spier

Spier is the Dutch word for muscle, the tissue that contracts to produce movement and force in animals. In humans, most skeletal muscles attach to bones via tendons and work with the skeleton to enable movement, maintain posture, and generate heat.

Muscle tissue is categorized into three main types: skeletal, smooth, and cardiac. Skeletal muscles are voluntary and striated; smooth muscles control the walls of organs and vessels; cardiac muscle powers the heart. A skeletal muscle consists of bundles of muscle fibers; each fiber contains myofibrils with sarcomeres, the contractile units built from actin and myosin.

Contraction follows the sliding-filament mechanism. A motor neuron triggers calcium release, exposing binding sites on actin. Myosin heads form cross-bridges, pull actin filaments, and hydrolyze ATP to power the movement, shortening the sarcomere. The neuromuscular junction transmits signals via acetylcholine.

Energy for contraction comes from ATP produced through aerobic respiration, anaerobic glycolysis, and phosphocreatine. Muscle fiber types range from slow-twitch (type I) to fast-twitch (type II), with differing endurance and force capabilities.

In health, regular exercise increases muscle mass and strength and supports metabolic health, while aging can lead to sarcopenia. Muscles can be affected by injury, myopathies, and muscular dystrophies, and by metabolic or neuromuscular disorders.