Sprinters can gain muscle mass in their upper and lower bodies.
Sprinting is a high-intensity form of running that pushes your cardiovascular system into overdrive by recruiting more muscle fibers than steady-state cardio training. When you first begin a sprinting routine, your legs might slim down due to a loss of fat, but the muscles beneath will grow. Over time, this muscle growth will give your legs shape and may increase their overall size.
Muscle Growth Process
To understand how running sprints will increase the size of your leg muscles, you first need to understand how muscle is built in the first place. Whenever you apply repeated resisting forces to muscles and cause them to work, damage to satellite cells kicks off a regeneration process that regrows muscle tissue, making it denser and larger over time. This process is known as hypertrophy, and it's one of the reasons why you need a lot of rest following an exhaustive sprint workout.
Since sprinting activates more muscle tissue than other forms of running, your heart has to work harder to keep you going. Sprinting pushes your heart rate above 80 percent of its maximum, kicking off your anaerobic energy-producing system. Your anaerobic system produces energy quickly through the break down of glycogen stores, but you can only keep up a constant energy loop in this zone for a few minutes at a time.
Sprinting and Muscle Growth
While steady-state cardio training releases adrenal hormones that catabolize muscle tissue, sprinters experience none of these wasting effects. This information was confirmed by a 2006 study conducted by Aristotle University of Thessaloniki, Greece. Researches attempted to find a link between exercise intensity and body composition. Although exercise intensity didn't appear to have a significant effect on total body mass, higher-intensity exercises resulted in less muscle loss.
The same aspects of sprint training that allow you to build muscle in your lower body also make it a high-impact form of exercise. Since sprinting has such an intense effect on your muscles, you shouldn't perform sprints on consecutive days to allow for proper rest and muscle regeneration. Additionally, overtraining in any form can actually result in a loss of muscle tissue over time, so pay attention to fatigue and take rest when you need it.