This article is from the Diet FAQ, by Claudia McCreary firstname.lastname@example.org with numerous contributions by others.
Hard enough to get your heart beating fast, but not hard enough to exhaust
you; this is the pace at which your muscles burn fat most efficiently.
Exercising harder than this causes carbohydrates (sugars) to be burned, not
fat. (For a detailed, easy-to-read discussion on this subject, check out The
New Fit or Fat, by Covert Bailey.) There are several ways to tell whether
you're exercising at the proper intensity:
* Heart rate: Determine your maximum safe heart rate by subtracting your
age from 220, then exercise hard enough to bring your heart rate to
65-80% of your maximum. For example, if you're thirty years old, your
maximum heart rate is 190, and you should aim for a heart rate of
123 to 152 while exercising. If you're not in great shape (just starting to
exercise, recovering from a minor illness, etc.), you should aim for the
lower end of your range. Taking your pulse during exercise can be tricky,
since you'll usually need to stop jogging, dancing, or whatever, to
accurately feel your pulse (at your wrist or at your carotid artery, which
is located at the side of your neck just under the jaw). Stopping for too
long, however, can cause your pulse to drop down out of your target range.
Measure your pulse briefly (Covert Bailey recommends 6 seconds, other
authorities recommend 10 or 5 seconds), then multiply that figure by the
correct amount (10, 6 or 4) to determine your average pulse per minute. (*
If your normal, resting heart rate isn't somewhere around 70-80 beats per
minute, the "maximum safe heart rate" formula above may not be an accurate
indicator of exertion for you; use the "talk test," explained below,
* Talk test": This method doesn't require that you stop exercising, but it
can earn you some odd looks out on the jogging track. :) Try speaking out
loud as you exercise--if you have enough breath to speak easily, without
gasping, but not enough to sing, then you're doing ust fine.
*Getting warm or working up a sweat: the least precise of these methods.
If you exercise in warm conditions, you should exercise hard enough to work
up a light sweat. In cold conditions, it's sufficient to work hard enough
to make yourself warm.
Keep in mind that as your fitness improves, you will have to work harder to
get your heart rate up, so keep checking your pulse (or using the talk test)
even if you've been exercising for some months.