ENERGY WAS JUST ENERGY UNTIL ENERGY MET ULTRABOOST 21. Prototype after prototype. Innovation after innovation. Testing after testing. Meet us in the hot pursuit of the pinnacle harmonization of weight, cushioning, and responsiveness. Ultraboost 21. Say hello to incredible energy return. SPECIFICATIONS Sock-like fit Lace closure adidas Primeknit+ upper Textile lining Supportive heel counter Boost midsole Weight: 340 g Midsole drop: 10 mm (heel 30.5 mm / forefoot 20.5 mm) Stretchweb outsole with Continental™ Rubber