LONDON, July 25 (Reuters) - Olympic athletes hoping to leave their mark in the record books in London may find themselves relying on the one thing they cannot control -- the British weather.
Whether you are a sprinter, distance runner, shot putter or discus thrower, the science behind athletics says your chances of living up to the Olympic motto of faster, higher, stronger could be decided by the unpredictable elements.
For example, if London returns to the cold and wet conditions of last week, the prospect of Usain Bolt breaking his sprint world records is slim -- though a cold spell would be welcomed by endurance athletes such as marathon runners.
"In terms of the hot and cold axis, you have the simple effect of the shorter the event, the warmer you need to be," Steve Ingham, a physiologist at the English Institute of Sport, told Reuters.
"If you are warm, your muscles will be operating more powerfully for the short explosive events. If you are persisting with an event, like an endurance event, then heat is not your friend. You need to stay cool."
This is borne out by the experience of previous Olympics.
The men's 100-metre sprint world record has fallen twice in the last four summer Games in warm conditions, while the men's marathon benchmark has not been lowered at the Olympics since Abebe Bikila achieved the feat in Tokyo in 1964.
The reason is relatively simple. Muscles contract with an improved efficiency when it is warm. The biology underpinning the theory, however, is more complicated.
"Within a certain limit, when it is warm, your muscle enzymes will operate faster, turning over energy and releasing it to the muscles," Ingham said.
"And when you are accumulating heat in endurance events, you are slowly but surely tipping yourself over (the optimum), the enzymes are getting too hot and it is starting to impair not only the enzyme's actions but you also become very uncomfortable."
The optimum temperature for what Ingham calls a "power performance" is a very un-British 26-32 degrees Celsius, whereas for endurance events it would be 12-18, which is roughly what Londoners endured last week.
"If you are out in the cold weather," Ingham said, "there is also a psychological element in that you are feeling like you should be shivering rather than relaxed and warm and ready to go."
Then, of course, there is rain. Britain has had the wettest June on record and the wet weather is set to return just in time for Friday's opening ceremony.
"The wet will provide a different limit to performance in that there is likely to be a biomechanical change," Ingham said.
"There is likely to be a slippage, albeit very subtly, at ground contact with your shoe, so that might mean you have to go for a different type of shoe or equipment."
Athletes competing in the throwing events will also be affected by the weather, but unlike the sprinters and distance runners, it is the effect of temperature on aerodynamics that could help or hinder their record attempts.
According to Professor Mont Hubbard, director of the sports biomechanics lab at the University of California, Davis, shot putters and hammer throwers will be hoping for heat, while in the discus and javelin they will be less concerned should Britain revert to its autumnal climate.
The reason has to do with forces, and specifically what forces come into play in different throwing events.
"A sphere, which is what the shot and the hammer are, only has drag," Hubbard told Reuters.
"Drag is the part of aerodynamics that slows you down."
Drag, he explained, is proportional to air density, and that is affected by temperature and air pressure.
Hubbard said: "To throw a sphere a long way you want low pressure, which is usually associated with stormy weather and high temperatures, both of which cause the density to be low."
This, however, is in contrast to the discus and the javelin.
These two events are affected by a force called lift, which is greater when the air density is high - and the density increases when the temperature falls.
Hubbard calculated that with all other factors being equal, a temperature drop of five percent results in a discus throw that is 38cm further.
Interestingly, Hubbard's calculations also showed that if the temperature drops, a thrower should alter the angle of his release by a small amount for optimum performance.
Not that he believes athletes could do this in the heat of competition.
"It is a question of how accurately they could control it," he said. "All these throwing events are very explosive and everything happens so rapidly and it comes with years of training to try to do it a certain way."
Reporting by Toby Davis; editing by Stephen Wood