Former Springbok captain Joost van der Westhuizen suffers from amyotrophic lateral sclerosis, a terminal motor-neurone disease characterised by slurred speech and creeping paralysis.

Joost van der Westhuizen is barely intelligible but extraordinarily expressive. He is physically diminished yet spiritually enriched. On this, rugby union's showpiece weekend in the northern hemisphere, he redefines notions of heroism and encourages his successors to pause and recalibrate.

The former Springbok captain suffers from amyotrophic lateral sclerosis, a terminal motor-neurone disease characterised by slurred speech, breathing problems and creeping paralysis. Victims rarely live longer than five years beyond diagnosis.

In the two years since Van der Westhuizen was diagnosed he has revealed the courage enshrined in the definitive moment of a storied career, the tackle which stopped Jonah Lomu in his tracks during Nelson Mandela's World Cup final in 1995.

At 42, his body is shutting down while his brain remains hyperactive. Basic actions, such as talking, chewing, standing and walking, are becoming increasingly difficult. The instinct is to recoil, withdraw. Yet instead of succumbing to debilitating weakness he has shown remarkable strength in counselling fellow sufferers.

Sport lends itself to lazy sentimentality and cheap praise. Yet when Van der Westhuizen appeared at a London dinner last week which raised £100,000 for his J9 Foundation, he generated a sense of awe and inadequacy. He could not take to the stage, but spoke through a pre-recorded video, which was received in rapt silence.

“It took me a year to make peace with myself,” he said during an equally humbling radio interview with Richard Keys and Andy Gray the following morning. “You learn to adapt and accept. You have a choice: are you going to lie down and die or are you going to live your last few months? Life is joyous.

“In the beginning you go through all the emotions. You start to ask yourself, 'Why me?' It's quite simple: why not me? I have to go through this to help future generations. There is a reason for it, and we don't have to know why.”

Rugby can be a destructive force, a sport of misplaced machismo and wanton aggression. It cherishes its warriors like Van der Westhuizen. He is, by his own admission, a flawed man transformed by adversity.

His perspective, painfully acquired, might be acutely personal, but it has a challenging relevance to those athletes who are encouraged to believe in the myth of their invulnerability. He was forced out of celebrity's bubble to confront himself.

He concedes he was wilful, arrogant and recklessly self-indulgent. Those around him made excuses for his excesses. He thought he had reached a nadir three years ago when he tearfully retracted denials that he had been captured on a sex tape involving a stripper.

Then Henry Kelbrick, his friend and doctor, noticed a lack of physical power and indistinct speech patterns. The immediate prognosis that he would be in a wheelchair within a year did not factor in the strength of his will.

Time is suddenly his most precious commodity. He is being injected with an expensive and experimental goat serum designed to alleviate some of the degenerative effects of the disease, which has no known cause.

His achievements, as the great scrum-half of his era, have already conferred an intimate form of immortality. Images of his speed and physicality, his opportunism under duress, will endure. He will continue to stimulate the imagination of those who watched and once wished they could be him.

His plight echoes that of baseball legend Lou Gehrig. He died in 1941, two years after being diagnosed with ALS. His valedictory speech - “Today I consider myself the luckiest man on the face of the earth” - remains one of the most emotionally charged moments in North American sport.

Van der Westhuizen visits other victims to militate against surrender: “Emotionally they are not in a good space. They feel sorry for themselves and deteriorate quickly. When you accept your condition you will live longer.”

As for his own fate, he is unequivocal: “I will decide when I go.” – The Independent on Sunday