If what I have been saying in this comment section has any value, then most people have already committed to muscle memory how long each of their fingers is relative to any other finger on the same hand.
If you just hit the D key and want to hit the T key next, on the Kinesis Advantage, in addition to having to know how far the D key is from the T key in the two horizontal dimensions, you have to know the delta in altitude (i.e., how far it is in the vertical dimension).
In addition to the "delta in altitude" information just described, almost every key on the Kinesis (the axis along which the key will travel when pressed) "points" in a different direction, which is two more degrees of freedom. So, for almost every key on the Kinesis, I would have had to "memorize" (i.e., commit to muscle memory) 5 "scalar values" in contrast to the 2 scalar values for every key on a normal keyboard.
I am not saying that no one can "memorize" that information; I am saying only that my estimate of the time and effort required for me to memorize it caused me to ditch my Kinesis for the standard planar arrangement.
Note that I used a Kinesis for 2 years. It was only after I realized that I would need to "memorize" where the keys are (and what direction they travel when pressed) to recover from RSI that I gave up on Kinesis.
And yes, I have been free from RSI in the six years since making this realization and ditching my Kinesis.
And yes, I think most of the ergonomic keyboards currently sold by Microsoft are bad for the same reason the Kinesis Advantage is bad -- particularly those keyboards that could be described as "wave shaped" or "hill shaped". (At least the Kinesis Advantage has decent keyswitches!)
ADDED. There is more wrong with the Kinesis than just the amount of information it requires a person to commit to muscle memory. I just got done stating that the DataHand is an even worse keyboard than the Kinesis, and the DataHand requires a person to have only a few "muscle memories". If I understand correctly and if the DataHand device is a good fit for the person's hand, for every character, the person need know only which finger to twitch and which of four directions (up, down, left or right) in which to twitch the finger.
The reason the DataHand is such a bad keyboard is that it causes the person to repeat over and over again a kind of movement that is very different from the kinds of movements people are used to making and are good at making.
An example of a kind of movement that humans are good at is striking something with a hand or a finger. There is a lot of (conscious and unconscious) "data processing" going on when a human is getting ready to hit something with a hammer or with his fist, hand or finger. My belief is that the best way to avoid RSI and other problems is to get the brain to devote a lot of "cpu cycles" to planning out the physics of every keystroke.
The DataHand (and to a lesser extent the Kinesis) has bad ergonomics because it makes it hard for the brain to do detailed physical planning. This planning would entail perceiving or remembering the exact location, relative to the striking finger, of the key to be pressed. In contrast, when all the keys are on the same plane, it is more likely the the brain will plan out "ballistic" movements resembling hops, or plan out a "backswing" before the "swing", and, again, we want to get the brain deeply engaged in planning movements.
Designing the keyboard so that every key is right next to a finger tip is bad ergonomics because the brain is poorly adapted to carrying out the resulting key-pressing movements.
To summarize, the most important things for avoiding RSI are (1) choosing a keyboard that encourages the right kind of movements (namely, the kind the brain is good at planning) and (2) reminding oneself during typing that pressing a key should be more like hitting a ball with a tennis racket than making a sign with a flag, baton or the hand. Part (1) consists mainly of avoiding so-called "ergonomic" keyboards like the DataHand, Kinesis and most modern Microsoft "ergonomic" keyboards. The Microsoft Natural Elite keyboard by the way was a good keyboard (well, besides the pretty crappy rubber-dome keyswitches) because even though it was a "split" keyboard, all of the keys designed to be hit with the left and lay in the same plane, as did all of the keys designed to be hit with the right hand. But they might be hard to get now. And like I said, the keyswitches are pretty atrocious even compared to the (scissor-style) keyswitches on the good modern Logitech keyboard like the K750.
