The last 10 to 20 metres of the 100 m dash are described as the deceleration phase due to fatigue(tiredness).
Even though it is said to be the deceleration phase it is being noticed that top athletes are becoming able to maintain
their maximum velocity for longer intervals allowing them to pass the finish line with a higher speed. Sprinters have improved
a great deal, but there is still deceleration (slowing down) at the end of the race. Just not as much.
The competition rules at the finish line are that the final time is calculated from when the trunk of the athlete passes
the finish line.
Therefore this is why we see athletes falling at the end of their race. They lean forward at the end. Because they are
travelling with a great deal of momentum that has built up over the race, they are propelled downward and fall.