Just a few hours a week of moderate exercise may not prevent all heart attacks, but it could make the difference in who survives one, Danish researchers say.

Over several decades, the study found that regular moderate exercisers were half as likely to die when they had a heart attack, compared to people who were sedentary.

Despite this benefit, exercise did not seem to protect heart attack survivors from dying or experiencing heart failure later on, the researchers write in the European Journal of Preventive Cardiology.

“We already know that exercise protects you from heart attack, as well as a number of other beneficial effects,” said senior author Dr. Eva Prescott of the University of Copenhagen and Bispebjerg Hospital.

“The main finding here is that among people who do get a heart attack, the ones who exercise more seem to be more likely to survive a heart attack than people who exercise less,” Prescott said by email.

“Exercise is good for you, we know that. These findings confirm that and help us understand why,” she said.

To examine the potential influence of exercise on heart attack survival, the researchers analyzed data on more than 14,000 people participating in the Copenhagen City Heart Study, none of whom had experienced a heart attack or stroke at the beginning of the study period.

Participants reported on their level of physical activity at the baseline assessment between 1976 and 1978 and researchers followed their health over the years through 2013.

Based on their leisure time physical activity at the start of the study, participants were categorized into one of three groups: sedentary, meaning they engaged in only light activity for less than two hours per week; light exercise, which meant they did two to four hours of walking or equivalent activity each week; or moderate/high exercise, which meant greater than two hours of vigorous activity like biking weekly.

Over the study period, 1,664 people had heart attacks and 425 died right away. The average age at which people had a heart attack was 71.

Among those who had a heart attack, the majority, 54 percent, were in the light exercise category - and they were 32 percent more likely to survive than people who were sedentary. People who were moderate or high exercisers were 47 percent more likely to survive.

Having been an exerciser did not guard against future heart failure for heart attack survivors, however. People who exercised were just as likely as sedentary peers to develop heart failure in the five years following a heart attack, the study also found.

The same was true for death over the longer term. Among the heart attack survivors, 83 percent died during the seven-year period following their heart attack. There was no difference in the mortality rate between people who had moderate/high levels of exercise and those who exercised less or not at all.

Nonetheless, regular exercise can help lower blood pressure, said Antonio Crisafulli, who studies exercise and heart function at the University of Cagliari in Italy.

"In short, if we have a low blood pressure, the work required to the heart is reduced,” said Crisafulli, who was not involved in the study.

Exercise is not a miracle cure and even active people may face heart attacks, Crisafulli noted, but active people are more likely to survive heart problems.

Exercise signals to the body to create more pathways to bring oxygen to the heart, Prescott added. In this way, even if an artery becomes blocked and causes a heart attack, the heart will still have other ways to get oxygen.

“Exercise is one of the best and smartest ways to keep our body healthy and to survive if something wrong happens in our cardiovascular system,” Crisafulli added.