This summer's Olympic games in London feature 14 different freestyle swimming competitions, by far the most races for any type of stroke. The world's elite swimmers can traverse a 50-meter pool in 22 to 26 seconds, yet they are divided over which of two variations of the stroke are more effective: the more powerful "deep catch" approach or the more streamlined "scull." And the physics behind the debate is fascinating.
In the deep catch approach, a swimmer puts his or her arm straight forward, then down as deep as possible into the water, and pushes that arm back as hard as possible, keeping the palms perpendicular to the direction the swimmer wants to move. In sculling, swimmers reach out but then bend their elbow, keeping it high in the water as their lower arm bends back past their body in an S-shaped pattern.
Is one stroke better than the other? The answer depends on the amount of power generated compared with the amount of drag caused by executing a particular stroke.
A team of researchers led by Rajat Mittal, vice chair of the mechanical engineering department at Johns Hopkins University, has used high-resolution video and underwater images to compare the two strokes. As seen in the video below, Mittal concludes that the propeller-like deep catch is more effective and more efficient than the sculling stroke.
Former Olympic swimmer Gary Hall, Sr., disagrees however, claiming that "drag trumps power" in the pool. In the video below, the two-time silver medalist explains, "The speed we can generate is directly proportionate to the power that we generate, but it's inversely proportionate to the frontal drag that we create during our swimming."
When looking at the strokes actual Olympic competitors use, the bottom line seems to be that the "best" stroke depends on the race distance and the athlete's preference. The straight-armed deep-catch approach seems to work best for sprinting, whereas the curved-arm scull approach saves energy and creates less drag, helping some swimmers in 100-meter races and many swimmers at longer distances. Of course, individual comfort for each swimmer has to be taken into account. Several years ago superstar Michael Phelps experimented with deep-catch to better compete in 100-meter races, but he later said he wasn't comfortable with the style and was disappointed with his performance using it.
Image courtesy of Chris 73, via Wikimedia Commons