The first study to show a relationship between strenuous exercise and upper-respiratory tract illness was published in the South African Medical Journal in 1983. Participation in the Two-Oceans Ultra-Marathon (56km) resulted in a 33 per cent incidence of illness symptoms in the two weeks following the race compared with a15 per cent incidence in a non-exercising control group.

The most important factor that predicted illness was running speed. Almost half of those who ran at a pace faster than 4 minutes, 17 seconds per kilometre became ill, while less than 20 per cent of the slowest runners (taking more than 5 minutes and 50 seconds per kilometre) became ill. A few years later, a study examining illness after the Los Angeles Marathon (42.2km), showed that in the build-up to the marathon, people training more than 97 kilometres a week had a twofold risk of becoming ill compared with those training less than 32 kilometres a week.

Now, don't get the wrong idea here and use these stats as evidence to support the argument for exercising slower and training less, because sedentary behaviour increases the risk of developing chronic diseases, such as cardiovascular disease and diabetes. For example, increasing television watching by two hours a day will increase the risk of type 2 diabetes by up to 14 per cent.

While the illness symptoms associated with strenuous exercise have typically been associated with infection, it is only present in about 30 per cent of cases. The symptoms may also indicate the presence of allergy or airway inflammatory response complicating the situation more than initially thought.

A mechanism proposed to explain increased "illness" symptoms is linked to the immune system response to tissue damage related to excessive exercise and lack of recovery. When tissue is damaged, it releases immune modulating molecules, called cytokines. In some individuals, these cytokines stimulate a branch of the immune system associated with airway allergies and inflammation. This mechanism explains how individuals can exhibit the symptoms of a "cold" without necessarily being infected.

If you have persistent muscle soreness in your build-up to endurance competition, build in extra recovery sessions or days off where you use nutrition, sleep, stretching, massage, cold water immersion, and compression garments to promote tissue healing.

Response by: A/Prof Andrew McKune, Nathan D'Cunha, and Dr Nenad Naumovski, Faculty of Health, University of Canberra

Brought to you by the Fuzzy Logic science show, 11am Sundays on 2XX 98.3FM. Send your questions to askfuzzy@zoho.com