PHYS-17 · Principle

I'm fast-twitch. Am I not built for endurance?

The mix is partly inherited and varies widely, but training turns fast fibres toward the fatigue-resistant IIa type; slow-fast conversion is debated. Train the event, not the label.

How the body works, agreed across the reviews below; a study entry shows what one experiment measured.

established physiologyApplies to everyoneEstablished — training shifts fast fibres from IIx toward IIa (IIA-IIX interconversion "well recognised", REF-308; also in single-fibre work, Plotkin et al. 2021), nerve activity being a major control of the fibre…

Practical Application

I'm fast-twitch. Am I not built for endurance? The mix is partly inherited and varies widely, but training turns fast fibres toward the fatigue-resistant IIa type; slow-fast conversion is debated. Train the event, not the label.

What it means for training: train for the event and judge by performance, without assigning a type (SYNTH-13); choose loads for strength or size, not to grow slow fibres (SYNTH-89).

What it does not mean: that a biopsy or gene test sets someone's event (an ACTN3 variant goes with power or endurance status only in group comparisons, REF-433), or that type I share predicts efficiency (r = 0.75 in 19 cyclists, REF-484; none in 40 men, REF-485).

Mechanism

Fibre identity is set in embryonic development and later modulated by nerve activity and hormones; the proportion varies significantly between people (Schiaffino & Reggiani 2011).

In one group's data, about 45% of the variance in type I share was inherited, 40% environmental and 15% sampling error; about a quarter of people had under 35% or over 65% type I fibres (Simoneau & Bouchard 1995, North American Caucasian adults).

Three months of heavy lifting cut myosin IIX from 9.3% to 2.0% in sedentary men, and three months off overshot it to 17.2% (REF-309); 16 weeks of strength training moved elite cyclists' fibres from IIX to IIA with fibre area unchanged (REF-075, 14 riders).

Fibres shift between hybrid and pure types and between slow and fast; how far, beyond 16 weeks, is unknown (Plotkin et al. 2021).

Common misreading

"Lifting turns my slow fibres fast"; "my fibre type decides my best event".

Backs

SYNTH-17 "% Type I fibres correlated with GE (r = 0.75) in 19 highly trained cyclists" (one side of a split) · SYNTH-89 "the one elite-cyclist strength trial with fibre data saw a IIX → IIA shift, not type I growth" · SYNTH-89 "Do not say it is proven to grow type I fibres specifically."