the.com/horseback

Humanity's first 70 mph, achieved on something that mostly wants to eat grass.

means On the back of a horse — the position you occupy when riding one, as in 'on horseback.'

from A plain Old English compound: 'horse' (hors) plus 'back' (bæc), the literal back of the literal animal. English has long loved this kind of stacking — 'horseback,' 'shoulder blade,' 'fingertip' — naming a part by simply bolting two everyday words together. The word does exactly what it says, which is rare and refreshing.

original cavalry: Mongol riders conquered more land than anyone in history
core strength: Riding works abs harder than the horse's legs
trust fall: You steer a half-ton animal with your hips
ancient flex: Stirrups arrived late, around 300 AD China
reading minds: Horses sense your heartbeat and mirror your nerves

for instance

buzkashi — afghan sport where teams on horseback compete for a headless goat carcass

pato — argentine game played on horseback with a ball in a net, dating to 16th century

reenactment at gettysburg — civil war battle site in pennsylvania where cavalry charges are recreated annually