Ask an Expert: Which is better in a workout: Intensity or duration?

Q: "I just purchased an elliptical trainer, and after three weeks of 30-minute routines every other day, I’m finding that staying in my ‘target heart rate’ zone is too easy. A higher level feels right – rigorous, but not too hard. The trouble is, at this rate, I am working at my ‘maximum heart rate’ – about 150 to 176. Would I be better off staying with my current high intensity level, or backing off on the intensity and doing a longer workout?"

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

The short answer is that you can benefit from a combination of both training methods, alternating shorter, high-intensity workouts with longer, moderately intense workouts. I’ll get into more detail about this in a bit, but first, let me clear something up.

Your question raises a common misconception about using the standardized target heart rate (THR) and maximal heart rate (HRmax) formula to determine how intensely you should exercise.  This formula estimates target and maximal heart rates as follows:
  • HRmax = 220 – age
  • THR = 65% to 85% of HRmax
Even though many fitness instructors – and even some fitness equipment manufacturers – use this formula to help people find their “optimal training intensity,” the formula has some significant flaws. 
Specifically:
  • The formula is based only on age and doesn’t take into account your gender, fitness level or health status.
  • There is no published record of research to back up the equation.
  • Current research shows that the formula overestimates maximal heart rate in young adults, and underestimates it in people over 40.
So what should you do instead to establish the right exercise intensity for yourself?
  • Exercise according to your own “rating of perceived exertion”
  • Instead of trying to keep your heart rate consistently within the range that the THR formula recommends, just continue to exercise at the intensity that feels right to you – rigorous, but not too hard.
You can monitor your intensity by using a simple, individualized scale called the Borg rating of perceived exertion (RPE). Developed by Dr. Gunnar Borg, the Borg RPE scale is used worldwide by professionals in medicine, exercise physiology and sports.

The RPE scale allows you to rate your own perception of your level of exertion on a scale of 6 (no exertion at all) to 20 (maximal exertion). A rating of 12 to 14 is considered “moderately intense.” Using this scale, you can learn to increase your intensity when your level of exertion feels light, and to slow down a little when your exertion feels too heavy. 

View the RPE scale and read more about how to use it in my answer to a previous question on exercising at the right intensity.

Go for a balance of intensity and duration
For a balanced fitness program, try alternating longer, moderately intense sessions (35 to 45 minutes at an RPE of 11 to 13, or “light to somewhat hard”) with shorter, higher-intensity sessions (20 to 30 minutes at an RPE of 13 to 16, or “somewhat hard to hard/heavy”).

Shoot for a total of four to five sessions per week. The higher-intensity workouts will help you expend more total calories per minute and improve your aerobic capacity. The longer, moderately intense workouts will build your exercise endurance, help maintain your cardiovascular fitness and help your body recover from the higher-intensity workouts.

Since you’ve been building your fitness level for several weeks, you should be able to safely start this alternating training program now. People who are just starting an exercise routine should work on increasing their duration (time spent in each workout) and frequency (number of workout sessions per week) before adding this alternating training method.

Using home equipment can be a rewarding and successful way to meet your fitness goals, and you’re off to a great start. Continue to exercise at the level that feels right for you by monitoring your RPE, and vary your intensity and duration from session to session.

It’s fine to check your heart rate, too – over time, you will find a target heart rate that is specific for you, based on your individual fitness level. Thanks again for your great question, and keep up the good work!

July 2007