How to measure exercise intensity?

The health benefits of working out are bountiful — increased energy and stamina, and reduced risk of cardiovascular disease, high blood pressure, diabetes, obesity, thinning bones (osteoporosis), and possibly anxiety and depression.

You can obtain these benefits by accumulating 30 minutes of moderately intense physical activity, such as walking briskly for 2 miles, housecleaning or pushing your mower across your lawn, on most days of the week, according to Health Canada. Or you may want to set the bar higher and establish an exercise routine — planned, structured, repetitive movement to boost your physical fitness.

Health Canada defines physical fitness as cardiorespiratory endurance, body composition, muscular strength and endurance, and flexibility. Studies of the benefits of exercise usually measure its effect on the cardiorespiratory system. During aerobic exercise, the body requires oxygen to keep moving. Aerobic exercise capacity is the body's ability to remove oxygen from the air and deliver it to working muscles. Because few people will walk into a sports medicine laboratory and have their oxygen use measured, alternative methods have been developed to gauge aerobic exercise capacity.

Target heart rate

As your body uses more oxygen during aerobic exercise, your heart rate increases. One method to estimate exercise intensity is target heart rate, which is a percentage of your maximum heart rate that varies with your age. By taking your pulse during exercise, doing some math or consulting a table, you can determine if you are reaching your target heart rate.

The Health Canada guidelines recommend a range of target heart rates depending on fitness level. For instance, someone who is unconditioned may want to start with a target heart rate of 55 percent of maximum heart rate, and a person who is at peak fitness might aim for 90 percent of maximal heart rate. But most adults who want to boost their fitness should aim for a range of 70 percent to 85 percent of their maximum heart rate, three to five times a week, 20 minutes to 30 minutes a session.

To take your pulse:
- Stop exercising and find your pulse within 5 seconds by placing two fingers on the thumb side of your wrist over the lateral (radial) artery. With practice, you may be able to find your pulse during some forms of exercise.

- Exert gentle pressure. Using a watch or clock with a second hand, count your pulse for 10 seconds.

- Multiply by six to calculate the rate for 1 minute.

- Compare this number with your target heart rate.

To calculate your target heart rate using the Health Canada recommendation for most adults:
- Find your maximum heart rate, which is approximately 220 minus your age.

- Multiply this number by 0.70, then by 0.85 to find the 70 percent to 85 percent range.

For example, if you are 40 years old, your maximum heart rate is 220 - 40 = 180. 180 X 0.70 = 126 and 180 X 0.85 = 153, giving you a target range of 126 beats to 153 beats per minute.

Find your zone

An alternative

Target heart rate has some disadvantages. It's a rough guideline that is less reliable as you grow older. For this reason some exercise specialists recommend that only young, healthy people use target heart rate to monitor their workouts.

Also, medications and chronic conditions such as diabetes and cardiovascular disease can change the heart's response to exercise. Beta blocker medications, which are prescribed to treat high blood pressure and heart conditions, can prevent the increase in heart rate that usually occurs during exercise. And many people find it cumbersome to stop their routine and take their pulse.

An alternative is the Borg rating of perceived exertion (RPE) scale, which relies on your perception of how hard you are working. With this method, you rate exercise intensity from 6 to 20. Using this scale, Ray Squires, Ph.D., director of the Cardiovascular Health Clinic at Mayo Clinic, Rochester, Minn., recommends that most adults exercise three to five times a week, 20 minutes to 30 minutes a session, at an RPE of 12 to 14.

Borg rating of perceived exertion

Proceed with caution

You may need a medical evaluation before undertaking a fitness or exercise program. People who should consult a physician include men age 45 and older, women age 55 and older, people with two or more risk factors for coronary artery disease, people with any major signs or symptoms of coronary artery disease, and people with disorders of the heart, lung or endocrine system, such as diabetes.