Sun, 19 Mar 2006

The hand is more complex than I thought

Refer to this numbering system for the metacarpals (bones of the palm) and phalanges (finger bones). Except for the carpals and associated joints, all of these joints are hinge joints. That is, they rotate in only one plane. The knuckles are the joints at the ends of the metacarpals.

You'll need to look at your own hand now. If you're blind, you'll have to do this by touch. If you have no hands, you'll have to borrow someone else's hand. Straighten out your hand so the phalanges and metacarpals are all in the same plane. Bend your knuckles, and you can see that they're all pretty-much co-linear. You can see that the metacarpals travel through parallel planes. If they were pressing Marquardt switches, the switches would be mounted in the same plane (although possibly at different heights)

Now relax your hand, as if resting on a ball. Those damn metacarpals move around! They rotate relative to each other. Given that the knuckles are hinge joints, and can only move in the same plane, and that plane has now rotated. That means that the distance between your fingertips depends on whether you're holding your palm relaxed or flat.

To see what I mean, form your palanges into a C shape with your palm (metacarpals) relaxed. Now, moving only your palm, and not your knuckles or your thumb, flatten out your palm. You may need to move your knuckles a little bit. You will see that your fingers spread apart.

The implication of all this is that the phalanges on a relaxed hand will tend to come closer to each other as you bring your fingertips towards your palm. It also means that the direction of movement converges. Because it converges, using coplanar-mounted switches is not best. That means that the Mark VI switch isn't going to work well. Nor does the Mark VII keyboard. The only way such a key mounting would align correctly with the finger movement is on a Mark IV keyboard (phalanges nearly in-line with metacarpals), which I've already decided is not comfortable.

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