 Doctors in the United States have reported an increase in such injuries as pulled calf muscles, Achilles tendinitis, and metatarsal stress fractures, which they attribute to barefoot runners attempting to transition too fast.The running shoe itself has also been examined as a possible cause of many injuries associated with shod running.One 1991 study found that wearers of expensive running shoes that are promoted as having special features, such as added cushioning or "pronation correction", were injured significantly more frequently than runners wearing inexpensive shoes.