Moving against gravity by running up hills places stress on your lower body muscles, which can help them to grow and become stronger. Acting as resistance, the incline requires a greater effort than running on flat surfaces; the physical challenge can help your leg muscles to develop and, in turn, allow you to run faster and for longer distances.
Running uphill will activate and increase the size of all of the muscles in your legs, which include the front of the thigh quadriceps, back of the thigh hamstrings, inner and outer thighs, the calves and glutes. The back of the body muscles, or those in the posterior chain -- the glutes, hamstrings and calves -- receive the most benefits from inclines.
Focus on Form
Maximizing muscle growth and development and avoiding injuries when running uphill requires using proper form. Hit the ground with the front of your foot rather than the heel. Shorten your stride and land your foot under your body rather than in front or behind. Engage your core and elongate your spine; lean slightly forward and push your shoulder blades down and away from your ears.