Whether you're a regular person who plays five-a-side with their friends once a week or a seasoned professional who's been doing it for 20 years on the biggest stage, when your time's up, you know it.
Maybe you're not able to walk straight for a week after playing as your muscles are tighter than a butcher's knot, perhaps your back goes when bending down to pick the ball up out of the net or, in Gary Neville's case, you've been given the run around by Peter Odemwingie and Graham Dorrans for 71 minutes. How can you ever come back from that?
Well, the answer to that is with great difficulty, as Martin Demichelis discovered during a Manchester derby in March 2016. While both sides weren't enjoying great seasons by their lofty standards - City were fourth, United sixth - as Leicester City took advantage and pulled off the underdog story of a lifetime, the stakes obviously remained incredibly high. Nobody told Marcus Rashford.
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The game was only 16 minutes old when the teenager marked his derby debut in style. Still wearing the No.39 on his back, he had managed to get goalside of right-back Bacary Sagna and was in a great position to take an incisive Juan Mata pass in his stride. Then, as simple as can be, a sudden burst of pace and a silky touch left a lunging Demichelis on his knees. With his next touch, Rashford calmly slotted the ball past Joe Hart to become the youngster Manchester derby scorer in the process.
Rashford was only 18 back then, playing in just his fourth Premier League match and, as had been seen with his brace in his league debut against Arsenal, the sense of occasion certainly didn't daunt him. At the Etihad, in enemy territory, he made a seasoned defender with over 500 appearances and several trophies to his name look as out of place as a Gianni Infantino selfie at a funeral of a beloved football icon.
It wouldn't be the only time he left his adversary red-faced. Demichelis was lucky not to give away a penalty as the rampant Rashford burst into the box only to be toppled to the ground by the centre-back's wardrobe-like frame. It was a clear body check with little attempt to play the ball, which Rashford was understandably not pleased with. He squared up to Demichelis and once Fernandinho got involved, Jesse Lingard did too, defending his young teammate.
Demichelis wouldn't last much longer. A sloppy back pass to Joe Hart resulted in the goalkeeper picking up an injury as he dealt with the danger. Three minutes after Willy Caballero came on to replace Hart, manager Manuel Pellegrini then hooked the defender, saying afterwards he was "nervous" and "not in a good day." United managed to hold on for the victory.
With that dubious last act, the curtain was closed on Demichelis' time at City and pretty much his career as a whole. Just shy of 14 months later, he would retire.
Many say Rashford actually retired him in that very moment. Like Old Yeller being put out of his misery, though maybe not as quickly or reluctantly. Social media was awash with United fans relishing in his blunder, claiming his career as a scalp for Rashford.
Never one to miss an opportunity to stick the boot in, Louis van Gaal served his own classic brand of brutal Dutch honesty: "Rashford is very quick and Demichelis looked like the years are catching up with him. He was a very good defender. He was my centre-back in Bayern, but that is the life of football."
“I know Martin well and he is a really professional player,” teammate and compatriot Caballero sympathetically said after the match, explaining how frustrated with himself Demichelis was. "Unfortunately, he feels really bad now about himself, and about the game, but he now has to go to Argentina for the international games, so I wish him all the best.
“We are a team, and when I have made a mistake and we have lost, I feel the same. The most important thing is not who makes mistakes, but who tries to react for the team.
“In this case it could have been Martin, but everyone makes mistakes during the season. We are a team and have to react, and to take our chances.”
That wasn't the last time he played for City but it was the last time he started. Demichelis would make just two more appearances for the Blues, being brought off the bench to play in defensive midfield. With the season concluded and Pep Guardiola bound for Manchester, Demichelis returned to Spain. He signed for Espanyol but played just twice before being released. Then there was a final swansong with former club Malaga, putting in a final 10 appearances. With two more caps for Argentina as well, Demichelis played just 16 more games after his encounter with Rashford.
“I want to be honest, I lost the strength in my legs and my concentration," he said when announcing his retirement from the game. "Unfortunately this day comes to us all. I've been deliberating it for some time, as I have always been very worried about the end of a professional football career."
Unlike his attempted tackle on Rashford, it was an elegant way for the City cult hero, not to mention a player with nine league titles to his name, to bow out. A touch of honesty and humility that a certain Portuguese now stranded in an irrelevant footballing backwater perhaps could have done with a smidgen of.
With another derby on the horizon and Rashford returning to the form that was so thrilling upon his introduction to the game, will he claim another victim?
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