We all know that if you're a couch potato, jumping into a strenuous workout out of the blue can be dangerous. Researchers say the same can be said of sporadic sexual activity.

It's one of the few studies to take a close look at this issue: Researchers warn if you're not physically fit, occasional physical and sexual activity can raise your risk for having a heart attack.

It's no surprise, but most Americans don't get enough daily exercise.

A survey involving 80,000 people found only about 5 percent of adults do something physically vigorous on any given day.

So a new report offers an ominous warning for the couch potato who attempts the occasional strenuous activity -- which can include sex.

"Episodic physical and sexual activity are associated with an increase in heart attacks and episodic physical activity is associated with an increase in deaths due to heart attacks," said Dr. Issa J. Dahabreh, Tufts Medical Center.

Researchers looked at 14 studies on men and women between the ages of 55 and 64. The less physical activity they had, the higher their risk.

"This triggering effect was actually significantly different depending on an individual's regular activity levels," said Dr. Jessica K. Paulus, Tufts Medical Center.

The report provided by the Journal of the American Medical Association finds the risk of a cardiac event dissipated an hour or two after being physically taxed.

While there's no evidence suggesting occasional physical or sexual activity is harmful to healthier people, it may be concerning to those who have cardiac risk factors.

"For those individuals who were unaccustomed to regular physical activity or who did not typically exercise, this risk was much higher than for those individuals who were regular exercisers," said Paulus.

The bottom line: You can get back in the game, but take it slow.

"If they are unaccustomed to exercising but they'd like to start becoming more physically active, they should do so very gradually and under the care and supervision of a physician," said Paulus.

And experts say regular exercise will pay off. Researchers say for every additional bout of physical activity a person regularly got each week, his risk of heart attack dropped 45 percent.