LONDON (Reuters) - Penpix of the leading candidates for the men's Olympic 100 metres which gets underway with opening-round heats on Saturday:
USAIN BOLT, Jamaica, age 25, pb (personal best) 9.58, sb (season's best) 9.76
When Bolt turned his head and smiled at the photo-finish camera as he scorched to a sensational world record of 9.69 to win Olympic gold in Beijing, many asked 'Could he have gone faster'?
The effortless way the Jamaican tormented his rivals in the Bird's Nest stadium left an indelible impression and Bolt proved he could indeed go quicker when he lowered his mark to 9.58 in Berlin in 2009.
Bolt won the 100, 200 and 4x100 relay treble in Beijing and when he repeated that feat at the world championships in Berlin, many believed he had become unbeatable.
Four years on he remains the man to beat in London despite lingering doubts over his fitness heading into the Games.
The biggest name in athletics now has a serious pretender to his crown in younger compatriot Yohan Blake, with the final on August 5 billed as 'The Lightning Bolt' against 'The Beast'.
Extrovert showman Bolt told the media on Thursday that he had completely overcome his back and hamstring issues, as he prepared for what no man has done before - successfully defend the 100 and 200 Olympic titles.
Bolt is using the disappointment of being disqualified for false-starting at last year's world championship final, along with defeats in the 100 and 200 by Blake in the Jamaican trials in June and July, as motivation.
"It's always a wake-up call to get beaten. It opened my eyes and I sat down to rethink a few things," he said.
YOHAN BLAKE, Jamaica, age 22, pb 9.75, sb 9.75
Nicknamed 'The Beast', Blake's rapid development has seen him emerge as the main threat to compatriot Bolt's 100 and 200 Olympic crowns.
Three years younger than his training partner and good friend, the 22-year-old lines up in London as the world 100 champion having taken the title last year in Daegu, South Korea in a then-personal best time of 9.82.
The gloss on that victory, when he became the youngest world 100 champion, was taken off by the fact Bolt was disqualified from the final for false starting but Blake has since proved his victory was no flash in the pan.
At the tail end of last season he ran an astonishing 200 at the Brussels Diamond League, clocking 19.26, the second fastest ever run over the half lap. Only Bolt (19.19) has gone quicker when he won the 2009 world title in Berlin.
This season Blake has continued in the same vein, stunning Bolt twice in three days by winning the 100 and 200 at the Jamaican Olympic trials, running a world-leading time and personal best of 9.75 in Kingston.
Like Bolt, Blake is coached by Glen Mills but unlike his friend, Blake is more reserved, eschewing the spotlight and concentrating on trying to live up to his nickname.
"They call me The Beast for a reason, knowing my work ethic," he said. "I'm always working. Even if I'm watching TV, I'm finding some work to do.
"That's me because I know what I want. I know what I can do and I know as long as I put in hard work anything can happen."
JUSTIN GATLIN, United States, age 30, pb 9.80, sb 9.80
Eight years after he won the Olympic 100 in Athens, Gatlin is back in the Games - but much has changed.
The 2005 world 100 and 200 champion was handed a four-year doping ban in 2006 after testing positive for too high a level of the male sex hormone testosterone.
The lengthy suspension not only wiped out a world record-tying 9.77 seconds the American had recorded in Doha in 2006, but sent him looking for income wherever he could find it, including unsuccessful tryouts with National Football League teams.
Gatlin returned to athletics in 2010, running primarily in small off-the-beaten-path meetings before beginning to hit his stride last year.
The sprinter made the American world championship team a year ago with a second-place finish in the U.S. trials but then went out in the semi-finals in South Korea.
He has looked more like his old self this season, winning the world indoor 60 title in Istanbul and until a Diamond League meeting defeat by Tyson Gay in Paris this month had been undefeated outdoors.
Gatlin won the U.S. Olympic trials with a personal best 9.80 and has been quietly preparing for his return to the Games.
Boos may accompany his introduction because of his doping ban but he has said he is looking forward to his first run in London in years.
ASAFA POWELL, Jamaica, age 29, pb 9.72, sb 9.85
Powell is the third fastest man in history and a former world record holder but is still waiting for a global track title.
Now regarded as the 'third' Jamaican behind Bolt and Blake, he is desperate to answer the critics who say he lacks the mental strength to be Olympic or world champion.
The 29-year-old must also prove he has finally put his injury concerns behind him, which have seen him and American Tyson Gay classed as the 'forgotten' men of sprinting.
Powell suffered a groin injury and missed last year's world championships and the problem has surfaced again this season with the Jamaican forced to pull out of a meeting in London earlier this month so he could rest.
He held the 100 world record between June 2005 and May 2008 but has never performed when it mattered in individual major championships.
