No scientific study into the relationship between sex and athletic performance has found a drop in speed or vigour on the field after a night of love - but there is increasing evidence that engaging in a brain-demanding task slows down cyclists and runners.

The latest research comes from Dr Clare MacMahon, senior lecturer in the department of biomedical and health sciences at Swinburne University. Working with German researchers - and 20 German runners - McMahon and company devised an experiment where the athletes had to run three kilometres. Each of their performances was timed.

Ahead of a second run, the runners spent 90 minutes staring at a screen. When different letters of the alphabet were displayed, the athletes had to respond by pressing various keys on a keyboard. In other words, they had to concentrate and perform a thinking task. A control group watched a television documentary.

The athletes then ran three kilometres again, sticking to a race plan where the first and last laps were faster.

On the second run, the cognitively fatigued group ran slower times than the control group - but according to blood tests and heart monitors, their level of physical effort remained the same as the first run.

In a paper titled Cognitive fatigue effects on physical performance during running, published in the Journal of Sport & Exercise Psychology, Dr McMahon writes: "On a physiological level, participants invested the same resources, but these corresponded to worse performance."

The key to this is what's known as "perception of effort" - how an athlete thinks they are performing, against what their body is actually capable of.

Perception of effort - and not the capacity of skeletal muscles, heart and lungs - determines when an athlete hits the wall of exhaustion. The brain gives out before the body does.

What McMahon's experiment suggests is that the brain needs to be rested - or at least spared a workout - before a crucial running race. Indeed, McMahon concludes that the findings have implications for how athletes prepare for events that require optimal performance.

Will young people on sporting scholarships be required to ditch their studies ahead of a big game? That's a hard one to think about.