There are different types of muscle fibers, some are best for strength (lifting/muscular) and some are best for endurance (running/marathons). You are born with some ratio of these muscle fibers and while you can make some bigger and improve their function you don’t really change this proportion.
This is why some people are fairly muscular without much effort while others have good endurance without much training. But no one is born an elite athlete and everyone can improve with training. There are also other traits that can be affected by genetics. The genetic differences only matter at the top ends of professional competition and even then technique and skill can often overcome them.
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