Aquatics

The world's top male freestyle swimmers can swim 50m in about 21 seconds, generating extraordinary speed and power. In backstroke, swimmers lie on their backs and use their arms to slide across the water's surface. In butterfly, the swimmers' arms move symmetrically, accompanied by a coordinated leg kick, evoking a flying butterfly. In breaststroke, the only stroke in which swimmers move their hands forward through the water after making a stroke, the key is to produce maximum thrust and minimum drag.

Olympic athletes must hone every detail of their technique, including the diving start, the timing of kicks and turns and the angles through which they move their arms.

Elite swimmers must also pay attention to pacing as a tactic. For example, a swimmer might advance to the final round by swimming quickly during the first half of a preliminary race to establish a dominant time. In the final, that same swimmer might hold back during the first half so that he or she can put in a burst of speed later. Such tactics are an essential part of the sport's appeal.

In individual medley events, a single swimmer competes using all four strokes in the following order: butterfly, backstroke, breaststroke, freestyle. Since each swimmer has particular strokes in which he or she excels, swimmers' relative positions on the leaderboard sometimes change as the stroke changes. These races are thrilling and fun to watch.

Medley relays differ from individual medleys in that the following stroke order is used: backstroke, breaststroke, butterfly, freestyle. Teams usually include the top-performing athlete for each stroke, creating all-star match-ups. In the 4×100m medley relay (mixed), which is a new event, teams consisting of two men and two women can choose who swims each stroke. Men and women may swim against each other at the same time, adding to the excitement.

In team relay events, it's important to shorten the changeover time – the time from one swimmer touching the wall to the next swimmer's legs leaving the starting platform. A poorly executed changeover can cause a team to lose their position in the race, or even to be disqualified if the outgoing swimmer sets off too early.

Constant evolution of technique is leading to ever greater levels of performance. Seven world records were set during the finals at London 2012 and Rio 2016.

The 100m breaststroke typifies this trend. At Beijing 2008, Kosuke Kitajima (Japan) became the first in the world to swim faster than 58 seconds. Kitajima's technique included a streamlined body position to minimize water resistance, lowering the position of his head after each breath for greater efficiency and a reduced number of strokes. This became the dominant style worldwide.

However, at Rio 2016, Adam Peaty (Great Britain) brought major change by achieving extraordinary speed through dynamic, fast-paced swimming that combined a large number of strokes with a powerful kicking movement. Peaty set a world record time of 57:13 and won the gold medal.

New techniques in freestyle, backstroke and butterfly are also emerging with each Olympic Games and driving ever faster times.

Recent years have seen more swimmers competing in multiple events. Michael Phelps (USA) and Katinka Hosszú (Hungary) both excel at the individual medley. At the same time, single-event specialists such as Peaty remain. This increasing division of the sport into multi-event swimmers and specialists promises to bring further innovation in the coming years.

In the past, one-and-a-half to two-and-a-half rotations from take-off to entry were standard for the three-metre springboard event. However, modern divers utilise the bounce provided by the springboard to leap even higher into the air, allowing them to complete three-and-a-half to four-and-a-half rotations and sometimes three twists before reaching the water.

Since platform divers don't benefit from any bounce from the board or height from their take-off, they must complete a series of small, fast revolutions before they enter the water. Consequently, top-ranked divers in the platform discipline tend to be shorter and more powerful, whereas springboard divers may be taller and leaner.

In both springboard and platform events, men and women seek to gain the highest total score for six and five dives respectively. The leaderboard can change dramatically as a diver who leads right up to the final dive may find themselves pushed out of top spot.

The platform event at the Beijing 2008 Games provided an example of this sort of drama when an Australian diver took first place only after the sixth and final dive, displacing a Chinese rival who had led throughout but finished with an imperfect entry. Each dive may only last an instant but the competition is thrilling to the very end.

The USA dominated Diving from as far back as the St Louis 1904 Games, but China began to emerge as a powerhouse from the Los Angeles 1984 Games for women and the Barcelona 1992 Games for men. At Beijing 2008, the country dominated seven out of the eight medal events. In an era when dives with two-and-a-half to three-and-a-half rotations had become the standard, Chinese divers pushed the level of difficulty to four-and-a-half rotations with almost no splash on entry.

The Tokyo 2020 Diving competition promises to be highly competitive as the USA has been regaining some of its former strength, along with Italy, Great Britain and Australia. To illustrate the level of competition across the world, Great Britain, the USA and China captured the gold, silver and bronze medals respectively in the men's three-metre synchronised diving competition at Rio 2016.

Divers will continue to produce ever higher and faster dives with more rotations and more beautiful water entries, adding to the unique spectacle offered by this engaging and exciting sport.

