Depending on how your winter training schedule is coming along, you may be getting close to some of your early season races and you might want to think about incorporating some speed work (or change up your current speed focus days). Treadmills are very handy for speed work, but there has to be a balance between indoor sessions and outdoor sessions if you truly want to see gains on the open road.
The limitation behind a treadmill is that you are really just trying to keep up instead of propelling your body mass forward. You do work very similar muscles, but it is harder to gain real-world speed. A treadmill is excellent for helping you work on quick leg turnover, and it can be a great tool when you are lacking motivation to push the pace outside, but if you really want to be able to maintain that leg speed during a race you need some outside time on a 400m track, long dirt trails, or smooth open road with no distractions. That said, getting inside for some speed workouts can limit risk of injury. In general, most treadmills offer more shock absorption than the open road (though we’ve been on a couple that feel worse) and you can easily fine tune your pace.
One thing, however, is that it almost always seems easier to hold that given pace OUTSIDE, than it does on the treadmill. You would think the opposite would be true, but we’ve always found that we have to work a lot harder to hit the machine’s pace readout, than to hold that same pace on the track. Again, we would bet that comes down your body mass being in motion, helping you maintain some speed.
Our favorite treadmill workout is to mix up pace and incline every 5′.
On a 60′ run, something like this:
10′ warm up low HR
5 x 5′ building in speed every 5 minutes (where last 5 or 10′ is faster than goal race pace)
Lower speed to mid range pace or even slower
5 x 3′ building in incline where HR begins to go anaerobic on the final couple intervals (helps to simulate more real world effort)
Straight into fastest pace possible for 2′ on a flat.
Cool down ~8′
Mix it up and play with the intervals.
Running on treadmill has different feeling from running outside, and the other reason is that running on treadmill is pretty boring.
Most treadmills lack the speed to help an individual increase his/her speed. Plus as you pointed out, they work only one part of the leg, but they do that using ideal circumstances, in real life an individual would naturally run on unlevel terrain, in ditches, up hills etc… a treadmill doesn’t accustomed the running to such circumstances, so for that reason, I’d say treadmills are more for appearance purposes rather than real fitness, which is the same for most gym equipment.
Well, most treadmills I've seen will do at least a 3 minute kilometer. There aren't many people in the world that need much faster than that to put together an extremely hard, and very beneficial speed workout. But yes, a mix of treadmill and real life terrain is ideal.
I train mostly on the road each day but I also use a treadmill as well. I agree with the above posters as you use complete different muscles groups outside as you do with a treadmill. However, a treadmill is good option to have when you are not motivated to go out running in the cold and rain.
@Jo Legion: That's a great comment, great work and I agree.