Why does hill running hurt so much? In part, because it takes more work. “You have to recruit more muscle fibers to get yourself up the hill, which causes those muscles to fatigue faster,” says Carwyn Sharp, Ph.D., assistant professor of exercise science at the College of Charleston in South Carolina. Plus, when you’re running on an incline, there’s a shorter distance for your foot to fall before it hits the ground.
http://www.runnersworld.com/article/1,7124,s6-238-263-264-13520-0,00.html
