Has there ever been...

[Didz]

But if so, then the speed of the horse's forward motion will decrease the force with which the naginata strikes the enemy, weakening the blow proportionally as the speed of the mount increases. Perhaps it was designed for use when relatively stationary?

Thats the point really, by striking backwards the impetus of the horse works with you to remove the blade from the victims body, if on the other hand you strike on the approach the impetus works against you and is likely to force the blade harder into the target. Needless to say, if the blade becomes imbedded in a dead weight whilst your mount continues to move way at a gallop something has to give, which probably means, your arm, your marriage to your horse or your weapon. None of which are particularly conducive to survival on a battlefield.

Another interesting question is how the naginata user would acheive purchase for any sort of swing without exposing himself to an enemy sword thrust. On foot defence is acheived by rapid movement and creating a space around the fighter by sweeping his weapon, but that become less practical to acheive on a horse. The horse is hardly going to be able to predict its riders intentions and may well have its own views on what is appropriate, whilst sweeping a pole weapon defensively becomes a lot more awkward with a horse between your legs and its head blocking the forward quadrant of the swing.

The French napoleonic drill manual for lancers shows various procedures for weilding a pole weapon defensively including one for defending yourself from an attack from your exposed left side by using a similar sweep like movement. This basically involved, raising the lance over your head with one hand, spinning it in your open palm so the point was facing over the horses rear and then sweeping it forwards across the left side of your horse and over your horse head in the hope of distracted your attacker. Trying to actually do that in any sort of swirling melee would have been pretty challenging and the results not particularly likely to succeed, even if you managed not to drop the lance during the spin. Not to mention the fact that sitting there with your arms raised over your head is just inviting someone to stab you in the chest.

Needless, to say the evidence is that such drills were rarely followed in actual battle and the most common solution seems to be to drop the lance as soon as the combat got messy and switch to your sword. Two-thirds of the men in all lancer regiments carried swords anyway, simply because anyone not in the front rank armed with a lance was more of danger to his friends than the enemy.

[Ulstan]

To me, the 'omg they'd cut the horses heads off' argument is one of the least plausible for why two handed weapons were not widely used on horseback.

I believe it is because they weren't necessary, and the benefit of a shield outweighed any advantage having a two handed weapon would give a mounted soldier.

An amusing bunch medieval knights must have been, always cutting off theri own horses heads and then crashing gloriously to the ground where the weight of their armor would prevent them from even getting up!

[Rebellious Waffle]

Hey, where else would they get the one horse's head in the Godfather?

[Didz]

An amusing bunch medieval knights must have been, always cutting off theri own horses heads and then crashing gloriously to the ground where the weight of their armor would prevent them from even getting up!

Thats another myth, beleived to have been based on the fact that Henry VIII needed to be winched onto his horse for jousting contests, mainly due to the gout in his legs.

Weapons Archeologists like Mike Loades have demonstrated repeatedly that not only could a knight in full plate get up off the ground, re-mount his horse and climb a seige ladder in full armour, but that they could run jump and even dance in armour if need required.

[Kobal2fr]

What he said. From Wikipedia :

While it looks heavy, a full plate armour set could be as light as only 20 kg (45 pounds) if well made of tempered steel. This is less than the weight of modern combat gear of an infantry soldier, and the weight is better distributed. The weight was so well spread over the body that a fit man could run, or jump into his saddle. Modern re-enactment activity has proven it is even possible to swim in armour.It is possible for a fit and trained man in armour to run after and catch an unarmoured archer. The notion it was necessary to lift a fully armed knight onto his horse with the help of pulleys is a myth originating in Mark Twain's "A Connecticut Yankee in King Arthur's Court", and only rarely occurred in the 19th century. Even knights in enormously heavy jousting armour were not winched onto their horses. This type of "sporting" armour was meant only for ceremonial lancing matches and the design had to be extremely thick to prevent severe accidents, such as the one causing the death of King Henry II of France.

Granted, the suits of armour worn by re-enactors are made out of better quality metal, and are probably a bit lighter than the real thing because of this, but on the other hand your run-of-the-mill re-enactor is a librarian, historian, scientist and so on, not a fit, trained and exercized military man.

This idea that plate armour was super-heavy stems from a complete misunderstanding of the whole point of platemail : the idea was not to take the hits like a WW2 tank armour would, but to deflect them harmlessly, mostly through the use of curved surfaces and clever design. So they didn't need to be that thick.

(To be 100% accurate, WW2 tank armour was often sloped, and thus also caused shots to glance off sometimes but it wasn't the reason it was sloped in the first place - it was so you had more "effective" thickness compared to a vertical plate, as the incoming shell would have to punch through more metal. So in essence, it was the brute force approach to armoring )

[Rebellious Waffle]

I wouldn't rule out the possibility that medieval armors were very light -- I know swords made during that period were dramatically lighter than most modern replicas (.9 to 1.3 kg, whereas modern replicas often weigh as much as 5.4 kg) so armor weight close to, or perhaps less than, that of modern replica armors might not be out of the question.