Origin, insertion, action, innervation — for every major muscle group. These mnemonics help you organize the muscular system into patterns so you stop memorizing and start understanding.
All three muscle types — skeletal, cardiac, and smooth — share these four fundamental properties. Contractility is the ability to shorten and generate force. Automaticity refers to the ability of cardiac and smooth muscle to contract without nerve stimulation. Extensibility means muscles can be stretched beyond resting length. Elasticity means they recoil to resting length after stretching.
Contractility
Ability to shorten and generate pulling force — unique to muscle
Automaticity
Cardiac and smooth can contract without nerve input
Stretching
Extensibility — can be stretched beyond normal resting length
Elasticity
Recoils to original resting length after being stretched
Three muscle types — how to distinguish them instantly
Skeletal muscle: striated, voluntary, multinucleate, attached to bone. Smooth muscle: non-striated, involuntary, single nucleus, found in organs and vessels. Cardiac muscle: striated like skeletal but involuntary like smooth, single nucleus, connected by intercalated discs with gap junctions. Only cardiac muscle has intercalated discs.
Skeletal
Striated, voluntary, multinucleate, attached to bone via tendons
Smooth
Non-striated, involuntary, single nucleus, organs and blood vessels
Cardiac
Striated, involuntary, branched, intercalated discs — heart only
Supraspinatus · Infraspinatus · Teres Minor · Subscapularis
The four rotator cuff muscles — one of the most tested groups in anatomy
The rotator cuff stabilizes the glenohumeral (shoulder) joint and enables rotation. SITS is the classic mnemonic. Supraspinatus initiates abduction (first 15°) — most commonly torn. Infraspinatus and Teres Minor externally rotate. Subscapularis internally rotates — the only one on the anterior scapula. All four attach to the greater or lesser tubercle of the humerus.
Supraspinatus
Initiates abduction (0–15°) — most commonly torn rotator cuff muscle
The quadriceps femoris group has four heads — all insert into the tibial tuberosity via the patellar tendon and all extend the knee. Rectus femoris is the only quad that also flexes the hip (it crosses both joints). Vastus medialis forms the "teardrop" shape visible on the inner knee. All are innervated by the femoral nerve (L2-L4).
Rectus Femoris
Extends knee AND flexes hip — only quad that crosses hip joint
Flex knee · Extend hip · Innervated by sciatic nerve
The three hamstring muscles — all cross both hip and knee
The hamstrings flex the knee and extend the hip. All three originate from the ischial tuberosity (sit bone) — why they're tight when sitting. Biceps femoris has two heads and inserts on the fibular head. Semimembranosus and Semitendinosus insert on the tibia. All are innervated by the sciatic nerve. Most commonly strained muscle group in athletes.
Biceps Femoris
Two heads — inserts on fibular head. Flexes knee, extends hip.
Semimembranosus
Membranous tendon — inserts on posterior medial tibia
Semitendinosus
Long tendon — part of pes anserine insertion on medial tibia
Origin
All from ischial tuberosity — the sit bone
Innervation
Sciatic nerve (L5, S1, S2)
Facial Expression Muscles
All Facial Expression Muscles = CN VII (Facial Nerve)
Key facial muscles — all innervated by cranial nerve VII
Every muscle of facial expression is innervated by CN VII (facial nerve). This is clinically important — Bell's palsy (CN VII damage) causes unilateral facial paralysis. Orbicularis oculi closes the eye. Orbicularis oris closes and purses the lips. Zygomaticus major draws lip corners up and back (smiling). Buccinator compresses the cheek (chewing, blowing). Frontalis raises the eyebrows.
Orbicularis Oculi
Closes eye — paralysis causes inability to blink (Bell's palsy)
Orbicularis Oris
Closes and purses lips — kissing muscle
Zygomaticus Major
Draws lip corners up and back — the smiling muscle
Buccinator
Compresses cheek — chewing and blowing
Frontalis
Raises eyebrows — CN VII upper division
Bell's Palsy
CN VII damage — unilateral facial droop, cannot close eye
Four muscles of chewing — all innervated by CN V (trigeminal)
All muscles of mastication are innervated by the mandibular branch of CN V (trigeminal nerve — V3). Masseter is the strongest muscle in the body relative to size — closes jaw. Temporalis elevates and retracts the mandible. Medial pterygoid closes jaw. Lateral pterygoid opens the jaw and protrudes the mandible — the only muscle that opens the jaw.
