What's a Great Rate?
How much heavy breathing can you take? The American College of Sports Medicine (ACSM) advises using the "maximum heart training range" formula to find out how hard you should work out to do your heart good, without working out so hard you do it harm.
Simply subtract your age from 220, then multiply by .6 for the low end of your range and by .9 for the high end.
Divide the product by six to translate into 10-second heart rates; count them through your pulse. (Don't use your thumb.) Check your rate during your workout. If it's too low, work harder; too high, ease up.