The Claim

In untrained young men, type I muscle fibers exhibit higher mitochondrial density than type II fibers at baseline, and this difference remains following 10 weeks of resistance training despite overall mitochondrial expansion.

Source: Myofibril and Mitochondrial Area Changes in Type I and II Fibers Following 10 Weeks of Resistance Training in Previously Untrained Men

What the research says

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Description
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In plain English

In young men who don't exercise, slow-twitch muscle fibers have more energy-producing parts than fast-twitch fibers to start with, and even after 10 weeks of weight training—which boosts these parts overall—the slow-twitch ones still keep their lead.

See the scientific wording

Type I muscle fibers in untrained young men have higher mitochondrial density than type II fibers at baseline, and this difference persists after 10 weeks of resistance training despite overall mitochondrial expansion.

Score breakdown, mechanism chain, raw evidence, ideal studies needed

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