The invention that helped medieval knights stay on horseback more effectively was the stirrup. Attached to the saddle by leather straps, a pair of stirrups gave a rider stable foot support for mounting, steering, and fighting.
The true stirrup developed in China during the Jin dynasty, probably by the 4th century, and was widely used there by the 5th century. It spread westward across Eurasia, reaching Europe by roughly the 7th or 8th century. Iron stirrups have been found in 7th-century Avar graves in Hungary.
Stirrups helped a mounted warrior keep balance while swinging a sword or leaning to either side. They are often credited with making the couched-lance charge possible, although historians debate how much the stirrup itself changed warfare; the saddle and existing cavalry tactics were also important. A stirrup is not the same as a spur, which urges a horse forward, or a lance, which is a weapon.