| Hamstrings (biceps femoris, semitendinosus, semimembranosus) | Flex the knee and extend the hip |
| Tibialis anterior | Dorsiflexes and inverts the foot |
| Gastrocnemius & soleus (triceps surae) | Plantar flex the foot via the calcaneal (Achilles) tendon |
| Fibularis (peroneus) longus & brevis | Plantar flex and evert the foot |