LEARNING OBJECTIVE: Identify primary muscle functions, muscle characteristics, types of muscle tissue, and important functional muscles.

Muscles are responsible for many different types of body movements. The action of the muscle is determined mainly by the kind of joint it is associated with and the way the muscle is attached to the joint. At one end of some muscles are long white tendons that attach the muscles to bone. The point of fixed attachment of a muscle to bone is called the origin. The more flexible attachments, especially attachments to a movable bone, are termed insertions.

Muscles seldom act alone; they usually work in groups held together by sheets of a white fibrous tissue called fascia. Muscles make up about one-half of the total body weight. Their main functions are threefold:

In addition, muscles are involved in such essential bodily functions as respiration, blood circulation, digestion, and other functions such as speaking and seeing.


Muscle tissue has a highly developed ability to contract. Contractibility enables a muscle to become shorter or thicker, and this ability, along with interaction with other muscles, produces movement of internal and external body parts. Muscle contraction in a tissue or organ produces motion and provides power and speed for body activity. A contracting muscle is referred to as a prime mover. A muscle that is relaxing while a prime mover is contracting is called the antagonist.


All muscles respond to stimulus. This property is called excitability or irritability. The mechanical muscular action of shortening or thickening (also called contraction) is activated by a stimulus sent through a motor nerve. All muscles are linked to nerve fibers that carry messages from the central nervous system.


The chemical action of muscle fibers consists of two stages, contraction and recovery. In the contraction stage, two protein substances (actin and myosin) react to provide energy through the breakdown of glycogen into lactic acid. In the recovery stage, oxygen reacts with lactic acid to release carbon dioxide and water.


When a muscle contracts, it produces chemical waste products (carbon dioxide, lactic acid, and acid phosphate) which make the muscle more irritable. If contraction is continued, the muscle will cramp and refuse to move. This condition is known as fatigue. If it is carried too far, the muscle cells will not recover and permanent damage will result. Muscles, therefore, need rest to allow the blood to carry away the waste materials and bring in fresh glucose, oxygen, and protein to restore the muscle protoplasm and the energy that was used.


Tonicity, or muscular tone, is a continual state of partial contraction that gives muscles a certain firmness. Isometric muscle contraction occurs when the muscle is stimulated and shortens, but no movement occurs, as when a person tenses his or her muscles against an immovable object. Isotonic muscle contraction occurs when the muscle is stimulated. The muscle shortens and movement occurs. An example would be lifting an object.


Muscles are also capable of stretching when force is applied (extensibility) and regaining their original form when that force is removed (elasticity).


During exercise, massage, or ordinary activities, the blood supply of muscles is increased. This additional blood brings in fresh nutritional material, carries away waste products more rapidly, and enables the muscles to build up and restore their efficiency and tone.

The importance of exercise for normal muscle activity is clear, but excessive muscle strain is damaging. For example, if a gasoline motor stands

idle, it eventually becomes rusty and useless. Similarly, a muscle cell that does not work atrophies, becoming weak and decreasing in size. On the other hand, a motor that is never allowed to stop and is forced to run too fast or to do too much heavy work soon wears out so that it cannot be repaired. In the same way, a muscle cell that is forced to work too hard without proper rest will be damaged beyond repair.

When a muscle dies, it becomes solid and rigid and no longer reacts. This stiffening, which occurs from 10 minutes to several hours after death, is called rigor mortis.


There are three types of muscle tissue: skeletal, smooth, and cardiac. Each is designed to perform a specific function.


Skeletal, or striated, muscle tissues are attached to the bones and give shape to the body. They are responsible for allowing body movement. This type of muscle is sometimes referred to as striated because of the striped appearance of the muscle fibers under a microscope (fig. 1-9). They are also called voluntary muscles because they are under the control of our conscious will. These muscles can develop great power.


Smooth, or nonstriated, muscle tissues are found in the walls of the stomach, intestines, urinary bladder, and blood vessels, as well as in the duct glands and in the skin. Under a microscope, the smooth muscle fiber lacks the striped appearance of other muscle tissue (fig. 1-10). This tissue is also called involuntary muscle because it is not under conscious control.


The cardiac muscle tissue forms the bulk of the walls and septa (or partitions) of the heart, as well as the origins of the large blood vessels. The fibers of the cardiac muscle differ from those of the skeletal and smooth muscles in that they are shorter and branch into a complicated network (fig. 1-11). The cardiac muscle has the most abundant blood supply of any muscle in the body, receiving twice the blood flow of the highly vascular skeletal muscles and far more than the smooth muscles. Cardiac muscles contract to pump blood outof the heart and through the cardiovascular system. Interference with the blood supply to the heart can result in a heart attack.


In the following section, the location, actions, origins, and insertions of some of the major skeletal muscles are covered. In figures 1-28 and 1-29 the superficial skeletal muscles are illustrated. Also note, the names of some of the muscles give you clues to their location, shape, and number of attachments.


The temporalis muscle is a fan-shaped muscle located on the side of the skull, above and in front of the ear. This muscle's fibers assist in raising the jaw and pass downward beneath the zygomatic arch to the mandible (fig. 1-29). The temporalis muscle's origin is the temporal bone. It is inserted in the coronoid process (a prominence of bone) of the mandible.


