Skeletal muscle

Skeletal muscle is a type of striated muscle, attached to the skeleton. Skeletal muscles are used to facilitate movement, by applying force to bones and joints; via contraction. They generally contract voluntarily (via nerve stimulation), although they can contract involuntarily.

Muscles have an elongated, cylindrical shape, and are multinucleated. The nuclei of these muscles are located just under the plasma membrane, which vacates the central part of the muscle fiber for myofibrils. This unique arrangement of the nuclei allows for higher efficiency. These muscles usually have one end (the "origin") attached to a relatively stationary bone, (such as the scapula) and the other end (the "insertion") is attached across a joint, to another bone (such as the humerus).

How skeletal muscle works

The strength of skeletal muscle is directly proportional to its cross-sectional area. The strength of a body, however, is determined by biomechanical principles (the distance between muscle insertions and joints, muscle size, and so on). Muscles are normally arranged in opposition so that as one group of muscles contract, another group relaxes or expands.

Skelatal muscle cells are stimulated by acetylcholine, which is released at neuromuscular junctions by motor neurons. Once the cells are "excited", their sarcoplasmic reticulums will release ionic calcium (Ca²⁺), this interacts with the myofibrils and, thus, induces muscular contraction (via the sliding filament mechanism). Besides calcium, this process requires adenosine triphosphate (ATP). The ATP is produced by metabolizing creatine phosphate and glycogen, which are stored within the muscle cells; as well by metabolizing glucose and fatty acids, obtained from blood.

Each motor neuron "controls" a group of muscle cells, known as "motor units". When more strength is required, than what can be obtained from a single motor unit, more units will be stimulated; this is known as "motor unit recruitment". If more strength is required than what can be obtained from the current degree of unit contraction, the motor neurons will send additional stimuli; this causes a process of contractile summation, which increases the degree of contraction. If a muscle is maximally contracted, it is said to be in a state of tetanic contraction.

Red and White Fibers

Skeletal muscles contain two types of fibers, used to produce ATP; the amount of each varies from muscle to muscle, and from person to person.

Red ("slow-twitch") fibers have more mitochondria, store oxygen in myoglobin, rely on aerobic metabolism, and are associated with endurance; these produce ATP more slowly. Marathoners tend to have more red fibers.
White ("fast-twitch") fibers have fewer mitochondria, are capable of more powerful (but shorter) contractions, metabolize ATP more quickly, and are more likely to accumulate lactic acid. Weightlifterss and Sprinterss tend to have more white fibers.