The adidas UltraBoost 21 'Core Black' applies a rudimentary palette to a performance running shoe that combines lightweight comfort with enhanced energy return. Recycled Primeblue materials, including Parley Ocean Plastic, goes into the construction of the knit upper, fortified with a midfoot cage and molded heel counte. An optimized Boost midsole, made with 6% more Boost capsules than the UltraBoost 20, features added responsiveness via a new Torsion system that adidas calls Linear Energy Push.