Paddles or Propellers? Delineating the Perfect Swim Stroke

Should a swimmer's arms serve as paddles or propellers? That question, abstruse as it might seem, underlies a long-running controversy in swimming about the best, most efficient technique for the freestyle and the backstroke. It also prompted a new study from a group of scientists at Johns Hopkins University that, in seemingly answering the question, is likely to provoke even more debate.

The concern about how best to position and move the arm during the freestyle stroke (also known as the front crawl) and its inverse, the backstroke, first gained prominence back in the 1960s, when James E. Counsilman, the famed Indiana University men's swimming coach known as Doc, decided to apply scientific principles of propulsion and fluid dynamics to swim techniques.

The physics of swimming are simple enough. To move through the water, you must generate thrust. To do so, you can use dragging or lifting forces. Drag is created by, unsurprisingly, dragging back against the water and, in the process, pushing an object, like the swimmer's body, forward.

Lift, on the other hand, is created mainly by the flow of fluid around an object moving at an angle through the water. The fluid flows faster around the more curved side of the object, lifting and thrusting it forward. Ship propellers work on this principle.

But until Doc Counsilman weighed in, it was widely believed that swimming, for humans, involved primarily drag forces. You pulled against the water, like someone paddling a canoe, your arm remaining straight, palm perpendicular to the body. This stroke technique is often called a "deep catch" style of swimming, since you pull long and deep against the water.

Coach Counsilman was convinced, however, that lift could and should provide a majority of the propulsion for human swimmers, and that the way to generate lift was to scull, or move the stroking arm through an S-curve underwater.

In his revised version of the freestyle, the arm, bent as it breaks the surface, pulls back against the water at first, as in a paddling stroke. But then the arm starts turning sideways in a gentle curve as it begins to trace an S shape, the thumb heading up as the palm turns parallel to the body. The arm reverses that motion to traverse a full S shape before emerging from the water.

Fluids would flow swiftly around the hand as it sliced through the water and, Coach Counsilman contended, create more lift than the deep-catch stroke.

[In the deep-catch stroke, illustrated at top, the hand pulls long and deep through the water. In the scull, below, the hand traces an S shape]

Coach Counsilman instituted this new stroke technique for his swimmers, first at Indiana University and later as head coach of the United States Olympic team. His swimmers, who included Mark Spitz, won more than 20 Olympic medals and 23 Big Ten Conference titles.

In the years since, sculling during the freestyle stroke and backstroke became commonplace among elite and recreational swimmers.

But many coaches continued to question whether lift, generated by sculling, was really the fastest, most efficient way for swimmers to reach the wall.

So the Johns Hopkins scientists, who before the 2008 Summer Olympics had studied how best to perform the butterfly stroke (their conclusion: have extremely flexible ankles and, if possible, big feet), decided now to put the two strokes to the test in a series of complex computer simulations.

They began by creating a virtual animated arm, using laser scans and motion-capture videos from Olympic-caliber swimmers. "We decided to separate the arm from the rest of the body so the we could study, in isolation, the underwater flow dynamics" around a swimmer's arm during the freestyle stroke or backstroke, says Rajat Mittal, a professor of mechanical engineering at Johns Hopkins and a devoted recreational swimmer, who oversaw the study.

They then gathered underwater videos of elite swimmers, supplied by USA Swimming, which they categorized as displaying either a sculling or a deep-catch stroke.

The scientists ran their animated arm through multiple simulations of each stroke, requiring thousands of hours of computer time.

The result was "a bit of a surprise," Dr. Mittal says. It turned out that lift was, as Doc Counsilman had maintained, important for efficient, and therefore fast, stroking. In all of the scientists' simulations, lift provided a majority of the propulsive force.

But sculling did not supply much lift. In fact, it impeded both lift and drag. "Our shoulders won't twist all the way around," Dr. Mittal says, meaning our arms won't lever about as ship propellers do, and the amount of lift we can create by sculling is small.

The better choice for human propulsion, he says, was the paddlelike deep-catch stroke, which actually produced more lift than sculling, along with a hefty dose of drag.

"All things being equal, our data show that the deep-catch stroke is far more effective," Dr. Mittal says.

Of course, races are not won or lost by disembodied arms, and as Dr. Mittal points out, "all things are not equal, most of the time." An effective deep-catch stroke requires considerable shoulder strength, which many swimmers lack, making a sculling-based stroke easier for them, at least until they develop robustly muscled shoulders.
"How you roll your body in the water with each stroke will also matter," he says, as will overall fitness. "Sculling is less fatiguing," so less-fit swimmers may opt to scull, he says.

But for fit, powerful swimmers, or those who aspire to become such, "my advice would be to use the deep-catch stroke," he says.

"Anecdotally, we've been told that more and more coaches are moving to the deep-catch," he continues, and his group's findings suggest that for most swimmers, whether elite or recreational, "that is the way to go.

