Introduction
The human body is capable of an extraordinary range of movements—from delicate finger motions to powerful athletic actions. These movements are made possible by the skeletal muscles, a specialized type of muscle tissue attached to bones by tendons. Skeletal muscles make up nearly forty percent of total body mass and serve as the engines of motion, stability, and posture.
Beyond movement, skeletal muscles also perform essential functions in maintaining body temperature, supporting joints, and protecting internal organs. Each muscle has a specific location, origin, insertion, and action that contribute to coordinated movement and stability.
This comprehensive discussion examines the major skeletal muscles of the human body, organized by region, and describes their structure, function, and significance in human physiology.
Overview of Skeletal Muscles
Skeletal muscles are voluntary, striated muscles controlled by the somatic nervous system. Each muscle consists of bundles of elongated, multinucleated fibers surrounded by connective tissue layers known as the epimysium, perimysium, and endomysium. These layers merge to form tendons that attach the muscle to bones.
When stimulated by a motor neuron, muscle fibers contract, pulling on bones to produce movement. Each muscle’s origin is the fixed attachment point, and its insertion is the movable point. The type of movement produced depends on the joint structure and the orientation of the muscle fibers.
Muscles often work in groups: the agonist produces the primary movement, the antagonist opposes it, and the synergists assist by stabilizing adjacent structures.
Muscles of the Head and Neck
The head and neck contain numerous muscles responsible for facial expression, mastication, eye movement, and head motion. These muscles are crucial for communication, sensory function, and feeding.
Muscles of Facial Expression
Facial muscles are unique because they insert into the skin rather than bone, allowing them to produce subtle expressions. They are all innervated by the facial nerve (cranial nerve VII).
Frontalis (Frontal Belly of Occipitofrontalis)
Located on the forehead, this muscle raises the eyebrows and wrinkles the forehead, expressing surprise or curiosity.
Orbicularis Oculi
Encircling the eye, it closes the eyelids during blinking or squinting.
Zygomaticus Major and Minor
Extending from the cheekbone to the corners of the mouth, these muscles elevate the lips to produce a smile.
Orbicularis Oris
Surrounding the mouth, it closes and purses the lips, functioning during speech and kissing.
Buccinator
Located in the cheek, it compresses the cheeks against the teeth, important in chewing and blowing.
Platysma
A broad sheet of muscle extending from the chest to the jaw, it tenses the skin of the neck and depresses the lower jaw.
Muscles of Mastication
The muscles of mastication are responsible for chewing and are innervated by the trigeminal nerve (cranial nerve V).
Masseter
A powerful jaw muscle that elevates the mandible, closing the mouth.
Temporalis
A fan-shaped muscle on the side of the skull that elevates and retracts the mandible.
Medial and Lateral Pterygoids
Located deep within the face, these muscles move the mandible side to side and assist in chewing and grinding movements.
Muscles of the Eye
The extrinsic eye muscles control eye movement and are among the most precise muscles in the body. Each eye has six muscles: superior, inferior, medial, and lateral rectus, and superior and inferior oblique muscles. These muscles coordinate movement to direct the gaze and maintain binocular vision.
Muscles of the Neck
The neck muscles facilitate head movement, swallowing, and speech.
Sternocleidomastoid
Extending from the sternum and clavicle to the mastoid process of the skull, it flexes and rotates the head. When both sides contract, they bend the neck forward; when one contracts, the head turns toward the opposite side.
Scalenes (Anterior, Middle, and Posterior)
Located laterally in the neck, they elevate the first two ribs during inhalation and assist in neck flexion.
Trapezius (Upper Fibers)
Although largely a back muscle, the upper fibers elevate the shoulders and extend the neck.
Muscles of the Thorax
The thoracic muscles are primarily involved in respiration, facilitating the expansion and contraction of the chest cavity.
External Intercostals
Located between the ribs, these muscles elevate the ribs during inspiration, increasing thoracic volume.
Internal Intercostals
Situated beneath the external intercostals, they depress the ribs during forced expiration, reducing thoracic volume.
