Reactive Strength represents the capacity to change quickly from eccentric to concentric exercises. An example would be landing on the ground and right away jumping upward. Another example is displayed when throwing punches in combination.
After throwing a right cross, you will instantly reverse the motion of your body as you follow with a left hook. Reactive strength utilizes the stretch shortening cycle. The stretch reflex begins with an eccentric phase, where the muscle increases in length under tension. During this stage, the body stores prospective kinetic energy. If a concentric action instantaneously follows the eccentric action, the kinetic energy will increase the force of the concentric contraction. The stretch shortening cycle is similar to the action of a rubber band.
When you stretch a rubber band (eccentric), you accumulate energy. As soon as you let go the band (concentric), the energy is utilized with a strong snapping action. The stored energy created by the tension enhances the strength of the following contraction.