You’re not a couch potato. You do exercise regularly. But are you as physically fit as you think you are—enough to protect your heart? How many push-ups you can do might give you an important clue to your heart health.

Because physical activity is one of the best ways to prevent heart disease and prolong life, the American Heart Association urges doctors to check their patients’ physical fitness as part of routine physical exams. However, in practice that usually means just asking patients how much they exercise—a predictably unreliable way to measure. Of course, doctors also use treadmill tests to assess cardiovascular fitness. But it’s not part of every regular checkup, and it’s also time-consuming and expensive. A recent study from Harvard’s School of Public Health has found an accurate way to assess physical fitness that’s also quick, easy—and free.

For the study, the researchers measured the exercise capacity of 1,000 active, healthy firefighters (average age 40)—who were slightly overweight on average but not obese—when they did push-ups and worked out on treadmills. They were then ranked by how many consecutive push-ups they could do…and for how long they could work out on a treadmill before reaching 85% of their maximum heart rate or were unable to continue (exercise tolerance). The men were followed for 10 years, noting the number of cardiovascular disease (CVD) events (diagnosis of coronary artery disease, heart failure or sudden cardiac death) that occurred.

Results: The more push-ups a man had been able to do at the start of the study, the lower his likelihood of having a CVD event during the following 10 years. Compared with men who had only been able to do 10 or fewer push-ups, those who had been able to do more than 40 had a 96% reduced risk for a CVD event over the 10 years of the study. In fact, of the 37 CVD events that occurred, only one was in a man who could do more than 40 push-ups, versus eight for men who could only do 10 or fewer. (Exercise tolerance on a treadmill also predicted CVD events, but not as directly as number of push-ups.)

More research is needed to see if these results might apply to women, older men or less physically active adults.

In the meantime, why wait for your next doctor visit? Drop now and do some push-ups. If you struggle to reach even 10, let it be a wake-up call to improve your fitness!