The Muscles - How the skeletal muscles work

The great range and variety of functions served by skeletal muscles can be suggested by naming just four: the diaphragm, used in breathing; the muscles that make the eye wink; the deltoid muscle that gives the shoulder its shape; and the tongue.

As with the bones, the body tends to make its greatest demands on muscle tissue in the area of the joints and the backbone. A smoothly functioning joint requires that bone, cartilage, and muscle all be sound and able to work together effectively.

Tendons and Ligaments

We often hear the words tendon and ligament used in the description of the knee or another joint. These are actually two types of skeletal muscles, distinguished as to their function.

A tendon can be described as a tight cord of muscle tissue that attaches other skeletal muscle to bone. For example, the Achilles tendon running down the back of the calf, the strongest tendon in the body, connects the muscles of the calf with the bone of the heel. A ligament is a somewhat more elastic band of muscle fibers that attaches bone to bone.

A tendon is not always evident in the connection of muscle to bone. Various groups and shapes of muscle fibers may be similarly employed, forming connective tissue without the formation of tendon.


Various associated tissues between or around skeletal muscles serve to reduce the wear and tear of friction in areas such as a joint, where muscle, bone, and cartilage may rub against one another. For example, the tendons that pass along the back of the hand from the wrist to the fingertips, as well as many other muscle groups throughout the body, are enclosed in lubricated sheaths. The muscle-sheathed bursas , lined inside with lubricating fluid, are also found in areas subject to friction, such as where a tendon passes closely over a bone.

Man's upright posture and two-legged locomotion subject the backbone to heavy stresses. It is buttressed, however, with scores of tightly packed bundles of muscle attached to either side of the spinal column.

Flexors and Extensors

Most of us probably first used the word “muscle” when, as children, we watched an older child or adult flex an arm and proudly display the bump of muscle between the crook of elbow and shoulder. This biceps muscle works together with the triceps muscle on the underside of the arm. The arm is bent at the elbow by contraction of the biceps, which makes this muscle get shorter and thicker; in this position, called flexion , the triceps muscle is relaxed. To return the arm to its normal straight position, called extension , the biceps relaxes and the triceps contracts. In this bit of muscle teamwork, the biceps, which bends the arm at the elbow joint, is called the flexor , while the triceps straightens the arm and is called the extensor . Similar flexor-extensor action can be observed at many body joints, including the fingers.

Muscles, Their Locations, and Exercises that Strengthen Them
Muscle Location Movement
Trapezius Upper back and each side of neck Shoulder-shrugging and upward-pulling movements
Deltoids Shoulders Arm raising and overhead pressing
Pectorals Chest Horizontal pressing and drawing arms across body
Latissimus dorsi Wide back muscle stretching over back up to rear Deltoids Pulling and rowing movements
Serratus Jagged sawtooth muscles between Pectorals and lattissimus Dorsi Pullover and Serratus leverage movements
Spinal erectors Lower length of spinal column Raising upper body from a bent-over position
Biceps Front portion of upper arm Arm bending and twisting
Forearms Between wrist and elbow Reverse-grip arm bending
Triceps Back of upper arm Pushing and straightening movements of upper arms
Rectus abdominals Muscular area between sternum and pelvis Sit-up, leg-raising, knee-in movements
Intercostals Sides of waist, running diagonally to Serratus Waist twisting
External oblique abdominals Lower sides of waist Waist twisting and bending
Buttocks Muscular area covering seat Lunging, stooping, leg raising
Leg biceps Back of thighs Raising lower leg to buttocks, bending forward and stretching
Frontal thighs Front of thighs Extending lower leg and knee bending
Calves Lower leg between ankle and knee Raising and lowering on toes

Muscles, Their Locations, and Exercises that Strengthen Them

Smooth Muscle

Beginning about midway down the esophagus, layers of smooth muscle line the walls of the 25 feet of digestive tract, extending into the stomach and through the intestines. These muscles keep the stomach and intestinal walls continually in motion, constricting and relaxing to push food along. Smooth muscle also effects the opening and closing of important valves, called sphincters , along the digestive tract.

Muscle Action in Forearm Movement

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