There's something I've been puzzling over for months now, ever since I decided to start doing a great deal of walking.

The question: why is it harder to use a treadmill than to walk outside?

My evidence: I have an outdoor route that is exactly four miles. I have a very fast walking pace and I do this route in about 65 minutes. Afterwards I feel tired, but a good tired, like I had a good walk and burned a few calories.

When I'm on the treadmill, set to 3.9 miles per hour, however, I feel like my feet and all joints up to my hips are going to explode. Okay, that's an exaggeration, but it's not far from it. I find I simply can't go the entire four miles that I do outside at a similar pace, or if I do, I feel like I'm in pain afterwards.

So what's the deal? If someone told me to pick which would be easier, I would have picked the treadmill. My mind is telling me that the ground is moving under my feet, giving me at least a small advantage over moving around outside, and the platform seems to have a slight amount of shock absorption. Outside the terrain and incline changes, and the path isn't straight ahead.

Am I crazy? Does anyone else feel the same way? Am I doing something wrong on the treadmill? If so, I'm happy to try to change it. At the moment I so prefer walking outside that I go no matter what the conditions, and if it's raining I walk up and down the six levels of our parking garage, all to avoid that damn treadmill...