Training can increase muscle mass and alter muscle fiber composition depending on the specifics of an athlete's training. (blog.bridgeathletic.com)
More than 150 genes have been linked to different aspects of physical performance, including oxygen uptake efficiency, aspects of heart performance, and muscle fiber composition. (geneticliteracyproject.org)
According to scientific studies, muscle fiber composition of the chest is 60 % fast - twitch and 40 % slow - twitch fibers. (fitnessandpower.com)