Overview of Major Skeletal Muscles of the Human Body

Muscular System: An Overview

The muscular system is composed of specialized tissues called muscles, which are responsible for movement, posture, and heat production in the human body. This study guide provides an overview of the major skeletal muscles, their locations, and their primary functions, focusing on both anterior (front) and posterior (back) views, as well as key muscles of facial expression.

Major Skeletal Muscles: Anterior View

The anterior view highlights muscles located on the front of the body, many of which are involved in flexion, adduction, and medial rotation of limbs, as well as movements of the trunk and face.

• Frontalis: Raises eyebrows and wrinkles the forehead.

• Orbicularis oculi: Closes the eyelids.

• Zygomaticus major and minor: Elevate the corners of the mouth (smiling).

• Orbicularis oris: Closes and protrudes the lips (kissing muscle).

• Sternocleidomastoid: Flexes the neck and rotates the head.

• Deltoid: Abducts the arm at the shoulder.

• Pectoralis major: Flexes, adducts, and medially rotates the arm.

• Biceps brachii: Flexes the elbow and supinates the forearm.

• Brachialis: Flexes the elbow.

• Brachioradialis: Flexes the forearm at the elbow.

• Rectus abdominis: Flexes the vertebral column and compresses the abdomen.

• External oblique: Rotates and laterally flexes the trunk.

• Sartorius: Flexes, abducts, and laterally rotates the thigh; flexes the knee.

• Quadriceps femoris group (Rectus femoris, Vastus lateralis, Vastus medialis, Vastus intermedius): Extends the knee; rectus femoris also flexes the hip.

• Tibialis anterior: Dorsiflexes and inverts the foot.

• Fibularis (peroneus) longus: Everts and plantarflexes the foot.

• Extensor digitorum longus: Extends the toes and dorsiflexes the foot.

• Gastrocnemius: Plantarflexes the foot and flexes the knee.

• Soleus: Plantarflexes the foot.

Major Skeletal Muscles: Posterior View

The posterior view displays muscles on the back of the body, many of which are involved in extension, abduction, and lateral rotation of limbs, as well as stabilization of the trunk and head.

• Occipitalis: Moves the scalp posteriorly.

• Trapezius: Elevates, retracts, and rotates the scapula; extends the neck.

• Deltoid: Abducts the arm at the shoulder.

• Infraspinatus: Laterally rotates the arm.

• Teres major and minor: Teres major adducts and medially rotates the arm; teres minor laterally rotates the arm.

• Latissimus dorsi: Extends, adducts, and medially rotates the arm.

• Triceps brachii: Extends the elbow.

• Gluteus maximus: Extends and laterally rotates the thigh.

• Gluteus medius: Abducts and medially rotates the thigh.

• Hamstring group (Biceps femoris, Semitendinosus, Semimembranosus): Flexes the knee and extends the hip.

• Gastrocnemius: Plantarflexes the foot and flexes the knee.

• Soleus: Plantarflexes the foot.

• Calcaneal (Achilles) tendon: Common tendon for gastrocnemius and soleus, attaches to the calcaneus (heel bone).

Muscles of Facial Expression (Anterior View)

Facial muscles are responsible for a wide range of expressions and movements. They are unique in that they insert into the skin rather than bone, allowing for subtle and complex facial movements.

• Corrugator supercilii: Draws eyebrows downward and medially (frowning).

• Procerus: Wrinkles the skin over the bridge of the nose.

• Nasalis: Compresses the bridge and depresses the tip of the nose; elevates corners of nostrils.

• Levator labii superioris: Elevates the upper lip.

• Levator anguli oris: Elevates the angle of the mouth.

• Risorius: Draws the corner of the mouth laterally (grinning).

• Depressor anguli oris: Depresses the angle of the mouth (frowning).

• Depressor labii inferioris: Depresses the lower lip.

• Thyroid cartilage of the larynx: Not a muscle, but an important landmark for neck muscles.

• Clavicle: Serves as an attachment point for several neck and shoulder muscles.

Key Terms and Definitions

• Origin: The fixed attachment point of a muscle.

• Insertion: The movable attachment point of a muscle.

• Flexion: Decreasing the angle between two body parts.

• Extension: Increasing the angle between two body parts.

• Abduction: Movement away from the midline of the body.

• Adduction: Movement toward the midline of the body.

• Rotation: Movement around an axis.

Example: Quadriceps Femoris Group

• Rectus femoris: Originates at the ilium, inserts at the patella via the quadriceps tendon; extends the knee and flexes the hip.

• Vastus lateralis, medialis, and intermedius: Originate at the femur, insert at the patella; extend the knee.

Summary Table: Selected Major Muscles and Their Functions

Muscle	Location	Primary Action
Deltoid	Shoulder	Abducts arm
Pectoralis major	Chest	Flexes, adducts, medially rotates arm
Rectus abdominis	Abdomen	Flexes vertebral column
Gluteus maximus	Buttocks	Extends, laterally rotates thigh
Gastrocnemius	Calf	Plantarflexes foot, flexes knee
Biceps brachii	Upper arm (anterior)	Flexes elbow, supinates forearm
Triceps brachii	Upper arm (posterior)	Extends elbow

Additional info:

• Muscles work in antagonistic pairs; for example, the biceps brachii (flexor) and triceps brachii (extensor) control elbow movement.

• Facial muscles are innervated primarily by the facial nerve (cranial nerve VII).

• Muscle names often reflect their shape, location, or function (e.g., rectus = straight, abdominis = of the abdomen).