Q - A friend of mine was training for his first marathon in the fall, and had been mainly using a treadmill and even for all of his long runs (up to three hours). He ran a 10K race a few weeks ago before the marathon, and was expecting to get a PB of under 41 minutes, since he had already run a few sub-41's on the treadmill. So he was disappointed with his finishing time of 41:45, which started him thinking about the difference between road running and treadmill running.
So here's my question: Are there any physiological differences between running on a treadmill and running outside? Does my friend NEED to run outside regularly in order to more closely approximate the race-day conditions for his fall marathon?
A - So the magic "carpet" didn't produce eh? Given that your buddy supposedly ran more than one sub-41, I too would say that the 41:45 was a disappointment - assuming of course that the 10 km route was flat and that the weather was reasonable on race day.
There was a revealing study conducted several years ago that demonstrated equality of the energetic cost of treadmill and outdoor running with the use of a 1% treadmill grade over a duration of approximately 5 min and at velocities between about 9:00 and 5:20 per mile pace, which includes your buddy's pace.
What this means is if you want to do the same amount of work in terms of oxygen usage on a treadmill, for a given running speed, you need to be running the speed at a slope of 1%.
[Here's the reference for the study: A 1% TREADMILL GRADE MOST ACCURATELY REFLECTS THE ENERGETIC COST OF OUTDOOR RUNNING Abbreviated Journal Title: J Sports Sci Date Of Publication: 1996 Aug Journal Volume: 14 Page Numbers: 321 through 327 - and you can read the study's abstract here]
The 1% slope requirement in part makes up for the fact that the runner while on a treadmill is not "pushing" through the atmosphere, and so isn't dealing with wind resistance.
It begs at least one question? How certain is your friend that he was running the pace he thinks he was? I say this because a treadmill's calibration can be off.
When I was involved in research and doing treadmill testing, the protocol required that rather than relying on a speedometer, we would determine and adjust the speeds by putting a tape marker on the belt and counting the number of times it appeared in a time period, mutilplied by the length of the belt, which in turn enabled the calculation of the belt's speed.
So it's always possible your friend's treadmill could be off, or if it's cheap or faulty, may even be affected by the runner's weight.
Another way of getting at this, may be for your friend to observe his heart rate while on the treadmill running at 41:00 minute pace, at a 1% slope, and then see what it is while running on an outdoor track at the same speed with no wind. If he is certain that the treadmill readings (slope too) are accurate and he's seeing differences in his heart rate responses it may be that he is more economic (using less oxygen) on the treadmill. There are slight differences in the biomechanics, which might account for this.
Another possible factor is the definate difference that exists between the cushioning of the foot strike on a treadmill versus on tarmac or cement. The impacts are quite different, and over the course of a lengthy run, a runner who has only done the long runs on the more forgiving surface (treadmill), may find his legs getting sore at a relatively earlier point in a road race on asphalt, posing a nasty limitation.
I see this is getting a tad long-winded, so I'll finish this answer by saying you want the real thing, at least a substantial amount of the time in your training. Certainly being accustomed (psychologically and physiologically) to striving under a wide variety of weather conditions, and dealing with the responsibility of executing the pace, is a strong argument for this.
Of course when the temperature is 10 degrees, and the wind is howling and the snow's on the ground, the treadmill can be the least worst of two evils. As a matter of fact there have been very successful, elite runners - Ingrid Kristiansen (former world marathon record holder of 2:21:06 from 1983) for example - who have successfully used treadmills in their training instead of dealing with harsh weather.