Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

All muscle cells work similarly, but the fibers of skeletal muscles that move our bones come in several varieties. At one extreme are slow-twitch fibers that do not contract rapidly or powerfully but use energy aerobically and don’t fatigue easily. These type I fibers are colloquially known as red muscle because of their darker tinge.28 At the other extreme are fast-twitch (type II) fibers, which come in two types: white and pink. White muscle (type IIX) fibers burn sugar to generate powerful and rapid forces but fatigue rapidly. Pink muscle (type IIA) fibers produce moderately powerful forces aerobically and thus fatigue at an intermediate rate. Altogether, red fibers are ideal for sustained low-intensity activities like walking or jogging a marathon, pink fibers are best for medium-intensity activities like racing a mile, and white fibers are essential for bursts of extreme power but short duration like sprinting a hundred meters.