Several  measures can help you easily determine whether you are exercising at a moderate level - one that is not overly intensive but still likely to produce heart-healthy benefits.

• 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: Another approach is to 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. Estimate your maximum heart rate by subtracting your age from 220. Thus, for a 50-year-old man moderate exercise would achieve a heart rate between 85 and 119 beats per minute (220 - 50 = 170 x 0.5 = 85 and 220 - 50 = 170 x 0.7 = 119).

My colleague Dr. Kerry Stewart, an exercise physiologist at Johns Hopkins, recommends one way to determine how much weight to use for resistance training. He suggests you find a weight you can lift comfortably for about 12 to 15 repetitions. That amount of weight will represent about 30 to 40 percent of your maximum lifting ability, he says.

Breath-holding inevitably accompanies weightlifting, but you should try to avoid holding your breath for an extended period.