The masseter muscle raises the mandible, or lower jaw, to close the mouth (fig. 1-28). It is the chewing muscle in the mastication of food. It originates in the zygomatic process and adjacent parts of the maxilla and is inserted in the mandible.


The sternocleidomastoid muscles are located on both sides of the neck. Acting individually, these muscles rotate the head left or right (figs. 1-28 and 1-29). Acting together, they bend the head forward toward the chest. The sternocleidomastoid muscle originates in the sternum and clavicle and is inserted in the mastoid process of the temporal bone. When this muscle becomes damaged, the result is a common condition known as a “stiff neck.”


The trapezius muscles are a broad, trapezium-shaped pair of muscles on the upper back, which raise or lower the shoulders (figs. 1-28 and 1-29). They cover approximately one-third of the back. They originate in a large area which includes the 12 thoracic vertebrae, the seventh cervical vertebra, and the occipital bone. They have their insertion in the clavicle and scapula.


Figure 1-28.—Anterior view of superficial skeletal muscles.

Pectoralis Major

The pectoralis major is the large triangular muscle that forms the prominent chest muscle (fig. 1-28). It rotates the arm inward, pulls a raised arm down toward the chest, and draws the arm across the chest. It originates in the clavicle, sternum, and cartilages of the true ribs, and the external oblique muscle. Its insertion is in the greater tubercle of the humerus.

Figure 1-29.—Posterior view of superficial skeletal muscles.


The deltoid muscle raises the arm and has its origin in the clavicle and the spine of the scapula (figs. 1-28and 1-29). Its insertion is on the lateral side of the humerus. It fits like a cap over the shoulder and is a frequent site of intramuscular injections.

Biceps Brachii

The biceps brachii is the prominent muscle on the anterior surface of the upper arm (fig. 1-28). Its origin is in the outer edge of the glenoid cavity, and its insertion is in the tuberosity of the radius. This muscle rotates the forearm outward (supination) and, with the aid of the brachial muscle, flexes the forearm at the elbow. 

Triceps Brachii

The triceps brachii is the primary extensor of the forearm (the antagonist of the biceps brachii) (fig. 1-29). It originates at two points on the humerus and one on the scapula. These three heads join to form the large muscle on the posterior surface of the upper arm. The point of insertion is the olecranon process of the ulna.

Latissimus Dorsi

The latissimus dorsi is a broad, flat muscle that covers approximately one-third of the back on each side (figs. 1-28 and 1-29). It rotates the arm inward and draws the arm down and back. It originates from the upper thoracic vertebrae to the sacrum and the posterior portion of the crest of the ilium. Its fibers converge to form a flat tendon that has its insertion in the humerus.


The gluteus (maximus, minimus (not shown), and medius) are the large muscles of the buttocks, which extend and laterally rotate the thigh, as well as abduct and medially rotate it (fig. 1-29). They arise from the ilium, the posterior surface of the lower sacrum, and the side of the coccyx. Their points of insertion include the greater trochanter and the gluteal tuberosity of the femur. The gluteus maximus is the site of choice for intramuscular injections.

Quadriceps Tibialis Anterior

The quadriceps is a group of four muscles that make up the anterior portion of the thigh. The four muscles of this group are the rectus femoris that originates at the ilium; and the vastus lateralis, v. medialis, v. intermedius (not shown), that originate along the femur (fig. 1-28). All four are inserted into the tuberosity of the tibia through a tendon passing over the knee joint. The quadriceps serves as a strong extensor of the leg at the knee and flexes the thigh. Additionally located in the quadriceps area is the adductor longus that adducts, rotates, and flexes the thigh.

Biceps Femoris

The biceps femoris (often called the hamstring muscle) originates at the tuberosity of the ischium (the lowest portion of the coxal bone, part of the pelvic girdle) and the middle third of the femur (fig. 1-29). It is inserted on the head of the fibula and the lateral condyle of the tibia. It acts, along with other related muscles, to flex the leg at the knee and to extend the thigh at the hip joint.


The gracilis is a long slender muscle located on the inner aspect of the thigh (figs. 1-28 and 1-29). It adducts the thigh, and flexes and medially rotates the leg. Its origin is in the symphysis pubis, and its insertion is in the medial surface of the tibia, below the condyle.


The sartorius is the longest muscle in the body. It extends diagonally across the front of the thigh from its origin at the ilium, down to its insertion near the tuberosity of the tibia (fig. 1-29). Its function is to flex the thigh and rotate it laterally, and to flex the leg and rotate it slightly medially.

Gastrocnemius and Soleus

The gastrocnemius and soleus (together commonly called the calf muscles) extend the foot at the ankle (figs. 1-28 and 1-29). The gastrocnemius originates at two points on the femur; the soleus originates at the head of the fibula and the medial border of the tibia. Both are inserted in a common tendon called the calcaneus, or Achilles tendon.

The tibialis anterior originates at the upper half of the tibia and inserts at the first metatarsal and cuneiform bones (fig. 1-28). It flexes the foot.


The diaphragm (not shown) is an internal (as opposed to superficial) muscle that forms the floor of the thoracic cavity and the ceiling of the abdominal cavity. It is the primary muscle of respiration, modifying the size of the thorax and abdomen vertically. It has three openings for the passage of nerves and blood vessels.