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The butterfly (shortened to fly^[1]) is a swimming stroke swum on the chest, with both arms moving symmetrically, accompanied by the butterfly kick (also known as the "dolphin kick") along with the movement of the hips and chest.^[2][3] It is the newest swimming style swum in competition, first swum in the early 1930s and originating out of the breaststroke.^[4]

The butterfly stroke boasts a higher peak velocity than the front crawl, owing to the synchronous propulsion generated by the simultaneous pull/push of both arms and legs. However, due to the pronounced drop in speed during the recovery phase, it is marginally slower than the front crawl, especially over extended distances. Furthermore, the butterfly stroke demands a different level of physical exertion, contributing to its slower overall pace than the front crawl.

The earliest mention of the butterfly kick can be attributed to George Corsan, who, according to the International Swimming Hall of Fame introduced it by demonstration to David Armbruster in 1911. Corsan called it a "fishtail kick".^[5] A perhaps independent discovery of the fishtail kick was made by Volney Wilson some time before 1938, Wilson was a young physicist and swimmer who had analyzed the motions of swimming fish.^[6] Another discovery of the butterfly kick was made by Jack Sieg, who, while being watched by David Armbruster, went underwater, lay on his side and performed the butterfly kick in imitation of a fish. Armbruster was reminded of the time in 1911 when George Corsan had previously demonstrated it to him, and noticed the speed that Sieg was able to achieve. Armbruster got Sieg to demonstrate it on his front, and Sieg was even faster. Sieg was even able to beat the best flutter kickers at the University of Iowa using the fishtail kick.^[7] Initially, Corsan and Armbruster thought that the "fishtail kick" could be a possible alternative to the flutter kick in freestyle, but in 1933 it began to dawn on Armbruster that it would perhaps be more effective in breaststroke.^[8] Armbruster began to experiment with Sieg on using the butterfly kick in breaststroke and had excellent results. Presumably to reflect this, he also began referring to the previously named "fishtail kick" as the "dolphin breaststroke kick".^[8] He published his results in 1935 in an article called "The Dolphin Breast Stroke" in The Journal of Health and Physical Education^[7] where, due to its potential to be faster than the traditional whip kick, Armbruster was enthusiastic to have it legalized within the breaststroke rules, to "offer this new type of stroke for exploitation as a competitive racing-speed stroke".^[7] Armbruster followed this appeal with another longer article in the 1937 NCAA Swimming and Diving Guide called "The New Dolphin Breast Stroke on Trial", which further urged the NCAA to allow the kick in the rules for the extra speed.^[9] Unfortunately for Armbruster, this would not come to fruition, as in a FINA meeting in October 1938, the bureau unanimously agreed to prohibit up-and-down movements of the legs in the vertical plane.^[10]

