An excerpt from Popular Mechanics article from Dec 18, 2009:
For all it demands in the way of cardiovascular fitness and sheer strength, swimming is really the ultimate technique sport. “Why are records being broken?” asks George Washington University professor Rajat Mittal, a leading researcher in hydrodynamics. “Swimming is the most technical sport.”
What makes a fast stroke fast?
In a word, efficiency. When researchers Rick Sharp and Jane Cappaert of the International Center for Aquatic Research studied the 1992 Olympics, they concluded that swimmers who made the finals actually had a power output that was 16 percent lower than those they beat. Russian champion Alexander Popov was estimated to be using 25 to 40 percent less energy than his rivals. The key to victory, Mittal explains, is not the athlete’s pure power output (as is the case in sports like track and cycling) but his efficiency. “It all comes down to how well you can transfer energy from your body to a complex fluid medium,” he explains.