Dr Benjamin Levine  was only 10 years old when the famous Dallas Bedrest and Training Study took place. Five young men were put to bed for three weeks and then put through endurance training for two months.

“Almost everything that we know about the response to deconditioning and conditioning began with that study,” says Levine, Professor of Internal Medicine/Cardiology and Distinguished Professor of Exercise Sciences at the University of Texas Southwestern Medical Center.

Thirty years later, Levine did a follow-up study with the same five men. The new study asked what happened to these men from 30 years of aging, and it turned out that not a single one of these volunteers was in worse shape 30 years later than they were after three weeks of bedrest when they were in their 20’s.

In a nutshell: Three weeks of bedrest were proven to be worse for the body’s ability to do physical work than 30 years of aging. This tells us that some component of what we typically think of as the inevitable consequence of cardiovascular aging is actually modifiable, reversible and related to physical activity.

Further work with Masters athletes and sedentary seniors by Levine and his team showed that a lifelong pattern of high intensity competitive exercise was able to prevent stiffening of the heart and blood vessels. A recent paper describes how the effects of sedentary aging on the heart can be completely reversed with exercise under the right circumstances.

Both the preventive and reparative exercise, however, has to become habitual before the heart stiffening process begins. There comes a point, typically around 65, past which the structure of the heart cannot be altered with exercise.

The question Levine and his team then asked was: how much exercise is enough? Their conclusion is that the right “dose” is about 4-5 days a week of exercise, and that a mix of types and durations of exercise is optimal.

One day should incorporate an exercise that lasts at least an hour and is done for fun. At least one session should be high intensity (for example the Norwegian 4X4 workout). Two to three days a week should be moderate intensity exercise—30 minutes of something intense enough to work up a sweat, a little shortness of breath, but not so hard that you cannot have a conversation. Finally, there should be at least one day a week of some kind of strength training, which can be separate or on the same day as one of the other sessions.

The idea that we should regularly exercise, says Levine, is not a surprising one. But he sees a large gap between the public’s general awareness of the benefits of regular exercise and the collective will to overcome the barriers to physical activity for most people.

“Many of these barriers are structural,” says Levine, who is also founder and Director of the Institute for Exercise and Environmental Medicine (IEEM), a partnership between Texas Health Presbyterian Dallas and UT Southwestern.

“They are related to how we build our communities. They are economic. Nobody pays for cardiac rehabilitation or exercise until you get a heart attack. We wait until it’s too late. But we’re trying to change that.”

UT Southwestern, partnering with Texas Health, is working on implementing a system called the Physical Activity Vital Sign, which was developed by Kaiser doctors in California. It involves doctors monitoring levels of physical activity the same way they do blood pressure or body weight. When patients are not getting the recommended amount of activity, doctors will discuss ways in which they can help them achieve the required level. They will also work with the insurance company to reimburse costs for exercise when it is still preventive, before there is a diagnosis.

“We need a comprehensive strategy,” he says. “We need to be referring patients to exercise professionals in the community, getting that cost paid for and reimbursed by health insurance, making the right incentives for institutions and for businesses to support their employees to be physically active, and creating spaces where that can happen in our communities.”

In many ways, says Levine, it’s useful to think of exercise like any other medicine. In the same way we prescribe medicine for high blood pressure and cholesterol, and insurance companies reimburse for that medication, we should prescribe and pay for exercise.

Exercise is also like medicine in that you can underdo it, but also overdo it. There is accumulating evidence that some endurance athletes can do so much exercise that they actually hurt themselves.

Levine has in the past studied endurance runner Charlie Engle’s heart, and is now in the process of studying endurance swimmer Ben Lecomte’s heart as he swims across the Pacific Ocean.

“We’re looking at his body composition, and in particular at the strength of his skeleton, since eight months of floating on the water is almost like spending time on a space station. Is he going to lose bone mass?”

Regardless of who you are, and what your roles are, Levine emphasizes that you have to figure out a way to incorporate exercise into your life. It should not be an add-on activity but a part of your routine.

“If you’ve got small children, put them in a jogging stroller or take them for a walk. Engage with them, ride a bike with them. Make it a social circumstance so that you get something out of it and your kid gets something out of it.”