Muscle Naming, Functions & Mechanics

How are skeletal muscles named?

Muscles may be named based on size, location, action/function, body region, fiber orientation, number of heads, shape, or structures they attach to.

Muscle naming terms — size

Major = larger

Minor = smaller

Longus = long

Brevis = short

Vastus = wide/large

Muscle naming terms — location/depth

Anterior = toward front

Posterior = toward back

External = toward outside

Internal = toward inside

Infra- = below

Supra- = above

Profundus = deep

Superficialis = nearer the surface

Muscle naming terms — action

Abductor = away from midline

Adductor = toward midline

Flexor = decreases joint angle

Extensor = increases joint angle

Levator = raises

Depressor = lowers

Pronator = turns palm posteriorly

Supinator = turns palm anteriorly

Muscle naming terms — fiber direction

Rectus = straight

Oblique = at an angle

Transversus = across/transverse

Orbicular = circular

Muscle naming terms — number of heads

Biceps = 2 heads

Triceps = 3 heads

Quadriceps = 4 heads

Examples of muscles named by shape

Deltoid = triangular

Quadratus = rectangular

Rhomboid = rhombus-shaped

Serratus = serrated/jagged

Trapezius = trapezoid-shaped

Agonist (prime mover)

The muscle that provides most of the force required for a movement

Antagonist

A muscle that opposes and slows the action of the agonist

Synergist

A muscle that works with the agonist to guide the movement and make it smooth

Fixator

A muscle that holds a bone in place, making movement more efficient and reducing risk of injury

Origin

The muscle attachment to the part that is more fixed

Insertion

The muscle attachment to the part that moves

How do origin and insertion relate during contraction?

A muscle usually pulls its insertion toward its origin

Origin and insertion can sometimes switch depending on the movement

What are the four parts of a lever system?

Lever = bone

Load = object/body part being moved

Force = muscle tension

Fulcrum = joint

First-class lever

Fulcrum is in the middle between the load and force

ex. atlanto-occipital joint

Second-class lever

Load is in the middle between the fulcrum and force

Uncommon in the body

ex. metatarsophalangeal joints

Third-class lever

Force is in the middle between the fulcrum and load

Very common in the body

ex. elbow joint using the biceps brachii

Mechanical advantage

A small force can move a large load

Mechanical disadvantage

A greater force is required to move the load