Muscles Acting on the Pectoral Girdle and Humerus

Muscles Acting on the Pectoral Girdle

  • The pectoral girdle consists of two bones: the scapula and the clavicle.
  • Muscles acting on the pectoral girdle insert onto one or both of these bones, originating from the axial skeleton.
  • There are six muscles in total: two anterior and four posterior.

Anterior Muscles

Pectoralis Minor
  • The term "minor" indicates relative size, meaning it's smaller than the pectoralis major.
  • Origin: Rib pairs three through five.
  • Insertion: Coracoid process of the scapula.
  • Action: Protracts and depresses the shoulder, rotating it forward and down (e.g., during forced exhalation).
Serratus Anterior
  • Named for its serrated knife-like edge.
  • Visible in very fit individuals.
  • Origin: Rib pairs one through nine.
  • Insertion: Medial border of the scapula.
  • Action: Multiple actions of the pectoral girdle; often called the "boxing muscle" or "throwing muscle" due to its role in punching and throwing motions.

Posterior Muscles

Rhomboideus Major and Minor
  • Again, "major" and "minor" refer to relative size.
  • The major is larger and located below the minor.
  • Insertion: Medial angle of the scapula.
  • Action: Retracts the pectoral girdle (draws the shoulders back, like when sitting up straight).
Levator Scapulae
  • Action: Elevates the scapula (lifting the shoulder joint).
  • Attaches above and pulls the girdle up when contracted.
Trapezius
  • Large, wing-like muscle on the dorsal surface of the back.
  • Extends from the nuchal region to the thoracic vertebrae and acromion.
  • Action: Numerous, depends on the part of the muscle contracting and which other muscles are working with it; involved in neck extension and shoulder movement.

Muscles Acting on the Humerus

  • These muscles carry out shoulder actions.

Pectoralis Major

  • Superficial muscle overlying the pectoralis minor.
  • Origin: Sternum and clavicle.
  • Insertion: Intertubercular sulcus of the humerus.

Latissimus Dorsi

  • Large, superficial muscle of the back.
  • Origin: Lower thoracic vertebrae, lumbar vertebrae, and ilium.
  • Insertion: Intertubercular sulcus of the humerus.
  • Action: Both the Pectoralis Major and Latissimus Dorsi allow for rowing actions of the shoulder joint and rotation.

Deltoid

  • The shoulder cap muscle.
  • Action: Prime mover for shoulder flexion, extension, and abduction (e.g., raising the hand).
  • Origin: Lateral scapula.
  • Insertion: Deltoid tuberosity of the humerus.

Rotator Cuff Muscles

  • Four muscles crucial for shoulder stability and movement.
  • Mnemonic: SITS
Supraspinatus
  • Posterior muscle; above the spine of the scapula.
Infraspinatus
  • Posterior muscle; below the spine of the scapula.
Teres Minor
  • Posterior muscle; muscle belly just below the spinatus group.
Subscapularis
  • Anterior muscle; located in the subscapular fossa.

Superficial Anatomy

  • Trapezius: Large wing-like muscle extending from the nuchal region to the thoracic region, inserting on the acromion.
  • Latissimus Dorsi: Very large dorsal muscle originating from lower lumbar vertebrae and the ileum, inserting on the intertubercular sulcus.
  • Deltoid: Shoulder cap muscle with its unique insertion point, the deltoid tuberosity, on the outer shaft of the humerus.
  • Infraspinatus: Rotator cuff muscle below the spine of the scapula.
  • Erector Spinae: Large muscle bundles on either side of the vertebral column, extending from the back of the head to the tailbone.

Muscles Acting on the Elbow

Biceps Brachii

  • Double-headed muscle; each head associates with an aspect of the scapula.
  • Insertion: Radial tuberosity (bypasses the humerus).
  • Action: Principal flexor of the elbow joint; also acts on the shoulder joint.

Brachialis

  • Prime flexor of the elbow.
  • More powerful than the biceps brachii during elbow flexion.

Brachioradialis

  • Fat-bellied muscle in the forearm.
  • Origin: Humerus.
  • Insertion: Radius.
  • Action: Synergist for elbow flexion, stabilizes the joint.

Triceps Brachii

  • Three-headed muscle group (long, short, and medial heads) on the back of the arm.
  • Insertion: Olecranon process of the ulna (bony elbow apex).
  • Action: Prime elbow extensor.

Trunk Anatomy

  • Trapezius: Located at the top.
  • Deltoid. Shoulder cap muscle.
  • Pectoralis major. Chest muscle.
  • Biceps brachii: Synergist with elbow flexion.
  • Triceps: Includes the lateral and long head.
  • Brachioradialis: Located on the forearm.

Forearm Actions

Supination and Pronation

  • Supinator muscle, pronator teres, and pronator quadratus facilitate forearm rotation.

