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Question: I recently had a test in a hospital fitness program where my aerobic fitness level was measured on a treadmill using some sophisticated analyzers. My score was 45, but I really don't understand what that means. Would you explain this number and what it means? Thank you.

Answer: The test you took is usually called a VO2max test, and the score is your VO2max. This score refers to the amount of oxygen your body can use at maximum work in milliliters per kilogram of body weight per minute (ml/kg/ min). The higher the number, the more energy you can produce and the more work (or exercise) you can do. World-class endurance athletes usually test from 80 to 90 ml/kg/min during max work; most sedentary adults are between 40 to 50 ml/kg/min. The values for women are usually 15 to 30 percent below those for men.

To measure your VO2max, we put you on a treadmill or bicycle ergometer and increase the workload each minute or two until you reach the point you can no longer continue. At the same time, we place a mouthpiece in your mouth and collect the air you breathe out in a container or pass it through some volume measuring equipment so we can measure the volume of air you breathe each minute during the test. We also analyze the expired air to see how much oxygen is left in it when it comes out. From this information we can calculate the amount of oxygen your body uses during each work stage and at maximum work.

For instance, let's say that at maximum work you breathed out 100 liters of air a minute, and the oxygen content of this air was 17 percent. This would mean that there was 17 liters of oxygen left in the expired air (0.17 X 100 liters/min

17 liters/min). Since the amount of air you breathed in is about the same as the amount of air you breathed out, and the oxygen in inspired air is always about 21 percent, we know that you breathed in about 21 liters of oxygen (0.21 X 100

21 liters/min).

The difference between the amount of oxygen you breathed in, 21 liters, and the amount you breathed out, 17 liters, is your VO2max, or 4 liters per minute. To get the score in the format you received, we multiply the 4 liters by 1,000 to get milliliters , and divide by your body weight in kilograms . So, for a 70 kg person (154 pounds), the VO2max would be 4,000 ml divided by 70 kg or 57 ml/kg/min.

Most research shows that VO2max can be increased about 20 to 25 percent as a result of training, so if you are untrained, you could increase your VO2max to about 56 ml/kg/min. This is the good news. The bad news is that you could probably never be a world-class endurance athlete because that would require a VO2max of 80 ml/kg/min or so. It seems obvious from this discussion that there is a rather large heredity factor in VO2max and that if you didn't inherit a large potential from your parents, that you may have to excel in some other sport that requires less oxygen for its performance. In fact, some twin studies looking at this question concluded that heredity alone accounted for up to 90 percent of the differences between people with normal VO2max and those with world-class values.

The most important factor in all of this is that you should train your VO2max as high as it can be, relative to your own potential, and then you will be able to enjoy life maximally and receive the health benefits associated with being fit.

Garth Fisher is director of the Human Performance Research Center at Brigham Young University.