Unless you're looking at Olympic sprinters, you're unlikely to see many women with ripped thighs, toned calves and tight butts from running alone. Running is an aerobic exercise that doesn't build muscle to the same degree that strength training does. Your legs should change noticeably after a few weeks on the track, becoming fuller, leaner and stronger. If your goal is overall weight loss and you notice bigger legs as a result, don't be alarmed; this means you're developing your lower body while still participating in a fat burning form of cardio.
How Muscles Grow
To understand why your legs are getting bigger from running, it helps to understand how your muscle mass will change from running. Cardio, like marathon running, works your cardiovascular system as well as your calf muscles, glutes, and hamstrings. Other forms of cardio, like high intensity interval training (HIIT), utilize your body weight and will do better to work fast-twitch muscles. These forms of cardio will burn carbs and are great to add to your workout routines, although your quads may get bigger from running because you are primarily working your lower body. However, if you’re looking to gain muscle, weight lifting is the way to go. Weight lifting will build muscle as well as burn fat. If you’re looking to have lean legs and lose weight, having a low weight high rep routine will help you to work slow-twitch muscles.
Impact on the Legs
The impact running has on your legs depends on the type of running you do. Long distance running (similar to light weight lifting) will work slow-twitch muscles and have an impact on the size of your legs. One of the most obvious benefits to running is the amount of calories you're sure to burn doing it. The Mayo Clinic states that you can burn between 600 and 1,300 calories per hour, depending on your weight and the pace you keep up during your run. You need muscular strength in the lower body to propel you forward throughout your run, which translates into muscle growth in your thigh muscles.
One of the reasons you don't see many distance runners carrying excess muscle is because they're taxing their cardiovascular systems more than their actual leg muscles. If you want to increase size and tone your legs more, try running at an incline, running at a faster pace or doing full-out sprints, which can help you build an impressive amount of muscle throughout your entire body. Another way to increase the size of your leg muscles is by doing exercises such as squats, lunges, deadlifts, and resistance training. Getting started with weight lifting to gain lean muscle or get slimmer legs can be difficult and many people will hire a certified personal trainer to get the proper form down.
Running alone will not change how big or lean your legs get. Every day you have to pay attention to your calorie deficit, the running sessions you engage in, and the types of muscle fibers you’re targeting in your workouts. If your goal is muscle building and having muscular legs, becoming a long-distance runner may not be for you. Losing body fat takes time and many factors must be considered but running is a great place to start and will change the size of your legs.