Newswise — Running with heavy shoes costs more energy than running with lightweight shoes. Does this also mean that you run slower with heavier shoes? To answer this question, researchers used three pairs of identical shoes and added small lead pellets to two pairs to make each shoe 100 and 300 grams heavier. (Note: an apple or deck of cards is about 100 grams.) Next, 18 runners ran a 3,000 meter time trial in each of these pairs once a week for three weeks. Unaware of the differences in shoe weight, the runners ran about one percent slower for each 100 grams added lead. Slowing down by one percent agrees well with previous laboratory treadmill studies showing that adding 100 grams of shoe mass costs one percent more energy. Changes in energy costs measured in the lab translate to changes in running performance.