THE ability of athletes to recover from illness or injury, then scale Olympian heights and push the boundaries of human endeavour are the stuff of sporting legend.

Hence the reception given Jessica Ennis when she claimed 2012 Olympic heptathlon gold in London. She had been a title contender four years earlier until injury excluded her from Beijing. So it was only fitting that we rained rapturous acclaim on her recovery to win the 2009 world title. Yet by the time she she defended that crown in 2011, her campaign had again been disprupted by injury, forcing her to settle for silver. It was likewise before the defence of her world indoor pentathlon title on the run-in to the Olympics last year. Again she had to accept silver.

Such setbacks made Olympic success all the sweeter, especially given the majestic manner in which she had shouldered the burden of being Olympic poster girl.

Now married and preferring the name Ennis-Hill, she is again derailed by injury. She withdrew from a UK Women's League appearance in Edinburgh on Saturday and has now pulled out of the hurdles and long jump at the Oslo Diamond League tomorrow.

She cites an Achilles injury (as in Edinburgh). It was an Achilles problem that forced her "precautionary" withdrawal from the UK trials for the European Indoor Championships in 2011. The seven weeks she missed then contributed to the failed defence of her world crown in Daegu.

Now, having missed an indoor season to recover from the Olympics, her attempt to regain that title in Moscow may be compromised by the latest injury. Alarm bells are already ringing. She has a heptathlon scheduled for Tallinn on June 29-30 but that's fraught on a suspect tendon so close to the worlds.

Every impact sport competitor from football through rugby to judo and wrestling lives with the reality of being one injury away from retirement. Very few competitors end their career on their own terms. Sir Chris Hoy is an exception.

Heptathlon, while not an impact discipline, places enormous demands on the body. We are not suggesting her career is under threat, but we should acknowledge that Ennis-Hill cannot survive at the top indefinitely. Her former rival, Carolina Kluft, was triple world champion, Olympic champion, double European champion but quit the event at the age of 25, just six months before Beijing. The Swede hinted at a return to challenge Ennis in London but, in 2009, she ruptured all three attachments to her right hamstring: career effectively over. Ennis-Hill is now 27.

Paula Radcliffe, 40 this year, established the world marathon record in 2003 but has only once subsequently come within five minutes of that. Though she took the world marathon title in 2005, she has completed only two marathons since 2008. Recurrent injuries have blighted her career, just as they did for Scotland's two greatest female endurance runners: Liz McColgan and Yvonne Murray.

There was gold at every turn for Sir Bradley Wiggins last year, but illness and a knee injury have ended Giro and Tour de France ambitions this year. Time will tell whether his cycle of success is merely stalled temporarily. Or ended, like Radcliffe's.

Just when we hoped Andy Murray might consolidate on the best year of his career, his back is giving rise to concern. It coincides with Rafa Nadal's recovery from a career-threatening knee injury, though, which will remind Murray of the virtues of persistence.

The career of Scotland's 1980 Olympic 100 metres champion, Allan Wells, was cut short by a toe injury. His Moscow contemporary, Linsey Macdonald (UK 400m record-holder) also retired prematurely; ditto her successor, Allison Curbishley, who claimed Macdonald's Scottish 400m record in 1997, just a month after her 21st birthday. She won Commonwealth silver the following year, but expected so much more. Injury cut her short, but her record of 50.71sec still stands.

Athletes need to feel an element of invincibility, even immortality. This helps them achieve goals which seem preposterous while cajoling bodies which are mortal and prone to breakdown.

I pray that Ennis-Hill wins more honours, Likewise, Wiggins and Murray, but we should not be greedy. We should savour great achievement and performance as it is delivered, rather than thrash ourselves to a frisson of expectation over a fragile future.

And another thing . . .

The British Athletes Club had no sooner endorsed the UK and World Anti-Doping Agency protocols last week than they were backing our excoriation of the International Olympic Committee for failure to strip the US relay squad of a title won with the help of a drug cheat.

"It is very disappointing that USA has been allowed to keep their gold medals won in the women's 4x400m relay at Athens 2004, despite one of their athletes, who ran in the heats, having admitted using performance-enhancing drugs at the time," said the BAC chair, four-time Olympic swimmer Karen Pickering.

"Great Britain's relay team have not only been denied their moment to stand on the podium at the Olympic Games, but with the decision that the race result will stand, they will not now even receive the bronze medals. It's a sad situation when it is clean athletes that are let down by the system . . . where there has been a violation of the rules, the results of a race and distribution of medals should be corrected accordingly."