The usual debate on treadmill versus outdoor running centres on bio-mechanical aspects. There are several popular misconceptions. One is that the treadmill "pulls you along" and so must therefore be easier than running outside. The force required to stop you from falling over is the same in both cases; the only difference is that the fixed point about which you rotate is your foot when you are running outside, while the belt of a treadmill rotates you about your centre of mass. You must still work against gravity and reposition your limbs both on the treadmill and outdoors. The second misconception is that you must lean from your ankles when you run on the treadmill, just as you would when running on the road. I specifically discussed this point in my post on "Treadmill running: to lean or not to lean?". As I discussed in that post, this is partly another way of saying the above: leaning is the same as rotating about a point - in one case you actively lean against the road and in the other, the belt "leans you". The other factor, of course, is that when you run outside you must overcome air resistance. There must be a compensatory horizontal force as the resultant of the ground resistance force and gravity, which is only possible if your centre of gravity is further forward than your point of contact. This is just a fancy definition of leaning.
There are some important bio-mechanical differences to running on the treadmill, however. The most important is the fact that the surface on which you run is perfectly flat and consistent. This means that every footstep will be exactly the same and will use exactly the same muscles the same way. The upshot is that an over-reliance on treadmill training lays you open to injury from weak stabilising muscles when trail running as well as to overuse injuries on the treadmill from stressing the same muscles in excess.
Air resistance is obviously an important factor as anyone who has ever been running on a windy day will atest to. Popular wisdom has it that a gradient of 1-2% on the treadmill will compensate for any extra work that would be done against air resistance. According to studies by Dr Mervyn Davies, however, at speeds greater than 18 kph the air resistance impacts the mechanics of running significantly. This means that there is really no substitute for track running at these kind of speeds although it has to be noted that, in race conditions, most of the athletes are somewhat shielded from the wind most of the time.
The aspect that people only seem to mention in passing is the aspect that is most important as far as I am concerned and that is heat. I am very sensitive to heat because I am on the large side (189cm, 85 kilos) and a lot of my energy is spent on evacuating heat. When you run outside even on a hot day with no breeze, you at least have a headwind equal to the speed at which you are running. If you work out how big a fan you would need to simulate this cooling effect on a treadmill then, according to my back of the envelope calculations you would need a 700W fan for a running speed of 16 kph!
I do a lot of my training these days on the treadmill and I started to wonder whether it really has the same training effect from a cardiovascular point of view. First of all, I compared a run I did on the treadmill at work of 20 minutes at 16 kph (followed by 4 lots of 200m at 19 kph) with a run I did on my home treadmill of 40 minutes at the same speed of 16 kph (actually 16.5 kph because my belt runs a little slow). As usual, I found it very uncomfortable running on the treadmill at work because it was so much hotter than in my cool basement with my huge 170W fan. What I expected to see was that the two graphs would start off at the same heart rate but that the one corresponding to my work treadmill would increase more rapidly. Instead, I noticed that they both increased at about the same rate but that my heart rate started off much lower at home. It may be that the work treadmill runs faster than it should or perhaps a 1% gradient is not the same on both treadmills but the difference is quite striking: it is as "easy" for me to maintain that pace for twice as long at home.
Then I thought I would compare my 40 minute run at home with the Half Marathon I ran (outside, obviously) at an almost exactly evenly paced 16 kph. There is really no doubt that these are two very different cardiovascular exercises.
This would appear to back up apparently contradictory findings that treadmill running is easier but outdoor running is faster. In other words, at a given pace the heart rate is lower on the treadmill but runners perceive treadmill running to be harder than running at the equivalent pace outside. This is sometimes put down to the "boredom" of running on a treadmill but I think that there is something more fundamental behind this. I am anything but bored when I am watching a good movie on the treadmill... Corporal temperature is undoubtedly one of the inputs to the Central Governor in the brain responsible for pacing us via sensations of exertion and exhaustion. What I don't know is to what extent this is a real reason to slow down corresponding to a real energetic cost. This was the eternal debate I had with my coach who prescribes training to heart rate: should I have started off my 40 minute run on the treadmill at 17 kph, say, and finished it at 15 kph or should I have run it at a constant 16 kph? Is the part of my heart rate that corresponds to the evacuation of heat reflecting the same work?
Especially as many VO2 Max tests are performed on a treadmill, I think this is something that warrants study - at least, I haven't managed to find anything definitive on the subject as yet. In terms of heart rate, is running on the treadmill similar to running in humid conditions due to the relative difficulty in evacuating heat? Does the gradient of the heart rate drift have any correlation with the size of the runner or with the ambient temperature? What are the differences in training effect between running to a constant heart rate on a treadmill and running at a constant pace? Will anyone be around to care after the World ends on the 21st of December?