Exercise - Heart Rate - Lightheadedness question

[phaseolus]

So I was at the YMCA the other night, doing my cardio stuff after my lower body strength stuff. I've just discovered that, although I can't do much running on the treadmills without having to rest a couple times before petering out shortly after running a mile, I can go on those elliptical things forever.

I'm 44 years old, and the charts and the digital displays on the machine claim I should be keeping my heart rate in the 140-150/min range. Trouble is that doesn't seem like I'm working at all... I'm pretty comfortable keeping it around 175 for pretty long periods of time, 30-35 minutes or so. Theoretically this is around the maximum my heart is capable of beating...

So all's going well for a while, after about 35 minutes I got slightly lightheaded, which I took as my cue to cool down and stop the workout. What's actually going on in my body when this happens -- is it an oxygen thing? Is there any potential for danger? And is there any reason why I can't keep my heart rate around 175, within reason?

[supernova]

1. You might want to talk to a professional trainer

or a doctor. The Target Heart Rate is mostly for exercise purposes. The kind of output you're talking about is usually reserved for atheletes in competition.

As for your body, yes it's partly oxygen depletion. You are not beating enough to keep up with your body.

[Joe_VB]

2. You might want to go to your Doctor and have schedule

a stress test.