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If you're a heart-attack survivor, it is important to get exercise, but not as much as you might think. In fact, you should be careful not to overdo it, according to a first-of-its-kind study that found that running more than seven kilometres per day or walking briskly for more than 10.6 kilometres per day puts you at heightened risk of having another heart attack or a stroke.

The study, published this week in the journal Mayo Clinic Proceedings, was led by Dr. Paul Williams, a staff scientist at the Lawrence Berkeley National Laboratory, and Dr. Paul Williams, a staff scientist at the Lawrence Berkeley National Laboratory in Connecticut.

While it's long been known that heart-attack survivors should get some regular exercise, up until now little has been known on what effects too much exercise might have.

This new study looked at 2,400 physically active survivors who were tracked for a decade. It confirmed the importance of getting some exercise: People ran less than 48 km a week (6.8 km per day) or walked less than 74 km per week (10.6 km per day) decreased their risk of death from heart attack and strokes compared with sedentary people.

But those who ran or walked just slightly more than that were found to be twice as likely to die from stroke or a heart attack compared to people who exercised less.

"These analyses provide what is to our knowledge the first data in humans demonstrating a statistically significant increase in cardiovascular risk with the highest levels of exercise," Williams and Thompson write in the journal.

In an accompanying article by three cardiologists, it is noted that "extrapolation of the data from the current Williams and Thompson study to the general population would suggest that approximately 1 out of 20 people is overdoing exercise."

They have coined the term "cardiac overuse injury" to describe "this increasingly common consequence of the 'more exercise is better' strategy."