But how fit is Robert Mugabe?

When Robert Mugabe came a cropper last week, stumbling at an airport and falling on his hands and knees, many saw it as evidence that Zimbabwe's nonagenarian president should quit and end the uncertainty over his successor.

But not the country's state-controlled media, which has attempted to spin the incident into a personal triumph for the world's oldest leader, arguing that his ability to bounce back is a sign of strength rather than weakness, of youthful virility even as he approaches his 91st birthday.

"President Mugabe's slip at Harare international airport last week must surely be a gift from God," said the Sunday Mail newspaper, widely regarded as a government mouthpiece. "This month, the man turns 91 years old and at that age, falling down - as with a young child - can result in not only embarrassment, but also serious injury.

"But President Mugabe showed that he is mentally agile by having the reflexes to break that fall, and that he is physically fit by walking away totally unscathed."

At the time of the tumble, a Mugabe spin doctor claimed that "even Jesus" would have tripped over an offending hump on the red carpet. Journalists claimed they were forced to delete photos of the incident by security agents at the scene. But some pictures slipped out and a cutout image soon went viral with the hashtag #MugabeFalls. They included spoof pictures of Mugabe in the middle of dancefloors or alongside singer Rihanna.

The Sunday Mail, however, took a dim view of the critics. "Anyone claiming that he is infirm and incapable of ruling because he slipped, broke a fall and walked away, all at age 91, can only be from that lunatic fringe of society that having failed to depose him via the ballot box now clutches at the most fantastical of straws," it said.

"This incident proved why Zimbabweans vote for him consistently: he is as mentally alert and physically capable as he has ever been. Any suggestion otherwise can only come from people who are as thick as elephant manure and evidently far less useful."

Mugabe was returning from an African Union (AU) summit in Ethiopia when he fell. He had been elevated to chair of the AU, the Sunday Mail noted, and sometimes worked until 5am while his rivals slept. "Surely, an incapable leader does not work until 5am!" it said.

He is the only leader independent Zimbabwe has ever known, having been in power since 1980, and a lavish celebration is planned in Victoria Falls for his 91st birthday on 21 February. There have been constant rumours over his health in recent years and speculation over whether his death could provoke fresh instability and violence. Mugabe's Zanu-PF party is deeply divided and last year his wife, Grace Mugabe, emerged as a possible successor.

Zimbabwe's main opposition, the Movement for Democratic Change, dismissed the Sunday Mail comment as wishful thinking. Obert Gutu, its spokesman, said on Monday: "When I read that editorial I actually had a very good laugh. I thought, now things are getting out of control.

"We sympathise with President Mugabe's fall. We appreciate that when you are that age your reflexes are not as agile as a man in his 40s, 50s or 60s. That said, it is clearly indicative of the fact Robert Mugabe is no longer fit for purpose. At 90 years of age he has clearly seen better days and, no matter what his spin doctors might think, he is already frail and not in the best of health."

It was "unfair and cruel" for Zimbabweans to demand that he continue in the job, Gutu added, especially with the country facing a "rash of economic challenges". But who will replace him remains unclear. "He is obviously an intelligent man who is aware that, if he steps down today, his party will collapse like a deck of cards. If it was a company and there was no successor as CEO, it would be untenable."

Zimbabwean state television regularly shows singers paying tribute to the president, while the state press is unswervingly loyal. The Herald newspaper published a collage of other world leaders who have stumbled, and listed incidents in which former US presidents George W Bush and Gerald Ford, Australia's former prime minister Julia Gillard and Queen Sofia of Spain all tripped.

Dewa Mavhinga, senior researcher for Zimbabwe at Human Rights Watch, said: "Clearly it's nonsense but it also shows that the Sunday Mail and Herald newspapers have effectively become propaganda tools for the government and Zanu-PF. They are people who stick their heads in the sand and don't want to accept the reality that he is too old and his health is failing."

Mugabe has repeatedly insisted that he is "fit as two fiddles". His hardline ally Emmerson Mnangagwa was named his first vice-president at December's Zanu-PF congress, while the long-time contender Joice Mujuru was sidelined and accused of involvement in an assassination plot. However, Mavhinga believes the race is far from over. "Mnangagwa may be one step closer and Mujuru one step further away, but it's in no way clear that Mnangagwa is going to take over. There are vultures in Zanu-PF and the most likely scenario is utter chaos."

Such concerns are shared by Ibbo Mandaza, an academic and author. He said: "An open and constitutional succession appears less certain. I have fears of an untidy if not violent succession given the aftermath of the so-called congress and the balance of factions. This explains the denialism about the president's age. The truth is, the old man is old. It's a thing to neither lament or laugh at, but we should take it seriously."