Weather conditions in Sunday's Olympic men's marathon will have as much to do with who crosses the finish line first, as the level and intensity of the competitors' training programs, according to results of a recent study.

A joint study by sports science researchers at Charles Sturt University and South Africa's University of Capetown shows that lightweight athletes have a distinct advantage over heavier competitors racing in long distance events in hot conditions.

"I think if the temperature in Sydney on Saturday is above 25 degrees, we are likely to see one of the smaller runners in the event crossing the line first, " according to CSU researcher Dr Frank Marino.

Sixteen highly trained runners weighing between 55 and 90 kilograms, took part in the study which involved measuring their responses during three separate long runs, in cool, moderate and hot temperatures.

"What we found was that at the low and moderate temperatures, there appeared to be little or no difference in the athlete's ability to complete the distance.

"However, in the hottest conditions, around 35 degrees, some of the heavier runners failed to complete the distance or ran at a much slower rate than the more light weight runners," Dr Marino said.

"What is significant about this research is that there is finally some scientific proof to what athletes and coaches have observed for many years. We have shown that lighter runners produce and store less heat and so are able to run faster or further before reaching their limiting body temperature," he said.

Examining the results of Olympic marathon competition over the last century, Dr Marino said the anecdotal evidence strongly supports the results of this latest research.

"If you look at all the winners in these events, they will invariably be the smaller competitors, particularly when the races have been conducted in hot conditions. The winner of the men's marathon at Atlanta, South African runner Josia Thugwane was a mere 46kg and only 1.45m tall."

Dr Marino plans to follow this research with a study to establish if racial origin has any impact on the capacity of long distance runners. The research will be published in the next edition of the highly respected Pfluger's Archives: The European Journal of Physiology.