You will see it described as the transient inability of a muscle to maintain optimal physical performance. Nor is it regarded as something than can be observed by others.
According to the medical profession, then, it would not be possible to look at Hearts' nest of young players and conclude that they have been overplayed to the point of exhaustion this season. That is the claim often made on their behalf, of course.
What about confidence, then? Surely they are short of that given that the last time they won a game, Methuselah was about of an age with most of their current players? "We're definitely not running short on confidence," said a bright and perky Scott Robinson, their central midfielder.
Hearts' young players continue to unintentionally undermine claims that they are being worked to the point of being knackered, and are so battered and beaten that the SPFL should lift its signing embargo so that reinforcements could be sought. Call it the impressibility of youth, but it is amusing to see them offered sympathy and compassion and swat them away without a care in the world.
"If we were totally lacking in confidence Motherwell would be coming here and winning 3-0 or 4-0," said Robinson. "I felt we should have got a draw. We're just not getting the breaks but that just seems to be the way it is right now."
This week the SPFL executive will consider a letter from Hearts' administrator, Bryan Jackson, calling for the current signing embargo to be lifted in order to spare their young players burn-out. It will not happen and Jackson knows it. The SPFL will not give the request enough consideration to look into the specific issues being claimed on behalf of the Hearts "kids" but, if they did, the performance against Motherwell on Saturday would count against the club's case rather than for it.
Hearts played with guts and energy at Tynecastle. Tired and lacking confidence? They were still battering away at Motherwell, joint second in the league, in the closing stages of the game. There was a moment in the second half when their sheer doggedness and resilience provoked a loud reaction from the stands. Chants went up including a "we'll support you ever more". It was a forceful show of unity and appreciation from the supporters.
The issue is not tiredness nor even confidence, it is the lack of quality throughout the side. Hearts make too many poor decisions and lack the technique to complete moves when they make the right ones. When Ryan Stevenson drifted from the attack back into the deeper roles, there was no-one up front who could hold the ball up (Dale Carrick was unable to) and Motherwell simply mopped up and came back at them.
There will be no reprieve for manager Gary Locke in terms of the embargo being lifted - when he was asked if he was optimistic he simply laughed - but sympathy for him comes naturally. Danny Wilson was out with tonsillitis, Callum Tapping and Gary Oliver were on the bench without being fully fit, Kevin McHattie came off with a hamstring strain and Jamie Walker will undergo a scan today to determine when he suffered a broken metatarsal.
"Disaster," said Locke. If Swansea City complete a £200,000 move for Adam King, Hearts' fervent wish is for him to be allowed to stay for the remainder of the season before going south in the summer, simply because they need bodies.
Relegation has become almost an afterthought, but it is inevitable. A seventh defeat in eight league games (the other was a draw) and Ross County's weekened point means Hearts are an irrecoverable 20 points adrift. "It's going to take a miracle to turn this around," said Robinson. "But until it's mathematically impossible we will keep fighting and giving our all. You could see the effort and commitment that the boys put in.
"We will keep fighting for the jerseys. We've also got a massive semi-final coming up [against Inverness Caledonian Thistle in the League Cup] and, if we can win that and reach another final, then it won't be the worst season in the world."
Motherwell battered away at Hearts in the first half. James McFadden and Lionel Ainsworth were a handful and Iain Vigurs released a pass from which John Sutton squeezed home a brilliant angled finish against his old club just before half-time. The victory made club history as the first time in 77 years that Motherwell had managed six consecutive top-flight wins.
Their record has been flawless since losing to Albion Rovers and then collapsing 5-0 at home to Celtic in consecutive matches.
On Saturday they travel to Parkhead. "Celtic have a great side and it will be a good game," said Vigurs. "But we're going there to win. There's no better time. We've clicked and the boys all know each other's best attributes."
He namechecked McFadden, Keith Lasley, Stephen McManus and Steven Hammell: "We've got experienced boys in our dressing room and they've pulled us through. There's definitely more to come from us."