When sportsmen or women hit their peak is something that even experts have never quite been able to put their finger on.
Objective analysis is often overtaken by subjective arguments.
Take former Northern Ireland striker Martin Paterson, for example. His peak came at 21-years-old - he just didn't know it at the time.
Next Friday marks the seventh anniversary of the highlight of his international career - he didn't know then either that he'd never top that moment in a green shirt.
Paterson's wonderful header from a pinpoint Niall McGinn cross gave Northern Ireland a 1-0 victory over Russia at Windsor Park and, in turn, brought Michael O'Neill his first win as manager in his 10th game.
It was just the start of an epic journey for O'Neill and his players. For Paterson, however, it was the beginning of something else - the beginning of the end.
Just eight months later he played what would be his last international, two months before his 27th birthday, and as O'Neill and his players were savouring the big international stage at Euro 2016, Paterson was on a family holiday after being released by Blackpool.
When his former international team-mates were going agonisingly close to a place at the 2018 World Cup finals - robbed by a horrendous refereeing decision in the play-off against Switzerland - Paterson was playing out the final days of his career at the age of just 30.
Hamstring and thigh muscle problems, plus countless knee operations - "I don't think I've any meniscus left in my knee," he says - robbed him of his best years, but not some great memories. Particularly that night against Russia in the August sunshine, after the original fixture had been wiped out by snow in the spring.
"I made my Northern Ireland debut as a substitute in Spain. They were just on their way to becoming European and world champions and I remember coming on and looking at players like Ramos, Fabregas, Xavi, David Silva, Iniesta and I was thinking, 'This is some start'. They only beat us 1-0 though," says Paterson, who was proud to represent the country of his grandfather's birth but regrets that he died before seeing his grandson play international football.
"I joined Huddersfield in the summer before the Russia game, but I knew straight away it wasn't for me and I couldn't see myself enjoying it.
"Mark Robins played me in the No.10 position. I was frustrated because I just wanted to play up front. I got to the Russia game and I got the chance to play up against two good physical centre-backs and I thought, 'You two are going to get it tonight'.
"It was a good night. I remember the goal and I remember I played well, I did my job for the team and I prided myself on that as much as, maybe more than, the goal itself.
"I never showed my best for Northern Ireland. Russia was a glimpse, but I was always in and out, injured and then coming back into the squad. My international career is a regret for me. I beat myself up about it. I enjoyed it, but I didn't perform anywhere close to what I know I was capable of. I feel like I let the country down.
"I believe if I had been fully fit and playing that I would have been on the plane to France. I really do. I think Michael was fond of me the way that I worked.
"I watched the games and I thought it was brilliant. It was painful as a player who wasn't on the plane and couldn't even get there."
Before toppling the Russians, Paterson had already made his mark in club football - in between injuries, which were to get more serious and more regular in later years.
After "living the dream on £45 a week" as a YTS kid at hometown club Stoke City he made his debut aged 18, but came close to missing out after incurring the wrath of irrepressible manager Tony Pulis when the idea that he would show his passion for the club he supported by dying his hair red and white almost backfired.
"The gaffer made me shave my head. He said, 'You won't play if you have that hair', " Paterson said with a wry smile.
"I'd no time, or probably no money left, to go to the barbers on a Friday. I went home and did it myself and it looked horrific, so I made my debut looking like a bald eagle."
When Pulis left and then returned for a second spell at the Potters, they were aiming for the Premier League and Paterson headed to Scunthorpe in the search of first-team football.
"Stoke actually offered me a better deal than Scunthorpe by a mile, but I'd been on loan at Grimsby Town and sampled proper football, the drug of scoring goals and being a proper professional," he said.
Five goals in the early months of the season earned him that first senior international call-up at the age of 20, and a tally of 14 for the season sparked interest from other Championship clubs as The Iron were relegated from the second tier.
He had been on the brink of signing for Norwich City, but one meeting with Owen Coyle was enough to take him to Burnley.
"I was substituted quite early in a game at Burnley and I sat and watched them play. There was just something about the way they were playing. There was a freedom to it. I thought I would score goals in that team," he said.
"Owen Coyle was really enthusiastic. I met him at the bottom of the Burnley steps. There are three flights of stairs up to his office. He's a bundle of energy and he said to me, 'I don't walk up stairs, I'll see you in the office'," and he sprinted up the three flights of stairs. My agent and I looked at each other and I thought, 'This will do me'.
"When we got into the office he said he'd got me a bacon sandwich. We'd just travelled two hours and I thought this was great and then he said, 'I thought we'd signed an athlete, but he's eating bacon butties'. It was things like that, I thought I like this guy and I still speak to him. I was on the phone with him the other day.
"Unfortunately that was probably the peak of my powers in the next two years. I scored 19 goals, we got beat in the League Cup semi-finals by Tottenham, lost in the FA Cup to Arsenal and won the play-offs.
"It was a dream come true to get to the Premier League. Everyone looks back on it, I'm sure, with great memories. I look back with a bit of sadness because that was me just starting to improve as a player, but really it was my peak - at the age of 21."
Paterson was only able to make 18 appearances in the top flight and his four goals were unable to help the Clarets stay up.
"We beat Manchester United 1-0 at Turf Moor. I was good that night. I played up front on my own and caused them problems. That was a real highlight for me because it showed that my style was able to affect good players," he said.
"I did my knee after that and it took me a while to get back. I came back and played on the right, and I think those last 12 games - I scored four goals - was probably the best I ever played."
In all, over three seasons, Paterson made only 78 appearances - although he did hit 21 goals in 41 displays in 2011-12 - before that ill-fated switch to Huddersfield.
"After the knee injury I felt there was an imbalance. I just felt something wasn't right with me. I couldn't run with that same speed. I was never really my true self," he said.
He knew the end was coming, but the path he took would lead to where he is now, making waves on the other side of the Atlantic as assistant coach at Fort Lauderdale CF, the reserve team of newly-formed MLS club Inter Miami owned by David Beckham.
"I went to Orlando and had four or five big injuries, came back to Blackpool and had 17 substitute appearances - not fit the whole year. Then I went to Port Vale and played a couple of months before I went to Tampa Bay Rowdies. I knew that I wasn't able to play properly and I was struggling. I thought it was the right time to leave English football because it had become too much for me," he said.
"Neil Collins, our centre-half at Tampa, got the manager's job and I was assistant manager for two years. I think Miami's sporting director Paul McDonagh, who signed me at Orlando, was always observing and watching what I was doing because he liked me as a character.
"I declined a new contract at Tampa and I believe that when Paul heard that he called me straight away. It was a very short phone call that went something like, 'Do you want to work for Inter Miami?' I said yes and that was it.
"David Beckham is in and around the training ground at times. I have seen him and spoken to him. I believe he is very active in the role as owner and I believe the club won't stop until they achieve success."
With his eye on a future as a head coach, neither will Paterson.