The demands of the terrain keep your muscles hungry for oxygen, and the uneven surfaces will work your body harder than a track or a street can. You'll be developing balance, agility, and stability around the ankles.
The quads, glutes, hamstrings, and calves do most of the work, but because you're constantly compensating for terrain, your core and upper body will be in dynamic motion.
Amp It Up
"The ego is a big motivator," says Chase, who co-authored The Ultimate Guide to Trail Running. That's why he likes to turn his workouts into a race with a friendly – but competitive – game of follow the leader. First, recruit some trail-running partners, then pick a leader. His job is to shake off the pack. Everyone else's job is to stay on his heels. Change leaders every three to five minutes; for more of a challenge, chase the best hill runner on the inclines and the most agile runner on the twistiest trails.