Other distinctive features are a relatively long, level croup, or top of the hindquarters, and naturally high tail carriage. The USEF breed standard requires Arabians have solid bone and standard correct equine conformation. Well-bred Arabians have a deep, well-angled hip and well laid-back shoulder. Within the breed, there are variations. Some individuals have wider, more powerfully muscled hindquarters suitable for intense bursts of activity in events such as reining, while others have longer, leaner muscling better suited for long stretches of flat work such as endurance riding or horse racing. Most have a compact body with a short back. Arabians usually have dense, strong bone, and good hoof walls. They are especially noted for their endurance, and the superiority of the breed in Endurance riding competition demonstrates that well-bred Arabians are strong, sound horses with superior stamina. At international FEI-sponsored endurance events, Arabians and half-Arabians are the dominant performers in distance competition.