The head muscles are usually grouped into two categories: facial or chewing muscles. The facial muscles include:
1. frontalis - covers frontal bone, runs from cranial aponeurosis to the skin of the eyebrows and allows you to raise eyebrows.
2. orbicularis oculi - has fibers that run in circles around the eyes and allows you to blink, squint, and wink.
3. orbicularis oris - the circular muscle of the lips. It closes and opens the mouth and is also called the kissing muscle.
4. buccinators - runs horizontally across the cheek and inserts to the orbicularis oris. It flattens the cheek and compressing it to hold food while chewing.
5. zygomaticus - extends to corner of mouth to cheek bone. Referred to as smiling muscle for raising corners of mouth upward.
The chewing muscles include:
1. masseter - runs from zygomatic process to mandible and covers the angle of lower jaw. It closes the jaw by raising the mandible.
2. temporalis - fan shaped muscle overlying the temporal bone. Inserts into the mandible and acts as a synergist of the masseter in closing the jaw.
Trunk & neck muscles
The trunk muscles has those that move the vertebral column, the anterior thorax muscles, and the abdominal wall muscles. They include:
1. pectoralis major - large fan shaped, covers upper chest, origin from sternum, shoulder girdle, and first six ribs. It inserts on proximal end of humerus, forms anterior wall of axilla, adducts and flex the arm.
2. intercostal muscles - deep muscles between ribs, external intercostal are important for breathing by helping raise your rib cage to inhale. Internal intercostal depress the rib cage and helps you exhale.
3. rectus abdominus - run from pubis to rib cage and they flex the vertebral column. They also compress abdominal contents during defecation and childbirth.
4. external oblique - make up lateral walls of abdomen. Fibers run down medially from last eight ribs and insert at ilium. They flex the vertebral column, rotate the trunk and bend it laterally.
5. internal oblique - Paired muscles deep to external obliques. Fibers run at right angles to external obliques, Arise from the iliac crest, and insert to last three ribs. They have same function as external obliques.
6. transversus abdominus- Deepest of abdominal muscle wall and it's fibers run horizontally across the abdomen. Arises from lower ribs and iliac crest and inserts into pubis. compresses abdominal contents.
7. latissimus dorsi - Two large, flat muscles that cover lower back. originate on lower spine and ilium then sweep superiorly to insert into the proximal end of humerus. Extends and adducts the humerus.
8. erector spinae - Prime mover of back extension. Provides resistance that helps control the action of bending over at waist.
9. quadratus lumborum - Form parts of posterior abdominal wall. Each muscle pair flexes spine laterally, extends the lumbar spine, arise from iliac crests, and inserts into upper lumbar vertebrae.
10. deltoid - fleshy, triangle-shaped muscles, form rounded shape of your shoulders. Bulky, a favorite injection site, origin winds across the shoulder girdle from spine of scapula to clavicle. Inserts to the proximal humerus. Main movers of arm abduction.
11. trapezius - forms a diamond or kite-shaped muscle mass. Origin runs from occipital bone down to the end of thoracic vertebrae. They flare laterally to insert on the scapular spine and clavicle. It extends the head and can elevate, depress, adduct, and stabilize the scapula.
The neck muscles move the head and shoulder girdle and are small and strap-like. They include:
1. platysma - single sheetlike muscle, covers anterolateral neck, from connective tissue of the chest muscles, inserts to area around the mouth, and pulls the corners of the mouth inferiorly, making it sag.
2. sternocleidomastoid - two headed, found on each side of neck, one arises from sternum and the other from the clavicle. They fuse before inserting into the mastoid process and when they contract it flexes your neck.
upper limb muscles
The upper limb muscles have three groups: one involves muscles arising from the shoulder girdle and crossing the shoulder joint, the second causes movement at the elbow joint, and the third includes forearm muscles inserting the hand bones and causing their movements. It includes:
1. biceps brachii - bulges when elbow is flexed, originates by two heads from shoulder girdle, and inserts to radial tuberosity. prime mover for flexion of the forearm and acts to supinate the forearm.
2. brachialis - Lies deep to biceps muscles and lifts ulna as biceps lifts radius.
3. brachioradialis - Fairly weak muscle that arises on the humerus and inserts to the distal forearm.
4. triceps brachii - only muscle fleshing out the posterior humerus. Its three heads arise from shoulder girdle and proximal humerus, it inserts to olecranon process and it is the prime mover of the elbow extension. Its known as the boxer muscle because it can deliver a straight arm knock out punch. Click to view larger image.
lower limb muscles
The lower limb muscles are the strongest, largest muscles in the body and they cause movement at the hip, knee, and foot joints. They include:
1. gluteus maximus - forms most of the flesh of buttock, a powerful hip extensor that brings thigh in a straight line with pelvis, it originates from sacrum and iliac bones, and inserts on gluteal tuberosity of the femur into the large tendinous iliotibial tract.
2. gluteus medius - runs from the ilium to femur beneath the gluteus maximus, a hip abductor and important for steadying walking. Important site for giving intramuscular injections and it overlies the large sciatic nerve.
3. iliopsoas - a fused iliacus and psoas major muscles. runs from iliac bone and lower vertebrae to insert on the lesser trochanter of femur. A prime mover of hip flexion and keeps upper body from falling backward while standing.
4. adductor muscles - forms muscle mass at medial side of each thigh and adduct or press the thighs together. Origin is on the pelvis and insert on proximal aspect of femur.
5. hamstring group - It has the biceps femoris, semimembranosus, and semitendiosus which originates on ischial tuberosity and inserts on both sides of proximal tibia. They're prime movers of thigh extension and knee flexion.
6. satorius - runs obliquely across the thigh from anterior iliac crest to medial side of tibia. a weak thigh flexor and acts as a synergist to bring about the cross-legged position.
7. quadriceps group - rectus femoris and three vastus muscles. the vastus muscles originate from femur and the rectus femoris from the pelvis. they all insert into the tibial tuberosity. They extend the knee powerfully and it can also help flex the hip.
8. tibialis anterior - on anterior leg, arises from upper tibia then parallels the anterior crest where it inserts by a long tendon. It acts to dorsiflex and invert the foot.
9.extensor digitorum longus - lateral to tibialis anterior, arises from the lateral tibial condyle, inserts into the phalanges of toes. Prime mover of toe extension.
10. fibularis muscles - Has the longus, brevis, and tertius muscles, are found on lateral part of leg, arises from fibula, inserts into metatarsal bones, they plantar flex and evert the foot.
11. gastrocnemius - two bellied muscle that forms the curved calf of posterior leg. It arises by two heads, one from each side of the distal femur and inserts through the large calcaneal tendon to the heel of the foot. prime mover for plantar flexion of foot.
12. soleus - Arises on tibia and fibula and inserts into calcaneal tendon and is a strong plantar of the foot.