Powell has finished fifth in the last two Olympic finals, although he did win relay gold in 2008, while he picked up bronze medals in the 2007 and 2009 worlds.
The Jamaican, who last week said he was fully fit, has said the emergence of Bolt and Blake has taken the pressure off him and he has vowed to perform well at his last Games.
"I'm pumped up," he said. "I'm not getting any younger. It's probably my last one. There's a lot of attention on Jamaica right now and everyone is pushing for the Olympics."
TYSON GAY, United States, age 29, pb 9.69, sb 9.86
Despite his global success Gay, the world's second fastest man, does not own an Olympic medal.
But after his second-place finish in the U.S. trials and a recent Diamond League victory over compatriot Gatlin, he is expected to be in the mix when Bolt and Blake square off in the final.
That the 2007 double world sprint champion is even in the Olympics is a mini-miracle.
Four months ago the humble American was unable to work out on the track as he recovered from hip surgery he underwent in July 2011.
Gay's first race in nearly a year came in early June and he has steadily improved since then.
His determination has never been questioned. Injuries, though, have often been a stumbling block.
After a sizzling but wind-assisted 100 metres in the 2008 American trials, Gay appeared ready to challenge new world record holder Bolt.
But a hamstring injury in the 200 at the trials left him less than fit and he went out in the 100 semi-finals at the Beijing Olympics.
Gay rebounded to take silver behind Bolt in the shorter race at the 2009 world championships and a month later ran 9.69 seconds, the second fastest performance of all-time.
He handed Bolt a stunning defeat in Stockholm to rank number one in the world in 2010 but missed the 2011 world championships with his hip problems.
Now he is hoping for an early birthday celebration in the 100 final which comes four days before he turns 30.
KESTON BLEDMAN, Trinidad & Tobago, age 24, pb 9.86, sb 9.86
Bledman is an outside bet for a podium spot but appears to lack nothing in confidence after clocking a personal best 9.86 to win his national title on June 23.
He became the fastest non-Jamaican this year with that run although American Gatlin went faster a day later.
"No disrespect to Bolt, Blake, my training partner Tyson Gay but when I go to the Olympics, I'm going to win. I ain't going there to play around. I have at least 9.7 in me," he said after his scorching run in Port of Spain last month.
Bledman is now joint second on the all-time Trinidad and Tobago performance list alongside quadruple Olympic medallist Ato Boldon.
Richard Thompson is number one with his 9.85 national record.
Bledman already has an Olympic medal in his collection after helping Trinidad & Tobago to silver behind the Jamaicans in the 4x100 relay in Beijing.
The 24-year-old will hope to make his mark individually on the big stage after only reaching the world championship semi-finals last year.
RYAN BAILEY, United States, age 23, pb 9.88 sb 9.93
The mystery man in the American squad, the tall Bailey booked his spot in London with an unexpected third-place finish in the U.S. trials.
A high school sprint champion, he has followed in the footsteps of former world record holder Maurice Greene by moving to Los Angeles to train with highly successful coach John Smith.
Athletics has been somewhat of a saviour for the 23-year-old.
His troubled childhood included living in a car with his ill mother at one point when she could not work and pay rent.
Bailey was frequently involved in gang fights and in 2006 was stabbed three times in the back and shoulder by a rival gang member.
An Oregon high school coach rescued him and Bailey became the state champion in both the 100 and 200.
He added a national junior college title in 2009 and in 2010 set his lifetime 100 best of 9.88 seconds.
Bailey moved to Smith's camp in late 2011 but was slowed by leg injuries until his Olympic trials breakthrough.
KEMAR HYMAN, Cayman Islands, age 22, pb 9.95, sb 9.95
The Florida State University student is the eighth fastest man this year of those competing in London.
One to watch for the future, rather than expect any great showing in London, the inexperienced Hyman heads into his first Games having dipped under 10 seconds for the first time on July 7 when he clocked 9.95 in Madrid.
Much is expected of Hyman who did not become a serious runner until his junior year of high school.
He enjoyed an outstanding 2012 indoor season when he earned All-American honours with a third-place finish over 60 metres at the NCAA Championships, clocking 6.59 seconds.
Hyman underlined his potential with his display in Madrid but failed to make his mark a week later at a damp and drizzly London Grand Prix, trailing in last of seven behind Gay in the rather pedestrian time of 10.25.
He is by no means the first sprinter of note to emerge from the Cayman Islands, following in the footsteps of Cydonie Mothersill who won Commonwealth gold in 2010, and Kareem Streete-Thompson, a world indoor silver medallist in 2001.
(Compiled by Justin Palmer and Gene Cherry, editing by Tony Jimenez)