Kontraktsioon

Kontraktsioon is a physiological process in which muscle fibers generate active tension and shorten to produce movement. In skeletal muscle, a motor neuron action potential triggers calcium release from the sarcoplasmic reticulum, Ca2+ binds to troponin C, and tropomyosin moves to expose myosin-binding sites on actin. Energized by ATP hydrolysis, myosin heads bind actin, perform a power stroke, and pull the filaments; detachment occurs when another ATP binds, allowing cycling to continue as long as calcium is elevated and ATP is available. The collective shortening of many sarcomeres yields a measurable muscle contraction. Contraction strength is graded by motor unit recruitment and firing rate, and by metabolic state and neural input.

Contractions are classified by changes in muscle length and force pattern. Isometric contractions generate force without length change; isotonic contractions change length and can be concentric (shortening) or eccentric (lengthening under load). The sliding filament theory describes how actin and myosin interact to produce contraction. Neural and metabolic factors regulate duration and strength; fatigue or disease can disrupt contraction.

Kontraktsioon also appears in linguistics as contraction, the shortening or fusion of two or more words (for example, I am -> I'm; cannot -> can't). Contractions are common in many languages and are governed by phonological and syntactic constraints. The term derives from Latin contrahere, to draw together.