Skeletal muscle is a type of striated muscle, which usually attaches to tendons. Skeletal muscles are used to create movement, by applying force to bones and joints; via contraction. They generally contract voluntarily (via somatic nerve stimulation), although they can contract involuntarily through reflexes. The whole muscle is wrapped in a special type of connective tissue, epimysium.

Muscle cells (also called fibers) are cylindrical, and are multinucleated (in vertebrates and insects). The nuclei of these muscles are located in the peripheral aspect of the cell, just under the plasma membrane, which vacates the central part of the muscle fiber for myofibrils. (Conversely, when the nucleus is located in the center it is considered a pathologic condition known as centronuclear myopathy.)

Skeletal muscles have one end (the "origin") attached to a bone closer to the centre of the body's axis and the other end (the "insertion") is attached across a joint to another bone further from the body's axis. The bones rotate about the joint and move relative to one another by contraction of the muscle (lifting of the upper arm in the case of the origin and insertion described here).

There are several different ways to categorize the type of skeletal muscle fibers (see below). One method uses the type of protein contained in myosin (one of the important proteins that is responsible for the ability of muscle to contract). Using this classification scheme, there are two major types of fibers for skeletal muscles: Type I and Type II. Type I fibers appear reddish. They are good for endurance and are slow to tire because they use oxidative metabolism. Type II fibers are whitish; they are used for short bursts of speed and power, and use both oxidative metabolism and anaerobic metabolism depending on the particular sub-type, and are therefore quicker to fatigue.

How skeletal muscle works

Skeletal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons. Once the cells are "excited", their sarcoplasmic reticulum will release ionic calcium (Ca²⁺) which interacts with the myofibrils to induce muscular contraction (via the sliding filament mechanism). This process also requires adenosine triphosphate (ATP). The ATP is produced by metabolizing creatine phosphate and glucose (stored as glycogen or absorbed from blood) within the muscle cells by mitochondria, as well as by metabolizing fatty acids obtained from the blood and within the cell. Each motor neuron activates a group of muscle cells, and collectively the neurons and muscle cells are known as motor units. When more strength is required than can be obtained from a single motor unit, more units will be stimulated; this is known as motor unit recruitment. This is spatial summation. If more strength is required than can be obtained from the current number of motor units, the motor neurons continue to recruit more motor units. When all the motor units are recruited, there will be no further increase in contraction strength. To increase the force of contraction, it is necessary to increase the frequency of neuronal firing. This results in tetanic contraction, which is a smooth contraction. This is temporal summation.