Fatigue reaches the timing before it reaches the force

The first thing a tired nervous system loses is not strength but the precision of when muscles switch on.

Ask a fatigued athlete to jump as high as they can and quite often the number holds up. Ask them to jump the moment a light changes and it falls apart. The strength was there. The scheduling was not, and scheduling is what most sporting movements are made of.

Force and timing come from different parts of the same system. Peak force depends heavily on the contractile machinery and on how many motor units can be recruited at once, and a well-motivated athlete can drag that near normal for a surprisingly long time. Rate of force development and reaction latency depend on the speed and coordination of the neural drive, and those degrade earlier. Under fatigue, muscle activation onsets drift later, co-contraction rises because the system stiffens everything as a crude insurance policy, and the gap between the stabilisers firing and the prime movers firing widens.

That gap is where joints get caught. A hip abductor that switches on thirty milliseconds late during a cut is not a weak muscle. It is a muscle that arrived after the ground reaction force did, which leaves the knee holding a position it was supposed to be helped into.

You can see the same principle in longer efforts. The runner whose stride starts to look scruffy in the last kilometre has usually not lost much peak force. What they have lost is the tight coupling between the phases of the stride, so ground contact lengthens slightly, the foot lands a little further ahead, and stiffness leaks out of the system. Their engine is fine. Their conductor is tired.

I think this is why so many fatigue markers based on maximal output disappoint. A maximal test invites the athlete to compensate, and they will, using effort and adrenaline to hide the exact deficit you are looking for. Submaximal, timed and reactive tasks are more honest precisely because there is nothing heroic to be gained from them.

There is also a decision layer sitting on top, and it fails in the same direction. Tired athletes take longer to read a developing situation, and the movement they eventually choose is initiated late and executed in a hurry.

Rushed movement is loaded movement. The tissue does not know the difference between a bad decision and a heavy weight.