Olympic swimmers need to ignore their intuition when it comes to the best ways to propel themselves through the water to glory next week.
Scientists have made several discoveries about the mechanics of swimming since the last summer Games four years ago, refuting some seemingly obvious assumptions.
Take freestyle swimming. It seems that forming each hand into a tight cup maximizes the force it exerts on the water, while keeping the fingers apart produces a wimpier stroke.
Not so, explains Adrian Bejan, professor of mechanical engineering at Duke University in Durham, N.C. Inspired by animal locomotion, he and colleagues analyzed the force that fingers and toes generate when pulled through the water.
When the digits are separated by between one-fifth and two-fifths of their diameter, the force exerted is 50 percent greater than when they are pressed together with no spaces in between, Bejan and his team will report in an upcoming issue of the Journal of Theoretical Biology.
The reason involves something called a boundary layer. When fingers move through water, viscosity causes a thin stream of fluid to stick to (and move with) them rather than flow between them. As a result, slightly separating the fingers creates a wider surface than keeping them close together. Just as a wider oar pulls with greater force than a narrower one, so does a wider hand.
The greater force pulls the swimmer both forward and, on the down stroke, upward. The higher the body is above the water, the faster forward it can go, since air resistance is less than water resistance.
In addition, the higher you are, the faster forward you fall. Since the swimmer's fall from above the water's surface has a horizontal component as well as a vertical one, the greater height achieved with a more powerful stroke translates into greater forward velocity.
Paddles or propellers?
When it comes to arm positioning, coaches and swimmers spent decades debating the best technique for freestyle and backstroke. Conventional wisdom views the propeller-like "sculling" motion as superior, but new studies show that a paddle-like "deep catch" stroke is the quicker route to the medals podium.
In the sculling stroke, the elbow of a freestyle swimmer is raised high as the hand enters the water as vertically as possible. The arm moves inward toward the body and then outward, making an S shape.
"The arm stays bent, the hand enters the water in line with the shoulder, and after moving to the center of the body it moves out again," explained Rajat Mittal, professor of mechanical engineering at Johns Hopkins University in Baltimore.
Whether in a boat or a plane, a propeller also moves perpendicular to the direction of motion - that is, right and left. Perpendicular motion creates a lift force that pushes the boat forward.
The sculling stroke was popularized in the 1960s by the late James "Doc" Counsilman, an American Olympic and college swimming coach who was among the first to apply science to the mechanics of swimming. Counsilman reasoned that just as a propeller-driven boat can move faster than a paddle boat by generating lift, so the sculling stroke can produce lift that is more effective than the paddle stroke, in which the swimmer simply drives the hand into the water and pulls it back as hard as possible.
To see whether the sculling stroke deserved its popularity, USA Swimming provided Mittal and his graduate student, Alfred von Loebbecke, with underwater videos of America's elite swimmers using either the sculling or paddle stroke. The scientists used animation software to scan the swimmers' arms, from which they created computer simulations.
From the simulations, the scientists determined the lift forces and drag forces the swimmers' strokes produced. They describe the findings in a paper scheduled for publication in the Journal of Biomechanical Engineering.
Bottom line? "The deep catch stroke is more efficient and effective than the sculling stroke," said Mittal. In the backstroke, the deep catch stroke produced 18 percent more thrust than the sculling stroke. How much extra speed a swimmer can get from the extra thrust depends on factors such as a swimmer's shape and technique.
That is not to say the sculling stroke should be retired. Long-distance swimmers prefer it because it's easier to maintain for long periods.
Sprinters, however, generally favor the deep catch stroke, as can be seen in American Michael Phelps, who won a record eight gold medals in Beijing. He employs a modified version, keeping his elbow slightly bent as it pulls against the water. The straighter the arm, the greater the shoulder strength required to drag it back through the water. A slight elbow bend conserves energy for a last-yards power sprint.
Mittal also analyzed the dolphin kick, performed at the start and after each turn in the freestyle, backstroke, butterfly and breast stroke.
Contrary to expectations that the largest leg muscles would be the source of the kick's power, it turns out that 90 percent of the thrust in a dolphin kick comes from the ankles and feet. "The more you can make the lower part of your leg floppy like a dolphin fluke, the better your thrust," Mittal said.