A gruelling mix of stamina and speed, triathlon is one of the newer sports in the Olympic Games.
The discipline combines swimming, cycling and running and has its modern roots in the United States in the 1970s.
Its rapid growth as a mass participation event helped triathlon make its Games debut at Sydney 2000.
That trend has continued with races now established from Hamburg to Hawaii - home to the famous Ironman Triathlon - and everywhere in between.
The exploits of double Olympic champion Alistair Brownlee and his younger brother Jonny, and Rio 2016 gold medallist Gwen Jorgensen, have attracted more people to a sport which remains firmly on the way up.
The origins of triathlon
The earliest recorded triathlon took place just east of Paris in 1901 with the "sportsmen of the time" completing a run, cycle and canoe.
From 1920, the canoe segment was replaced by a swim with a newspaper listing the annual event as comprising a 3km run, 12km bike ride and a crossing of the River Marne.
While other 'Les Trois Sports' events were staged in Marseille and La Rochelle before World War II, the sport only garnered minority interest.
Two years after Shorter's success, two members of San Diego Track Club - Jack Johnstone and Don Shanahan - staged the Mission Bay Triathlon on 25 September 1974 with 46 people paying one dollar apiece to take part.
Johnstone, who had competed in run-swim duathlons, wanted to put on his own event with Shanahan keen to add a cycling element.
Neither man had previously heard of the triathlon events held in France earlier in the century.
The race started with a three-mile (4.8km) run on Fiesta Island followed by a five-mile (8km) bike ride on the mainland around SeaWorld San Diego.
Then followed a swim-run hybrid comprising approximately 500 yards (460m) swimming and another three miles (4.8km) running with the competitors having to crawl up a steep bank to the finish.
It quickly became an annual event with other triathlons starting to spring up across America.
The birth of Ironman
Two of the 46 competitors at the first and second Mission Bay Triathlons were U.S. Navy Commander John Collins and his wife Judy.
After moving from California to Hawaii's main island O'ahu in 1975, they were involved in a sprint run-swim event before having the idea of creating something for endurance specialists.
In October 1977, the Collins announced the first Around The Island Triathlon to be held the following February comprising three existing competitions - the Waikiki Rough Water Swim, the Around-Oahu Bike Race and the Honolulu Marathon - but taking place one after the other.
John Collins famously said, "Whoever finishes first, we'll call him the Iron Man."
The Ironman tag stuck with a line from the instructions to competitors also living on to this day:
"Swim 2.4 miles! Bike 112 miles! Run 26.2 miles! Brag for the rest of your life."
Just 15 took part in the first event on 18 February 1978 with Lyn Lemaire becoming the first female competitor the following year.
The Collins then handed control of the race to Hank Grundman and Valerie Silk with the latter taking full ownership following their divorce in 1981.
That year, Silk moved what had become the Ironman World Championship to the less built-up Hawaii Island with the February 1982 race putting the sport on the map.
California college student Julie Moss looked set for women's victory but collapsed due to dehydration in the closing stages.
Moss was determined to finish and crawled over the line with millions around the world seeing her brave and dramatic effort.
That sparked further interest in the race with organisers having to use a lottery to allocate the 1,000 available places from then on.
Silk sold Ironman in 1990 to Dr. James P. Gills who established the World Triathlon Corporation (WTC) which set up the first events outside of the Ironman World Championship.
There are now around 40 Ironman events worldwide plus many more Ironman 70.3 (half Ironman) races since they were launched in 2001.
The growth of Triathlon in Europe
The success of the Ironman movement prompted interest elsewhere with the first European triathlons being held in 1980.
The Nice Triathlon in November 1982 - switched from Monaco due to the death of Princess Grace - was a success despite a number of swimmers requiring hospital treatment due to water temperatures of 14 degrees Celsius in the Mediterranean.
The inaugural Nice race comprised 1500m swimming, 100km cycling, and a marathon run of 42.2km with the swimming and cycling differing from the respective Ironman distances of 3.9km and 180km.
That definitely favoured the runners with the distances changed the following year to 3000m swim, 120km cycling and 32km running.
The European Triathlon Union was founded in 1984 and became one of a number of continental triathlon federations which formed the umbrella International Triathlon Union (ITU) in 1989.
Led by Canadian Les McDonald, the main objective of the ITU was a place in the Olympic Games with then IOC President Juan Antonio Samaranch already keen to include the sport.
In August 1989, the first Triathlon World Championship was held in Avignon, France over the set 'Olympic' distance of a 1500m swim, 40km cycle and 10km run.
The ITU continued to oversee triathlons over a range of distances and set up the World Cup Series in 1991.
And in 1994, the IOC added triathlon to the Olympic Programme with the sport making its debut at Sydney 2000.
A day later, Canada's Simon Whitfield took the men's gold ahead of Germany's Stephan Vuckovic and Czech athlete Jan Rehula.
The Olympic Years
Triathlon's establishment of an Olympic sport saw it become even more of a global phenomenon.
Launched in 1997, the London Triathlon has long been the world's biggest with more than 12,000 participants over a number of distances.
The Noosa Triathlon in Queensland, Australia is the most popular for a single (Olympic) distance.
After some protracted disagreements at the start of the 21st century, the ITU and WTC have started to cooperate on anti-doping issues and devise rules together on set distances.
