John McKown cautioned me that he found that the "thumb low" position (used in Mark V) created extra fatigue. That hasn't been my experience with the prototypes. I think I know why.
I've been learning far more about hand dynamics than I ever thought I would care about. For example, you really only have two sets of muscles controlling the curvature of the fingers. The third joint is connected to the same tendon as the second joint. The only reason you have a joint there is to help you wrap your hand around objects. Thus, you can only have a maximum of two keys per finger. That is, incidentally, the problem with the Mark III keyboard: you have to coordinate multiple muscles to press a key, whereas the standard chordite keyboard uses just one muscle.
The biggest muscle in controlling the thumb pulls it in to the middle of the palm; in the direction of curvature of the thumb. Thus, for the least fatigue, the keyboard should accept pressure from that direction, and redirect it as needed through a lever.
The direction of forces in a keyboard is complicated. The bulk of the force comes from the fingers pressing on the keys. This rotates the keyboard towards the base of the palm. A support at the base of the palm is supposed to counter that force. However, the force doesn't go away. You end up with the base of the palm being the fulcrum of a third-class lever. Your fingers are pushing on the key at the end of the lever, which forms the load. The effort must come from your thumb.
However, your thumb is not bearing directly towards your palm. Your thumb's direction of rotation (and thus the vector of force from the thumb's largest muscle) is pointing at the pinky, not at the middle of the palm (which is where the force is expressed, on average. In actuality, each finger produces a different rotational force on the keyboard. For now, we'll pretend that the direction of rotation is the same -- towards the base of the palm. Each finger, though, is pressing on this lever at a different distance from the thumb.
So, what kind of a lever do we have here? The fulcrum is the first knuckle, which has the keyboard support bearing on it. The effort is applied to the shorter distance (the distance between the pad that the thumb bears on, and the first kuckle support), and is expressed against the load from the sum of all the finger presses.
Too complicated! I need a picture here:
The fingers generate the load at the end of the green arrow. The fulcrum is the head of the green arrow. The force is being applied against the green lever by the red lever at the point of intersection of the green and red arrows. The thumb lever is a little more complicated, because the lever is split in two, connected by a rigid linkage (the keyboard). The thumb bears on the thumb saddle at the end of the red arrow. The fulcrum is the red dot, which sits top the first knuckle.
You can see that you have two class three levers. A class three lever requires more force but less motion. Two class three levers one bearing on the other requires lots more force but very little motion. The problem here is that there isn't any motion. This is all static force. You don't want to use a class three lever if you can help it.
I think that the Mark V provides better leverage. The thumb is bearing in the direction its largest muscle moves. The effort point of the lever is closer to the length of the load port of the lever, so it's closer to being a class two lever than a class three. The thumb and fingers are designed to grab things, so the closer the keyboard comes to something you can grab (like a rock), the easier it will be to use.