A lightweight Boost engineered midsole is embedded into the Solar Boost 19. The boost foam compound provides endless energy return from heel strike through to the toe-off phase of the gait cycle. It features hundreds of visible energy capsules that store and unleash endless energy every time your foot hits the ground, meaning that every step is charged. When your heel hits the floor, the boost particles compress and store kinetic energy; as you transition through to mid-stance, the boost particles relax and release their stored energy, which propels you forward with an explosive take off. The boost midsole will change your runs forever, thanks to its energy-returning properties. In addition, when the boost capsules compress to store energy, they cushion the foot from impacts. The boost particles only compress so far, which means that they also provide light support whilst on the road. A Solar Propulsion Rail works with boost to increase stability for a smooth, distraction-free run. The rail helps to guide the foot through the gait cycle for a more dynamic run and better performances. In addition, a midfoot torsion system moves with the foot for a smooth, supportive transition from landing to push-off. The system exists to increase stability and allow the forefoot and rearfoot to move independently for added protection, control and an easy heel-to-toe transition.