Q. When I run (usually on a treadmill), I'm exhausted afterwards. But when I run outdoors, I feel stronger after I'm done. I think this is because I slow down noticeably when I run outdoors. But is it actually better that way? If I'm exhausted after a workout, am I overdoing it?

Q. When I run (usually on a treadmill), I'm exhausted afterwards. But when I run outdoors, I feel stronger after I'm done. I think this is because I slow down noticeably when I run outdoors. But is it actually better that way? If I'm exhausted after a workout, am I overdoing it?

A. The funny thing about running on a treadmill is that it should be easier than taking it to the streets — that rotating rubber belt propels you forward, requiring less energy. But, in reality, when man meets machine, the results can be disastrous, says Mike Broderick, co-owner of Running Strong, a D.C.-based coaching service.

"On a treadmill ... people think they ought to run a certain pace. So they'll crank it up and hold on for dear life," he says. But when you're outside, without numbers flashing in your face, it's easier to follow the rhythms of your body: "You don't need to hit anything. You just slow down." That's likely your story. When you're away from the treadmill, you're more attuned to your perceived exertion and adjust accordingly.

The easiest way to check if you're working enough is by calculating your heart rate. For most training runs, Broderick recommends staying between 65 and 75 percent of your max (which is usually about 220 minus your age). Once or twice a week, he gives you permission to go all out and hit 80 to 85 percent, but those runs should be shorter, and they require more recovery time.

He suspects you're subconsciously doing the former outside and the latter on the treadmill, which explains why you're so spent after those indoor runs. "You should feel tired, but invigorated," Broderick says.

"If you feel tired and overwhelmed, you've probably done too much."