Masseter
Strongest relative to size — elevates mandible (closes jaw)
Temporalis
Fan-shaped — elevates and retracts mandible
Medial Pterygoid
Closes jaw — works with masseter
Lateral Pterygoid
Only muscle that OPENS the jaw — and protrudes mandible
Which intercostals breathe in and which breathe out
External intercostals run obliquely downward and forward — they elevate the ribs during inspiration. Internal intercostals run downward and backward — they depress the ribs during forced expiration. Think: External = Elevates = Inspiration. Internal = forced expiration. The diaphragm does most of the work in quiet breathing — intercostals assist with deeper breathing and forced expiration.
External
Fibers run downward and forward — elevate ribs — INSPIRATION
Internal
Fibers run downward and backward — depress ribs — forced EXPIRATION
Diaphragm
Primary muscle of breathing — contracts and flattens during inspiration
Accessory
SCM and scalenes — used during heavy breathing/respiratory distress
Gluteal Muscles
Max Med Min — Big Medium Small
Gluteus Maximus · Gluteus Medius · Gluteus Minimus
Three gluteal muscles — size, action, and clinical importance
Gluteus Maximus is the largest and most powerful muscle in the body — extends and externally rotates the hip. Used for climbing stairs and rising from sitting. Gluteus Medius and Minimus abduct and internally rotate the hip. Medius is the preferred IM injection site in adults. Trendelenburg sign = Medius weakness — pelvis drops on unsupported side when standing on one leg.
Gluteus Maximus
Largest muscle — extends and externally rotates hip. Stairs, rising from chair.
Gluteus Medius
Abducts hip — preferred IM injection site in adults. Trendelenburg if weak.
Gluteus Minimus
Abducts and internally rotates — deep to medius
Trendelenburg
Pelvis drops on unsupported side — gluteus medius weakness
📝 Exam Prep
5 Common Exam Questions
Frequently tested concepts — know these cold before your exam.
❓ What are the three types of muscle tissue and how do they differ?
✅ Skeletal muscle: voluntary, striated, multinucleated, attached to bones via tendons. Smooth muscle: involuntary, non-striated, single nucleus, found in hollow organs (GI tract, blood vessels, bladder). Cardiac muscle: involuntary, striated, intercalated discs, found only in the heart.
❓ What is the rotator cuff and which muscle is most commonly injured?
✅ The rotator cuff is SITS: Supraspinatus, Infraspinatus, Teres Minor, Subscapularis. These muscles stabilize the glenohumeral joint. The supraspinatus is most commonly torn — it's compressed under the acromion during abduction. Supraspinatus tears cause a painful arc between 60–120° of abduction.
❓ What is the origin and insertion rule in muscle anatomy?
✅ Origin = the more fixed, proximal attachment point (usually on bone or cartilage). Insertion = the more mobile, distal attachment point that moves when the muscle contracts. When a muscle shortens, the insertion moves toward the origin.
❓ What are the agonist, antagonist, and synergist muscle roles?
✅ Agonist (prime mover) = muscle primarily responsible for a movement. Antagonist = muscle that opposes the agonist (relaxes while agonist contracts). Synergist = helps the agonist and stabilizes the joint. Example: bicep curl — biceps = agonist, triceps = antagonist, brachioradialis = synergist.
❓ What is the difference between isometric and isotonic muscle contractions?
✅ Isometric = muscle generates force WITHOUT changing length (holding a position, like a plank). Isotonic = muscle changes length while generating force. Concentric = muscle shortens (lifting phase of curl). Eccentric = muscle lengthens under load (lowering phase) — produces greatest muscle damage and soreness.