There is one thing that can strike fear into the heart of any cross country runner: hills. Hilly courses are often hated because going uphill is hard, however for a long time I have been telling myself that what goes up must come down, and running downhill is ridiculously easy. The best example of this that I can think of is the Bowdoin Park course in Poughkeepsie, NY. This is widely regarded as one of the hardest XC courses in the state. It's a 5 km course that winds its way up and then down a large hill. The very top of the hill is exactly the half way mark of the course so there's 2.5 km on the "uphill" section and 2.5 on the "downhill" section, and the start and finish line are at exactly the same elevation. According to google earth there is a 65 meter elevation change from the start/ finish to the 2.5 km mark (although that seems a little low to me). Most runners hate this course because of all the work they must do against gravity in the first half of the race, but they forget that in the second half gravity is doing work on them. For example, since work=force*displacement, a 70 kg runner must do 4550J of work against gravity going uphill, but gravity does 4550J of work on them going downhill so net work (as far as climbing hills goes) for the course is 0 joules. Now obviously not even close to all the energy used running is returned, and running downhill isn't free fall so energy must used by the runner on the way down. Yes, hills are challenging but but I think that people need to stop complaining and accept that during races gravity helps them almost as much as it hurts them.