Muscle Anatomy

Types of Movement
     All muscles exert their force by pulling between at least two points of attachment. The movement that results from contraction is called the action of the muscle. Typically, one attachment remains stationary and is called the origin and the other attachment moves and is called the insertion.
     Almost all muscles cross at least one joint (moveable connection between two bones) and cause an action across that joint. The type of movement that results depends upon the nature of the joint.
     Common types of movement that result from muscle contraction include:
Flexion – A movement that decreases the angle between two bones at the joint.
Extension – A movement that increases the angle between two bones at the joint. However, if the angle goes past 180o (a straight line) in the direction opposite flexion the movement is called hyperextension.
Rotation – Rotation is a movement that results in movement of one bone around its longitudinal axis.
Abduction – A movement that results in the part moving away from the midline.
Adduction – A movement that results in the part moving toward the midline.
     Special types of movement occur at particular joints and include:
Dorsiflexion and plantar flexion – These movements only apply to movements of the foot at the ankle joint. Dorsiflexion is when the foot is raised as when you dig in your heels. Plantar flexion is when you lower your foot as when you lift yourself onto the balls of your feet.
Inversion and eversion – These movements also apply only to the feet. Inversion is when you turn your feet inward so that your soles are facing one another. Eversion is when you turn your feet outward so that your soles are facing laterally.
Supination and pronation – These movements apply to the forearm. Pronation is rotation of the radius across the ulna that results is your palms facing backwards. Supination is movement in the opposite direction that uncrosses the radius from the ulna to cause the palms to face forward.
Opposition – This movement enables us to be skillful tool users. Opposition is the movement of the tip of the thumb that enables it to touch the tips of the other fingers.

Naming Skeletal Muscles
     Memorization of specific muscles is easier if you have some understanding of how the muscle was named. Conversely, deciphering the significance of a muscle’s name will give you information about the muscle. Muscles are named according to a number of criteria:
Direction of muscle fibers
     A muscle may be given a name based on the direction of muscle fibers, or the muscle as a whole, relative to the longitudinal axis of the muscle, or body part with which the muscle is associated. The term rectus implies that the fibers or muscle is oriented straight with respect to the longitudinal axis. The term oblique means that the orientation is at an angle to the longitudinal axis.
Relative size
     When more than one muscle are in the same location they may be distinguished by their relative size. These terms include:
major for bigger and minor for smaller;
maximus for biggest, intermedius for middle and minimus for smallest;
longus for longest and brevis for shortest.
     Some muscles are given a name based on the bone they are associated with or the region of the body where they are found. For example, the pectoralis major is found in the chest region and the rectus femoris is found near the femur.
Number of origins
     A muscle that has more than one attachment at locations that do not move, called origins, and only one attachment at the location that does move, called the insertion, is said to have more than one head. The term biceps means two heads; the term triceps means three heads; and the term quadriceps means four heads.
Location of the origin and insertion
      A muscle may be named for their attachment sites. For example, the brachioradialis attaches to the humerus (origin) and the radius (insertion).
      Many muscles are named according to their shape. For example, deltoid means triangular in shape.
     Muscles are often named for their primary action. For example, flexor, extensor, adductor and abductor are names associated with the action of the muscle.

