The AMA Supercross Championship is an American motorcycle racing series. Founded by the American Motorcyclist Association (AMA) in 1974, the AMA Supercross Championship races are held from January through early May. Supercross is a variant of motocross which involves off-road motorcycles on a constructed dirt track consisting of steep jumps and obstacles; the tracks are usually constructed inside a sports stadium. The easy accessibility and comfort of these stadium venues helped supercross surpass off-road motocross as a spectator attraction in the United States by the late 1970s.