Muscle Anatomy and Function: Study Notes for ANP College Students

Muscle Anatomy and Function

Basic Principles of Muscle Action

Muscles are responsible for producing movement by contracting and pulling on bones. Each muscle must cross a joint to move that joint, and has two points of attachment: the origin (stationary, usually proximal) and the insertion (mobile, usually distal).

• Muscles only pull; they never push.

• Movement occurs when a muscle contracts and pulls the insertion toward the origin.

Muscle Roles: Agonist, Antagonist, Synergist, Fixator

Muscles work together in groups to produce smooth and coordinated movements. The main roles are:

• Agonist (Prime Mover): The muscle that produces the most force during a specific movement (e.g., biceps brachii during elbow flexion).

• Antagonist: The muscle that opposes the prime mover (e.g., triceps brachii during elbow flexion).

• Synergist: A muscle that assists the prime mover (e.g., deltoid in shoulder flexion).

• Fixator: A muscle that stabilizes the origin of the prime mover (e.g., muscles stabilizing the scapula during arm movements).

Muscle Naming Conventions

Criteria for Naming Muscles

Muscles are named based on several characteristics:

• Location: e.g., Biceps brachii (upper arm)

• Number of Origins: e.g., "biceps" means two origins or heads

• Origin and Insertion: e.g., Sternocleidomastoid (originates at sternum and clavicle, inserts on mastoid process)

• Size: e.g., Gluteus maximus (largest gluteal muscle)

• Shape: e.g., Deltoid (triangular shape)

Location of Flexors and Extensors

• Flexors: Usually located on the ventral (anterior) side, except at the knee.

• Extensors: Usually located on the dorsal (posterior) side, except at the knee.

Facial Muscles

Unique Features of Facial Muscles

Facial muscles typically insert into the skin rather than bone, allowing for a wide range of facial expressions. They are innervated by the Facial Nerve (Cranial Nerve VII).

• Orbicularis oculi: Closes the eyes.

• Orbicularis oris: Closes and puckers the mouth.

• Zygomaticus major: Inserts into the corner of the mouth; responsible for smiling.

• Buccinator: Compresses cheeks, holds food between teeth, and is used in whistling.

• Mentalis: Pulls down the lower lip.

Facial Nerve Damage

Damage to the facial nerve can result in flaccid paralysis of facial muscles, causing the face to droop on one side. Bell's Palsy is a common, usually reversible, condition involving facial nerve dysfunction.

Muscles of Mastication

Masseter

The masseter is a major muscle involved in chewing (mastication). It does not insert into the skin, but rather into the mandible, and is innervated by the Trigeminal Nerve (Cranial Nerve V).

Neck Muscles

Sternocleidomastoid and Scalenes

The sternocleidomastoid muscle flexes the neck and rotates the head to the opposite side. The scalenes are deep neck muscles that assist in breathing by elevating the rib cage.

Muscles of the Shoulder and Upper Back

Trapezius

The trapezius muscle is a large, superficial muscle of the upper back and neck. It is involved in moving, rotating, and stabilizing the scapula and extending the neck.

Pectoralis Major and Latissimus Dorsi

The pectoralis major is a large chest muscle responsible for flexion, adduction, and medial rotation of the humerus. The latissimus dorsi is a broad back muscle responsible for extension, adduction, and medial rotation of the shoulder.

Rotator Cuff Muscles (SITS)

The rotator cuff is a group of four muscles that stabilize the shoulder joint and allow for circumduction and rotation:

• Supraspinatus: Shoulder abduction

• Infraspinatus: Lateral rotation of the shoulder

• Teres minor: Lateral rotation and adduction

• Subscapularis: Medial rotation

Scapular Stabilizers

The rhomboid muscles stabilize the scapula, while the serratus anterior pulls the scapula forward and downward.

Deltoid

The deltoid is the prime mover for shoulder abduction. It has anterior (flexion), posterior (extension), and superior (abduction) fibers.

Muscles of the Upper Arm

Biceps Brachii

The biceps brachii has two heads (long and short) and crosses both the shoulder and elbow joints. It is responsible for flexion of the elbow and supination of the forearm.

Brachialis

The brachialis lies deep to the biceps brachii and is a strong flexor of the elbow.

Triceps Brachii

The triceps brachii is the main extensor of the elbow joint.

Abdominal Muscles

Major Abdominal Muscles and Functions

The abdominal wall is composed of several layers of muscle that provide trunk flexion, rotation, and compression of the abdomen:

• External oblique: Compresses abdomen, rotates trunk

• Internal oblique: Compresses abdomen, rotates trunk

• Rectus abdominis: Flexes the vertebral column

• Transverse abdominis: Compresses abdomen

Back Muscles

Erector Spinae

The erector spinae group (spinalis, longissimus, iliocostalis) runs parallel to the spine and is responsible for extending and laterally flexing the vertebral column.

Muscles of Respiration

Diaphragm

The diaphragm is the primary muscle of breathing, contracting to increase thoracic volume and draw air into the lungs.

Hip Muscles

Flexors and Extensors

The iliopsoas (iliacus + psoas major) is the prime mover for hip flexion. The gluteus maximus is the prime mover for hip extension.

Hip Flexion Synergists

• Sartorius: Spans two joints, assists in hip flexion and knee flexion.

• Rectus femoris: Part of quadriceps femoris, also spans two joints, assists in hip flexion and knee extension.

Quadriceps Femoris

The quadriceps femoris group consists of four muscles (rectus femoris, vastus lateralis, vastus medialis, vastus intermedius) and is the main extensor of the knee.

Hip Abductors and Adductors

• Abductors (lateral thigh): Gluteus medius (prime mover), tensor fascia latae

• Adductors (medial thigh): Adductor magnus, adductor longus, pectineus

Iliotibial Tract (IT Band)

The iliotibial tract is a thick band of fascia running along the lateral thigh, providing stability to the knee and hip.

Hamstrings

Hamstring Muscles

The hamstrings are located on the dorsal side of the thigh, span two joints, and are responsible for hip extension and knee flexion. The group includes:

• Semitendinosus

• Biceps femoris (lateral)

• Semimembranosus

Antagonistic Muscle Groups

The hamstrings and quadriceps femoris are antagonistic muscle groups: hamstrings flex the knee and extend the hip, while quadriceps extend the knee and flex the hip.

Lower Leg Muscles

Major Muscles and Functions

• Gastrocnemius: Flexes the knee and plantar flexes the ankle (via Achilles tendon).

• Soleus: Plantar flexes the ankle (only).

• Tibialis anterior: Dorsiflexes the ankle (anterior compartment).

Summary Table: Major Muscle Groups and Actions

Muscle Group	Primary Action	Location
Biceps brachii	Elbow flexion, forearm supination	Anterior upper arm
Triceps brachii	Elbow extension	Posterior upper arm
Deltoid	Shoulder abduction	Shoulder
Pectoralis major	Shoulder flexion, adduction, medial rotation	Chest
Latissimus dorsi	Shoulder extension, adduction, medial rotation	Back
Quadriceps femoris	Knee extension, hip flexion	Anterior thigh
Hamstrings	Hip extension, knee flexion	Posterior thigh
Gastrocnemius/Soleus	Plantar flexion of ankle	Posterior lower leg
Tibialis anterior	Dorsiflexion of ankle	Anterior lower leg