Muscle fiber type (power vs endurance)

ACTN3 builds a protein in fast-twitch muscle fibers used for explosive power. A common variant switches it off, and people without it are less represented among elite sprinters. The popular idea that the same variant boosts endurance is not well supported.

Possible results

Power and sprint type. You have two working copies of ACTN3, which builds a protein in the fast-twitch muscle fibers used for short, explosive efforts. This power version is more common among sprinters and strength athletes, though everyday fitness depends far more on training than on this gene.

Mixed power and endurance. You have one working and one switched-off copy of ACTN3, so you sit between power and endurance. In practice this gene nudges your muscle make-up only slightly, and training matters much more.

Less explosive-power tendency. Both your copies of ACTN3 are switched off, so your fast-twitch fibers don't make this particular protein. This version is under-represented among elite sprinters and strength athletes, so it leans slightly away from explosive power. The common claim that it gives an endurance advantage is not well supported, and training matters far more either way.

Evidence & sources

Across global populations, about 20% of people carry two copies of this variant, and about 32% carry none.

This variant appears in 1 published genetic study: PubMed 36402844.

Related

Endurance / aerobic capacity (PPARGC1A) · Eye color (blue, green/hazel, brown) · Freckling / sun sensitivity · Hair thickness / shovel-shaped incisors · Height · Red hair / fair skin (MC1R gene) · Skin pigmentation (light vs dark)