Michael Johnson makes his living by telling people where they went wrong.
Whether it be as one of the most respected pundits at BBC Sport or at his Michael Johnson Performance centres, analysing the minute details of an athlete's performance pays the American's bills.
It is particularly cruel, therefore, that he faces the future with no such clarity about his health.
Four months ago Olympic sprinting great Johnson had a stroke.
It was a mini stroke from which the 51-year-old has made a miraculous recovery - he is already spending his days paddle-boarding, rowing, cycling and running.
But despite multiple tests and analysis from doctors, he admits he is living in fear because of one nagging question - why?
"We've sort of concluded that we'll probably never find that cause," Johnson tells BBC Sport.
"It's something I've had to come to terms with. If I knew what the cause was, I could potentially do something about it and feel much better knowing that this is what caused it and now I have eliminated that potential issue. Unfortunately, I don't have that luxury.
"Obviously that could create a little bit of fear, that I was doing all the right things before the stroke and now am doing all the right things again, even more. Yet it could happen again."
Fear of the unknown is not the only scary concept Johnson has had to face in recent months. He has also had to embrace a completely alien emotion - vulnerability.
Over a glittering career during which he won four Olympic golds, and set world records in the 200m and 400m, Johnson was happy to talk the talk and walk the walk.
One of his most famous soundbites was: "They don't give you gold medals for beating somebody. They give you gold medals for beating everybody."
And at the 1996 Olympics in Atlanta he raced to double glory in golden spikes.
Neither the words, nor actions, of a man keen to show weakness.
'I don't like sympathy'
"I can't say 100% I am in a situation now where I'm totally comfortable being vulnerable," says the sprinter who was known as 'Superman' during his career - and at the 1997 World Championships wore a shirt bearing the comic book character's 'S' on a victory lap.
"It's highlighted that it's an issue that I have, this need to be superman, so to speak. This need to feel that people perceive me as being very fit, strong and able to do all these different things is a personal issue that I have got to still work through.
"My persona, personally and publicly, has been that I have got everything under control and I don't need anyone else's help, don't need anyone's sympathy. I don't like sympathy or empathy.
"I think one of the real lessons for me in this situation was being faced with the vulnerable position of not being able to walk. Needing help to do that and some of the regular normal daily activities of life was tough for me.
"But I realised that in order to get back to where I needed to be, people's help was something I was going to need, it was something I needed to open up to."
As Johnson is quick to point out, the unwillingness to show vulnerability is not all bad. Such steeliness helped deliver those four Olympic gold medals after all, and he believes it has helped him complete a remarkable recovery.
It was early September when Johnson suffered a transient ischaemic attack (TIA). It is a mini stroke caused by a temporary disruption in the blood supply to part of the brain and results in a lack of oxygen to the brain.
He lost mobility and co-ordination in his left side and in the days afterwards it took him 15 minutes to walk 200m - the same distance he ran in 19.32 seconds to win Olympic gold at the Atlanta Games of 1996.
Four months on, he has already returned to his previous fitness regime. But for a move from San Francisco to Malibu, he would have already enjoyed one of his favourite winter pastimes - skiing.
An important emotion during that rapid recovery was anger. It was briefly aimed at the predicament he found himself in, having adhered to all of the guidelines for avoiding strokes (eating healthily, drinking alcohol moderately, exercising regularly and watching your weight).
Within 24 hours that frustration was channelled elsewhere.
'My story can help other people'
"Having been doing all the right things like training every day and keeping myself in good shape while other people aren't, and they are fine and I am laying in a hospital bed having suffered a stroke, obviously you are going to be angry about that," he says. "And I was a little bit, for about a day.
"I am able to recognise and know for myself that that is not going to do me any good and is going to make me feel worse.
"Telling yourself not to be angry is only going to make yourself more angry; you have to replace that with something and I was pushing my doctors to let me get into physical therapy as soon as possible."
As soon as possible meant - remarkably - twice-daily workouts within a week of the stroke. It's an approach he is also using to spread the word about the cause, working with the American Heart Association on an awareness campaign.
"My story can help people to be more aware," he says. "One of the most important things with strokes is recognising the symptoms and getting to the hospital as soon as possible.
"Having gone through the experience I could very well see that an individual might choose not to go to the hospital. I didn't experience any sort of jolting moment of 'oh my god something just happened to me'.
"I just didn't feel quite right so I just chose to go to the hospital as opposed to waiting it out or sleeping it off to see if it would go away. That could have been catastrophic."
'I have no interest in running marathons'
Understandably, Johnson says he and his wife were glad to see the back of a life-changing 2018 which, in the midst of his stroke, also saw them evacuate their home for five days because of the deadly California wildfires.
But while Johnson is trying to live more in the moment, and to be thankful every day, much of his life will be unchanged. He'll still try to abstain from the junk food in the BBC Sport studio. And he certainly won't be taking on any wild challenges like the London Marathon.
"I passed on most of the snacks in the studio to begin with," he says. "I usually brought my own instead so I will just continue to do that. Colin [Jackson] is the worst so without him in the studio any more, things got much better.
"I had no interest in running marathons before the stroke and that hasn't changed. The stroke didn't affect me to a point to where I am now crazy enough to go actually run a marathon.
"The best thing for me is to keep the risk factors at bay by continuing to eat right and continue to keep myself in great shape. To watch the different factors like heart rate, blood pressure and diet, take the medication I've been instructed to, and then just move on with life.
"I have approached my recovery with the same drive as my athletics career. But it obviously takes on more importance because you are talking about your livelihood and your quality of life. It's not an accolade. It's serious business."