OK, I’m going to try and keep this as simple as possible. Within all muscle groups, there are a subset of fast and slow twitch muscle fibers. What type of exercising you’re performing will determine which muscle fibers you’re developing.
Slow twitch muscle fibers are aerobic which means “with oxygen,” so they are used for long distance exercise. These particular muscles are elongated and small. For example, those of you who enjoy jogging or spend hours of cardio, your body type will be more lanky without much muscle definition, but your cardiovascular system will be in good shape.