A sole structure for an article of footwear is provided. The sole structure extends longitudinally from a back edge to a front edge of the article of footwear and transversely from a medial side to a lateral side of the article of footwear. The sole structure includes an outsole having a ground-contacting layer. A first rib projects upward from the ground-contacting layer. The rib has side walls and an end wall. A channel is defined by the side walls and end wall. The channel opens downward through the ground-contacting layer and has a depth extending above the ground-contacting layer. The rib may have a multi-stage vertical stiffness profile. The sole structure may include a midsole attached to the outsole. An article of footwear having the sole structure attached to an upper is also provided.