Energy Return System (E.R.S.) is a technology created by Reebok where a series of cylinders composed of the plastic DuPont Hytrel are placed under the midsole of an athletic shoe. The system was introduced in 1997 to compete with Nike Air, and was gradually phased out of production upon the arrival of Reebok’s Hexalite cushioning [1]. Reebok claims that the cylinders act as springs, releasing energy after a foot strikes.