When you’re running, your entire body is moving forward with a certain momentum, which means you must apply a certain impulse (force, over some period of time) to get it moving. So every time you speed up or resume running from a stop, you’re doing work that wouldn’t be done on a treadmill.
It’s kind of a red herring, since the treadmill is adjustable; if it seems easier, just crank up the speed and/or incline…now it’s harder. Really, you should run by heart rate, anyway. The real issue this time of year is safety: outside, you gotta worry about cars, bicyclists, and last but not least, the heat. Collapse from the heat? Better hope a good samaritan wanders by. Twist an ankle in an unseen hole? Better not be too far from home. BTW, I ain’t buying the “soft and cushiony” nature of treadmills; sure, they’re easier on your joints than pavement, but compared to grass there’s no comparison. Also, the issue of boredom is another thing that depends on how you look at it. If you’ve got a TV or music to watch/listen to, for many people that’s less boring than watching the trees go by. Also, treadmills give you the option of zoning out: as long as you’re still on the treadmill, there’s no problem. Outside, you risk problems if you zone out, since awareness of your surroundings is key to staying out of the hospital.
I prefer a treadmill because there aren’t any excuses.
Its never too hot or too cold to run. There’s always plenty of light if you have to run at night. Its never too windy and you’re not in danger of getting struck by lightning or a truck.
And I agree with Burrido. The treadmill forces you to keep running at a certain pace. If you’re outside, you can slack off your pace quite easily without even knowing it.
Your legs and feet might react differently to running on a treadmill or running outside, but your heart and lungs won’t know the difference.
There’s nothing wrong with treadmills, they allow for aerobic exercise when weather is inclement, etc. But “two miles” on a treadmill is nowhere near as tough as two miles outside - running and actually moving one’s body through space.
I’m of the opinion that the treadmill is only to be used when weather or time constraints keep me from running outside.
That said, I can just about do 12mi on the gym’s treadmills before the timer runs out (99 min), but when I run the same 12mi outside, my pace suffers and it takes 10-20 minutes longer for the same run.
Boredom is not a factor since I have been up and down the same canal towpath so many times that it is not much more exciting than the gym. I just make sure to have good tunes on.
I guess it’s like the free weights vs. machines issue in the weightroom.