Research
Mixed
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.
High-volume endurance training will make your muscles more efficient at using oxygen by increasing Type I fibers. If you are an endurance athlete, expect this shift. If you are a strength athlete, be aware that excessive endurance work might reduce your peak power, but you can mitigate this with proper tapering.
GoodSupportsHIGH confidence
Endurance training generally induces a fiber type shift toward a more oxidative phenotype... a shift toward type I fibers was apparent with higher volume endurance training.
Why this rating
Supported by multiple longitudinal studies using single-fiber analysis, though long-term elite data is scarce.
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
- 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.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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