Muscle Anatomy and Function: A Comprehensive Guide

Posted on Aug 6, 2024 in Physical Education

Muscle Anatomy and Function

Types of Muscles and Muscle Actions

Prime movers are the large muscles in the body that are responsible for large, gross movements, such as the gluteus maximus. Antagonists are also prime movers, but they oppose the action of other prime movers. For example, the bicep brachii and tricep are antagonists to one another. Synergists are small muscles that work with prime movers to add a little bit more force or reduce unnecessary movement. Fixators are synergists that stabilize the origin of a muscle so that you can make the desired movement more smoothly.

Muscle Fiber Arrangements:

  • Parallel: Fibers run in the same direction as the long axis of the muscle (e.g., sartorius muscle in the thigh, the longest muscle in the body).
  • Fusiform: Spindle-shaped muscles with a larger muscle belly in the center that tapers off at the ends (e.g., biceps brachii).
  • Pennate: Short fibers that attach obliquely to a central tendon running the length of the muscle (e.g., rectus femoris).
  • Convergent: Fibers converge from a large origin to a small insertion (e.g., pectoralis major).
  • Circular: Fibers are arranged in a circle (e.g., orbicularis oris).

Lever Systems

First-class lever system: Load-fulcrum-effort (e.g., holding your head up on the vertebral column; the vertebrae become the fulcrum, and the head is the load).

Second-class lever system: Fulcrum-load-effort (e.g., when you stand on the balls of your feet).

Third-class lever system: Load-effort-fulcrum (e.g., using a dumbbell to do a bicep curl).

Major Muscles of the Body

Upper Limb Muscles

Pectoralis Major:

  • Origin (O): Sternal end of the clavicle, sternum, and ribs 1-6
  • Insertion (I): Fibers converge to insert by a short tendon into the intertubercular sulcus of the humerus
  • Action (A): Prime mover of arm flexion, rotates the arm medially, adducts the arm
  • Innervation (IV): Lateral and medial pectoral nerves

Deltoid:

  • O: Embraces the insertion of the trapezius muscle
  • I: Deltoid tuberosity of the humerus
  • A: Prime mover of arm abduction when all its fibers contract
  • IV: Axillary nerve (C5 & C6)

Latissimus Dorsi:

  • O: Indirect attachment via lumbodorsal fascia into the spine
  • I: Spirals around the teres major to insert in the floor of the intertubercular sulcus
  • A: Extends, adducts, and medially rotates the arm
  • IV: Thoracodorsal nerve

Triceps Brachii:

  • O: Long head – infraglenoid tubercle of the scapula, lateral head – posterior humerus, medial head – posterior humerus
  • I: By a common tendon into the olecranon process of the ulna
  • A: Powerful forearm extensor
  • IV: Radial nerve (C6-C8)

Biceps Brachii:

  • O: Short head – coracoid process of the scapula, long head – supraglenoid tubercle of the scapula
  • I: Radial tuberosity of the radius
  • A: Flexes the elbow joint and supinates the forearm
  • IV: Musculocutaneous nerve (C5-C6)

Head and Neck Muscles

Muscles of Facial Expression: 11 muscles in the face; facial expressions are innervated by cranial nerve #7 (Facial Nerve). They insert into the skin.

  • Orbicularis Oculi:
    • O: Frontal and maxillary bones
    • I: Tissue of the eyelid
    • A: Closes the eye
    • IV: Facial nerve (CN VII)

Masseter:

  • O: Zygomatic arch and bone
  • I: Angle and the ramus of the mandible
  • A: Prime mover of jaw closure
  • IV: Trigeminal nerve (CN V)

Sternocleidomastoid:

  • O: Manubrium of the sternum and medial portion of the clavicle
  • I: Mastoid process of the temporal bone and the nuchal line of the occipital bone
  • A: Flexes and laterally rotates the head
  • IV: Accessory nerve (CN XI)

Scalenes:

  • O: Transverse processes of cervical vertebrae
  • I: Anteriorly on ribs 1-2
  • A: Elevates ribs 1-2, laterally flex the neck
  • IV: Cervical spinal nerves