Olympic gold and victory at the Ironman World Championship are the two most coveted honours a triathlete can win, but they make very different demands on an athlete with the full Ironman four times as long as the Olympic distance.
As a result, triathletes tend to focus on Olympic competition first before building up stamina for the test of the Ironman.
Sydney silver medallist Michellie Jones did so successfully, finishing runner-up in Ironman in 2005 before taking gold a year later.
He retained his world title 12 months later before completing his third victory in 2019.
Some athletes choose to take another path entirely.
Reigning Olympic champion Gwen Jorgensen took time out after Rio to have a baby before quitting triathlon for the marathon.
After undergoing a heel operation, the American announced that she will try and qualify for Tokyo 2020 in the 10,000m on the track.
After that, she will look to run the marathon at the Paris 2024 Olympic Games.
Spirig took silver behind Jorgensen in Rio and is weighing up whether to compete at a fifth Olympic Games after Tokyo 2020 was postponed by a year.
She will be 39 if she lines up in the Japanese capital.
The current world star of triathlon is Britain's Alistair Brownlee who took gold on home soil at London 2012 before retaining his crown in Rio.
His younger brother Jonny took bronze in London 2012 and silver four years later, although the pair gained as much attention for the finish of the 2016 World Cup Final on the Mexican island of Cozumel a month after the Rio Games.
Jonny was leading the race inside the final 600 metres but started to wilt in the heat before grinding to a halt.
South Africa's Henrik Schoeman passed him to take the win but Alistair stopped to help his sibling and practically carried him through the closing stages before pushing him over the line for second place.
While Jonny is fully focused on Tokyo 2020 - and perhaps winning two medals with the mixed team relay event making its debut - Alistair has not yet committed to bidding for a hat-trick of golds.
The 32-year-old switched to training for the Ironman after Rio and finished 21st behind Jan Frodeno in last year's Ironman World Championship.
He bounced back to break the course record in Western Australia six weeks later but has since hinted that he would defend his Olympic title.
How to get started in triathlon
Triathlon is essentially three sports in one with the bicycle being the most expensive piece of equipment.
While any road bike will do for a beginner, professional athletes will use a specialised tri bike with aerodynamic handlebars like those seen in time trials.
Also essential for the cycle are a helmet and water bottle.
The opening swim takes place in open water so a wetsuit is obligatory in cold conditions but forbidden in international competitions when the water temperature is above 20 degrees Celsius (22 degrees for long-distance events).
Normal surfing wetsuits tend to be too heavy and restrictive around the shoulders for smooth swimming so thinner wetsuits have been designed specifically for triathlons.
Swim caps and googles are often provided by the event organiser.
High-level athletes will wear a tri suit underneath the wetsuit which they keep on throughout the race.
On the transition from the swim to the bike ride, athletes discard the wetsuit and put on tri shoes which are similar to cycling shoes although these may have holes in the bottom to enable water (from the swim) to drain away.
Those shoes are then swapped for standard running shoes on the final transition although these may again have small holes to get rid of water.
Tying laces takes time so most professionals will have some sort of elastic lace system and socks tend only to be worn in long-distance events where comfort outweighs the time lost to put them on.
While the distances may differ, the essential rules of triathlon remain the same.
No outside assistance is permitted and the rules were extended ahead of the 2018 season to outlaw assistance from fellow athletes such as Alistair Brownlee helping brother Jonny over the line in Cozumel.
No flippers or propulsion aids are allowed in the swim and there are strict rules during transition phases.
Helmets must be in place before the bike has been mounted and only removed after being dismounted with disqualification the penalty for non-compliance.
Also, the bike must only be mounted after crossing the transition line with Jonny Brownlee incurring a 15-second penalty for riding too soon in the London 2012 triathlon.
Other time penalties can be awarded for unsportsmanlike conduct, littering on course and slipstreaming (drafting) rivals in the majority of amateur races where slipstreaming is not permitted.
And as British triathletes Georgia Taylor-Brown and Jess Learmonth found to their cost in the Tokyo 2020 test event last August, trying to cross the line together for a "contrived tie" will result in disqualification.
While triathlon is traditionally an individual event, relay races have become popular in the last decade.
So much so that an Olympic mixed team relay event will be held for the first time at Tokyo 2020.
This consists of two women and two men each completing a super-sprint distance - 300m swim, 8km bike ride and 2km run - before tagging their team-mate.
Mixed team, mixed NOC relays have featured at all three Youth Olympic Games and provided fast and furious action.
Your first triathlon
Triathlon clubs are easy to find nowadays and they can help with training as well as entering races.
The recommendation for a first race would be over the novice distance of a 400m swim, 10km bike ride and 2.5km run.
Unless you are in a warm climate, a wetsuit will almost certainly be required for the swim.
For most first-timers, transitioning from the water to the bike will be a novel experience and it might be worth simply getting dry before changing into running gear, socks and trainers which you can wear for both the bike ride and the run.
Transitioning from cycling to running can take some getting used to and it is a good idea to practise that, even in the gym moving from an exercise bike to a treadmill, to get used to the 'jelly legs' sensation.
The other distance worth considering for an early triathlon attempt is the sprint distance of a 750m swim, 20km cycle and 5km run which is half the Olympic distance.