How much is enough? One of the biggest problems with endurance (aerobic) exercise is that it is easy to overdo. Inexperienced and misinformed exercisers think they have to work very hard for exercise to do any good. Exhaustion, sore muscles, painful joints and shortness of breath are often the results of jumping in too hard and too fast. As a result, many people discontinue their exercise programs feeling discouraged or thinking that exercise is just not meant for them.

Here's the rub — there is no scientific formula for determining how much exercise you need. The most important thing to remember is that some activity is better than none. If you start slowly and increase your efforts gradually, it is more likely that you will maintain your exercise program as a lifelong habit. Keep in mind that it is better to begin your conditioning program by "undertraining" rather than overexerting. Here are some rough guidelines to help you decide how much exercise is enough for you.

Several studies suggest that the upper limit of benefit is about 200 minutes of moderate intensity aerobic exercise per week. Doing more than that doesn't gain you much, although the extra fatigue can increase your risk of injury. On the other hand, doing 100 minutes of exercise per week gets you about 90% of the gain, while 60 minutes of aerobic exercise per week yields about 75% of the gain. Sixty minutes is just 15 minutes of mild aerobic exercise four times a week! Let's take a closer look at some general guidelines for the frequency, duration, and intensity of aerobic exercise.

1. Frequency: Three or four times a week is a good schedule for aerobic exercise. Taking every other day off gives your body a chance to rest and recover. We recommend that you rest at least one day per week.

2. Duration: Start with just a few minutes, and gradually increase the duration of your aerobic activity to about 30 minutes a session. You can safely increase the amount of exercise time by alternating intervals of brisk exercise with intervals of rest or easy exercise. For example, after 3 to 5 minutes of brisk walking, do 1 to 2 minutes of easy strolling, then another 3 to 5 minutes of brisk walking. Eventually, you can build up to 30 minutes of activity. Then gradually eliminate rest intervals until you can maintain 20 to 30 minutes of brisk exercise. If 30 minutes seems too long, consider two sessions of 10-15 minutes each. Either way appears to improve health significantly.

3. Intensity: Safe and effective endurance exercise should be done at no more than moderate intensity. High intensity exercise increases the risk of injury and causes discomfort, so not many people stick with it. Exercise intensity is measured by how hard you work. For a trained runner, completing a mile in 12 minutes is probably low intensity exercise. For a person who hasn't exercised in a long time, a brisk l 0 minute walk may be of moderate to high intensity. For others with severe physical limitations, 1 minute may be of moderate intensity. The trick, of course, is to figure out what is moderate intensity for you. There are several easy ways to do this.

Talk Test
Talk to another person or to yourself, sing, or recite poems out loud while you exercise. Moderate intensity exercise allows you to speak comfortably. If you can't carry on a conversation or sing because you are breathing too hard or are short of breath, you're working too hard. Slow down. The talk test is an easy way to regulate exercise intensity.

If you have lung disease, the talk test might not work for you. If that is the case, try the perceived exertion test.

Perceived Exertion
Another way to monitor intensity is to rate how hard you're working on a scale of 0 to 10. Zero, at the low end of the scale, is lying down, doing no work at all. Ten is equivalent to working as hard as possible, very hard work that you couldn't do longer than a few seconds. Of course, you never want to exercise that hard. A good level for your aerobic exercise routine is between 3 and 6 on this scale. At this level, you'll usually feel warmer, that you're breathing more deeply and faster than usual, that your heart is beating faster than normal, but you should not be feeling pain.

Heart Rate
Unless you're taking heart-regulating medication (such as the beta-blocker propranolol), monitoring your heart rate while exercising is one way to measure exercise intensity. The faster the heart beats, the harder you're working. (Your heart also beats fast when you are frightened or nervous, but here we're talking about how your heart responds to physical activity.) Endurance exercise at moderate intensity raises your heart rate into a range between 60 and 80 percent of your safe maximum heart rate. Safe maximum heart rate declines with age, so your safe exercise heart rate gets lower as you get older. You can follow the general guidelines in the Age-Exercise Heart Rate Table, or calculate your individual exercise heart rate. Either way, you need to know how to take your pulse.

Take your pulse by placing the tips of your middle three fingers at your wrist below the base of your thumb. Feel around in that spot until you feel the pulsations of blood pumping with each heartbeat. Count how many beats you feel in 15 seconds. Multiply this number by 4 to find out how fast your heart is beating in one minute. Start by taking your pulse whenever you think of it, and you'll soon learn the difference between your resting and exercise heart rates.

How to calculate your own exercise heart rate range:

1. Subtract your age from 220
   Example: 220 - 60 = 160
2. To find the lower end of your exercise heart rate range, multiply your answer in step 1 by 0.6
   Example: 160 x 0.6 = 96
3. To find the upper end of your exercise heart rate range, which you should not exceed, multiply your answer in step 1 by 0.8
   Example: 160 x 0.8 = 128

The exercise heart rate range in our example is from 96 to 128 beats per minute. What is yours? Most people count their pulse for 15 seconds, not a whole minute. To find your 15 second pulse, divide both the lower end and upper end numbers by 4. The person in our example should be able to count between 24 (96/4) and 32 (128/4) beats in 15 seconds while exercising.

The most important reason for knowing your exercise heart rate range is so that you can learn not to exercise too vigorously. After you've done your warm up and 5 minutes of endurance exercise, take your pulse. If it's higher than the upper rate, don't panic. Slow down a bit. Don't work so hard.

At first, some people have trouble keeping their heart rate within the "ideal" heart rate range. Don't worry about that. Keep exercising at the level with which you're most comfortable. As you get more experienced and stronger you will gradually be able to do more vigorous exercise while keeping your heart rate within your "goal" range. But don't let the target heart rate monitoring become a burden. Recent studies have shown that even low-intensity exercise can provide significant health benefits. So use the "ideal" heart rate range as a rough guide, but don't worry if you can't reach the lower end of that range. The important thing is to keep exercising!

If you are taking medicine that regulates your heart rate, have trouble feeling your pulse, or think that keeping track of your heart rate is a bother, use one of the other methods to monitor your exercise intensity.

Bryant Howard BA, MA, CFT, SPN — Bryant is certified through the International Sports Sciences Association. He is both a Fitness Trainer and Performance Nutrition Specialist. He also holds certification in CPR and First Aid. He has trained competitive athletes for ten years and offers personal and group training sessions in the Portland, OR area. Bryant Howard owns Performance Fitness Training