Genetics and Muscle Performance

Understanding how your body responds to different types of exercise helps you train more effectively. Genetics influences muscle composition, endurance capacity and recovery — shaping whether you naturally lean toward power-based activities like sprinting and weightlifting, or endurance sports like distance running.

Fast-twitch and slow-twitch muscle fibres

Skeletal muscle contains two broad fibre types, and the proportion you have influences what you find easier.

Fast-twitch (Type II) — generate rapid, forceful contractions but fatigue quickly. They rely largely on anaerobic metabolism, producing energy without oxygen.
Slow-twitch (Type I) — contract more slowly, resist fatigue, and use oxygen to sustain prolonged effort. These dominate in endurance activity.

Everyone has both. The ratio varies between individuals and between muscles, and while training shifts fibre characteristics to a degree, the underlying distribution has a substantial genetic component.

The ACTN3 gene

ACTN3 encodes alpha-actinin-3, a structural protein found specifically in fast-twitch fibres. A common variant, R577X, introduces a stop signal that prevents the protein being produced.

People with at least one functional R copy produce the protein and are somewhat over-represented among elite sprint and power athletes
People with two X copies — roughly 18% of the population — produce none, and are more common among elite endurance athletes

Worth stressing: the effect on any individual is modest. Plenty of powerful athletes carry two X copies, and the variant tells you nothing about what you can achieve with training.

Training for power

Sprinting and hill sprints
Olympic lifts — cleans, snatches, jerks (with proper coaching)
Plyometrics — box jumps, bounds, medicine ball throws
Heavy strength work at low repetitions with full recovery between sets
High-intensity intervals

The endurance side

Muscular endurance is the ability to sustain repeated contractions over time — central to distance running, cycling and swimming. It depends on slow-twitch fibre proportion, mitochondrial density and VO2 max, the body's capacity to deliver and use oxygen.

Several genes have been studied here, including PPARA and PPARGC1A, both involved in fat metabolism and mitochondrial development, and the ACE insertion variant. Findings across studies have been mixed, and no single variant reliably predicts endurance capability.

Training for endurance

Long steady-state runs, rides or swims
Tempo work at a sustained, controlled effort
Rowing and cross-training
Higher-repetition, lower-load resistance circuits
Gradual weekly volume increases — no more than around 10% at a time

Genetics is a starting point, not a ceiling

This is the most important part. Genetic variants influence your starting tendencies and how quickly you adapt to particular stimuli. They do not set limits on what you can achieve.

The difference between a trained and untrained version of the same person vastly exceeds any difference created by these variants. Consistency, progressive overload, sleep, nutrition and recovery remain the dominant factors — by a wide margin. If you enjoy endurance running and carry "power" variants, keep running. Enjoyment predicts adherence, and adherence predicts results.

Testing with Rightangled

Rightangled's DNA tests examine markers relating to muscle fibre type, endurance and power response, recovery and injury risk. Our blood tests can check iron, vitamin D and thyroid function — all of which directly affect training performance, and all worth ruling out if progress has stalled. Results are reviewed by our clinical team, which includes GPhC-registered independent prescribers, with medical oversight from our doctor, Dr Abdullah.

Rightangled is a UK-registered online pharmacy — verify us with the General Pharmaceutical Council (registration number 9011933), see our LegitScript certification, and read independent patient reviews on Trustpilot.

Unlocking Your Athletic Potential: The Role of Genetics in Muscle Performance