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Muscle Fiber Types and Training

Researchers investigate potential for type shifting.

Illustration of muscle fiber types

Experts debate whether and how muscle fiber types can be shifted through training. And training matters, according to a review study available in Sports (2021; 9, [9], 127).

To review, type I are slow-twitch fibers with fatigue resistance, type IIa are higher twitch fibers but more fatigable, and type IIx are fast-twitch and highly fatigable. Elite endurance or power athletes tend to have a genetic predominance for one or the other muscle fiber types.

Researchers from CUNY Lehman College in the Bronx and Auburn University in Alabama reviewed current studies to determine best training practices. Some research suggests that slow-speed resistance training with high load can shift IIx and IIx/IIa hybrids to more IIa fibers. Sprint, power and plyometric training can shift to more IIa fibers but people with more IIx fibers to start do not experience much of a shift. In general, endurance training produces a fiber type shift toward more type I slow, oxidative fibers. Detraining, however, leads to a shift back toward IIa, which may explain why taper training for endurance athletes before a race is helpful to boost the capacity for “kicking” at the end of a race.

Most studies lack longitudinal data. For example, programs of 6–16 weeks shifted predominantly type IIx fiber athletes toward type IIa, but the impact of longer-term training cannot be determined. Interesting findings include that in one individual, different muscle regions—deep versus superficial and origin versus insertion—may have different fiber patterns. Additionally, certain muscles have a higher propensity toward fiber type shifting than others, and certain individuals may have more ability toward fiber type shifts.

Study authors note that understanding fiber type shifts holds promise for improving the ability to tailor training for different body parts and regions for specific tasks.

See also: Skeletal Muscle Hypertrophy



Shirley Archer-Eichenberger, JD, MA

Shirley Archer, JD, MA, is an internationally acknowledged integrative health and mindfulness specialist, best-selling author of 16 fitness and wellness books translated into multiple languages and sold worldwide, award-winning health journalist, contributing editor to Fitness Journal, media spokesperson, and IDEA's 2008 Fitness Instructor of the Year. She's a 25-year industry veteran and former health and fitness educator at the Stanford Prevention Research Center, who has served on multiple industry committees and co-authored trade books and manuals for ACE, ACSM and YMCA of the USA. She has appeared on TV worldwide and was a featured trainer on America's Next Top Model.

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