You might be avoid them, but hills offer significant benefits to your running workout. Hill drills can help you burn more calories, improve your efficiency and speed, ward off certain injuries and enhance stamina. Running uphill is a form of resistance training that may increase the size of your leg muscles, depending on the frequency and intensity of your training and your genetics.
To build muscle strength, you have to work against resistance. Hills offer resistance to a runner who must propel herself against gravity. Your legs become stronger, which transfers to faster flat running.
Stronger legs don't always mean larger muscles. To build muscle, you need to work against heavy resistance consistently. Getting huge legs usually requires a combination of strength moves, such as stepups, squats, deadlifts and lunges, a high-calorie diet and the right genetics. You might augment your leg strength workouts with short sprints up very steep hills. A beginner might see a change in the size of her leg muscles from longer runs uphill because the muscles are being challenged beyond the point to which they are accustomed and are responding to the stimulus. If you run or strength-train regularly, moderate hill runs once or twice a week will not greatly increase the size of your legs. If you significantly increase the volume and frequency of your hill-climbing runs -- for instance, if you are training for a mountain trail run -- your leg muscle size might increase.
Genetics and Muscle Balance
Running uphill requires hamstring and calf strength. If large calves and legs run in your family, running uphill could increase their girth. Include downhill running, which recruits your quadriceps, along with uphill running to encourage muscle balance.
Hill training is beneficial when done in moderation. Run hills too often and you risk burning out and overdeveloping your hamstrings at the expense of other lower body muscles. Stick to just one or two focused hill-training sessions per week for the most benefit. Max hill sprints as part of an overall strength-training regimen designed to build length strength should also be done only once or twice a week. Your body needs time to recover and grow between these workouts.