Prior to 1935, the breaststroke rules from both FINA and the NCAA stated that the arms had to be simultaneously pushed forward on recovery.^[8] Since the rules did not state that the arm recovery had to be underwater, a few swimmers came up with the idea of an overarm recovery, which was initially called the "flying fish".^[11] The International Swimming Hall of Fame credits Sydney Cavill with creating the overarm recovery,^[12] but it is also possible that Erich Rademacher independently discovered it in 1927 Germany. Rademacher generally used the flying fish coming out of the turns,^[13] which was dubbed the "flying breaststroke turn" and at the end of the race, which was dubbed the "flying finish".^[14] Rademacher insisted that this did not break the rules, and continued to use it in the late 1920s and early 1930s in the US.^[8] Sources conflict as to whether he used it in the 1928 Olympics, with the International Swimming Hall of Fame saying he did^[13] but Francois Oppenheim's book "The History of Swimming" saying he didn't.^[15] In 1935, presumably for the removal of ambiguity, the NCAA changed the rule regarding the breaststroke recovery to use the word "moved", rather than "pushed".^[16] Other early adoptees of the "flying breaststroke turn" were Walter Spence and then Wallace Spence, who went on to dominate the breaststroke events using it.^[8] The next notable swimmer to further innovate on the butterfly arms was Henry Myers, who in early December 1933 used the flying fish for the whole of the breaststroke leg of a three-stroke medley head, winning against Wallace Spence in spectacular fashion.^[8] The reaction to this performance was mixed. Though the officials did not disqualify him, the Brooklyn Central YMCA's magazine wrote a "vitriolic article" that made disparaging remarks about "the sportsmanship of young Myers who observed the letter but not the spirit of the breaststroke rules".^[17] Furthermore, Wallace Spence withdrew from the finals of the event alleging that Myers was swimming freestyle and he should "learn how to swim breaststroke properly".^[18] Despite this, Myers was very proud of his creation, saying: "It [is] uninteresting to watch a breaststroke race, in time, the old breaststroke would have become as passe as the English sidestroke, as far as racing is concerned. A butterfly-breaststroke race is a very exciting race to watch. The splashing and violent arm-motion seems to be quite conducive to spectator enthusiasm."^[18] Myers interested two teammates of his: Lester Kaplan and Paul Friesel, in the overarm recovery. Kaplan and Friesel went on to set new personal bests in breaststroke by over 3 seconds using it.^[18] As the overarm recovery caught on more, a swimmer called John Herbert Higgins, in February 1936 at the 1936 National AAU Indoor Swimming Championships swam a time of 1:10.8 for the 100 metre breaststroke event, and became the first swimmer to set a world record using exclusively the flying fish.^[8] This technique of continuous flying fish would come to be known as "butterfly-breaststroke". Later at the 1936 US Olympic Trials, Higgins set a new 200 metre American Record of 2:44.1.^[19] In the 1936 Berlin Olympic 200m breaststroke finals, Higgins finished fourth while using the butterfly-breaststroke technique.^[20][21] Though the butterfly-breaststroke was gaining traction, it was almost prohibited as a style of breaststroke in an October 1938 FINA Executive Bureau meeting where it was "supported by four votes out of seven to prohibit the butterfly arm recovery in the breaststroke event. However, since alterations to the technical rules required a two-thirds majority before adoption, the motion was declared not carried".^[10] In the 1948 London Olympics, all but one of the finalists used the butterfly-breaststroke technique, and the only finalist who did not use it (Bob Bonte of the Netherlands) finished last.^[22] This dominant display of butterfly-breaststroke led to the idea of separating breaststroke into two breaststroke events, consisting of traditional breaststroke and butterfly-breaststroke.^[23] However, the IOC president Sigfrid Edstrom decided not to add any events to the 1952 Helsinki Olympics, due to the chaotic situation after World War II.^[8] Despite this, FINA did add a definition of the two styles of breaststroke to the rules which named traditional breaststroke style "A" and butterfly-breaststroke style "B". A consequence of this was competitors were not able to swim both of the styles in the same race.^[24]

By 1952, the argument for separating the "A" and "B" styles of breaststroke was gaining momentum,^[25] and hence in 1953, breaststroke was separated into two different events. Furthermore, while traditional breaststroke required the whip kick, the new butterfly stroke allowed the use of butterfly kick. Now the butterfly stroke was its own separate event.^[26] The butterfly stroke was first seen in the Olympics at the 1956 games, where the Men's 200 metre butterfly event was won by William Yorzyk, and the Women's 100 metre Butterfly event was won by Shelley Mann.^[27]

The butterfly technique with the dolphin kick consists of synchronous arm movement with a synchronous leg kick.^[28] Good technique is crucial to swim this style effectively. The wave-like body movement is also very significant in creating propulsion, as this is the key to easy synchronous over-water recovery and breathing.

The swimmer is face down at the start, arms out front, and legs extended behind.

The butterfly stroke has three major parts, the pull, the push, and the recovery. These can also be further subdivided. The arm movement starts very similarly to the breaststroke from the initial position. In the beginning, the hands sink a little bit with the palms facing outwards, and slightly down at shoulder width, then the hands move out to create a Y. This is called catching the water. The pull movement follows a semicircle with the elbow higher than the hand and the hand pointing towards the center of the body and downward to form the traditionally taught "keyhole".

