The impact of vigorous exercise during childhood may have been underestimated, with new research showing it could determine whether or not they survive a heart attack later in life.

An Australian study conducted on rats has shown that when exercise begins early in life the number of muscle cells in the heart multiply.

This is significant as it's always been assumed the heart stops producing these protective cells, known as cardiomyocytes, at birth, lead researcher Associate Professor Glenn Wadley from the Institute for Physical Activity and Nutrition at Deakin University said.

With Australian children considered to be among the least active in the world, Prof Wadley says the study, recently published in The Journal of Physiology, gives parents another reason to get their kids off the couch and huffing and puffing; and the younger the better.

"Clearly, endurance exercise is beneficial for the heart at any age, but it appears that a window of time exists in the younger heart whereby exercise might be able to grow more cardiomyocytes," Prof Wadley said.

The window of opportunity to grow the extra muscle cells is from childhood up until puberty, Prof Wadley said.

Cardiomyocytes are responsible for the contraction of the heart, working together they produce around 100,000 hearts beats per day, every day.

The more cardiomyocytes a person has in reserve the better, particularly for heart attack survivors, Prof Wadley explained.

A heart attack kills tens of millions of irreplaceable cardiomyocytes instantly.

"The amount of muscle cells you lose will have an effect on how well you'll recover from that heart attack," Prof Wadley said.

For the study, rats were placed into four groups, one group remained sedentary and the others completed a four-week exercise program but at different stages of life.

The three active groups completed four weeks of exercise between either five to nine weeks of age (juvenile); 11-15 weeks of age (adolescent) or 20-24 weeks of age (adult).

The three active groups ran on treadmills, doing a daily workout at a moderate pace and then their hearts were examined microscopically to determine changes in the structure, function and shape.

"Our research showed that after just four weeks of exercise, the juvenile rats had an increase of 40 per cent in heart muscle cell number, but also an increase in the size of the cells, compared to the sedentary rats," Prof Wadley said.

"However, the most exciting discovery is that these additional muscle cells were still present in the juvenile exercised rats when we examined them later in adulthood, despite them being couch potatoes for a long time after the training had stopped," he said.

While more research is needed, Prof Wadley said there's no reason to think these findings wouldn't translate into humans.