Running uphill and running on a flat surface both activate different muscles to varying degrees.
When running uphill, the primary muscles used are the glutes, quadriceps, hamstrings, and calves. These muscles have to work harder to push the body up the incline, resulting in greater activation and development of these muscles. Running uphill also requires more power and explosiveness, as well as increased range of motion at the hips, knees, and ankles.
Running on a flat surface, on the other hand, primarily uses the quadriceps, hamstrings, and calves, but to a lesser extent than running uphill. This is because running on a flat surface requires less power and explosiveness, as well as less range of motion at the joints.
In terms of the benefits to your muscles, running uphill can be a great way to strengthen and develop the muscles of the lower body, including the glutes, quadriceps, hamstrings, and calves. It can also help to improve endurance, speed, and power, due to the increased demands on the muscles.
Running on a flat surface, on the other hand, can be a good way to maintain cardiovascular fitness, and can also be useful for developing speed and endurance. However, it may not be as effective for developing and strengthening the lower body muscles to the same degree as running uphill.
It's important to note that both types of running can be beneficial for overall health and fitness, and incorporating a variety of running surfaces and terrains can help to provide a well-rounded workout for the muscles.