"If you want to run a road race train on a road." This is sage advice but like most things in life there's room for a little grey. Sports coaches and trainers of all sorts talk about training and cross training. There are training days where the athlete is completely focused on performing the exact same movements of the sport he is training for. There are also rest days. The purpose of a rest day is to give the muscles sufficient time to recover and regenerate. For committed athletes full rest may be undesirable so they will perform a complimentary activity. This is called cross training.
For a runner, cross training maybe yoga, swimming, cycling or strength training. I've recently started adding trail running as a form of cross training, and want to share with you why I think it falls into this category of training. First I want to describe the environment of trail running. I'm specifically referring to trails that are known as single track and are most often associated with mountain biking. These trails typically are littered with roots, rocks, extremely short up hills and down hills, log piles, wooden bridges and assorted flora that will get in your way.
Here is why I consider trail running a form of cross training. Running a single track trail in the woods will slow you down. This is a good thing. You may run an 'easy' run on the road at 8:30 but running a trail will slow you to 11:00 and it will feel like 8:30. So you get all the cardio benefits of running 'fast' but the actual movements are considerably slower with short range of motion. This will allow your running muscle groups some rest.
Trail running requires a much higher level of neuromuscular engagement that challenges the entire kinetic chain and all the muscles associated with it. The runner will use his muscles in ways he could not by road running alone. Obstacle avoidance, foot placement, push off, and highly controlled landings require muscle recruitment not encountered on the road. The short and often steep ascents and descents challenge the glueal and lower leg muscles respectively. The twists and turns keep your mind in a constant state of decision making and thus further challenge the neuromuscular systems.
In short, trail running is a completely different animal when compared to road running and in my opinion can be called cross training. So get out to the woods and kick up some dust.