The ACTN3 gene encodes the protein alpha-actinin-3, which is primarily expressed in fast-twitch (Type IIa & IIx) skeletal muscle fibers. These fibers are responsible for generating rapid, powerful contractions used in activities like sprinting, weightlifting, and jumping. However, they fatigue more quickly than slow-twitch (Type I) skeletal muscle fiber.
Often dubbed the “sprinter gene,” ACTN3 influences muscle performance, particularly in elite athletes.
There are three variants of the ACTN3 gene. One produces a high level of alpha-actinin-3, one produces a low level, and one produces none at all.
RR (homozygous R): Both copies of the gene produce alpha-actinin-3. Associated with enhanced sprint/power performance.
RX (heterozygous): One copy produces alpha-actinin-3, with intermediate effects.
XX (homozygous X): No alpha-actinin-3 is produced. Believed to be beneficial for endurance and resistance to cold.
The RR variant is most common in sub-Saharan Africa, while the XX variant is most common in cold climates.
One paper by researchers in Spain and Australia claims that sub-Saharan Africans primarily have the RR variant because they tended to live in a tropical “species-rich” environment. By species-rich, they mean that they did not need long hunts that require a higher level of endurance.
Researchers have also associated the differences in the utility of horse breeds with differences in the ACTN3 gene in horses. The gene is extremely nuanced in horse, with dozens of variations.