The push propels the palm backwards through the water, starting beneath and ending at the side of the body. The swimmer only pushes the arms 1/3 of the way to the hips, making it easier to enter into the recovery and making the recovery shorter and making the breathing window shorter. The movement increases speed throughout the pull-push phase until the hand is the fastest at the end of the push. This step is called the release and is crucial for recovery. The speed at the end of the push is used to help with the recovery.

In the recovery, the arms are swung sideways across the water surface to the front, with the elbows straight. The arms should be swung forward from the end of the underwater movement; the extension of the triceps in combination with the butterfly kick will allow the arm to be brought forward quickly and relaxedly. In contrast to the front crawl recovery, the arm recovery is a ballistic shot, letting gravity and momentum do most of the work. The only way to lift the arms and the shoulders out of the water would be by dropping one's hips. Therefore, the recovery, at least the acceleration of the arms, is in no way relaxed ^{[clarification needed]}. It is important not to enter the water too early because this would generate extra resistance as the arms move forward against the swimming direction. However, during longer distances, this is not easy to avoid, and it is more important to avoid dropping one's hips. A high elbow recovery, akin to that observed in the front crawl, would prove detrimental to the efficiency of the butterfly stroke due to the resultant undulations and the diminution of momentum generated by the triceps extension. Limitations of shoulder movement in the human body make such a move unlikely. The hands should enter the water with a narrow V shape (at 11 and 1 o'clock, if viewed like a clock) with thumbs entering first and pinkies last.

The arms enter the water with the thumbs first at shoulder width. A wider entry loses movement in the next pull phase, and if the hands touch, it will waste energy. The cycle repeats with the pull phase. However, some swimmers prefer to touch in front as it assists them in grasping the water. As long as they can perform this action efficiently, they do not incur any disadvantage.

The legs move together using different muscles. The shoulders rise with a strong up and medium down kick, then lower with a strong down and up kick. A fluid undulation connects the motion.

The feet are pressed together to avoid loss of water pressure. The feet naturally point downwards, give thrust downwards, move up the feet, and press down the head.

There is no stipulation in competitive butterfly rules that a swimmer makes a fixed number of pulses in butterfly–the swimmer may kick as little or as much as they wish.^[29] While competitive rules allow such a choice, the typical method of swimming butterfly is with two kicks.

As butterfly originated as a variant of breaststroke, it would be performed with a breaststroke or whip kick by some swimmers. While breaststroke was separated from butterfly in 1953, the breaststroke kick in butterfly was not officially outlawed until 2001.^[30] However several Masters swimmers were upset with the change since they came from a time when butterfly was usually swum with a breaststroke kick.^[31] FINA was then convinced to allow a breaststroke kick in Masters swimming.^[32] Given the option, most swimmers use a dolphin-kicking action. However, there still is a small minority of swimmers who prefer the breaststroke kick for recreational swimming and even for competition.

There is only a short window for breathing in the butterfly. If this window is missed, swimming becomes very difficult. Optimally, a butterfly swimmer synchronizes the taking of breaths with the undulation of the body to simplify the breathing process; doing this well requires some attention to butterfly stroke technique. The breathing process begins during the stroke's underwater "press" portion. The body naturally rises toward the water's surface as the hands and forearms move underneath the chest. The swimmer can lift the head to fully break the surface with minimum effort. The swimmer breathes in through the mouth. The head goes back in the water after the arms come out of the water as they swing forward over the surface. If the head stays out too long, the recovery is hindered.

Normally, swimmers take a breath every other stroke, which can be maintained over long distances. Often, breathing every stroke slows the swimmer down. (At a certain level, a breathing stroke becomes just as fast as a nonbreathing stroke; therefore, highly experienced competitors, such as Michael Phelps, may breathe every stroke.) Elite swimmers practice breathing intervals such as the "two up, one down" approach. They breathe for two successive strokes, then keep their head in the water on the next stroke for easier breathing. Swimmers with good lung capacity might also breathe every 3rd stroke during sprints for the finish. Some swimmers can even hold their breaths for an entire race (assuming it is short). To swim with the best results, keeping one's head down when taking a breath is important. If the swimmer lifts their head too high, the swimmer's hips often drop, creating drag, thus slowing the swimmer down. The closer one's head is to the water; the better one swims the general technique used by swimmers.

