An athlete's ideal weight can be a hard thing to pinpoint. Too much weight and the body spends more energy carrying it around, too little weight and there's not enough fuel to power the body at which point muscle tissue gets used up instead.
Most athletes find that they have to experiment through trial and error to find their ideal weight. Even though an athlete may want to be as slim as possible when it comes time to compete, a body weight that is too low can mean that the athlete's body will not have sufficient reserves to efficiently power the system and will start taking it out on the muscles.
Dr. Mark Tarnopolsky, a physiologist and muscle metabolism researcher at the University in Ontario says that people's physiology can vary so much that no formula can accurately determine their perfect weight. During his time in graduate school, Dr Tarnopolsky, who is also a nationally ranked athlete and competes in winter triathlons, saw first hand how the delicate balance was between losing too much weight and losing just enough. He, along with his colleagues at the university, would experiment with various levels of weight loss and measure the subsequent VO2 max, which is a measure of how much oxygen one's body is delivering to the muscles during exercise.
Dr Tarnopolsky found that his body reached a point of peak VO2 max where a kg more or a kg less in body weight would cause a decrease in VO2 max - in his case, his ideal weight hovered around 70.7 kg. More than likely, he said, his body had reached the point where it was beginning to burn its own muscle protein in order to fuel it. Even though he was lighter, he was weaker and he performed more poorly.
"You could see on the VO2 machine what your body knew was right," said Dr. Tranopolsky. "You'd feel tired, stale, lethargic when you tried to drive your weight down."
For many athletes, it takes years to figure out their optimum weight level. Dathan Ritzenhein, an American runner who last year broke the national 5,000 meter race record and was the top American finisher of the Beijing Olympic marathon in 2008, took 12 years of trial and error to determine his optimal racing weight, which was around 55.3 kg (his height is 1.7m). The tennis legend Andre Agassi with the help of his trainer Gil Reyes determined that Mr. Agassi's best weight lay between 80.5 and 82.5 kilos (Agassi's height being 1.82m).
Tennis players place significantly different physical stresses on the body than runners do, since tennis players never know how long each match may last, when they have to rally their resources, or what type of challenge they face from their opponent. The best tennis players have to physically ready for any eventuality.
For top bicyclists competing in such grueling events as the Vuelta de España and the Tour de France, finding their ideal weight can be even more critical as even a small amount of excess weight can lead to poor performances when climbing mountains but not enough and they won't have the energy to go the distance.
Andy Hampsten, the only American to ever win the Giro D'Italia in 1988, knows how crucial this balancing act can be. "I knew from experience and results that I had an ideal weight - or what I thought was ideal. If I set too low of a weight goal, I would be weak and stressed. If I weighed 4 or 5 pounds (1.8 - 2.3 kg) more than ideal, I could see I was slower than my competitors."
At 1.75m, Mr Hampsten aimed for a weight at race time of around 62.1 kg, down from 65.8 kg in the off season, usually by reducing his food intake two months prior to the start of the racing season.
According to Dr. Tarnopolsky, exercise science is nowhere near making good predictions for specific athletes. "I know an individual who is one of the fittest ultra-sport athletes", he added. "She competes in 100 milers (161 km), and her body fat is close to 20 percent. Yet she is one of the most talented athletes I have ever seen."
In the end, it is up to each individual athlete and not science to determine their ideal weight for competing in their particular discipline and the only way to find out is through a process of trial-and-error and experimenting with different weight levels.