Wes, over at a Code Geek’s Tail, asks what’s the difference between a treadmill and pavement? He has been running on a treadmill for about 5 years, but has not spent much time running outside. He is thinking of moving his running outdoors and wants to know what the differences are so that he can account for them. Here are a few differences that should be accounted for off of the top of my head:

• Treadmills are softer. You are going to take more of a pounding on asphalt than you would on a treadmill. If you jump into it too fast, you could hurt yourself. However, running on a grass or dirt trail could be even less impact than on the treadmill while still allowing yourself to stretch out a bit and avoid the boredom.
• Running outdoors encourages stronger bone growth. Because there is more of an impact, there is more pressure on your muscles and bones which will lead to faster results when you are training.
• You don’t have to break the wind on a treadmill. While you will not spend as much energy pushing air out of the way as you might on a bicycle, it still constitutes a large percentage of the energy that you are using. Air may not seem like much, but it takes a lot of work to get it out of your way so that your body can pass through it, and this is only made more difficult when there is a headwind pushing that air towards you. On a treadmill, you aren’t actually moving, so you don’t have to push nearly as much air out of the way.
• Running outdoors gets you somewhere. Running is meant as a means of transportation from one place to another by moving over the surface. Running on a treadmill is having the surface move beneath you. It is not quite natural.
• Running outdoors is more exciting. I have a definite bias against running on treadmills, mostly because it is so boring. When you run outside, you need to pay attention to where you put your feet, avoiding traffic and other pedestrians, and enjoying mother nature. There are more distractions that can keep you from dwelling on how tired you are, and to give you something to ignore when preparing for a race and learning how to focus.
• Treadmills allow more opportunity to learn something new while working out. While you could listen to an educational podcast no matter where you run, you don’t have to worry as much about your environment and your safety when you are on a treadmill. The treadmill also offers opportunities to watch television which isn’t something that’s available outdoors.
• Most treadmills do not allow you to run downhill. There are very few treadmills that allow for a negative incline when you are running. For proper working out of the muscles, you need to work downhill and uphill workouts into your training. Always going along a flat or slight incline is not a natural way to move or to train.
• The 5 most common treadmill mistakes do not apply outdoors:
  1. Taking it too easy
  2. Stepping off of a moving treadmill
  3. Using the handrails
  4. Unnatural stride length
  5. Not realizing how much easier it is
• Treadmills are safe from cars, but you are not likely to get thrown off of the ground like you can from a treadmill. Getting hit by a car and being thrown by a treadmill are both experiences I do not really care to repeat.
• It is tougher to hyper-extend your back outside than on a treadmill. If you do not use at least a 1%-2% grade on the treadmill, you can wind up leaning back as the belt kicks your legs backwards and you can hyperextend your back and hurt yourself. Using a small grade on the treadmill forces you to lean forward, which helps to protect your back.^*
• It can sometimes be easier to fit in a workout on a treadmill than it is outside. Inclement weather, health concerns, or having to stay home to watch young children can make it difficult to get outside to run. Often times, a treadmill workout can still be fit in under the circumstances as long as one is available.

I may update this post or write another one at some point as I think of more comparisons between treadmills and running outdoors, but this will do for a quick list off of the top of my head. As you can tell, I much prefer running outdoors. I tend to limit my treadmill training to my 1 mile warmups before lifting weights, when I can help it. Extremely bad weather or health problems might sometimes also send me indoors, but I’d still prefer to be outside most of the time.

My advice for new runners are to skip the treadmill and stick to working up their mileage outdoors. My advice for Wes is to keep running on the treadmill and to slowly work up the outdoor mileage. Having a treadmill base, he would probably be all right just starting to run outdoors but there is a higher risk of hurting himself as his body adjusts. Replacing a run or two a week at first, and gradually moving up to running more and more often outdoors, will allow himself to acclimate to running on a new surface and in different environmental conditions.

(Update: The new last point references a comment by Lee Miller D.C. at the Complete Running Network.)