called the "Big Lick". The customary style for rider attire and tack is saddle seat. Horses are shod in double and triple-nailed pads, which are sometimes called "stacks". In the early 21st century, this form of shoeing is now prohibited at shows governed by the National Walking Horse Association (NWHA), and the United States Equestrian Federation (USEF). Artificially set tails are seen in "performance" classes, on full-grown horses in halter classes, and in some harness classes, but generally are not allowed in pleasure or flat-shod competition. Horses in western classes wear equipment similar to that used by other breeds in western pleasure classes, and exhibitors may not mix English and Western-style equipment. Riders must wear a hat or helmet in western classes. Tennessee Walkers are also shown in both pleasure and fine harness driving classes, with grooming similar to the saddle seat horses. In classes where horses are turned out in saddle seat equipment, it is typical for the horse to be shown in a single curb bit with a bit shank under , rather than the double bridle more common to other saddle seat breeds. Riders wear typical saddle seat attire. Hats are not always mandatory, but use of safety helmets is allowed and ranges from strongly encouraged to required in some pleasure division classes. Horse Protection Act The showing, exhibition and sale of Tennessee Walking Horses and some other horse breeds is governed by the Horse Protection Act of 1970 (HPA) due to concerns about the practice of soring. This developed during the 1950s and became widespread in the 1960s, resulting in a public outcry against it. Congress passed the Horse Protection Act in 1970, declaring the practice to be "cruel and inhumane". The Act prohibits anyone from entering a sored horse into a show, sale, auction or exhibition, and prohibits drivers from transporting sored horses to a sale or show. Congress delegated statutory responsibility for enforcement to the management of sales and horse shows, but placed administration of the act with the Animal and Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA). Violations of the HPA may result in criminal charges, fines and prison sentences. The USDA certifies certain Horse Industry Organizations (HIOs) to train and license Designated Qualified Persons (DQPs) to complete inspections. APHIS inspection teams, which include inspectors, investigators, and veterinary medical officers, also conduct unannounced inspections of some horse