Scientists have proof that people who seldom take exercise may be best off avoiding sudden bursts of vigorous physical activity.
Many people aren't fit enough
University of Essex researchers found infrequent strenuous exercise poses a serious risk of a heart attack.
It keeps the heart racing for a long period - extending the time when problems could occur.
The researchers said it was not only those with known heart problems who should take care.
Dr Valerie Gladwell and Samantha Dawson compared the recovery times of volunteers after high-intensity and moderate intensity exercise.
Dr Gladwell said: "Following high-intensity exercise, heart rate remained significantly higher than normal for up to 30 minutes.
"In fact up to an hour after exercise heart rate still hadn't completely returned to normal."
Gentle is best
But with more gentle, moderate exercise, heart rate returned to normal within 15 minutes - only slightly slower than with the lowest intensity exercise.
Dr Gladwell added: "The quicker your heart rate recovers, the smaller the period of time within which heart problems can occur.
"Basically, you're hoping that the activity of nerves that control heart rate quickly return to normal."
The researchers said exercise training was an important treatment for patients who had experienced heart problems, like a heart attack, but it should be carefully controlled to avoid any dangers.
However, they said it was not just heart patients who needed to consider the risks of strenuous exercise.
Dr Gladwell said: "The study suggests that short bouts of unaccustomed high-intensity exercise increases the chances of heart problems.
"For example, shovelling snow is well-known for causing heart attacks.
"It is unaccustomed exercise, it is hard work and it occurs in cold conditions - all factors contributing to an increased risk."
The researchers said moderate exercise and training could help reduce the risk of experiencing heart problems when doing heavier activities.
The results of the study were presented at the Physiological Society's conference at the University of Cambridge.