Extrinsic Tongue Muscles:

  • Genioglossus: Protrudes and retracts the tongue
  • Hypoglossus: Primarily depresses the tongue
  • Styloglossus: Elevates and retracts the tongue
  • IV: All innervated by cranial nerve #12 (Hypoglossal Nerve)

Trunk Muscles

Trapezius:

  • O: Vertebrae of the thoracic spine
  • I: Acromion and spine of the scapula, clavicle
  • A: Stabilizes, elevates, and retracts the shoulder, lifts the head up
  • IV: Accessory nerve (CN XI)

Quadratus Lumborum:

  • O: Iliac crest and lumbar fascia
  • I: Transverse processes of L1-L4
  • A: Laterally flexes the vertebral column
  • IV: T12 and upper lumbar spinal nerves

Rhomboids (Major and Minor):

  • O: Spinous processes of C7-T1 (minor) and T2-T5 (major)
  • I: Medial border of the scapula
  • A: Stabilize the scapula, retract the scapula
  • IV: Dorsal scapular nerve (C4 and C5)

Levator Scapulae:

  • O: Transverse processes of C1-C4
  • I: Medial border of the scapula superior to the spine
  • A: Elevates the scapula, adducts the scapula
  • IV: Cervical spinal nerves and dorsal scapular nerve

Pectoral Minor:

  • O: Anterior surfaces of ribs 3-5
  • I: Coracoid process of the scapula
  • A: With ribs fixed, draws the scapula forward and downward
  • IV: Medial and lateral pectoral nerves (C6-C8)

Muscles of the Neck:

  • Major head flexor: Sternocleidomastoid
  • Synergists to head flexion: Suprahyoid and infrahyoid muscles
  • Lateral head movements: Sternocleidomastoid and scalene muscles
  • Head extension: Deep splenius muscles and trapezius

Trunk Movements:

  • Prime mover: Erector spinae group (iliocostalis, longissimus, spinalis)
  • Lateral bending of the back: Unilateral contraction of the erector spinae muscles, semispinalis muscles, and quadratus lumborum

Muscles of Respiration:

  • Primary function of deep thoracic muscles: Promote movement and breathing
  • External intercostals: More superficial layer that lifts the rib cage, increases thoracic volume, and allows inspiration
  • Internal intercostals: Deeper layer that aids in forced expiration
  • Diaphragm: Most important muscle in inspiration

Muscles of the Abdominal Wall:

  • Composed of 4 paired muscles: Internal obliques, external obliques, transverse abdominis, and rectus abdominis, along with their fasciae and aponeuroses
  • Fascicles run at right and oblique angles to one another, giving the abdominal wall added strength
  • Form the abdominal wall and are involved with lateral flexion and rotation of the trunk
  • Help promote urination, defecation, and childbirth

Muscles Crossing the Shoulder and Elbow

Muscles Crossing the Shoulder:

  • 9 muscles cross the shoulder and insert into the humerus
  • Prime movers include:
    • Pectoralis major (arm flexor)
    • Latissimus dorsi (arm extensor, adductor, and medial rotator)
    • Posterior fibers of the deltoid (arm abductor)
  • Rotator cuff muscles (supraspinatus, infraspinatus, teres minor, and subscapularis): Function mainly to reinforce the capsule of the shoulder joint
  • Secondary actions of synergists and fixators: The coracobrachialis and the teres major act as synergists but do not contribute to the reinforcement of the shoulder joint

Muscles Crossing the Elbow:

  • Forearm extension: The triceps brachii is the prime mover of forearm extension, and the anconeus is a weak synergist
  • Forearm flexion: The biceps brachii, brachialis, and brachioradialis are the prime movers of forearm flexion; the brachioradialis also helps stabilize the elbow

Muscles of the Hand

Intrinsic Muscles of the Hand:

  • Lie in the palm
  • Move the metacarpals and fingers
  • Control precise movements
  • Are the main abductors and adductors of the fingers
  • Produce opposition (bringing the thumb and little finger together)