The Endorphin Speed 6 Running Shoe swaps in PWRRUN PBX, a lighter, softer foam with 2 millimeters more of it underfoot, and pairs it with a reshaped nylon plate for tempo runs and speed days. The plate adds propulsion and smoother transitions as the pace climbs. What Our Gearhead(R) Experts Are Saying:"On speed days, the mesh upper held my foot in place as the pace climbed, so my stride stayed consistent from the first interval to the last. The breathable mesh also kept the foot cool through the final repeats."