Skeletal muscle is a type of striated tissue under largely voluntary control. Motor neurons of the somatic nervous system drive contractions that move the skeleton. Most skeletal muscles attach to the skeleton via collagenous bundles called tendons, which anchor muscle to bones and transmit force across joints. Skeletal muscle is one of three principal muscle types, along with cardiac muscle and smooth muscle.

Microstructure and organization

At the cellular level, skeletal muscle is composed of elongated multinucleated cells called muscle fibers or myocytes. Fibers are bundled into fascicles by connective sheaths. Each fiber contains many myofibrils, themselves built from repeating sarcomeres — the basic contractile units made of interdigitating actin and myosin filaments. The regular arrangement of filaments produces the characteristic striated appearance under the microscope.

Mechanism of contraction

Contraction begins when a nerve impulse in a motor neuron reaches the neuromuscular junction and releases neurotransmitter. The resulting electrical signal travels along the fiber membrane and into transverse tubules, triggering calcium release from internal stores. Calcium binding permits cross-bridge cycling between actin and myosin; ATP provides the energy for repeated cycles that shorten sarcomeres and generate force. Relaxation follows calcium reuptake and cessation of neural drive.

Energy supply and metabolism

Skeletal muscle uses multiple metabolic pathways to meet energy demands. At rest and during sustained, low-intensity activity, fibers rely on aerobic metabolism and oxidative enzymes. During brief, intense effort, fibers increasingly use anaerobic pathways. Mitochondria, stored glycogen and intramuscular fat are important fuel sources; blood flow and capillary supply adapt with training and activity level.

Development, repair and fiber types

Muscle develops from embryonic mesoderm by fusion of precursor myoblasts into multinucleated fibers. In adulthood, satellite cells act as resident stem cells that assist growth and repair after injury. Muscle fibers are commonly classified by contractile and metabolic properties: slow, oxidative fibers are fatigue resistant and suited to posture and endurance; fast fibers are specialized for rapid force production and vary in fatigue resistance. Activity, training, aging and disease influence fiber composition and size.

Physiological roles and examples

Skeletal muscle enables locomotion, stability and many fine motor tasks. It controls breathing through the diaphragm, supports posture and facial expression, and is a major organ for whole-body metabolism and thermogenesis. Functional units range from large postural muscles to small, precisely controlled muscles of the hand and eye.

Clinical relevance

Conditions affecting skeletal muscle include inherited disorders, inflammatory myopathies, metabolic myopathies and problems of neuromuscular transmission. Age-related loss of muscle mass and strength (sarcopenia), immobilization atrophy and disuse are common clinical concerns. Assessment may involve clinical examination, electrophysiological testing, imaging and laboratory studies. Treatment and prevention emphasize activity, rehabilitation and addressing underlying causes.