Ultramarathons tend to be on trails for a simple logistical reason: few intersections that have to be controlled; the softer surface and rolling terrain are also easier on joints, so runners seem to prefer them to 50+ miles on asphalt. Any place there's 30-100 miles of trail will probably be pretty rugged and there seems to be a pride factor among ultrarunners of doing the toughest courses. Each course has its own unique challenges, but long hills and "technical" - mostly tree roots and rocks - are ubiquitous.
When I first tried ultras, some people struck me as being born to run tough courses; they did less than half the training of others and they flew over terrain where I had to choose every step carefully. I now see things differently. While I still say that nothing prepares one for trail racing like running on trails, there are people getting huge benefits from cross-training.
Often, running hills in the local parks, I'll see people charging hills at breakneck speed and invariably find that they're training for cross-country skiing (okay, once it was a trail runner). There are a lot of skiers in trail races; they've done all the work of running over the winter with little of the pounding and they have to develop good balance and agility, plus the leg strength for downhills (a day of alpine skiing is tough on the quads). Surprisingly, snowshoeing, which seems closer to running, doesn't seem to have the same effect; perhaps it's still seen more as an activity than a sport.
There are a lot of cyclists, too, among the ultra trailrunners. Pierre Ostor famously sometimes rides his bike to a race, runs the race and then bikes back home again, as if several hours on the trail are not enough. Cyclists tend to have great leg strength.[Theres a small group of Olympic-caliber cyclists who also are Olympic-caliber speed skaters; they have phenomenally muscular legs, I've noticed.] Cycling is a great aerobic sport and it also seems to be easier on leg joints than running - unless you do it like I do - so a lot of ultraruners who are doing low mileage are supplementing with a lot of biking.
Diane Farmer (Hi, Diane) became famous for finishing the tough Superior 100 Mile, having done no run longer than 20 miles and doing all of her running on a treadmill. This made no sense to me at all until I learned that she wasn't mentioning her hiking trips, where she'd spend several hours each day hiking in mountains. The difference between fast hiking and running disappears during hundred milers. In fact, I think an hour spent hiking is better training for trail running than an hour spent running on a road. Hiking teaches one to handle the long hills that are ever-present in ultras.
Some runners have claimed to run great 100 milers on minimal mileage and not done any cross-training. The few cases I've looked into turned out to be men who did all their training on steep hills or on treadmills set at 15 degree (or greater) incline. Having just taken more than 5 hours to run 17 miles up and down a hill, my mileage and speed won't impress many, but it was very good training for a hilly race.
Technical requires balance and agility, which become more difficult after, say 20 hours of running. Some people are better co-ordinated than others, but training on trails improves one's skill level at this. Something else which seems to help came as a bit of a surprise to me, though: yoga. Running long all the time decreases one's range of motion (such as the shuffling I mentioned last post), but yoga helps to maintain it, which can be important when one has to make eccentric motions to maintain balance. More importantly, yoga seems to enhance one's kinesthetic sense, one's knowledge of exactly where one's parts are, which is important in overcoming rocks and roots without stumbling. Yoga can work as an early alarm system; one will feel small aches and twinges before one would notice them in running.
Two caveats: Doing yoga for exercise is like going to a Catholic mass for the kneeling; it's about more than that. And I still don't ski, bike, hike and rarely do yoga; I like to run.
6 days ago