CH10: Muscular System Gross Anatomy

Facts About the Skeletal Muscles

  • Most skeletal muscles extend from one bone to another & cross at least one joint

  • Muscles contract —> pull on one of the bones toward the other bones across a moveable joint —> movement


Muscle Attachment

a) Origin (head):

  • the stationary end of the muscle

  • some muscles have multiple origins

  • e.g. biceps & triceps brachii

b) Insertion:

  • the end of the muscle attached to the moveable bone

c) Belly:

  • bulging part of the muscle between the origin & insertion


Agonist and Antagonist

Muscles typically come in antagonistic pairs (do opposite things)

a) Agonist (“contest”):

  • a muscle that accomplishes a certain movement

  • e.g. biceps brachii —> flexes elbow

b) Antagonist:

  • a muscle acting in OPPOSITION to an agonist

  • e.g. triceps brachii —> extends elbow


Functional Grouping

a) Synergists:

  • muscles working together to produce a movement

  • Prime mover:

    • the muscle (among a group of synergists) that’s mainly responsible for a movement

b) Fixators:

  • muscles that stabilize the actions of the prime movers

  • stabilize by holding the bone in place while another bone is moved


Muscle Types

- Unipennate Muscle

  • Muscle fibers arranged on 1 side of a central tendon

  • Resembles a feather

  • Ability to pack more fibers into a smaller space

    • generates greater force

  • Ex: Extensor digitorum longus (lateral, lower leg), flexor pollicis longus

- Bipennate Muscle

  • Muscle fibers are arranged on both sides of a central tendon (oblique pattern)

    • designed for strength and power

    • increased muscle cross-sectional area

  • Resembles a feather

  • Ex: Rectus femoris (quadriceps muscles), Gastrocnemius (calf muscle), and Deltoid (shoulder muscle)

- Multipennate Muscle

  • Multiple rows of muscle fibers with each row inserted into a separate tendon

    • tendons then converge to form a single, larger tendon

    • helps distribute force evenly across the tendon

      • reduces stress

      • improves efficiency

  • Ex: Deltoid muscle (shoulder), Triceps brachii muscle (arm), and Rectus femoris muscle (thigh)

  • Generates high force production due to increased physiological cross-sectional area

- Parallel Muscle

  • Has fascicles (bundles of fibers) that run parallel to the long axis of the muscle

    • allows for a wide range of motion & speed

    • less force compared to pennate muscles

  • Strap-like

    • long, thin muscles with constant diameter

  • Fusiform

    • spindle-shaped

    • larger diameter in the middle, tapering to tendons at each end

  • Fan-shaped

    • broad base that tapers to narrow tendon

  • Ex: Biceps brachii, Rectus abdominis, Sartorius

- Circular Muscle

  • Contracts to narrow or close a passage, and relaxes to widen or open it

    • this regulates the flow of substances through the body

  • Crucial in forming sphincters (circular muscle thickening)

    • rings of muscles

      • controls the movement of substances like food, urine, and feces

  • Ex: Gastrointestinal Tract, Urinary Tract, Blood Vessels, the Iris

- Convergent Muscle

  • Fascicles converge toward a single attachment point

    • meets at a central tendon or aponeurosis

  • Fan-shaped or triangular appearance

  • Generates powerful contractions over a wide range of motion

    • produces force in multiple directions

  • Ex: Pectoralis major (chest muscle), Gluteus Maximus (buttock muscle), Rectoris femoris (quadriceps muscle)


*Note: Pennate muscles typically have a lower range of motion and shortened speed compared to parallel muscles

Muscle TypesArm Muscles