person's peak exercise capacity as measured on a treadmill test is a more powerful predictor of how long someone will live than are risk factors like heart disease, high blood pressure or smoking, a study says.

The study, by researchers from the Veterans Affairs Palo Alto Health Care System/Stanford University, amounts to some of the strongest evidence yet of the importance of physical fitness.

"We're now beginning to prove the hypothesis of Darwin's whole `survival of the fittest' category in that people who are fitter tend to do better and live longer," said Dr. Gary J. Balady, a Boston Medical Center cardiologist.

For the study, patients with heart trouble and without it were given treadmill tests, which are routinely used to check for heart trouble.

In treadmill tests, patients are hooked up to sensors — including a mask to measure the amount of oxygen and carbon dioxide in each breath — and walk on a treadmill at gradually increasing speed and angle. They continue until they are exhausted, reach their maximum heart rate, or develop chest pain or other symptoms of heart disease.