Ask an Expert: Exercising at the right intensity

Q: What intensity should I aim for in my cardio workouts?

Answer provided by Mike Boggs, BS, MBA, CSCS (certified strength-conditioning specialist), fitness specialist, Providence Fitness Services:

The health benefits of participating in a consistent cardiovascular exercise program include increased energy and stamina, and decreased risk of cardiovascular disease, obesity, diabetes and high blood pressure. To reap these benefits, the American College of Sports Medicine (ACSM) recommends getting 30 minutes of moderately intense physical activity on most, and preferably all, days of the week.

It’s easy to determine what the ACSM means by “30 minutes” or “most days,” but “moderately intense” can be harder to understand and measure. Methods like target heart rate (THR) and, especially, the rating of perceived exertion (RPE) scale can help identify your ideal level of intensity for cardiovascular exercise.

Aiming for the “fat burning zone,” on the other hand, is less beneficial than many people think. The target heart rate formula THR is a recommended range of exercise intensity that’s based on your maximal heart rate (HRmax). HRmax is an age-related estimate of the fastest your heart can beat (maximum number of beats per minute) at your maximal level of activity. Here’s the formula for determining your THR:
  • 220 – Age = HRmax
  • HRmax x .65 = Bottom of THR range
  • HRmax x .85 = Top of THR range
According to the formula, a 40-year-old exerciser would have a THR of 117–153 beats per minute (220 – 40 = 180; 180 x .65 = 117; 180 x .85 = 153). However, this formula has several flaws:
  • It is based only on age and doesn’t take into account an individual’s gender, fitness level or health status.
  • There is no published record of research for this equation – the origin of the formula is an estimate based on observations of raw and mean data compiled in 1971. (Fox III, S.M. Naughton, J.P. and Haskell, W.L. Physical activity and the prevention of coronary heart disease. Ann Clin Res1971;3:404-432.)
  • Current research shows that the formula overestimates maximal heart rate in young adults, and underestimates it in people over 40.
  • Some medications, such as beta-blockers, can change the heart’s response to exercise, so an exerciser taking beta-blockers may never reach his or her target heart rate according to the formula.
If you are in particularly great or poor shape, therefore, your ideal exercise intensity might be higher or lower than the THR that this formula sets for your age group. At best, target heart rate is only a rough guideline, and it becomes even less reliable as you grow older. Experienced exercisers can experiment with a specific THR percentage or range (e.g., 85 percent or 80–90 percent) to see how they feel at that intensity level, and can adjust the target depending on their specific sport and desired result. But a better way to measure your intensity level is the rating of perceived exertion (RPE).

A better method: Rating of perceived exertion
The Borg RPE scale (named for its inventor, Dr. Gunnar Borg) may be the most versatile method of measuring exercise intensity for all age groups, genders and fitness levels. This is an individualized rating based on your own perception of how hard you’re exerting yourself when you are exercising. The scale lets you rate your intensity level from 6 (no exertion) to 20 (maximal exertion). An RPE of 12 to 14 is considered moderately intense – and in line with the ACSM recommendations – for most adults.

Borg RPE chart:
Rating                    Interpretation
6                              Rest - no exertion at all
7                               Extremely light
8                                *
9                                Very light
10                              *
11                              Light
12                              *
13                              Somewhat hard
14                              *
15                             Hard
16                              *
17                             Very hard
18                              *
19                             Extremely hard
20                             Maximal exertion  

To use the RPE scale, rate your own perception of your exertion while engaged in physical activity. Your rating should reflect how strenuous the exercise feels to you, combining all feelings of physical stress, effort and fatigue. According to the Centers for Disease Control and Prevention, a 9 on the RPE scale corresponds to “very light” exercise – like a healthy person walking slowly at his or her own pace for a few minutes. 13 is “somewhat hard,” but it still feels OK to continue. 17 is “very hard” and strenuous – you can still go on, but you’re very tired and you really have to push yourself. 19 is “extremely strenuous” – for most people, this is the most strenuous exercise they have ever experienced.

If you are shooting for moderate intensity (12 to 14) but you feel like you’re not quite there, push a little harder. If you feel like you’re beyond that and you want to keep it moderate, ease off a little. While it’s helpful to be familiar with both the THR and RPE methods, the RPE scale is your best guide for measuring exercise intensity.

Fat-burning zone vs. cardio zone
Many health clubs have charts listing “fat-burning zones” and “cardio zones” posted on their walls and on the cardiovascular machines. These ranges mislead people to believe that they are burning more fat, or only fat, when exercising at certain heart-rate intensities. There is a lower-intensity “fat burning zone” in which you utilize a greater percentage of calories from fat. However, the total number of calories burned at that rate is less than at higher intensities, so the total fat calories burned generally ends up being less, as well. For example, let’s say that you jog at a low intensity and burn 100 calories, 75 of which come from fat. At first glance, that looks good – that’s 75 percent of calories burned from fat! But if you were to jog at a higher intensity for the same period of time, you could burn 200 total calories, with 125 of them coming from fat. Although your percentage of fat calories burned drops to 62.5 percent, you still burn more total calories and more fat calories overall at the higher intensity.

The bottom line: for sustained weight loss, you have to burn more calories than you take in. Focusing on burning “fat calories” won’t help. Calories are calories – get out there and burn them.

Updated July 2007