Usain Bolt's stunning win in the 100m final at the Beijing Olympics in 2008 will live forever in the memory of those who saw it live. For sports fans, it was one of those "where were you?" moments, like Cathy Freeman's 400m gold medal in Sydney and the John Aloisi penalty that sent the Socceroos to the 2006 World Cup.
While the chest-beating, crowd-saluting audacity and world record breaking speed of Bolt's 100m win captured the world's imagination, his victory in the 200m a few days later franked his status as an all-time great. Gone was the showboating, replaced by a gritty resolve that he would not be beaten in his pet event. It paid off - again in world record time.
A year later, Bolt repeated the record-setting double at the world championships in Berlin, becoming the first man to simultaneously hold both the 100m and 200m Olympic and world titles.
The Jamaican's dominance of his contemporaries combined with his history-making times raises an obvious question: is he the greatest athlete of all time? The answer is not immediately no - more than can be said for most champions of track and field - so let's consider the contenders.
Daley Thompson and Roman Sebrle
The decathlon asks its champions to excel at several track and field disciplines. Bolt cannot hold a candle to its two finest exponents, Daley Thompson and current world record holder Roman Sebrle, when it comes to discus or javelin but neither was as overwhelmingly dominant as Bolt. It could also be argued that the need for decathletes to be jacks of all trades but masters of none eliminates them from the running. Women's heptathlon world record holder Jackie Joyner-Kersee is another all-time who fails to measure up to Bolt by the same score.
Bob Beamon and Mike Powell
Similarly, great one-off performances such as those by long jumpers Bob Beamon and Mike Powell cannot compare to Bolt's feats. Beamon's leap of 8.90m in Mexico City at the 1968 Olympics was the world's best for almost 23 years, before Powell topped it at the Tokyo world titles in 1991 with an 8.95m jump. It remains the world record 20 years later, but both men were fallible in their event on any given day. Bolt, at full fitness, is not.
If we turn our attention to those athletes who dominated over a long period, two names stand out: Edwin Moses and Carl Lewis. Moses' record of going undefeated in 122 successive 400m hurdles races from 1977 until 1987 is unparalleled in track and field history. He won two Olympic gold medals, two world championships and broke four world records in that time.
However, the global cohort of 400m hurdlers is minute compared to the 100m sprint. Boys and girls in schoolyards around the world do not test themselves by jumping over a series of fences lined up around an oval; they run in a straight line from one end of the oval to the other. Likewise, history's greatest pole vaulter Sergey Bubka, triple jumper Jonathan Edwards and discus thrower Al Oerter - a four-time Olympic gold medallist - were the best in a relatively limited field. They join Moses as all-time greats, but the greatest? No.
The Finnish champion won nine Olympic gold medals between 1920 and 1928 in distances ranging from 1500m to 10,000m. He was clearly the finest distance runner of his generation, and certainly one of the greatest of the 20th century. His crowning achievement came at the Paris Games in 1924, when he won five gold medals, including two in an hour in 1500m and 5000m finals which were scheduled less than 30 minutes apart. Nurmi's successes came in an era when athletics was hardly a global pursuit. To illustrate, just one African - Moroccan Mohamed El-Sayed - competed in the 1500m in Paris. The Flying Finn is an all-time great, but he did not beat the world as today's champions have done.
Lewis presents a more difficult case to deny. Longevity? Tick: four successive Olympic long jump titles. Versatility? Tick: those long jump golds, plus the sprint double at the 1984 Olympics, backed up by gold in the 100m in Seoul four years later. World records? Tick: twice in the 100m, with times of 9.92 and 9.86sec. Had Lewis been the one to break Beamon's mark, rather than Powell, it would be nearly impossible to deny him the title of history's greatest. But he did not, thus leaving one significant flaw in his otherwise impeccable CV.
That brings us to the final and perhaps the strongest contender: Michael Johnson. He claimed his first world title in the 200m in Tokyo in 1991 and dominated the long sprints for much of the following decade. He became the first man to win the 200/400 double at any major meet when he claimed both titles at the 1995 worlds in Gothenburg, but his career peak came the following year.
At the US Olympic trials in 1996, Johnson broke the 17-year-old world record in the 200m. He bettered his world mark in Atlanta later that year as he completed the 200 and 400m Olympic double for the first time in history. And he completed his ascent to greatness at the Seville world titles in 1999 when he finally eclipsed Butch Reynolds' 400m world mark.
So what is Johnson's flaw? Arguably, only the margins by which he broke his records. His first 200m record of 19.66 surpassed Pietro Mennea's 1979 record by just 0.06sec, although he later improved his best by 0.34sec to 19.32 in Atlanta. His 400m mark of 43.18 was just 0.11sec faster than Reynolds' best.
Why Bolt could be the best
While Bolt's 2008 record of 19.30 in the 200m just scraped past Johnson's best, he positively obliterated the previous 100m world record. In 12 months, Bolt took 0.16sec off Asafa Powell's previous world mark of 9.74 - a margin that it had taken six previous holders more than 16 years to shave from the record.
It is beyond doubt that Usain Bolt is a once-in-a-generation athlete, but it is arguable that both Lewis and Johnson are his equals in history. If the Jamaican champion repeats his sprint double in London, that debate is weighted even more heavily in his favour. If he breaks his 100m and 200m world records at a second successive Olympics, the debate will cease to exist.
The views in this article are those of the author and not necessarily those of BigPond Sport.