By Ian Ransom

MELBOURNE (Reuters) - Taekwondo's battle for relevance at the Olympics continues at the Rio de Janeiro Games, with tournament organizers introducing colored pants and a mixed martial arts flavor in a bid to spice up the competition.

The fringe sport, which made its Olympic debut at the 2000 Sydney Games, is assured of its place until Tokyo in 2020 but is feeling the heat to modernize as the International Olympic Committee looks to appeal to a younger demographic.

Taekwondo is far from a blood sport, but appears to have taken a cue from mixed martial arts by changing its square fighting mat to an octagon, the shape of the Ultimate Fighting Championship's cage.

Competitors will also be allowed to wear colored pants with their national flags at the Aug. 17-20 tournament at the Carioca Arena 3, which may upset purists accustomed to the starched-white trousers of Olympics past.

"The introduction of colored pants for athletes will transform the look of competition, adding to the vibrant, carnival atmosphere in Rio, and will give athletes an even greater sense of national pride as they compete for their countries," World Taekwondo Federation (WTF) president Chungwon Choue said.

The WTF has had its critics in the past for being aloof and reactive but earned praise at London for introducing a new scoring system and instant video reviews which banished much of the refereeing controversy that blighted previous tournaments.

In Rio, fighters will now be fitted with electric sensors in their protective head gear to help referees detect scoring shots, adding to the torso sensors introduced at London.

Competitors will also enter the arena to theme music of their choice, adding a bit of theater to a discipline which has its origins in ancient Korean martial arts.

The improved scoring system at London saw all eight titles go to different nations and the field is expected to be wide open again.

Chinese double Olympic champion Wu Jingyu will bid for a hat-trick of golds in the women's 49kg flyweight division, while South Korea's Oh Hye-ri is under pressure to continue the country's reign over the 67kg category which was won by Hwang Kyung-seon at London and Beijing.

The men's heavyweight 80kg category also offers plenty of intrigue.

England-born former world number one Aaron Cook, who was controversially omitted from Team GB at his home London Games, will compete under the Moldovan flag at Rio, with top-ranked Iranian challenger Mahdi Khodabakhshi, nicknamed 'The Terminator', another strong medal chance.

(Editing by Frank Pingue)