A game consists of four quarters, each of which lasts eight minutes. Other than the goalkeeper, players may only handle the ball with one hand, and must shoot for goal within 30 seconds of starting an attack; if they fail to do so, possession passes to the other team. A key is for teams to take advantage of the full 30 seconds of possession to get the ball to the centre position and, from there, to attack the goal. In this most physically demanding of sports, rolling substitutions are allowed.

Competitors can move freely within the playing area, which measures 30m by 20m and is at least 2m deep. They stay upright by treading water and lift their upper body high above the surface to pass and shoot. The ball can travel at speeds of up 70 kilometres per hour as players leap to make a shot. These dynamic and thrilling manoeuvres, together with brilliant passing and counterattacking to break down opponents' defensive formations, make Water Polo exciting both to play and to watch.

Water Polo is a contact sport and a characteristic feature is the large number of fouls. These include ‘ordinary fouls’, where possession passes to the opposition, and ‘personal fouls’ which can lead to a penalty shot or a player spending 20 seconds in an exclusion zone next to their own team's goal. This situation requires the short-handed team to vary their tactics to avoid conceding a goal.

Physical contact with a player not in possession of the ball is called as a foul, but contact is permitted with players in possession. Also, much contact takes place under water, largely out of the view of the referee, where players will seek to gain an advantage over their opponents.

Due in part to the fact that Water Polo originated in England and first became popular in Europe, men's teams from this continent have captured most of its Olympic medals. Hungary, which has a professional league, captured a total of nine gold medals from Sydney 2000 through to Beijing 2008. The countries of the former Yugoslavia, where water polo is extremely popular, consistently vie with each other for dominance, with Croatia winning gold at London 2012 and Serbia doing so at Rio 2016. Italy, which has won a total of three gold, two silver and three bronze medals, is also a perennial contender.

Women's Water Polo has been an Olympic sport since Sydney 2000, where Australia won gold following an effort to develop a team for its host Games. The USA, which introduced an Olympic development programme to discover new players, won back-to-back gold medals at the London 2012 and Rio 2016 Games. In addition to these two countries, the Netherlands won gold at Beijing 2008 and Italy at Athens 2004.

Water Polo's growth will see new powers emerging, including countries that can field fast offences that draw on their athletes' swimming skills and others such as Japan, with a pass line defence counterattack strategy. New developments include an International Swimming Federation World League that provides chances for countries to compete at a variety of levels and for coaches and referees to hone their skills worldwide.

Possible rule changes will reduce the size of the playing area and time in possession, in order to increase the pace of the game without damaging its traditional appeal as an intensely physical and demanding sport.

Artistic Swimming became an Olympic discipline at the Los Angeles 1984 Games. Since then, the sport's regulations have changed a number of times. Initially, the sport consisted of two events: a solo routine (one performer) and a duet (two). A team event (with eight athletes) was held at the Atlanta 1996 Games, then the duet returned at Sydney 2000. Subsequent Games have featured both duet and team events.

Performers are scored by two panels, each comprising five to seven judges. In the technical routine, one panel of judges scores athletes' technical execution, while the other scores their choreography, use of music, synchronisation, difficulty and presentation.

In the free routine, one panel of judges scores athletes' execution, synchronisation and difficulty, while the other scores their choreography, musical interpretation and presentation.

More time is allotted to the free routine performance, which offers greater freedom in terms of choreography. A routine must nevertheless show a high level of expressive power and artistry, arguably making it more difficult than the technical routine.

Competitors use techniques such as sculling, in which they move their hands through the water to hold position or move; and an eggbeater kick, in which they do the same with their legs to propel themselves up out of the water. They develop an impressive amount of power in that moment, rising waist-high above the surface. Another technique enables swimmers to turn upside down underwater so that only the lower half of their bodies is visible.

A key requirement for athletes is to be able to execute moves while their head is underwater for an extended period of time. Some athletes can perform leg techniques for more than 30 seconds while holding their breath.

However, performances that are merely technically rigorous may result in a lower score if they do not include carefully executed synchronisation. It is also likely that fine detail, such as the extended position of fingertips and toes, will become an even more important element in future.

Artistic Swimming emerged from ornamental swimming and theatrical water ballets in the late 19th and early 20th centuries, also drawing on techniques from life saving and swimming. The first contests were for men, but Artistic Swimming became more associated with women after Australian Annette Kellerman performed in a glass tank at the New York Hippodrome in 1907.

When first staged at the Los Angeles 1984 Games, the sport involved fewer rigorous techniques and emphasised choreography designed to highlight supple, attractive movements. By Sydney 2000, dynamic lifts and jumps began to be included, followed soon after by twisting movements - all of which have become a key attraction.

Every aspect of performance is now considered: even the splashing of the water is brought under control when swimmers strike or trace circles on the surface with their hands.

Changes in choreographic style have seen different countries gain prominence. The United States and Canada reigned supreme when the discipline was first included in the Olympic programme, but European countries including Russia, Spain, France and Ukraine are now highly competitive. Asian athletes have also been successful, with China recently joining Japan as a leading nation.