As necessary—Some runners train on treadmills only when inclement weather keeps them from running outdoors. The treadmill can also be a good fallback option when you have certain minor injuries, as it allows you to crank up the gradient and go slow for a lower-impact workout.
Seasonally—Some runners in northern climates train primarily or exclusively on treadmills during the winter, when conditions make it difficult to run outdoors, and run primarily or exclusively outdoors in the warmer months. That was 2000 Olympic Trials Marathon winner Christine Clark's routine in Alaska.
Habitually—Some runners do certain runs on the treadmill routinely. For example, Rod DeHaven used to do his easy morning runs on a treadmill in his basement and his afternoon runs—including all of his high-intensity and long runs—outdoors. Others like to use the treadmill to do particular key workouts that the treadmill accommodates well: for example, long, steady uphill runs, which are hard to do outdoors unless you have a mountain handy.
Exclusively—A few treadmill runners like Rick Morris run primarily or exclusively on treadmills by choice. If that's your thing, go for it, but it is probably not the best way to train for optimal race performance.
Most of the workouts you do outdoors can also be done on a treadmill. But the treadmill is actually better suited than the outdoors to a few specific types of workouts. Here are three such sessions.
The Endless Hill
One of the favorite workouts of many Kenyan and Ethiopian runners is to run from the bottom to the top of a mountain and back down. This very effective workout is difficult to do if you do not live near a mountain, but you can do the climbing part of it on a treadmill. As a substitute for a regular weekend long run, hop on a treadmill and run for the same duration on an upward gradient. You can choose a steady gradient of 6 to 8 percent or make it more interesting by changing the gradient periodically, even going up to 10 to 15 percent for short periods.
Warm up with 1 mile of easy jogging and then run anywhere from 4 to 12 miles (depending on where you are in the training process) at your ideal marathon pace. Doing this workout on a treadmill enables you to lock right on to that pace and stay there.
The workout format that exercise physiologists commonly use to determine VO2Max is also useful as a powerful (if painful) fitness-boosting workout. Start by hopping on the treadmill and running easy for five to 10 minutes. Next, increase the belt speed by 0.5 mph and run for one minute at that speed. Now increase the belt speed by another 0.5 mph, hold the new speed for another minute, and continue in this fashion until you feel unable to run any faster. Reduce the belt speed and cool down. Note the maximum speed you attained and try to beat it when you repeat the workout in three or four weeks.
Active Expert Matt Fitzgerald is the author of Iron War: Dave Scott, Mark Allen & The Greatest Race Ever Run (VeloPress 2011) and RUN: The Mind-Body Method of Running by Feel. He is also a coach and training intelligence specialist for PEAR Sports. Learn more at mattfizgerald.org.