Last reviewed: May 12, 2026 Last updated: May 12, 2026

Written by: Jay Hastings , CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance. His work has included scaling healthcare startups, leading CLIA lab integrations, and helping expand consumer access to precision health tools.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

This article is part of the PlexusDx Education Hub — your resource for evidence-based guidance on genetic health. Browse all genetic health education

ACTN3 is nicknamed "the sprint gene" for good reason: it is one of the clearest examples in human genetics of a single variant that tracks with a measurable physical trait. Whether you lean toward explosive power or steady endurance is shaped by many factors, but ACTN3 is a well-studied piece of that puzzle.

The gene codes for a protein called alpha-actinin-3, found specifically in fast-twitch muscle fibers — the ones responsible for rapid, forceful contractions. A common variant switches that protein off entirely, and researchers have spent two decades mapping what that means for performance.

What ACTN3 does in muscle

Human skeletal muscle contains a mix of fiber types. Slow-twitch (Type I) fibers are fatigue-resistant and suited to endurance; fast-twitch (Type II) fibers contract quickly and powerfully but tire faster. Alpha-actinin-3, the protein ACTN3 makes, is part of the structural scaffold inside fast-twitch fibers, helping them generate and transmit force efficiently. It is essentially specialized hardware for power output.

The R577X variant and the XX genotype

The key ACTN3 variant is rs1815739, known as R577X. It introduces a "stop" signal that prevents the muscle from producing functional alpha-actinin-3. People inherit two copies, giving three genotypes: RR (full production), RX (partial), and XX (none). Roughly one in five people worldwide carry the XX genotype and produce no alpha-actinin-3 at all — yet they are perfectly healthy, because a related protein, alpha-actinin-2, compensates. What changes is the fine-tuning of fast-twitch fiber performance.

What the research shows about power and endurance

The landmark finding, from Australian researchers studying elite athletes, was that the RR genotype is overrepresented among sprint and power athletes, while the XX genotype appears more often in some elite endurance athletes. Follow-up studies across many populations broadly support a power-versus-endurance tilt: RR and RX carriers tend to have a slight edge in explosive tasks, and XX carriers may show a modest lean toward endurance and, in some studies, faster recovery. The effect is real but small — one contributor among hundreds.

Why genetics is not destiny in sport

No one has ever been elite because of a single gene. Athletic performance emerges from training, coaching, nutrition, recovery, biomechanics, psychology, and dozens of other genes affecting oxygen delivery, muscle growth, and metabolism (ACE, for example, is another commonly studied performance gene). Plenty of XX-genotype athletes excel at power sports, and plenty of RR carriers thrive in endurance. ACTN3 nudges a tendency; it does not assign you a sport or set a ceiling.

How to use ACTN3 insight

Knowing your ACTN3 genotype can be an interesting input for tailoring training emphasis or understanding your natural leanings, but it should never override how your body actually responds to training. The Precision Peptide Genetic Test analyzes 14 pathways, 49 peptides, 150+ genetic insights, including variants like ACTN3 that shape muscle and performance pathways. Genetics is a guide, not a guarantee — the most reliable feedback still comes from consistent training data and, where relevant, guidance from a coach or qualified provider.

Frequently Asked Questions About ACTN3

What does the ACTN3 gene do?

ACTN3 codes for alpha-actinin-3, a structural protein found only in fast-twitch muscle fibers, where it helps generate and transmit the force behind rapid, powerful contractions. Because fast-twitch fibers drive sprinting and lifting, ACTN3 is studied as a genetic contributor to explosive power versus endurance leanings.

Is the ACTN3 XX genotype bad for athletes?

No. The XX genotype means you produce no alpha-actinin-3, but a related protein compensates and you remain perfectly healthy. About one in five people carry it. It is associated with a modest lean toward endurance and, in some studies, recovery — not a disadvantage, just a different tendency. Many power athletes carry XX.

Can genetic testing tell me which sport to play?

No. ACTN3 is one small contributor among hundreds of genes and many non-genetic factors like training, nutrition, and coaching. Genetic testing can reveal your genotype and general leanings, but it cannot assign a sport or predict elite performance. Real training data is a far better guide to what suits you.

Can I change my ACTN3 genotype through training?

No. Your genotype is fixed for life, but it does not limit what training can achieve. You cannot add alpha-actinin-3 if you carry XX, yet you can build power and endurance through consistent, well-designed training. Genetics sets a starting tendency; effort and programming shape the outcome.

Curious how your muscle and performance pathways are set up? Take the Precision Peptide Genetic Test to see how your DNA shapes the pathways discussed here — and bring that context to your provider.

This article is part of the PlexusDx Education Hub. Browse all genetic health education

Compliance note: The Precision Peptide Genetic Test analyzes how your genes influence peptide-related biological pathways. It does not diagnose, treat, cure, or prevent any disease, and it does not recommend, prescribe, or determine which peptides you should use. Consult a qualified healthcare provider before beginning any peptide protocol or making changes to your care.

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance. References are included at the end of the article when scientific, medical, or health-related claims are discussed.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.