On the other hand, there are a few disadvantages of training for a triathlon on a treadmill. For starters, the surfaces of treadmills are soft and reduce shock, and they're not always similar to outdoor surfaces traversed in a race. Some running experts say that the variability of the ground -- whether it's dry, wet, hard or soft -- actually enhances your muscles' ability to react quickly to different surfaces in a race [source: Morris]. So, it's a good thing to train outside when possible, but don't be afraid to head indoors during icy weather.
Treadmill training can also take a toll on the runner's form. While running in place on a treadmill, it's often difficult to maintain the slightly forward lean that's necessary to keep pace without wasting energy. Too much treadmill training may result in a more bouncy form that isn't as efficient. Another problem associated with treadmills is the absence of wind. When you're running outdoors, you have to work harder to battle the resistance created by moderate wind. This difference can usually be remedied by setting a 1 or 2 percent incline, which creates a similar resistance [source: Morris].
Treadmill training can also be potentially harmful if the runner doesn't know how to use the machine or pushes himself to maintain a speed that's too fast. Mentally speaking, staying engaged while training on a treadmill can be a challenge. Since the runner doesn't feel as if he's going anywhere, it's easy to get bored or anxious. Triathlon trainers suggest to break up workouts in smaller intervals, watch TV or listen to music to avoid boredom.
For lots more information on training for a triathlon, see the links on the next page.