SELECTED SKELETAL MUSCLES OF THE HUMAN BODY

There are over 600 muscles in the human body. You are to learn the names, locations, and actions of 28 superficial muscles.

Terms to remember when learning actions of muscles:

  1. Skeletal muscles are attached to bones via tendons.
  2. The origin of a muscle is the stationary point of attachment.
  3. The insertion of a muscle is the movable point of attachment.
  4. When a muscle contracts, it shortens so that the insertion moves toward the origin. That movement is defined as the action of the muscle.

MUSCLES OF FACIAL EXPRESSION: (See Fig. 7-11)

  • orbicularis oris: circular muscle of the lips

Action: closes lips, protrudes lips

(This is your kissing muscle.)

  • orbicularis oculi: a circular sphincter muscle around the orbit and in eyelid

Action: blinking, squinting.

(This is the muscle that causes "crow’s feet" creases lateral to the eyes.)

  • frontalis: forehead and anterior scalp muscle

Action: raises eyebrows, wrinkles the forehead, as in "surprise"

MUSCLES USED IN CHEWING: (See Fig. 7-11)

  • masseter: extends from zygomatic arch to the angle of the mandible

Action: closes jaw

  • temporalis: fan-shaped muscle covering parts of temporal, frontal and parietal bones

Action: closes jaw

MUSCLES THAT MOVE THE HEAD: (See Fig. 7-11)

  • sternocleidomastoid: paired fleshy muscles in anterolateral region of neck

Action: rotates head, or when both are used simultaneously, flexes head

MUSCLES USED IN BREATHING: (See Fig.7-14 and 7-16.)

  • external intercostals: 11 pairs, between the ribs

Action: elevate the ribs to aid in inspiration

  • internal intercostals: 11 pairs, deep to the external intercostals

Action: depress the ribs to aid in forced expiration

  • diaphragm: dome-shaped; separates thoracic and abdominal cavities

Action: flattens when contracted, acting as the prime mover of inspiration; also increases intra-abdominal pressure

MUSCLES OF THE ABDOMINAL WALL: (See Fig. 7-14.)

Four paired flat muscles are layered to form the lateral and anterior abdominal walls:

  1. external oblique
  2. internal oblique
  3. transversus abdominis
  4. rectus abdominis

Action: compression and protection of abdominal contents; aid in flexion of vertebral column

(When you contract the diaphragm and abdominal muscles and close the glottis, you are performing the Valsalva maneuver which is employed to promote urination, defecation, childbirth, coughing, vomiting, etc.)

MUSCLES OF THE POSTERIOR THORAX: (See Fig. 7-16.)

  • trapezius: triangular muscle in shoulder, posterior neck and thorax

Action: stabilizes scapula, used to "shrug" the shoulders, assists in head extension

MUSCLES THAT MOVE THE ARM: (See Fig. 7-10 a and b.)

  • deltoid: forms rounded cap over shoulder joint

Action: prime mover in arm abduction; portions of the deltoid may also flex, extend or rotate the humerus

(The deltoid is a common site for intramuscular injections.)

  • latissimus dorsi: large, flat muscle of the lower back

Action: prime mover in arm extension, used in powerful downward movement of the arm as in swimming, rowing or hammering.

  • pectoralis major: fan-shaped muscle of the chest

Action: prime mover in arm flexion; also may rotate or adduct arm

MUSCLES THAT MOVE THE FOREARM AND WRIST: (See Fig. 7-18.)

  • biceps brachii: crosses both the elbow and shoulder joints

Action: flexes elbow joint and assists in supination of forearm, weakly flexes arm at shoulder joint

  • triceps brachii: only muscle of the posterior compartment of the arm

Action: prime mover of forearm extension

Note: Several muscles located in the forearm act on the wrist and fingers. Those in the anterior compartment of the forearm mostly flex the wrist and fingers, while those in the posterior compartment of the forearm mostly extend the wrist and fingers.

MUSCLES OF THE BUTTOCKS: (See Fig. 7-19.)

  • gluteus maximus: forms the bulk of the buttocks

Action: major exensor of the thigh; used mostly in forceful movements such as climbing stairs and running

  • gluteus medius: thick muscle; posterior portion covered by gluteus maximus

Action: abducts and rotates thigh

(This muscle is an important site for intramuscular injections; preferred over the gluteus maximus because there is less chance of damaging the sciatic nerve.)

MUSCLES OF THE POSTERIOR COMPARTMENT OF THE THIGH: (See Fig. 7-20.)

(There are actually three muscles referred to as the hamstring muscles. They all cross both the hip and knee joints. We will examine only the biceps femoris.)

  • biceps femoris: most lateral hamstring

Action: extends thigh at hip, flexes leg at knee

MUSCLES OF THE ANTERIOR COMPARTMENT OF THE THIGH: (See Fig. 7-20.)

quadriceps femoris group:

    1. rectus femoris (a site of intramuscular injection)
    2. vastus lateralis (a site of intramuscular injection)
    3. vastus medialis
    4. vastus intermedius (located deep to the rectus femoris)

Action: powerful knee extension as in climbing, running, and rising from a seated position

MUSCLES THAT MOVE THE FOOT AND TOES: (See Fig.7-21.)

  • tibialis anterior: located laterally to the anterior margin of the tibia

Action: prime mover in dorsiflexion of foot

(Paralysis of this muscle causes "foot drop".)

  • gastrocnemius: responsible for the shape of the calf of the leg

Action: plantar flexion of foot

(Plantar flexion lifts the entire weight of the body, as in standing on tip-toes. The gastrocnemius muscle inserts on the calcaneus via the Achilles’ tendon, the largest tendon in the body.)