Muscular System

The head and neck muscles provide movement for chewing, swallowing, and facial expressions.

The muscles of the head include:

• Occipitofrontalis: Forehead muscles that include the epicranial aponeurosis, frontalis (1 on each side), and occipitalis (1 on each side). They wrinkle the forehead, raise the eyebrows, and create surprised facial expressions.
• Temporoparietalis: Found on each side of the head above the ears. They tighten the scalp and raise the ears.
• Orbicularis oculi: Circular muscles around each eye. They protect the eyes by closing the eyelids.
• Zygomaticus: Located on each side of the face. They control the movement of the mouth and create smiles.
• Orbicularis oris: Found around the lips. They control the movement and shape of the lips.
• Risorius: Located on both sides of the lip. They move the lip upward and outward to create a smile.
• Depressor anguli oris: Run from the bottom lip to the outer sides of the lower chin. They pull the lower lip downward and move the corners of the mouth.
• Depressor labii inferioris: Found at the front of the chin, on either side of the face. They move the lower lip downward to show the lower teeth and help shape facial expressions such as smiling.
• Mentalis: Found at the center of the chin. It moves the lower lip and chin to help shape facial expressions like pouting.
• Buccinator: Found on either side of the jaw, between the upper and lower jawbones (maxilla and mandible), beneath the risorius muscle. They press the cheeks against the teeth to help with chewing and controls airflow in the mouth.
• Auricular muscles: These include the anterior, superior, and posterior muscles on each side of the head, around the ears. They move the ears and scalp upward and forward.
• Platysma: Thin, wide muscles running from the lower jaw to the collarbone (clavicle). They move the lower lip and jaw to help create facial expressions.

The muscles of the neck include the longus capitis, longus colli, anterior/middle/posterior scalene, and sternocleidomastoid. These muscles run from the base of the skull to the upper part of the back and chest. Together, they allow the head to turn, nod, and tilt. They also help with breathing.

The back and shoulder muscles allow movement of the neck, shoulders, abdomen, and back. They help with standing, staying upright when sitting, bending over, twisting, and turning your head. All of these muscles are paired, meaning there is 1 on each side of the body.

Muscles in the back include:

• Trapezius: A large muscle that runs from the back of the head and neck to the middle of the back. It has 3 sections: upper, middle, and lower. It controls movement of the head, neck, and shoulders.
• Latissimus dorsi: The largest muscle in the upper half of the body. It runs from the lower back to the upper arm bone (humerus).
• Deltoid: Sits at the top of the shoulder. It raises the arm away from the body and keeps the joints in place.
• Serratus posterior: Made up of 2 parts, inferior and superior. The inferior muscle sits below the latissimus dorsi. It helps control the lower ribs when breathing out. The superior muscle sits between the shoulder blades, close to the spine. It helps control the upper ribs when breathing in.
• Erector spinae: A group of muscles deep in the back with 3 parts:
  • Iliocostalis: Runs from the ribs to the sacrum. It helps the spine to bend.
  • Longissimus: Found between the iliocostalis and the spinalis. It stabilizes the back.
  • Spinalis: Runs from the skull to the sacrum. It’s the smallest muscle of the 3 parts. It helps to extend the back and neck.

The rotator cuff muscles in the shoulder include:

• Infraspinatus: A thick muscle that covers most of the scapula. It allows the arm to rotate.
• Teres minor: A small muscle that runs along the scapula and extends to the top of the humerus bone. It helps to rotate and keep the shoulder joint in place.
• Supraspinatus: A small triangle-shaped muscle that sits under the trapezius muscle. It helps to lift the arm away from the body.
• Subscapularis: A triangle-shaped muscle that sits between the scapula and the humerus bones. It helps to move the shoulder inward when it is raised.
• Major and minor rhomboid: Found in the upper back near the edge of the shoulder blades. These muscles work together to keep the scapula in place and attach it to the spine.

The muscles in the arms control movements such as bending, straightening, and rotating the arm. Muscles in the forearm and hand allow for gripping, pinching, and fine finger movements.

The muscles of each arm include:

• Biceps: Made up of 2 parts (heads), long and short. This muscle runs from the scapula to the radius in the forearm. It helps keep the shoulder joint in place, rotates the forearm, and helps bend the elbow.
• Brachialis: Found under the biceps on each arm. It helps bend the elbow.
• Triceps (brachii) (one on each sidearm) Made up of 3 parts (heads), located on the back of the upper arm. It straightens the forearm at the elbow.

