ACTN3: Why Some Sprint, Others Endure

Why can Usain Bolt explode down the track with unmatched speed, while Mo Farah can glide through 42 kilometres of a marathon without breaking stride? And why do footballers like box-to-box midfielders seem to have the best of both worlds, sprinting, tackling, and covering endless ground in a single match?

The answer lies partly in training, nutrition, environment, and mindset, but there’s also a fascinating genetic story at play. One of the most studied genes in sports science is ACTN3, often nicknamed the “speed gene.” It doesn’t decide your destiny, but it helps explain why some athletes are naturally inclined toward sprinting, others toward endurance, and some toward a balance of both.

What is ACTN3 and alpha-actinin-3?

The ACTN3 gene provides instructions to make a protein called alpha-actinin-3. This protein is found only in fast-twitch muscle fibers, the ones that contract quickly and generate explosive power. These fibers are crucial for activities such as sprinting, weightlifting, and powerful football movements like sudden sprints, headers, or tackles.

When ACTN3 is working normally, it helps fast-twitch fibers act like a “high-power engine.” But a common variation in the gene, called R577X, can change how much alpha-actinin-3 the body produces. This leads to three possible outcomes:

• RR (two working copies): Full production of alpha-actinin-3. Muscles are primed for power, speed, and explosive strength.
  
• RX (one working, one non-working copy): Some alpha-actinin-3 is produced. Athletes may balance both power and endurance traits.
  
• XX (two non-working copies): No alpha-actinin-3 is produced. Muscles lean more toward endurance characteristics, though strength is still present through compensation by another protein, alpha-actinin-2.
  

Interestingly, around 18–20% of the global population carries the XX genotype, meaning they completely lack alpha-actinin-3, and yet, they live completely normal, healthy lives. This is not a disease; it’s simply a natural variation that shapes muscle function.

What does research say about performance?

The story of ACTN3 became famous in 2003, when a landmark Australian study showed that elite sprinters had a much higher frequency of the R allele. In fact, none of the female Olympic sprinters in that study carried the XX genotype. A 2024 systematic review covering over 14,000 athletes across 13 countries confirmed that RR and RX genotypes were significantly more common in power athletes compared to both endurance athletes and non-athletes.

In endurance sports, the picture is more nuanced. Some early studies suggested that the XX genotype could be an advantage for long-distance runners because of its link with a shift toward slow-twitch, fatigue-resistant fibers. However, more recent large-scale studies have shown no consistent advantage for endurance performance, reminding us that success in marathon running depends on far more than one gene.

Team sports like football fall somewhere in between. A 2012 European study involving over 200 elite athletes showed that ACTN3 genotype distribution was similar between footballers and non-athletes, unlike sprinters, where the RR genotype dominated. This makes sense: a footballer needs both explosive sprints and long-lasting endurance, along with technical skill, tactical awareness, and mental strength. In other words, ACTN3 cannot fully explain the performance of a box-to-box midfielder; it is one piece in a vast puzzle.

Beyond performance: recovery and injuries

The influence of ACTN3 extends beyond sprint times or distances covered. Studies have shown that athletes with the XX genotype often suffer greater muscle damage after intense or eccentric exercise (like downhill running or long marathons). For example, a 2017 study in European Journal of Applied Physiology found that marathon runners with the XX genotype showed higher levels of muscle damage markers.

Similarly, injury risk seems tied to this gene. Research on football players in Italy demonstrated that XX players were 2.6 times more likely to suffer injuries compared to RR players, and the injuries tended to be more severe. Another study on elite ballerinas found that XX individuals were nearly five times more likely to suffer ankle injuries.

On the other hand, not all findings point in the same direction. A 2021 Brazilian study on downhill running reported that RR athletes actually showed greater strength loss and higher markers of muscle damage compared to X-carriers. This highlights that the relationship is complex: environment, training type, and even the demands of the sport can flip the script.

So, putting it all together, the ACTN3 R577X variation gives us a fascinating glimpse into the biology of performance.

• RR tends to favour sprinters, weightlifters, and athletes needing explosive power.
  
• RX provides a blend, which may be useful for athletes in sports demanding both power and endurance, like football.
  
• XX shifts muscles slightly toward endurance, but may also increase the risk of muscle damage and injury under certain conditions.
  

Still, it’s crucial to remember that ACTN3 is only one of over 200 genes linked to athletic traits. Training, nutrition, coaching, psychology, and opportunity shape athletes far more than one genetic variant. Usain Bolt is not just his ACTN3 genotype; Mo Farah is not only his slow-twitch fibers; and midfielders are not genetic accidents, they are the result of years of training layered on top of biology.