Too much high-intensity exercise may not be beneficial for the heart, according to two recent reports Live Science reported.

One study conducted by German researchers found that overdosing on high-intensity exercise can increase the risk of death from a heart attack or stroke in people who already suffer from a pre-existing heart condition. Another study by Swedish researchers found that young men undertaking endurance exercise for more than five hours a week may increase their risk of developing an irregular heart rhythm in later life.

"Both sets of findings indicate a J-shaped curve for the health benefits of exercise, with more not always meaning better, and raise questions about the intensity and duration of physical activity at different times of life," according to a linked editorial.  

For the first study, researchers followed more than 1,000 patients with heart disease for 10 years and found that those who performed strenuous exercises daily were more than twice as likely to die of a heart attack than those who exercised only two to four days a week. Those who exercised rarely or never had the worst outcomes, the Wall Street Journal reported.  

In the second study, researchers surveyed more than 44,000 men between the ages of 45 and 79 about their leisure time physical activity patterns at the ages of 15, 30, 50, and during the past year. They then tracked their heart health for an average of 12 years to gauge how many developed an irregular heartbeat or atrial fibrillation- a known risk factor for stroke.

They found that men who had exercised intensively for more than five hours a week were 19 percent more likely to have developed the condition by the age of 60 than those exercising for less than one hour a week.

This level of risk rose to 49 percent among those who did more than five hours of exercise a week at the age of 30, but who subsequently did less than an hour by the time they were 60.

Both studies were recently published in the journal Heart.