Gross Anatomy
  Facial Muscles
     The facial muscles are associated with facial expressions. These muscles include:
     The frontalis muscle lies over the forehead (Latin – frons). Posteriorly, it attaches to an aponeurosis and anteriorly it attaches to the skin of the eyebrows. Contraction of the frontalis wrinkles the forehead and raises the eyebrows.
Orbicularis oculi
     The fibers of the orbicularis oculi encircle the eye. This muscle enables you to close your eyes, squint, blink and wink.
Orbicularis oris
     This is another circular muscle that is found within the lips. It closes the mouth and protrudes the lips.
     This muscle has a name that associates it with a “horn blower”. Its fibers run horizontal in the cheeks and attach to the orbicularis oris. The contraction of this muscle flattens the cheeks as during sucking and whistling or when blowing on a trumpet. The compression of the cheeks also helps to hold food between the teeth during chewing.
     This muscle's fibers extend from the cheek bones to the corners of the mouth. Contraction of the zygomaticus raises the corners of the mouth as when smiling.
  Chewing Muscles
     The chewing muscles enable you to chew your food by moving the upper and lower teeth against one another. These muscles include:
     The masseter attaches to the zygomatic process of the temporal bone and inserts on the mandible. This muscle elevates the mandible during chewing.
     This muscle, as its name implies, attaches to the flat part of the temporal bone. Its fibers converge to attach to a process on the mandible. This muscle also works with the masseter  to elevate the mandible during chewing.
  Neck Muscles
     The neck muscles move the head and the shoulder girdle. These muscles include:
     The platysma is a broad sheet-like muscle that attaches to the connective tissue of the skin of the upper chest and inserts on the mandible and the skin at the corners of the mouth. Contraction of the muscle depresses the mandible, depresses the corners of the mouth and tightens the skin of the neck.
     The name of this muscle describes its attachments. This is a two headed muscle in which one head attaches to the sternum and the other head attaches the clavicle (cleido). The two heads fuse and attach to the mastoid process of the temporal bone. When both muscles contract together the head flexes as when bowing the head in prayer. If only one muscle contracts the head turns to the opposite side.
  Anterior Trunk Muscles
Pectoralis major
     This muscle is a large fan-shaped muscle that covers the upper part of the chest. The muscle inserts on the proximal part of the humerus and acts to adduct and flex the arm.
Intercostal muscles
     The intercostal muscles are found between the ribs (costa). These muscles aid in ventilating the lungs by raising and lowering the ribs during forceful breathing.
  Muscles of the Abdominal Girdle
     These muscles support the wall of the abdominal cavity. They also are involved with flexion of the vertebral column. These muscles are:
Mike Sorrentino, The Jersey Shore
Rectus abdominis
     The rectus abdominis muscles are a pair of strap-like muscle on either side of the midline that goes through the “belly button”. Lean and well-toned muscles give rise to the ever popular “six pack”. The fibers attach to the rib cage and the pubis of the hip bones. Contraction causes flexion of the vertebral column and, when the vertebral column is held in position, compression of the abdominal cavity.
External oblique
     The fibers of this muscle attach to the lower eight ribs and spiral downward and medially to attach to the hip bone. This muscle also flexes the spine and compresses the abdomen but in addition it also rotates and laterally flexes the spine.
Internal oblique
     The internal oblique is a muscle that is deep to the external oblique and has fibers that crisscross those of the external oblique. Its actions are similar to those of the external oblique.
Transversus abdominis
     This is the deepest muscle of the abdominal wall and it has fibers that run transversely. Its main action is to compress the abdominal cavity.
  Posterior Trunk Muscles
     The trapezius muscles are superficial muscles of the neck and upper trunk. Taken together they form a diamond shape. Because of their broad attachments they have a number of actions that include extending the head and elevating, depressing, adducting and stabilizing the scapula.
Latissimus dorsi
     This is another flat, superficial muscle that covers the lower back. It inserts on the proximal end of the humerus and its primary actions are to extend and adduct the humerus. Because of these actions this muscle is sometimes called the “swimmer’s muscle”.
Erector Spinae
     These muscles are the deep muscles of the back. As the name implies, they are extensors of the back. Spasms of these muscles is a common source of back pain.
  Upper Limb Muscles
     The deltoid muscles are fleshy, triangular muscles that give the shoulder its round contour. The muscle has origins on the pectoral girdle and inserts on the lateral side of the humerus. The primary action of this muscle is abduction of the humerus but it also contributes to flexion and extension of the humerus and lateral and medial rotation of the humerus at the shoulder joint.
Biceps brachii
     This muscle, as its name implies, has two heads. Both heads have an origin on the scapula and insert on the radius. This muscle acts to flex the forearm at the elbow joint and also supinates the forearm. It forms a noticeable bulge on the anterior side of the arm when it contracts.
     This muscle lies deep to the biceps  and flexes the forearm with the biceps brachii.
     This muscle has its origin on the distal part of the humerus and its insertion on the distal part of the radius. Hence its name derives from the origin and insertion. It is a weak flexor of the forearm and the muscle lies mainly in the forearm.
Triceps brachii
     This muscle is responsible for most of the flesh on the posterior side of the arm. It has three heads as the name implies and inserts on the point of the elbow. It is the main extensor of the forearm at the elbow joint.
  Hip Joint Muscles
Gluteus maximus
     This muscle contributes to most of the flesh of the buttocks. It is a powerful extensor of the thigh. It is used primarily when the hip is already flexed as when rising from a sitting position or climbing up stairs.
Gluteus medius
     The gluteus medius lies underneath the gluteus maximus. This muscle is an abductor of the thigh at the hip joint and steadies the pelvis during walking. The gluteus medius is the preferred muscle for intramuscular injections.
     This muscle is totally hidden from view as its has its origin on the anterior side of the ilium and the lower vertebrae and inserts on the proximal part of the femur. It is the main flexor of the thigh at the hip joint.
Adductor Muscles
     The adductor muscles form the fleshy mass on the medial side of the thigh. As the name implies they adduct the thigh at the hip joint.
  Knee Joint Muscles
Hamstring Muscles
     The hamstring muscles are a group of three muscles, the biceps femoris, semitendinosus and semimembranosus. These muscle form the fleshy posterior part of the thigh. These muscle are flexors of the leg at the knee joint but also contribute to extension of the thigh at the hip joint.
     This is a thin, strap-like muscle that attaches to the hip bone and crosses the anterior side of the thigh to attach to the medial side of the tibia. It is a weak flexor of the leg at the knee joint but also flexes and laterally rotates the thigh as when crossing the knee. Its name means “tailor” because a tailor would cross his legs while sewing.
Quadriceps femoris
     This muscle has four heads each with their own name. The rectus femoris attaches to the hip bone and the vastus lateralis, vastus intermedius and vastus medialis attach to the femur. All four heads fuse and insert on the proximal tibial. The patella or knee cap forms in the tendon of this muscle. The quadriceps femoris is a powerful extensor of the leg at the knee joint. The rectus femoris also contributes to flexion of the thigh at the hip joint.
  Ankle and Foot Muscles
Tibialis anterior
     This muscle is a superficial muscle on the anterior leg. It attaches to the tarsal bones on the medial side of the foot and dorsiflexes and inverts the foot.
 Extensor digitorum longus
     The name of this muscle says its action. It is an extensor of the toes (2-5) but also dorsiflexes the foot.
      This muscle has two heads and forms the curved calf of the posterior leg. The gastrocnemius attaches to the distal end of the femur and inserts onto the heel bone by way of the calcaneal (Achilles) tendon. It is a plantar flexor of the foot but because it crosses the knee joint, also contributes to flexion of the leg at the knee joint.
     This muscle lies deep to the gastrocnemius muscle. It attaches to the tibia and joins the gastrocnemius at the calcaneal tendon. It is also a plantar flexor of the foot at the ankle joint.