Some people are born to be sprinters or weightlifters. They seem to have muscles made out of springs, they're so powerful. You might also notice that someone this explosive also gets tired quickly.
Your muscle fiber type has a lot to do with how powerful your muscles are, and how quickly they tire out. Someone who is naturally strong and can bench press a lot of weight has a high concentration of fast twitch muscle fibers in the chest and other muscles that contribute to the bench press.
Your muscle is made out of thousands of muscle fibers, little strands of muscle tissue than come together to form one big muscle. Your muscle fibers are grouped into motor units. A motor unit is a bunch of muscle fibers, usually around 300, that are controlled by one nerve.
This nerve sends an electric signal down into the muscle fibers and makes them contract. There are different types of motor units: slow twitch, fast twitch and fast intermediate.