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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

What Are Slow and Fast-Twitch Muscles? 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.

Here's how it works: During aerobic exercises such as running or swimming, slow-twitch fibers are the first to contract.Fiber recruitment affects oxidative recovery measurements of human muscle in vivo. Crowther GJ, Gronka RK. Medicine and science in sports and exercise, 2003, Mar.;34(11):0195-9131. When the slow-twitch fibers become tired, fast-twitch fibers begin to take over.

There are significant benefits to working to the point of temporary fatigue—and therefore making sure fast-twitch fibers have been recruited. For instance, if you're looking to increase muscle mass, and improve strength, using fast-twitch fibers is the only way to do it.Slow-twitch fiber glycogen depletion elevates moderate-exercise fast-twitch fiber activity and O2 uptake. Krustrup P, Söderlund K, Mohr M. Medicine and science in sports and exercise, 2004, Aug.;36(6):0195-9131. On the other hand, aerobic exercises, those that mainly use slow-twitch fibers, can increase stamina and the oxygen capacity of your muscles, allowing the body to burn energy for longer periods of time. A high proportion of slow-twitch fibers has also been associated with low blood pressure.Muscle fiber-type distribution as a predictor of blood pressure: a 19-year follow-up study. Hernelahti M, Tikkanen HO, Karjalainen J. Hypertension, 2005, Apr.;45(5):1524-4563. Previous research has also shown that women may have a greater distribution of type I muscle fibers and lower distribution of type II muscle fibers than men.

Your Action Plan

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.

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