The invention is a system for modifying characteristics of an article of footwear, particularly the sole structure. The footwear may include a plurality of discrete, vertically-projecting, columnar elements that extend between upper and lower portions of a cavity formed in the sole structure. At least one of the columnar elements includes a void that is configured to receive an insert. The void is accessible from the exterior of the footwear and the insert may be interchanged with an alternate insert that has different physical characteristics, thereby modifying the characteristics of the footwear.