Quote Originally Posted by Tachikaze
The advantage of stirrups for archery is very significant. In the days before stirrups, lances were often used for overhand thrusting, not cradled across the lap. This technique minimizes the benefits of stirrups.
The lances used by the Sarmatians were described by the Greeks as 'barge poles' implying a longer and heavier weapon, that would make overhead thrusting awkward if not impossible. In his Aenid, Virgil also refers to a cavalry display at a funeral, with riders couching their lances, indicating that the technique wasn't unknown centuries before the invention of the stirrup.

If from the experience of modern jousters we are to accept that built-up saddles are the key to effective couched charges, rather than stirrups, then there's evidence from the Roman era that implies again that the couched charge long predates the stirrup: the four-horned Roman military saddle and saddles shown on Trajan's Column with high pommels and cantles, for example, exactly the sort of saddle design that would increase the effectiveness of a couched charge given modern experience.

Stirrups definitely aid mounted missile combat, whether archery or firing an Ak-47 - I recall seeing a short clip years ago showing a Mongolian horseman riding at full speed, standing in his stirrups with his upper body effectively rock steady, despite the rise and fall of the horse, firing his rifle at a series of targets as they galloped past. Stirrups also aid close-quarters combat, allowing the rider to again stand in the stirrups, thus granting greater flexibility of movement to dodge blows or extend reach, plus adding height and therefore impact to his own blows, etc. So if anything, stirrups would be more important to overhead thrusting than couched charges.

Overhead thrusting was probably used because it allows the rider to more easily strike to either side - avoiding the need to lift the weapon over the horses head - and to fight at closer quarters than might be possible wielding a spear underhand, without having to shorten your grip and thus unbalancing the weapon. Once a spear becomes too long, its weight would make it unwieldy for overhead thrusting, however at the same time it becomes more effective for couched charges since you can strike the target while still out of reach of his weapon.