Muscles of the Anterior Forearm (Flexors)

  • These muscles have tendons that insert into the palm of the hand and extend along the digits.

Terminology

  • Digitorum: Refers to tendons inserting into digits two through five (pointer finger through pinky).
  • Carpi: Indicates that the muscle inserts into the wrist.
  • Pollicis: Indicates insertion to the thumb (digit number one).

Muscle Group

  • Brachioradialis: Largest, most lateral muscle belly on the thumb side (radial side).
  • Flexor Carpi Radialis: Tendon stops at the wrist on the radial (lateral) side.
  • Flexor Palmaris Longus: Continues as the palmar aponeurosis (flat tendon in the palm).
  • Flexor Carpi Ulnaris: Most medial muscle; tendon stops on the pinky side (ulnar side).

Muscles of the Posterior Forearm (Extensors)

  • These muscles allow for straightening the hand and fingers.

Muscle Group

  • Extensor Carpi Ulnaris: Tendon ends in the wrist region on the ulna side.
  • Extensor Digiti Minimi: Inserts its tendon into digit number five, the little finger.
  • Extensor Digitorum: Splits into insertions for digits two, three, four, and five (pointer, middle, ring, and pinky).
  • Surface Anatomy: Extensor digitorum tendons are visible on the back of the hand.

Hip Joint Muscles

  • Muscles are typically located on the side where the action occurs.

Hip Flexors

  • Located on the front of the body.
  • Prime Hip Flexor: Iliopsoas (deep muscle group).
    • Iliacus: Originates from the iliac fossa.
    • Psoas: Originates from the lumbar vertebrae.
  • Insertion: Lesser trochanter of the femur via a tendon.

Hip Extensors

  • Located on the back of the hip joint.
  • Prime Hip Extensor: Gluteus maximus (large buttock muscle).

Hip Adductors

  • Located on the inner thigh muscles (medially).
  • Origin: Pubis region of the oscoxae.
  • Insertion: Along the length of the femur.
  • Most Medial: Gracilis; acts as an adductor and also crosses the knee joint to aid in knee flexion.

Knee Joint Muscles

Knee Extensors

  • Located on the front of the thigh (ventral surface).
  • Prime Knee Extensors: Quadriceps Femoris (four heads).
    • Rectus Femoris: Most superficial; if you place your hand in your lap, you're resting on the rectus femoris.
    • Vastus Lateralis: On the outside of the rectus femoris.
    • Vastus Medialis: On the inside of the rectus femoris.
    • Vastus Intermedius: Behind or Deep to the rectus femoris
  • Insertion: Via the patellar ligament to the tibial tuberosity.

Knee Flexors

  • Located on the back of the thigh (hamstring muscles).
  • Hamstring Muscles (three).
    • Biceps Femoris: Two-headed
    • Semitendinosus:
    • Semimembranosus
  • Prime Action: Knee Flexion
  • Tendons are Located on either side of the Popliteal Region

Muscles of the Crural Region (Leg)

  • Transverse Section.

Muscle Organization

  • Anterior Group.
    *Lateral Group.
  • Posterior Compartment (Deep vs. Superficial).

Foot Actions

  • Dorsiflexion: Heel strike (toes lifted, heel pushed down).
  • Plantar Flexion: Standing on tiptoe (toes pointed down, heel lifted).
  • Inversion: Soles of feet face one another.
  • Eversion: Soles of feet face away from one another.

Muscles of the Anterior Leg (Extensors)

  • Dorsiflexion is used to point your toe up.
  • They are Located in the FRONT!
  • Support the leading leg during walking.
    • Extensor Digitorum: Inserts into digits two through five.
  • Extensor Hallicus Longus: Inserts into digit number one (big toe or hallux).
  • Tibialis Anterior: Works with the extensor digitorum
  • Dorsiflexion is used to point your toe up.

Posterior Leg (Plantar Flexors)

  • Two Common Superficial Muscles.
    • Gastrocnemius. Large.
    • Soleus. Flat.
  • Insertion: Calcaneal (heel bone) via the Achilles tendon. These are the muscles people refer to when they talk of calf muscle
  • Plantar Flexors (lagging foot in locomotion)

Deep Group of Plantar Flexors

  • Tibialis Posterior:
    • Cause of shin splints, due to tears of muscle and connective tissue from the bone.

Lateral Compartment of the Leg (Eversion and Plantar Flexion)

  • Fibularis Longus and Brevis.
  • Associated with the fibula (lateral).
  • Plantar flexion and eversion, which can cause a sore shin.
  • Support the Lateral (outside) Edge of foot up and off the ground. You have have this if you're clumsy.

Surface Anatomy of the Curl Region

  • Belly of the tibialis anterior (front of the shin).
    *Fibularis longus (lateral).
  • Lateral malleolus (outer ankle knob, distal end of the fibula).
  • Soleus and lateral head of the gastrocnemius (posterior calf muscles).