Paddle vs. Propeller: Which Olympic Swimming Stroke is Superior?

stroke on my friends
Johnny Boy

Be A Cheetah (not a "cheater"!)

Back in the 1960s, researchers in Africa clocked the wild cheetah as it ran and determined that at full gallop, a cheetah reached a top speed of about 65 miles per hour, making it easily the world's fastest land mammal. No other quadruped or biped comes close. Galloping quarter horses top out around 47 miles per hour, while sluggish humans, in the person of the world record 100-meter sprinter Usain Bolt, have attained a top speed of less than 28 miles per hour. Even the bullet-train like greyhound, similar in build and running style to the cheetah, doesn't surpass 40 miles per hour.

So what is it about the cheetah and its particular physiology or running form that allows it to set such a blazing pace? And can a better understanding of cheetah biomechanics help humans to move faster?

Those were the questions that motivated a group of scientists at the Structure and Motion Laboratory at the University of London, who decided to compare the cheetah with one of its near rivals in speed, the greyhound.

"The two animals are quite alike in terms of body mass and running form," says Alan H. Wilson, a professor at the Royal Veterinary College at the University of London, who led the study, which was published in The Journal of Experimental Biology.

Both animals employ a running form known as the rotary gallop. Their legs churn in a circular motion, the animal's back bowing and its hind legs reaching almost past its ears at full stride. (Tongues tend to loll, too, but there's no indication that this attribute affects speed.)

"Up to a speed of about 40 miles per hour, there's very little difference," Dr. Wilson says. "But what happens after that," when the cheetah finds another gear and accelerates to 65 miles per hour, "is something we'd like to understand. We believe it can help us to better understand the determinants and limits of speed itself."

But closely studying cheetahs in the wild is logistically challenging, especially if you want exact measurements of running force and stride. So the researchers turned to captive cheetah populations at a zoo in Dunstable, England, and a sanctuary in Pretoria, South Africa. The animals were extensively measured and filmed.

Then some of the English cheetahs were taken to the performance lab and encouraged to chase a chicken-meat lure along a 90-meter track dotted with force plates to chart their strides. Meanwhile, high-speed cameras recorded their every movement from multiple angles.

The researchers repeated the experiment with trained racing greyhounds, then compared the two animals' pace and form.

The first thing they noted was that captive cheetahs are relatively slow, compared with their wild brethren. The galloping zoo-bred cheetahs topped out at a little less than 40 miles per hour, slightly lower than the top speed for the greyhounds.

"That finding was not really unexpected," Dr. Wilson says. Cheetahs that live in zoos do not have to feed themselves. They have less motivation to run hard, even when a chicken lure is waggled enticingly in front of them.

"They also don't necessarily learn to gallop as fast," Dr. Wilson says. "There is almost certainly some amount of speed that depends on learning" to be swift enough to bring down prey.

Dr. Wilson is now collecting data on wild cheetahs, but even in the zoo-bred animals, there were hints of their capabilities. When the cheetahs "felt like it," Dr. Wilson says, their leg turnover rate spurted and their pace dramatically increased. They began bringing their legs around faster and faster, their strides lengthening, even as the frequency of their strides increased.

The greyhounds, on the other hand, maintained a fairly even stride frequency throughout their entire run.

The cheetahs also hit the force plates differently from the greyhounds, their paws remaining on the ground slightly longer -- an action that presumably allows the legs to absorb more of the forces generated by the pounding stride.

"One of the limits to speed is that, at some point, you can generate more force than the muscles can withstand," Dr. Wilson says. Striking the ground with such shattering oomph can cause muscles to shred. The cheetahs reduced this risk by letting their paws linger a fraction of a second longer on the ground than the greyhounds did.

The lessons for human runners are somewhat abstract, since we have only two legs and, with rare exceptions, cannot curl them up past our ears, as cheetahs and greyhounds do. "The cheetah's back functions as an extension of its hind legs," Dr. Wilson points out, its spine coiling and extending with each stride, as ours cannot.

But there are tips we can glean from the cheetah. The speed with which a creature brings its leg back around into position appears to be one of the main determinants of speed, Dr. Wilson says. The faster you reposition the leg, the faster you'll move.

But swift leg turnover requires power. "Compared to the greyhound, the cheetah has bulky upper legs," Dr. Wilson says. Its powerful thigh muscles allow its legs to pump more rapidly than the spindly greyhound's can.

So strengthen your thighs.

And perhaps invest in lightweight racing shoes. "Having less weight in the lower portion of the leg aids in swift repositioning" of the limb, Dr. Wilson says.

Finally, while a dangling lure is optional, being hungry, Dr. Wilson says, at least metaphorically, "probably helps quite a bit."

So, the message? Run fast, but beware the "cheetah"...

Johnny Boy