A-Boundis a recycled, environmentally friendly midsole compound for resilient underfoot protection.Dual Layer EVAmidsole foam for durable cushioning.Natural Ride System (NRS)design includes a more natural, foot-shaped last, zero millimeter heel to toe drop, and metatarsal-specific shapes in the outsole to assist in proper foot mechanics and allow for more natural foot movement.Natural Foot Design follows the natural shape of the foot for ultimate comfort. The Altra Olympus 2.5 is Altra's most cushioned trail shoe. With sticky traction and all-day cushioning, this is a great choice for ultramarathons or rugged trail runs.