Measuring 'Gas Exchange'

If you want a more advanced gauge of your fitness than the simple tests described in the accompanying article, a few facilities in the area have the equipment needed to measure your "gas exchange": the degree to which your body converts oxygen to carbon dioxide as exercise intensity increases.

The ratio between oxygen inhaled and carbon dioxide exhaled at different work levels is one of the most basic measures of aerobic fitness.

At a light effort, perhaps three-fourths of the oxygen inhaled is converted to carbon dioxide; the remainder isn't needed to support the work being done. As the work gets harder, breathing becomes more rapid and the volume of oxygen inhaled increases, as does the percentage converted to carbon dioxide.

If the effort is intense enough, the ratio hits 1-to-1: All the oxygen is used. That is the "anaerobic threshold," the point at which the aerobic system is doing as much as it can and the body's anaerobic system (a less efficient process that breaks down carbohydrates in the absence of oxygen) kicks in.

Oxygen use tells a lot about individual fitness. The peak amount of oxygen a person can use, for example (a statistic called VO2 max), is a good measure of overall aerobic conditioning: The more oxygen your body can take in and use relative to body weight, the better shape you are in. Changes in the carbon-dioxide-to-oxygen ratio, when correlated with heart rate, can be used to establish training zones that emphasize different goals.

Local gyms that offer this sort of testing include the Life Time Fitness chain (assessments are available for non-members) and Sport and Spine Athletics in Fairfax. Both use technology developed by New Leaf, whose Web site (http://www.newleaffitness.com) has a Zip code finder to locate facilities that offer testing.

The test involves wearing a face mask while using a treadmill, stationary bike or elliptical trainer.

-- Howard Schneider