Research
Mixed
Resistance, sprint, and power training induce a shift from fast-twitch IIx fibers to the fast-twitch IIa phenotype, with minimal change in pure type I fibers.
If you engage in heavy resistance training, expect your muscle fibers to become more efficient at generating power (shifting from IIx to IIa), but do not worry about losing your slow-twitch endurance fibers. This adaptation supports better performance in power activities without compromising your base endurance.
GoodSupportsHIGH confidence
The current literature indicates that resistance training performed at slower speeds due to the use of relatively high loads (>70% of one-repetition maximum) produces a shift from IIx and IIx/IIa hybrids to more of a pure IIa phenotype and less shift in pure type I fibers, at least in the longitudinal timeframes that have been observed [13,14].
Why this rating
Based on multiple longitudinal studies using single-fiber analysis, though sample sizes are often small.
Source
Muscle Fiber Type Transitions with Exercise Training: Shifting Perspectives
Daniel L. Plotkin et al. · Sports · 2021
DOI 10.3390/sports9090127
narrative_reviewCited 249×
Read the paper DOI resolved against Crossref · corpus check 2026-06-10
More from this paper
- Endurance training induces a shift toward a more oxidative phenotype, characterized by an increase in Type I fibers and a decrease in Type IIa fibers.Good
- Tapering after intense training induces a 'Type IIx overshoot,' where fast-twitch fiber characteristics and power output increase beyond pre-training levels.Moderate
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