The APS (Antipronation Shock Absorption System) were the first running shoes offering customized cushioning. Localised in the heel area, the system was visible through a window on the bottom of the outsole. A special cassette incorporated in the midsole contained polyurethane rods placed on a thermoplastic polyurethane shaft. The cushioning could be regulated by a key added to the shoes. Turning the key clockwise draw the rods together, making the system harder, turning it anticlockwise made the midsole softer. Thanks to these features it was possible to adapt the cushioning to individual preferences and the weight of the runner, and also to the terrain where he was running. The APS were designed especially for overpronators, that is the group of runners who turned their feet to the inside while running. Therefore, the rods on the inside were harder than on the outside of the system, to provide the runner with as much stability as possible. The set absorption level could be checked through the window in the outsole.
A couple of Adidas APS photos (courtesy of adifansnet):
Below you have a few photos of the original leaflet added to Addas APS, precisely describing the structure of the shoe and the system (courtesy of nicetrainersmate.co.uk) - click on the photos to enlarge them:
Shoes offering runners such possibilities of adjusting the cushioning to their individual requirements didn't appear on market long after the Adidas APS. In 2003 a retro was released. It featured a slightly changed shape and a "APS" signature stitched on the heel of the shoe:
