Running is usually done as a method of increasing cardiovascular fitness and burning calories to slim down. If you're just started to up the intensity, or are beginning running for the first time, you'll experience muscle soreness after running, in much the same way you would after lifting weights, and might be wondering if this has the same effect of increasing leg muscle size.
The Experience Factor
Going from couch potato to regular jogger in the blink of an eye could result in some muscle growth and bigger legs, due to the fact your muscles go from doing virtually nothing, to suddenly training on a regular basis. If you're already fairly active, however, or used to lifting weights, running won't make your legs bigger. In fact, you'll need to be careful you don't lose muscle and make your legs smaller. Running is calorie-intensive, and to preserve muscle you need to increase your calorie intake to match expenditure, notes New York-based sports dietitian Lauren Antonucci.
The Key Players
Several different muscles are involved when you run, but the main ones are your quads, hamstrings, calves, glutes and hip flexors. If any muscles are going to increase in size, it's these. However, as running is more of a cardiovascular exercise than a strengthening one, the intensity you're working at generally isn't high enough to build much muscle mass.
Muscles grow when they're exposed to a stimulus. This stimulus causes the muscle tissue to break down, to be built up again bigger and stronger, thus increasing muscle size. Your leg muscles are undoubtedly working hard when you run, but not in quite the same way as they would be with a bodybuilding or strength routine. Generally, low reps build strength, sets of six to 15 reps build muscle mass and anything over 15 reps per set starts to venture into endurance territory, notes strength coach Will Scudder on the Muscle and Strength website. As distance running is a low-intensity activity performed for a sustained duration, the reps are much, much higher and therefore you're unlikely to build muscle.
It's All in the Type
Long, steady runs won't build much in the way of leg muscle, but you could increase leg size with different types of running. Hill sprints may actually build muscle, according to trainer John Alvino. A short, maximum-intensity sprint up a steep hill is much more taxing that a steady jog and places a greater demand on your muscles. Likewise, doing weighted sprints wearing a weight vest or running while pulling a sled places more strain on your quads, hamstrings and calves and could help muscle gain.