Athleticism is a complex trait that is influenced by both genetic and environmental factors. The strength of muscles used for movement and the type of fibers that make up muscles play a significant role in our athletic ability. Skeletal muscles are composed of slow-twitch (ST) fibers and fast-twitch (FT) fibers.
ST muscle fibers contract slowly and do not fatigue easily. FT muscle fibers contract quickly but tire rapidly. Many studies compare muscle fiber composition of endurance athletes (i.e., marathon runners) versus power athletes (i.e., sprinters or power lifters). Most endurance athletes require a higher proportion of ST muscle fibers compared to FT muscle fibers; the opposite is true in power athletes.
This begs the question: genes determine whether we become a marathon runner, sprinter, or couch potato? The average person is born with an equal amount ST vs. FT fibers, but this is not universally true. Some people are born with a significantly higher proportion of one fiber type, which may indicate a predisposition for either endurance or power-based activities.
Regardless, studies have shown that with proper training, we can remodel our predominant fiber type. Not everyone is able to train until they are a world-class marathon runner– if that were the case, we would all be one. However, the role of environmental factors (training time, availability of resources, support system, etc.) should not be undermined.
Giuseppe Lippi, Umile Giuseppe Longo, Nicola Maffulli, Genetics and sports, British Medical Bulletin, Volume 93, Issue 1, March 2010
Lievens, E., Klass, M., Bex, T., & Derave, W. (2020). Muscle fiber typology substantially influences time to recover from high-intensity exercise. Journal of Applied Physiology.