One way to determine how fit you are is to measure how much oxygen your body can efficiently consume-a measurement of how hard your muscles are working. Your VO2 max is the measurement of how much oxygen you consume in a minute, while exercising at sea level. The greater your oxygen consumption, the better your cardiovascular health.

Hospital fitness programs measure a person's VO2 max by having a person exercise under conditions that push the heart's ability to deliver blood to its maximum. For instance, you might run on a treadmill that increases in speed and/or steepness at regular intervals. While you are exercising, you breathe out through sensors that measure air volume and oxygen concentration. Computer analysis determines how much oxygen your body is using. As the workload increases, so does your oxygen consumption-up to a point. At that point, exercising harder shifts the muscles into anaerobic activity. Soon after this point, the muscles will fatigue and you will have to stop exercising.

VO2 max can be expressed as an absolute number-the number of liters of oxygen a person consumes in a minute. Or it can be expressed as a relative number-the number of milliliters of oxygen a person consumes in a minute divided by the person's weight in kilograms.

The average relative VO2 max of an untrained man in his mid-30s is about 40­45 ml/min/kg. After an endurance exercise program, that same man might have a VO2 max of 50­55 ml/min/kg. An Olympic champion 10,000 meter runner might have a value approaching or over 80 ml/min/kg. And if you think jockeys just get to go along for the ride while their horses do all the work, think again. One study found that a jockey's VO2 max can be as high as 69 ml/min/kg.