Babies' muscles are naturally developed under gene induction. Adults and teenagers want to develop muscles from two aspects: the first is to increase the number of muscle fibers, which is suitable for developing teenagers. Proper exercise during development can stimulate muscle development and increase the number of muscle fibers. The second method is to increase the thickness of muscle fibers. Suitable for adults, through increasing nutrition and repeated chronic stimulation of exercise and fitness, muscle fibers will gradually develop and thicken, but the number of muscle fibers will remain unchanged. Both methods can promote muscle formation. But people have different body shapes and muscle shapes. Skinny people also have slender muscles, so it is difficult to achieve the same muscle group as stocky people through exercise.