What is trail running?

Quote from Salomon websites:

Trail running is simply defined as going for a run in the heart of nature. It’s an extremely all-round non-stadia sport that, of course, works the legs, but also the upper body. Lastly, it requires concentration and attention.

Usually the track of trail running includes ascent and descents, rough terrain, hills, and many more so it make the sport more interesting yet challenging. Its not like a road running that always run and run, trail running usually combine running and hiking.

When the ascent part, runners often running because its make your leg get tired too fast. Except the elite runner that has reserve heart LOL.

When the descent part, this part the runners often full send to shorten or catch up the time that taken during the ascent part.