Muscle Composition

Your result, the one DNA position behind it, and what the research says.

6/10 confidence.

About Muscle Composition

ACTN3 builds alpha-actinin-3, a structural protein found almost only in fast-twitch muscle fibers - the ones behind short, explosive bursts of force. A single common variant introduces a stop signal that switches the protein off, and roughly one in five people worldwide make none at all. That's why this is one of the most-studied 'sports genes,' repeatedly linked to the split between power and endurance athletes.

Good to know: Your result is a tendency, not a limit - training drives far more of your performance. Power-leaning bodies often take to sprints and lifting; endurance-leaning ones to distance and stamina. About 1 in 5 people worldwide make no alpha-actinin-3 at all - completely normal.

Fun fact: Almost every elite Olympic sprinter ever tested carries at least one working copy of ACTN3.

Discovery: The power-versus-endurance link was first reported in 2003 in elite Australian athletes.

The Variant

The single variant behind this trait

Gene: ACTN3

Variant (rsID): rs1815739

Location: Chr 11:66,560,624

dbSNP rs1815739

The studies

A common nonsense mutation results in alpha-actinin-3 deficiency in the general population (North KN et al., Nature genetics, 1999)

ACTN3 genotype is associated with human elite athletic performance (Yang N et al., American journal of human genetics, 2003)

Mitochondrial DNA and ACTN3 genotypes in Finnish elite endurance and sprint athletes (Niemi AK et al., European journal of human genetics : EJHG, 2005)

ACTN3 R577X genotype and athletic performance in a large cohort of Japanese athletes (Kikuchi N et al., European journal of sport science, 2016)

The relationships between ACTN3 rs1815739 and PPARA-α rs4253778 gene polymorphisms and athletic performance characteristics in professional soccer players (Bulgay C et al., BMC sports science, medicine & rehabilitation, 2023)

Other lifestyle traits

Muscle Enzyme Baseline: Two positions that each raise how much creatine kinase sits in your blood at rest, which is one reason healthy people differ from each other on this test.

Endurance Wiring: A change in the master switch for building new mitochondria, the process that underlies aerobic endurance and the response to endurance training.

Endurance or Power Leaning: A marker for the classic ACE insertion/deletion variant, the most-studied genetic difference in sports performance.

Back to All Traits

Content updated September 2026.

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