Too much high intensity exercise may be bad for the heart, two new studies suggest.

The first study from Germany found that people with heart disease may have an increased risk of dying from a heart attack or stroke if they undertake too much high intensity exercise.

The results were based on a 10-year study, during which over 1,000 people with stable heart disease were monitored. Most of the participants were in their 60s and all had attended a cardiac rehabilitation programme which encouraged regular exercise to reduce the risk of a stroke or heart attack.

Current guidelines recommend that people with heart disease undertake up to 60 minutes of moderate intensity aerobic exercise - such as brisk walking - at least five times a week.

The study found that four in 10 of the participants were active between two and four times a week, while 30% were more active than this and 30% were less active than this. One in 10 admitted to rarely or never exercising.

The researchers found that those who were the least active were twice as likely to suffer a heart attack or stroke compared to those who were active on a regular basis. They were also four times more likely to die from all other causes.

However, the researchers were surprised to also find that those who undertook the most strenuous exercise on a daily basis were four times as likely to die from all causes, including a heart-related event.

The second study from Sweden involved over 44,000 people aged between 45 and 79. They were questioned about their past activity levels and had their heart health monitored for an average of 12 years.

They found that men who had undertaken endurance training for more than five hours per week early in their life - during adolescence or early adulthood - were more likely to go on to develop an irregular heart rhythm later in life.

In fact, they were 19% more likely to have developed an irregular heart beat (atrial fibrillation) by the age of 60 compared to those who exercised for less than an hour per week.

An irregular heartbeat is known to increase the risk of a stroke.

Furthermore, the level of risk rose to almost 50% among those who undertook more than five hours in intensive exercise every week at the age of 30, but who did less than an hour per week by the time they were 60.

However, those who walked briskly or cycled for at least an hour a day at the age of 60 were 13% less likely to develop an irregular heartbeat than those who did next to no exercise.

Details of these findings are published in the journal, Heart. In an accompanying editorial, Spanish doctors emphasised that while perceptions of how much exercise is good for us may change, doing no exercise at all should never be an option.

"A thin line separates accurate information and unnecessary alarmism, leading to inactivity and consequent heart disease.

"The benefits of exercise are definitely not to be questioned; on the contrary, they should be reinforced. The studies reviewed here, and future studies, will serve to maximise benefits obtained by regular exercise while preventing undesirable effects - just like all other drugs and therapies," the team from Hospital Clinic de Barcelona said.

For more information on heart health, see our Heart Disease Clinic here