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.


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. (Illus­tra­tion by

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.

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.”




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