What is "speed endurance"?
"Speed Endurance" can be defined as the ability to maintain speed after an athlete has hit their maximum top speed.
An olympic sprinter running sub 10, will typically hit top speed around 70m. A college sprinter running sub 10, will typically hit top speed around 60m. Everyone else will hit top speed around 40-50m.
After the point a sprinter reaches their maximum speed, everyone begins to decelerate. This is where speed endurance training can help an athlete reduce the amount of speed they lose.
For example, let's say an athlete with poor "Speed Endurance" loses 5% of their speed after they reach top speed.
Now let's say they correctly implemented "Speed Endurance" training and instead they only lose 3% of their speed. That would be an example of successfully improveing SE.