Stiffness jumps…
Depending on your event. For a short distance sprinter I would suggest a rebound jump (drop back off a box and jump back onto it). The box should be at least twice the height that the centre of gravity would traverse during a sprint which would be 5 cm for slower sprinter and less for faster sprinters. That means the box should be at least 10 cm in height for the slower sprinters. The goal is to rebound with as short a ground contact time as possible to develop the skill of efficient recruitment patterns during pre-activation and activation as well as providing stimulus for the tendons and connective tissue surrounding the muscle to adapt. In addition the rebound jump serves the purpose of enhancing the skill/processing of feedback from the muscle spindles and golgi tendons.