A boxer can do a hundred abdominal crunches, a thousand press-ups and dance with a skipping rope all day. He can learn the art of attack and defence. But one question, particularly pertinent to Amir Khan, continues to vex even the wisest boxing brains (and that is not an oxymoron) - can anything be done about a glass chin?

“If we knew the answer, that would be like solving the biggest mystery in the history of all boxing,” Emanuel Steward, one of the sport's premier trainers, once said. Angelo Dundee, who guided the great Muhammad Ali, sees no great mystery. “You can't train a chin,” he said, simply.

Yet that is what Khan, 21, will be attempting under the tutelage of Freddie Roach at his Wild Card Gym in Los Angeles when the young Briton heads west next week to rebuild his career. The trainer is devising a series of exercises, based on martial arts techniques, to “deaden the nerves on the jaw”, or at least try to condition them so that one big blow does not again leave Khan resembling a puppet cut loose from its strings. “Is it sound? I'm not sure,” Roach said. “But we've learnt some Thai techniques working with Manny Pacquiao [the Filipino world lightweight champion]. He was suffering with blows to his body, so, using martial arts, he got us to hit him with a stick.

“I'm not saying we are going to be hitting Amir Khan with a stick, but there is a belief you can deaden the nerves using pressure, tension, wrestling exercises with the chin on the ground. We'll try and deaden, or toughen, those nerves on the tip of his jaw.”

Most of the time will be spent trying to alter Khan's style so that he is not hit so often and does not suffer the sort of knockout blow that cost him a surprise 54-second defeat by Breidis Prescott in Manchester last month. It was his third knockdown in 19 professional bouts.

But the gym work on Khan's suspect jaw does raise, again, the question of whether a boxer can physically alter his ability to withstand a heavy blow or whether, for better or worse, he is stuck with his chin. “Not every great boxer has a great chin,” Roach said. “It is like a big puncher. They can be improved, but you generally get what you are born with.”

That has not stopped Roach embracing those martial arts techniques, usually employed to toughen up shins and feet for kickboxing and similar disciplines. There have long been theories, their effectiveness unproven, about strengthening the jaw muscles or conditioning the neck.

According to Dr Barry Jordan, a neurologist who was once chief medical officer for the New York State Athletic Commission and has researched boxing's effects on the brain, there are several factors to consider when weighing up why some boxers collapse while others, from George Chuvalo to Antonio Margarito, seem capable of withstanding sledgehammer blows.

Jordan believes that anticipating the punch and possessing strong neck muscles assist a boxer. There is little doubt that Khan not only has a suspect chin but sticks it out, too. “I think he's made mistakes looking for the big knockout because then you put yourself in harm's way,” Roach said. “He started out knocking people down with one punch, but the higher you climb, you need to protect yourself.”

Yet Jordan believes that, while training can make a boxer better able to roll with the punches, the glass chin is a weakness that is handed out at birth. “We know that there is a gene that makes certain boxers liable to neurological impairment over the long term and, while no one has ever conducted detailed research on the effects of one punch, a good chin is about a fighter's genetic predisposition to tolerate punishment,” he said. “In layman's terms, the blow, the sudden acceleration and rotation of the head, causes a disconnection. The ability to withstand that may alter during the career of a boxer.”

But, as far as he can tell, a boxer's tolerance cannot be greatly affected in the gym. If Khan is stuck with this weakness, his best hope is to not get hit, which is why he is spending the next six weeks with Roach. “They say Willie Pep didn't have a great chin, so he changed his style,” Roach said. “And we are talking about one of the greatest boxers of all time. We will be talking to Amir about the way he stands, holds his hands, everything.”

Khan will spar with Pacquiao, the world champion who faces Oscar De La Hoya in December, which should help him to become accustomed to heavy punches and please those who believe that a glass jaw is a matter of heart as much as chin.

No amount of work, though, may solve the abiding mystery of the porcelain jaw. “After that sort of knockdown, some guys roll over and die,” Roach said. “Some get better. Amir seems pretty positive to me, but I really won't know - no one will - until he's back in the ring.”

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