Introduction of the stirrup

[Dan Howard]

It seems to me that the only way you could use stirrups to add the weight of the horse to the weight of the rider on impact with a lance is if you leant forwards in the saddle, pushing down and backwards with
the stirrups. But you'd need stirrup straps that were long enough and were positioned far enough back on the saddle that you could swing your legs backwards on impact. That could add some of the weight of the horse to the rider, but I don't know if it was ever done.Ambrosius

If you look at the Bayeux Tapestry you see the opposite occurring. The legs are forward of the saddle and the knees are almost straight.