Sport on TV: Johnson crosses the line and races to his conclusions

The subject of race in sport is fraught with danger. The arenas must be free of such considerations, and you are bound to be walking on eggshells. Or in Michael Johnson's case, running very fast over them, so fast that he barely cracks a single one. With the sonorous voice of the American South and four Olympic gold medals in his pocket, there can be no arguing with his credentials to tackle the taboo.

Michael Johnson's Survival of the Fastest (Channel 4, Thursday) puts forward the theory that the vast majority of great sprinters are descended from slaves. Those who were shipped from Africa and survived the atrocious conditions to reach the United States were by necessity the strongest. Or they were left in Jamaica, the last-ditch dumping ground for the most difficult captives. Hence it is no freak of nature that the tiny Caribbean island, where 90 per cent of the population are descended from slaves, is the USA's chief rival in the sprinting stakes.

"Slavery was kind of an unnatural selection, a speeded-up evolution, a mass murder of the weakest, only the fittest able to survive and pass on their genes," says the former 200m and 400m Olympic champion. His theory is somewhat scuppered when he asks a couple of Jamaican teenagers why their island is so successful at sprinting and the answer comes: "The yam and the bananas." Another says: "They will realise we are black people, that we are just as good as whites," which is closer to the mark. But it's a psychological imperative left over from colonial days, not a physical one.

He meets a number of academics including Professor Bill Amos, who studies elephant seals in California. Hunted to apparent extinction, they returned having discarded their weaker traits because only the strongest survived. But sprinters are not descended from seals – the flippers would be no use at all.

More than a scientific inquiry, this programme resembled a tragic episode of Who do you think you are? Johnson unearthed a picture of his great great great great aunt, Irella Battle. But going further back in time, there is nothing. Johnson may or may not have come from a line of Senegalese kings. But the nature of slavery is that there is no hinterland on the African side because the slaves' identity was systematically stripped away.

Slaves were rounded up from all parts of the continent, creating "an incredibly rich gene pool with the widest range of talent available". But there's no way to back up such theories with hard facts. So you really cannot draw any conclusions from the terrible darkness shrouding the slave trade. It's just not that black and white.

* Two former sporting titans are cycling from Olympia to the Olympic Park in London to raise a mighty £2m for charity. On Flintoff and Dallaglio's Big Ride (Discovery, Thursday), "Freddie" challenges "Lol" to a race at Lake Garda, the former cricketer going around it on a bike and the ex-rugby player going across it in a boat. He's rowing for the first time in his life, with the help of TV habitué James Cracknell. The boat gradually fills with water and Dallaglio spends all his time trying to empty it. It's a good job they had made it to Italy, because a bail-out in Greece would have been very hard work.