ACTN3 – The Muscle Gene

Here’s what you need to know about this tricky gene and its variants.

The ACTN3 gene is in large part responsible for the type of muscle tissue that you have. Not only the muscle fiber dominance, but also the various qualities of the tissue itself – how well it handles muscle damage, how much mTOR is activated, its capacity to use oxygen for fuel, etc.

You have two main types of ACTN3. The X variant is better suited for endurance activities while the R variant is better suited for speed and power. Some people are all of one type (RR or XX) and some people have one of each (Rx, RX or rX).

Let’s look at the characteristics of both variants:

ACTN3 RR – “Genetic Freaks”

  • Greater mTOR activation from resistance training, which means more protein synthesis, more muscle growth
  • Higher fast-twitch fiber ratio
  • Lesser muscle damage from hard training, which means it’s easier to recover from strength training
  • Reduced risk of injury
  • Possibly less extensibility, which can come with a greater, more powerful stretch reflex

Someone with an ACTN3 RR type would be the hyper responders to training. They have a monster mTOR activation from training. They would also have more fast twitch fibers which not only make them stronger, faster, and more explosive, but also give them more potential for growth. They can also grow easily from heavy weights because they repair damage easily.

ACTN3 XX – “Hardgainers”

  • Lower mTOR activation from resistance training, which means lower protein synthesis, less muscle growth
  • Lower fast-twitch fiber ratio
  • Higher capacity to use oxygen to produce energy
  • More muscle damage from hard training, which means recovery takes longer and is harder
  • Increased risk of injuries
  • Possibly more extensibility, but less powerful stretch reflex

Someone who is ACTN3 XX would be your typical hardgainer. Very low response to training for muscle size due to both a low mTOR activation and slow muscle damage repair. They would also have fewer fast twitch fibers, which means their strength, power, and muscle growth potential is lower.

Remember, mTOR is what regulates protein synthesis. The more you turn it on, the higher protein synthesis is and the more muscle you can build.

Note: If you fall into this hardgainer category, check out The Hardgainer Prescription for training advice.

Mixed Type – Most People

If you’re a mixed type (Rx, RX or rX), which is around 50-70% of the population, you’d have a response that’s somewhere in between these two extremes.




  1. Pickering C et al. ACTN3: More than Just a Gene for Speed. Front Physiol. 2017;8:1080. PubMed.