There are three ways to run according to speed:

(1)100m and 200m are speed events. Exercise is the white muscle fiber in muscle, and lasting exercise can increase the volume of muscle. Which is what you call "getting stronger".

(2) Endurance events above1500m. Exercise is the red muscle fiber in the muscle, and lasting exercise can reduce the size of the muscle. Coupled with the consumption of subcutaneous fat and intramuscular fat, the legs will become thinner and thinner. Of course, there is a limit to this fineness.

(3) 400m and 800m are speed endurance events. The movement effect (thickness) is between the two and tends to sprint.

Tip: if you want to make your legs thinner, it is recommended to choose an item above 3000 meters. It is best not to be lower than 1500m.