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...

Kind of related, I'm the same way with my bike on a trainer. Perhaps it's due to the very stationary state of the bike, versus the natural side to side motion that you can have on the road and trail. Perhaps it's a deeper psychological impact due to being indoors?

On a mechanical device, the motion is likely to be nearly identical from step to step (or cycle to cycle), leading to repetitive stress on specific muscles.

Outdoors, things are far less uniform, spreading the wear and tear around some more, as the motion varies from step to step.

That's my perception of it.

I think Mark's point is valid. I think it's also why doing things outdoors is often better exercise than doing things indoors. More muscles get stresses in different ways leading to overall better conditioning. At least that's my perception. I have seen quite a few people who are regulars in the gym not fare all that well on the trail.

Boredom? That is, if you're outside, you're probably more engaged in your environment, whereas on the treadmill, you're in a static environment, where you can focus on all your little aches and pains.

Otherwise, I'm with Mark's assessment. The same sort of thing holds for free weights, versus a weight machine (depending on the machine, of course). The former requires all of the surrounding stabilizer muscles, while the latter inhibits motion so that they never come into play (or do so in a comparatively reduced fashion).

I would guess off the tread mill you vary your speed at least somewhat.

Is your treadmill perpetually uphill?

I find the treadmill much easier than running outside, but considerably more boring - so I almost never do it.

Even when I crank things up high I never feel like a treadmill is helping me exercise, it just gives me an hour of 'stomp stomp stomp' - yeah, I'm a heavy runner. At least outdoors I can vary things, run down by the river on soft soil, then up the hill on a road etc.