David Leggat: What drives the great to come back?

Only those sports competitors who have achieved great things on the grandest stages could understand this.

Those of us whose sport was played at more modest levels can only wonder what makes these heroes - and that's what they are to millions - reluctant to walk away for good.

It's nothing new. Sir Stanley Matthews, wing wizard for England, Blackpool and Stoke City, was still turning tricks in the old first division at 50, 33 years after making his debut at the top.

He was a teetotal vegetarian, so he wasn't exactly the life and soul of the party, but he remains the only footballer knighted while still playing.

What on earth was he doing risking life and limb at 50, remembering those were years when leniency was the order of the day towards the hatchet men who prowled the English game with brutal intent.

Bjorn Borg couldn't bring himself to sling his trusty wooden rackets on a bonfire after establishing a record to sit alongside the finest of his game. He ended up being torpedoed by players not good enough to lace his boots, but armed with a graphite thunderstick.

Mark Spitz, winner of seven golds at the Munich Olympics, unsuccessfully tried out for the Barcelona Games 20 years later.

A few weeks ago, Lance Armstrong did the Tour de France gig again. He'd won it seven times and still he was back for more. Finished a hugely creditable third too, which proved it was still worth his while. He could still do it. But what was to gain at the age of 37?

Which brings us to Michael Schumacher. His comeback has a small question mark over it, as he battles a pinched neck.

In sporting terms, necks are more important in motor racing than many other pursuits. The strains of high-speed cornering, the G-forces brought to bear on an essentially fragile piece of skin, gristle and bone, put fierce pressure on a Formula One driver's neck.

Schumacher, seven times world champion, has returned - admittedly on a short-term deal - to fill in for the injured Felipe Massa at Ferrari in this month's European Grand Prix at Valencia.

He's not driven this year's F60 model; has not raced the Valencia street circuit; and has been out of the game for three years. But listen to this: "It feels as if a flush of positive energy is coming over me," Schumacher said this week. Perhaps he's been to one of those shonky lifestyle hucksters.

No one questions that the German, even at 40, has the ability, and there's equally no questioning his love of sport remains intact. The question, again, is why? After all he's done, the baubles and trinkets and standing he has accumulated, what's going on up top?

Hopefully, Schumacher's head won't be turned by Bernie Ecclestone, the Hitler admirer who runs Formula One. He hinted Schumacher could not only win, but might be persuaded to return full-time. Certainly you could suggest - as the fossil-like Ecclestone did of Hitler - that in his time Schumacher got things done.

Maybe it's simple; it's an inability to walk away, to be satisfied. Taste the glory and never be sated. They don't all have it, but it's the mental burden many of the true greats have to bear.