Diaphragm
A dome-shaped muscle separating the thoracic and abdominal cavities. When it contracts, it flattens, increasing thoracic cavity volume and drawing air into the lungs. It is the primary muscle of respiration and is innervated by the phrenic nerve.
Muscles of the Abdominal Wall
The abdominal muscles play key roles in protecting internal organs, maintaining posture, and generating movements such as flexion, rotation, and compression of the trunk.
Rectus Abdominis
The prominent “six-pack” muscle running vertically along the midline. It flexes the vertebral column and compresses the abdomen during defecation, childbirth, and forced expiration.
External Oblique
The most superficial lateral abdominal muscle. It compresses the abdomen and rotates the trunk to the opposite side.
Internal Oblique
Located beneath the external oblique, it performs similar functions but rotates the trunk to the same side.
Transversus Abdominis
The deepest abdominal muscle, running horizontally. It compresses abdominal contents and stabilizes the core.
Together, these muscles maintain intra-abdominal pressure and protect the viscera.
Muscles of the Back
The muscles of the back support the vertebral column, maintain posture, and move the spine, shoulders, and arms.
Superficial Back Muscles
Trapezius
A large, diamond-shaped muscle extending from the skull and cervical spine to the scapula and clavicle. It elevates, retracts, and rotates the scapula, and assists in neck extension.
Latissimus Dorsi
A broad muscle covering the lower back, extending to the humerus. It extends, adducts, and medially rotates the arm. It is active in movements such as swimming, rowing, and climbing.
Deep Back Muscles
Erector Spinae Group (Iliocostalis, Longissimus, Spinalis)
These three muscle columns run parallel to the spine. They extend the vertebral column and maintain erect posture. When one side contracts, the spine bends laterally.
Quadratus Lumborum
Located in the lower back, it aids in lateral flexion of the vertebral column and stabilizes the pelvis.
Muscles of the Shoulder and Upper Limb
The muscles of the shoulder and upper limb allow for a wide range of motion at the shoulder, elbow, wrist, and fingers.
Muscles of the Shoulder and Upper Arm
Deltoid
A thick, triangular muscle covering the shoulder. It abducts the arm and assists in flexion and extension. It is a common site for intramuscular injections.
Pectoralis Major
A large, fan-shaped chest muscle that flexes, adducts, and medially rotates the arm. It is vital in pushing and lifting actions.
Latissimus Dorsi
As mentioned earlier, it contributes to arm extension and adduction from the back.
Rotator Cuff Muscles (Supraspinatus, Infraspinatus, Teres Minor, Subscapularis)
These four small muscles stabilize the shoulder joint and enable rotation and abduction of the arm. Injuries to this group are common in athletes.
Biceps Brachii
Located on the anterior arm, it flexes the elbow and supinates the forearm.
Brachialis
Situated beneath the biceps, it is the primary flexor of the elbow.
Triceps Brachii
Located on the posterior arm, it extends the elbow and stabilizes the shoulder joint.
Muscles of the Forearm
The forearm muscles control wrist, hand, and finger movements. They are divided into anterior flexor and posterior extensor compartments.
Flexor Group (Anterior Compartment)
Includes muscles such as flexor carpi radialis, palmaris longus, and flexor digitorum superficialis, which flex the wrist and fingers.
Extensor Group (Posterior Compartment)
Includes muscles such as extensor carpi radialis and extensor digitorum, which extend the wrist and fingers.
Muscles of the Hand
The hand contains numerous intrinsic muscles that enable fine motor movements. These include the thenar muscles, which move the thumb, and the hypothenar muscles, which control the little finger. The lumbricals and interossei assist in precise finger positioning.
Muscles of the Pelvic Region
The muscles of the pelvis support internal organs and control movements of the hip and thigh.
Iliopsoas (Psoas Major and Iliacus)
These muscles flex the hip joint, allowing movement of the thigh toward the body. They are crucial for walking and running.
Gluteus Maximus
The largest and most powerful muscle of the body. It extends and laterally rotates the hip, providing propulsion during climbing and running.
Gluteus Medius and Gluteus Minimus
Located beneath the gluteus maximus, they abduct the thigh and stabilize the pelvis during walking.
Tensor Fasciae Latae
Assists in hip abduction and stabilizes the knee via the iliotibial tract.
