The Brooks Glycerin GTS 21 has super-soft cushioning for supreme comfort. Now with more super-soft nitrogen-infused DNA LOFT v3 cushioning that is optimized to be lightweight, responsive, and durable for push comfort. With improved fit from the engineered warp knit upper which is breathable and allows for a flexible, accommodating fit while still soft against the foot. The Guiderails support your body in its natural motion path while keeping excess motion in check. The new RoadTack rubber is environmentally friendly, with a recycled silica content. Other recycled materials used is equivalent to 5 plastic water bottles.