Why Do Legs Get Bigger When Running?

Muscles respond to exercise by getting stronger. To get stronger, muscles need to get bigger. Running works predominantly muscles in your leg, which will get bigger in response to the stimulus provided by exercise. The overall effect of running doesn’t necessarily have to make your legs bigger, but the way you train and eat will have an effect.

  1. Muscle Fibers

    • The legs get bigger when running because the muscles are getting stronger. To get stronger, muscle cells get bigger. Inside skeletal muscle cells there are muscle fibers and any exercise, including running, can lead to increasing the number of muscle fibers in a cell. This can account for 20 to 30 percent of muscle growth.

    Mitochondria

    • Mitochondria are the engine of muscles. Your muscle cells use mitochondria to convert sugar and fats into ATP, the energy source of muscles. Exercising can increase the number of mitochondria in your muscle cells. In fact, low-intensity exercise such as jogging is the best way to increase the number of mitochondria in your muscle cells. Increasing the number of mitochondria in your muscles can account for 20 to 30 percent of increase in leg size.

    Cell Volume

    • Your legs also get bigger when running because the volume of muscle cells can increase. High-intensity exercise such as sprinting is the best type of exercise to increase cell volume. The sarcoplasm, the fluid that fills your cells, will increase to match the increased demands that exercise places on the body. Increased cell volume can account for 20 to 30 percent of muscle growth.

    Muscle Groups

    • Running targets predominantly muscles in your lower body and it's the muscles in your leg that will respond to the stimulus provided by running and grow. The gluteus maximus, hamstrings, quadriceps and calves are the largest muscle groups that are used while running. It is these muscle groups, in the legs, that will get bigger when you run.

    Distance Versus Short

    • Sprinting and jogging provide the body with two very different stimuli and will develop muscles differently. Sprinting, being a high-intensity effort, relies more on the burning of sugar as a fuel, whereas, low-intensity exercise will burn more fat as an energy source. Going for long runs at the a moderate intensity will help you burn fat, from all over the body, including the legs and could result in a leaning effect.

    Nutrition

    • What you eat plays an important role in the growth of legs from running. Eating lots of protein will help you build muscle fibers, but you also need sugars for the energy required to build muscle and fats to expand the cells. Controlling calories and nutrient intake properly, eating less than you burn, allows you to build more muscular and leaner legs with running by burning fat stored in the thighs and getting stronger muscles.