The proper utilization of the core muscles, in conjunction with precise timing and fluid body movement, greatly facilitates the execution of the butterfly stroke. The body moves in a wave-like fashion, controlled by the core, and as the chest is pressed down, the hips go up, and the posterior breaks the water surface and transfers into a fluid kick. During the push phase, the chest goes up, and the hips are at their lowest position. In this style, the second pulse in the cycle is stronger than the first one, as the second one is more in flow with the body movement.

Although butterfly is very compatible with diving, the resulting reduction in wave drag does not lead to an overall drag reduction. In the modern style of the butterfly stroke, one does only a little vertical movement of the body.

Butterfly uses the regular start for swimming, if in a competition the swimmer would start off a diving block. After the start, a gliding phase follows underwater, followed by dolphin kicks swimming underwater. Swimming underwater reduces the drag from breaking the surface and is very economical. Rules allow for 15 m underwater swimming before the head breaks the surface and regular swimming begins.

The turn and finish requires the swimmer to touch the wall "with both hands separated and simultaneously".^[3] Both hands must simultaneously touch the wall during turns and finish while the swimmer remains swimming face down. The swimmer touches the wall with both hands while bending the elbows slightly. The bent elbows allow the swimmer to push themself away from the wall and turn sideways. One hand leaves the wall to be moved to the front underwater. At the same time, the legs are pulled closer and moved underneath the body towards the wall. The second hand leaves the wall to be moved to the front over water. It is commonly referred to as an "over/under turn" or an "open turn". The legs touch the wall, and the hands are at the front. The swimmer sinks underwater and lies on the breast, or nearly so. Then the swimmer pushes off the wall, keeping a streamlined position with the hands to the front. Like the start, the swimmer is allowed to swim 15 metres underwater before the head breaks the surface. Most swimmers dolphin kick after an initial gliding phase.

There are four styles of the butterfly stroke.

Two main styles of butterfly stroke seen today are: "arm pull up simultaneous with dolphin kick" and "arm pull down simultaneous with dolphin kick".^[33]

"Arm pull up simultaneous with dolphin kick": After the head goes underwater, both arms go underwater but are still higher than the head. After the first dolphin kick, pull both arms immediately with downward motion. While pulling, arms and legs are relaxed, and both knees and waist are slightly bent to prepare the dolphin kick. After the arms push the water backwards, pull the arms up simultaneously with a dolphin kick. In this style, the turning point from drowning to floating ^{[clarification needed]} is at the time of downward arm motion.

"Arm pull down simultaneous with dolphin kick": After the head goes underwater, both arms go underwater until lower than the head. After the first dolphin kick, raise both arms with relax. While rising arms, bend both knees and waist to send the body back to the surface and prepare a dolphin kick. Pull both arms downward while executing the dolphin kick. After this sequence, immediately push the water backward. In this style, a turning point from drowning to floating is at the time of the waist bend.

Two additional styles of butterfly stroke are similar to the two styles above but without a "second" dolphin kick.^[34] This allows the swimmer to conserve energy and be more relaxed.

As of the 1st January 2023, the FINA rules for the butterfly stroke state the following rules:^[3]

SW 8.1 From the beginning of the first arm stroke after the start and each turn, the body shall be kept on the breast. It is not permitted to roll onto the back at any time, except at the turn after the touch of the wall where it is permissible to turn in any manner as long as the body is on the breast when leaving the wall.

SW 8.2 Both arms shall be brought forward simultaneously over the water and brought backward simultaneously under the water throughout the race, subject to SW 8.5.

SW 8.3 All up and down movements of the legs must be simultaneous. The legs or the feet need not be on the same level, but they shall not alternate in relation to each other. A breaststroke kicking movement is not permitted.

SW 8.4 At each turn and at the finish of the race, the touch shall be made with both hands separated and simultaneously, at, above or below the water surface.

SW 8.5 At the start and at turns, a swimmer is permitted one or more leg kicks and one arm pull under the water, which must bring him to the surface. It shall be permissible for a swimmer to be completely submerged for a distance of not more than 15 metres after the start and after each turn. By that point, the head must have broken the surface. The swimmer must remain on the surface until the next turn or finish.

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