Type I vs Type II muscle fibers

Skeletal muscle fibers are broadly classified into Type I (slow-oxidative) and Type II (fast-glycolytic and fast-oxidative-glycolytic) on the basis of myosin heavy chain isoform expression, metabolic profile, and contractile speed. Type I fibers are fatigue-resistant, mitochondria-dense, and reliant on oxidative metabolism; they dominate endurance activity and Zone 2 training stimulus. Type II fibers — subdivided into IIa (intermediate) and IIx (fast, least oxidative in humans) — generate higher force and power but fatigue more rapidly and are preferentially recruited during heavy resistance exercise and sprinting. With aging, Type II fibers show selective atrophy and denervation before Type I, contributing to dynapenia and fall risk; resistance and power training selectively preserve and hypertrophy these fast fibers.