Key Terms
Origin
The fixed end; attaches to the bone that does NOT move.
Insertion
Sternum and ribs 5-7. Divided by three tendinous intersections (collagen bands) = the "six-pack" appearance.
Example
Sternocleidomastoid = sternum + clavicle (origin) to mastoid process (insertion).
Prime mover (agonist)
The principal muscle responsible for a movement.
Synergist
Assists the prime mover; makes the action more efficient.
Fixator
A synergist that stabilizes the origin so the prime mover can work. Antagonist: opposes the prime mover; controls and br
Fascicle
A bundle of muscle fibers wrapped in perimysium. How fascicles are arranged determines force output and range of motion.
Parallel
Fascicles run along the long axis of the muscle. Most common arrangement in the body.
Circular (sphincter)
Fascicles arranged concentrically around an opening. Contract = opening closes; relax = opening widens.
Convergent
Broad origin, fascicles angle toward a single attachment point (tendon, aponeurosis, or raphe). Example: pectoralis majo
Pennate (feather-shaped)
Fascicles angle into a central tendon. Less range of motion but more fibers packed in = more tension per unit size.
Fusiform
Spindle-shaped; wide belly tapers to tendons at both ends. Subtype of parallel.
Location
Frontalis = on the frontal bone. Shape: orbicularis = circular; deltoid = triangular (Greek delta).
Axial muscles
Muscles of the trunk and head. Appendicular muscles: muscles of the limbs.
Masseter
Prime mover of chewing; elevates the mandible to close the mouth. Temporalis: assists; retracts the mandible.