How Do You Know If You're Fit?

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Harvard Medical School
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How Do You Know If You're Fit?

Brigham and Women's Hospital

We all know that keeping physically active and exercising regularly are good for your health. Thirty minutes of moderate-intensity exercise most days of the week:

  • Lowers blood pressure
  • Decreases the risk of heart disease
  • Helps prevent type 2 diabetes
  • Reduces the risk of certain types of cancer
  • Adds to bone strength
  • Assists in weight control

Regular exercise also improves your chances of living longer with more stamina. But how do you know if you are getting enough exercise? This is where fitness comes in.

Defining Fitness

We often use the term fit to describe someone who looks healthy and trim with muscular definition. But the more technical definition refers to cardiovascular fitness. The level of cardiovascular fitness is determined by how efficiently the heart, lungs, blood vessels and red blood cells supply muscles with oxygen during sustained exercise, along with the ability of the muscles to use that oxygen. It is also known as cardiorespiratory fitness or aerobic fitness.

Defining fitness more precisely means taking some measurements of bodily functions. In special fitness laboratories, technicians can get the most accurate assessments by measuring maximum oxygen consumption (VO2 max for short) during strenuous exercise. This requires breathing through a mask that measures the volume of air inhaled and the concentration of oxygen in the exhaled air while cycling on a stationary bike or running on a treadmill. It's a complicated and expensive procedure, and so is usually limited to research or training elite athletes.

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Using METs To Gauge Fitness

An easier way to gauge cardiovascular fitness is to measure it in metabolic equivalents (METs). One MET is the amount of oxygen used when you are completely inactive, such as sitting still or sleeping. Average healthy but non-athletic middle-aged men and women have peak exercise capacities in the range of 8 to 10 METs; marathon runners can have values as high as 18 to 24.

The easiest way to find out where you currently stand is to go to a gym and ask to get on an exercise machine that displays your MET level. Many models of treadmills, elliptical trainers and rowing machines have built in MET calculators. If an exercise machine with a MET display is not available, you can estimate your exercise intensity level in METs according to the type of physical activities you do. For example, walking at 3 miles per hour puts your level at about 3.5 METs. Jogging at a pace of 5 miles per hour (12-minute miles) brings the intensity level to 8 METs.

Tables listing different physical activities and their associated intensity levels are readily available.


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What Is the Right Amount of METs for Me?

Similar to maximum heart rates adjusted for age, exercise researchers have developed a simple to use calculation to find out your target MET level:

  • For women, MET level = 14.7 - (0.13 x your age in years)
  • For men, MET level = 14.7 - (0.11 x your age in years)

For example, a 45-year-old woman has a target MET level of almost 9 METS.

Hitting your target METs or higher indicates very good to excellent cardiovascular fitness. Falling under 100% is associated with diminished health status.

In the August 4, 2005, issue of the New England Journal of Medicine, researchers from Rush University Medical Center in Chicago reported results of an eight-year study on 5,721 women healthy enough to walk on a treadmill. They recorded each woman's exercise capacity in METs. Women whose exercise capacity at the start of the study in 1992 was less than 85% of the predicted value for their age were twice as likely to have died over the next eight years compared with those who achieved 85% or better.

High fitness, hitting your target METs or higher, is clearly good. Medium, greater than 85% but less than 100%, is okay, but you should aim for improvement. Low fitness, less than 85% of your target, means you need to start exercising more.

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Raising Your METs

Unlike some measures of health, fitness and exercise capacity are not set in stone. No matter where you are today, you can boost it with regular physical activity that challenges your body. That means working your body hard enough to speed up your heartbeat and breathing. Any increases in exercise intensity and the time you spend doing it will move you in the right direction.

A recent University of Florida study suggests some answers to the duration and intensity questions. Researchers recruited almost 500 sedentary women and men for an exercise experiment. All were asked to walk for 30-minute sessions at different intensities and frequencies:

  • Moderate intensity — 45% to 55% of their maximal heart rate
  • High intensity — 65% to 75% of their maximal heart rate
  • Moderate frequency — three to four times a week
  • High frequency — five to seven times a week

After two years, all the subjects who followed their exercise prescriptions improved their cardiovascular fitness. Those who walked at moderate intensity for 30 minutes five to seven times a week and those who walked at high intensity three to four times a week had substantially greater fitness improvement over the low intensity, low frequency group. No surprise, the group that stuck with high intensity and high frequency did the best.

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Bottom Line

Thirty minutes of dedicated exercise at moderate intensity most days of the week is likely to achieve your target MET level. Doing higher-intensity workouts on fewer days also works, but people stick with this one less often. To know for sure that you are hitting your target, measure your METs now and track your progress during the coming weeks.

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Howard LeWine, M.D., is chief editor of Internet publishing, Harvard Health Publications. He is a clinical instructor of medicine at Harvard Medical School and Brigham and Women's Hospital. Dr. LeWine has been a primary care internist and teacher of internal medicine since 1978.

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Last updated July 09, 2013

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