Explain how "elastic potential energy" dramatically changed the sport of pole vaulting when flexible fiberglass poles replaced stiffer wooden poles.

Wooden poles are rigid. That means that if you apply a force on the top of it, energy will travel directly to the ground (and you will just feel a very strong normal force) and the pole won't store any energy. An elastic pole will flex, storing energy that won't go directly to the ground, but as it recovers it's original shape, half of this stored potential energy will transforme to kinetic energy for the athlete, giving them energy to jump higher.

Explanation:

Explained above.