Type I vs Type II muscle fibers

Your skeletal muscle has two broad fiber types: Type I (slow) and Type II (fast). They differ in their myosin type, metabolism, and contraction speed. Type I fibers are fatigue-resistant, packed with mitochondria, and run on oxygen. They dominate endurance work and Zone 2 training. Type II fibers come in subtypes (IIa, intermediate; and IIx, fast; humans lack the rodent IIb type). They produce more force and power but tire faster. Your body recruits them mainly for heavy lifting and sprinting. Here is the aging catch: Type II fibers selectively shrink and lose their nerve supply before Type I do. That drives the loss of power (dynapenia) and raises fall risk. The fix: resistance and power training selectively preserve and grow these fast fibers.

Last reviewed: June 22, 2026

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