Piriformis and Deep Rotators
Rotate the femur laterally and stabilize the hip joint.
Muscles of the Lower Limb
The lower limb muscles are among the strongest in the body, responsible for locomotion, balance, and supporting body weight.
Muscles of the Thigh
The thigh muscles are divided into anterior, medial, and posterior compartments, each with distinct functions.
Anterior Compartment
Quadriceps Femoris Group (Rectus Femoris, Vastus Lateralis, Vastus Medialis, Vastus Intermedius)
These four muscles extend the knee. The rectus femoris also flexes the hip. Together they form the front of the thigh and insert into the patella via the quadriceps tendon.
Medial Compartment
Adductor Longus, Adductor Brevis, and Adductor Magnus
These muscles adduct the thigh, bringing it toward the midline.
Gracilis
A long, thin muscle that adducts the thigh and flexes the knee.
Posterior Compartment
Hamstring Group (Biceps Femoris, Semitendinosus, Semimembranosus)
These muscles extend the hip and flex the knee. They are critical for running and jumping.
Muscles of the Leg
The leg muscles, located below the knee, control foot and ankle movement and are divided into anterior, lateral, and posterior groups.
Anterior Compartment
Tibialis Anterior
Dorsiflexes and inverts the foot, preventing toes from dragging during walking.
Extensor Digitorum Longus
Extends the toes and dorsiflexes the foot.
Lateral Compartment
Fibularis (Peroneus) Longus and Brevis
Evert and plantarflex the foot, providing stability while standing and walking.
Posterior Compartment
Gastrocnemius
The prominent calf muscle, responsible for plantarflexion of the foot and flexion of the knee. It provides the power for jumping and running.
Soleus
Located beneath the gastrocnemius, it assists in plantarflexion and is essential for posture and endurance activities.
Tibialis Posterior
Supports the arch of the foot and inverts it during movement.
Flexor Digitorum Longus and Flexor Hallucis Longus
Flex the toes and assist in gripping the ground during walking.
Muscles of the Foot
The intrinsic foot muscles maintain the arches and assist in precise toe movements. These include the flexor, extensor, and lumbrical groups, which work to stabilize the foot during locomotion.
Functional Significance of Major Muscles
The arrangement of skeletal muscles allows the body to perform complex and coordinated movements. Each muscle contributes to specific actions, but efficient movement requires the cooperation of multiple muscles acting in synergy.
Posture and Stability
Muscles such as the erector spinae, abdominal group, and lower limb muscles maintain upright posture and balance.
Locomotion
The coordinated actions of the gluteals, quadriceps, hamstrings, and calf muscles enable walking, running, and jumping.
Manipulation and Dexterity
The fine control of hand and forearm muscles allows for precise tasks such as writing or instrument playing.
Breathing and Circulation
Muscles like the diaphragm, intercostals, and skeletal muscle pumps assist in respiration and venous return.
Heat Production
Muscle contractions generate heat as a byproduct of metabolism, maintaining core body temperature.
Major Muscle Groups and Their Coordination
For functional analysis, the body’s major muscles can be grouped by region and primary action:
- Head and Neck Muscles: Facial expression, mastication, and head movement.
- Trunk Muscles: Respiration, posture, and core stability.
- Upper Limb Muscles: Movement of the shoulder, arm, and hand.
- Lower Limb Muscles: Locomotion, posture, and weight support.
These groups operate in coordination under neural control, forming a complex network of motion and balance.
Muscle Injuries and Adaptations
Muscles are susceptible to strain, overuse, and disease but also capable of remarkable adaptation.
Muscle Strains and Tears
Result from overstretching or excessive force. Recovery depends on severity and involves inflammation and repair by satellite cells.
Atrophy
Occurs when muscles are not used, leading to decreased size and strength. It is common in immobilization or paralysis.
Hypertrophy
An increase in muscle size due to strength training or mechanical stress. It enhances power output and endurance.
Muscle Fatigue
Results from energy depletion and accumulation of metabolic byproducts during prolonged activity.
Proper exercise, nutrition, and recovery are essential for maintaining healthy skeletal muscle function.
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