Know any HIIT aces who struggle to run a mile? How about a marathoner who can’t nail a box jump? The difference might lie in the makeup of their skeletal muscles.
People have two general types of skeletal muscle fibers: slow-twitch (type I) and fast-twitch (type II). Slow-twitch muscles help enable long-endurance feats such as distance running, while fast-twitch muscles fatigue faster but are used in powerful bursts of movements like sprinting.
What You Need to Know
Fast-twitch muscles break down into two categories: moderate fast-twitch (type IIa) and fast-twitch (type IIb or IIx).The effects of endurance, strength, and power training on muscle fiber type shifting. Wilson JM, Loenneke JP, Jo E. Journal of strength and conditioning research / National Strength & Conditioning Association, 2012, Dec.;26(6):1533-4287. Moderate fast-twitch muscles are thicker, quicker to contract, and wear out more rapidly than slow-twitch. Fast-twitch, the most powerful and lowest in endurance, are activated when the body nears maximum exertion.
There’s no conclusive evidence that muscle fibers can transform from slow- to fast-twitch or vice versa.The effects of endurance, strength, and power training on muscle fiber type shifting. Wilson JM, Loenneke JP, Jo E. Journal of strength and conditioning research / National Strength & Conditioning Association, 2012, Dec.;26(6):1533-4287. In other words, while you can improve at sprinting or other explosive movements, it may not be because you’re converting muscle fiber types. Still, fast-twitch (type IIb) fibers can be converted into moderate fast-twitch (type IIa), or vice versa, through increases in endurance or resistance training.Muscle mechanics: adaptations with exercise-training. Fitts RH, Widrick JJ. Exercise and sport sciences reviews, 1996, Oct.;24():0091-6331. And keep in mind that fast-twitch muscle depletes with age considerably more than slow-twitch, so increasing muscle power is less feasible later in life.Human aging, muscle mass, and fiber type composition. Lexell J. The journals of gerontology. Series A, Biological sciences and medical sciences, 1996, Jan.;50 Spec No():1079-5006.
Still, generally speaking, when muscles are forced to work differently and you get out of your comfort zone, you get a better workout. Keep mixing your workouts up to recruit all types of muscles fibers—and to fight dreaded gym boredom.
Originally published September 2011. Updated August 2015.