Having had a little time to think some more on this , I believe I left a few things out of my initial reply.

Brent; There usually is a back plate to go with that breast plate so the voider will cover that area also.

Lafayette; I have fought in full harness from horse with hand weapons and I have to tell you it's not as easy as it looks. Most of what we did was with "arming" swords for counting blows as we all can pretty much agree that a swords edge is pretty ineffective against plate (and were out out have some fun, not kill each other). It is difficult enough to get horses within striking distance for that alone! To use a mace or hammer from horse you have to be extremely close and you are almost wrestling from the saddle at that point. I cannot imagine using a "longsword" as a short lance or spear from horse due to the ranges involved and any technique to make this effective would have to be done at speed. I believe that the end result would be more like a head on collision that anything else. Perhaps Rittmiester Frye or Gordon Clark would like weigh in on this.

Well, in this case we have to remember that I'm not talking from personal experience, only from Jean de Joinville's account of the something-th Crusade. The one that got stuck in Damietta. That being said, I did find that the longsword's grip is more convenient for carrying it in a couched position--although I've never used it to really hit anything that way.