When it comes to exercising, the trick is to work out hard enough to produce heart-healthy benefits and yet not so fiercely that you do yourself harm. Luckily, there are several easy ways to determine whether your level of activity is helping or harming you:

• The talk test. You have exceeded the moderate-intensity level if you are breathing so hard that you cannot carry on a normal conversation.
  
  
• The "8 to 20" scale. Consider the intensity of exercise on a scale from 8 to 20, where 8 is barely moving and 20 is the most extreme exercise you can imagine. On this scale, moderate-intensity exercise would be rated as about a 12.
  
  
• Heart rate. During moderate-intensity exercise, your heart rate should remain between 50 percent and 70 percent of your maximum heart rate. You can estimate the range of your maximum heart rate by subtracting your age from 220 and then multiplying that figure first by 0.5 (that is, by 50 percent) and then by 0.7 (70 percent). Thus, a 50-year-old man, to assure he's getting moderate exercise, would aim for a heart rate between 85 (220 - 50 = 170 x 0.5 = 85) and 119 (220 - 50 = 170 x 0.7 = 119) beats per minute.

If you prefer lifting weights, my colleague Dr. Kerry Stewart, an exercise physiologist at Johns Hopkins, recommends this way to determine how much weight to use for resistance training: Find a weight you can lift comfortably for about 12 to 15 repetitions. That amount of weight will represent about 30 percent to 40 percent of your maximum lifting ability, he says. (Another tip: Breath-holding inevitably accompanies weightlifting, but try to avoid holding your breath for extended periods.)