Well, yes this is all supposition I will grant that your arm might be more strained if it was straight but I'm conerced that bending it slightly would effect stability when recieving blows, you may have apoint though. try this, instead of holding the arm straight have your forarm parralel to the ground and your upper arm at an angle so that your upper arm is what holds the shiled away from your body.

Actually that may have been how they did it, rather than a straight arm. That would give you stability but it would also put most of the strain of taking blows on your shoulder and upper arm. I could phone John if I could find his number but it really would be better if I could talk to him face to face with at least some form of scutum.