Contrary to popular belief, a new study finds that too much exercise can be bad for your health and strains the heart, especially for those who have recently suffered from a heart attack.   

Studies have highlighted that less than half of the U.S. population meets the recommended amount of weekly physical activity. Health experts frequently emphasize the importance and benefits of exercising, but the public chooses their own routines for fitness goals, or lack thereof. This has led people to question why physicians don't prescribe exercises in the same manner as they recommend drugs for their patients.

The answer is simple. To write a prescription, it is important to know exactly how much activity produces a positive result, and whether there's an upper limit, the point at which helpful effects begin to do more harm than good.

Paul Williams, a staff scientist at Lawrence Berkeley National Laboratory, conducted a study with group of heart-attack survivors who had been enrolled in either the National Runners' Health Study or the National Walkers' Health Study.

"The notion that there was increased risk for people at high exercise has been around for a while, but the first thing that came into my mind was that there was something unusual and maybe something wrong about the data," Williams said in a press release. "So I was hesitant to proceed."

As the study reached its final stages, researchers found that exercise, like any other prescription, could be dangerous in high doses. Those who have experienced a heart attack and ran more than 30 miles a week, or spent more than six hours weekly doing vigorous activities, were at an increased risk (by up to twofold) of dying from a heart attack.

"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," said Williams. "Results suggest that the benefits of running or walking do not accrue indefinitely and that above some level, perhaps 30 miles per week of running, there is a significant increase in risk. Competitive running events also appear to increase the risk of an acute event." However, they point out that "our study population consisted of heart attack survivors and so the findings cannot be readily generalized to the entire population of heavy exercisers."

Findings were published online in the journal Mayo Clinic Proceedings.