The muscles, found at the base of the thumb, control thumb movement. These include:

• Abductor pollicis brevis: Found on the outer side of the thumb. It moves the thumb outward, away from the palm.
• Adductor pollicis: The inner muscle of the thumb. It has 2 parts (heads) and moves the thumb inward toward the palm.
• Flexor pollicis brevis: A small muscle that covers the outer and inner parts of the thumb. It helps the thumb bend.

The muscles in the abdomen and chest help support posture, stabilize the core, and allow movements such as bending, twisting, and breathing.

The main muscles in this area include:

• External oblique: Found on each side of the abdomen. These muscles run diagonally downward from the lower ribs to the hip bones (iliac crest). They help the trunk bend and twist.
• Internal oblique: Found under the external obliques. They run diagonally upward from the hip bones (iliac crest) to the lower ribs. They also help the trunk bend and twist.
• Transverse abdominis: The deepest abdominal muscle layer. It runs horizontally across the abdomen, from the spine to the midline. It helps stabilize the core and support breathing.
• Rectus abdominis: These long, flat muscles run vertically from the chest to the pubic bone. They help bend the body forward and support the internal organs.
• Pectoralis major and minor: Found on each side of the chest. The pectoralis major is a thick muscle that helps the upper body push and lift up. The pectoralis minor lies beneath it and helps move the shoulder forward and downward.

The hip and pelvis muscles stabilize the hip joint and allow movements such as walking, running, and climbing.

The main muscles in this area include:

• Gluteus maximus: The largest muscle in the buttocks. It extends and rotates the hip outward.
• Gluteus medius: Found beneath the gluteus maximus. It helps lift the leg to the side and helps keep the pelvis stable when walking.
• Gluteus minimus: The smallest of the buttock muscles. It lies deep in the buttocks and helps keep the pelvis stable during movement.
• Tensor fasciae latae (TFL): A small muscle on the outer side of the hip. It runs from the top of the hip bone (iliac crest) to the side of the thigh. It helps with hip rotation and balance.

The muscles of the upper leg and knee provide strength, stability, and movement for everyday activities like walking, standing, climbing, and sitting. They also support the knee and hip joints.

The quadriceps, or quads, are 4 muscles on the front of each thigh. They work together to straighten the knee and help flex the hip. The 4 muscles are:

• Sartorius: The longest muscle in the body. It runs from the outer hip across the thigh to the inner knee across the outside hip to the knee joints. It helps bend the hip and knee and rotate the thigh outward.
• Vastus medialis: Found on the inner side of the thigh. It helps keep the kneecap in place.
• Vastus lateralis: The largest of the quadriceps, found on the outer side of the thigh. It works with the other thigh muscles to straighten the knee joint, keeps the kneecap (patella) in place, and flexes the hips.
• Rectus femoris: Runs from the hip to above the kneecap. It helps flex the hip.

The hamstrings are 3 muscles on the back of each thigh that work together to bend the knee and straighten the hip. They are:

• Semimembranosus: Found on the inner thigh and attached to the lower leg. It extends the hip, bends the knee, and rotates the leg.
• Semitendinosus: Found toward the middle of the back of the thigh. It helps bend the knee, extend the hip, and rotate the leg.
• Biceps femoris: Found on the outer side of the back of the thigh. It helps extend the hip and bend the knee. It is important for movements and stability.

The muscles in the lower leg and foot help with standing, walking, balance, and movement of the ankles and toes. They control how the foot points, flexes, and supports the body’s weight.

The muscles on the front (anterior) and back (posterior) of the lower leg include:

• Tibialis anterior: The main muscle on the front of the lower leg. It’s found along the front outer side of the shinbone (tibia). It runs from the top of the tibia to the ankle. It helps pull the foot up toward the shin and supports the ankle.
• Gastrocnemius: A large, 2-headed muscle at the back of the lower leg. It runs from the thighbone (femur) to the Achilles tendon. It allows the foot to point downward and lifts the heel upward.
• Soleus: Found beneath the gastrocnemius muscle. It helps point the toes downward, supports the ankle, and helps maintain posture when standing.

The muscles of the foot include:

• Extensor hallucis brevis: A thin muscle on the top of the foot that connects to the first toe. It helps move the big toe.
• Extensor digitorum brevis: A thin muscle on the top of the foot that connects to toes 2 through 4. It helps lift and straighten the toes when walking.
• Interosseus muscles (first through fourth): A group of small muscles between the bones of the foot. They help spread the toes apart, bring them together, and stabilize the foot during movement.