Ultimately, the ergonomic goal of a tool like a keyboard which is used for extended periods should be to encourage use with all joints in a relatively neutral position, with very low static load on muscles.
The thing that splitting the keyboard (while keeping it flat) helps with is reducing “ulnar deviation” of the wrist (outward rotation in the plane of the hands). It is also possible to avoid ulnar deviation on a standard keyboard by changing typing style a bit, which is why some people have less problem than others with a single-piece keyboard.
What helps most is not splitting the keyboard per se, but tilting the two sides upward toward the center (the ideal is probably something like a 45° tilt, but even a bit of “tenting” helps significantly).
Having the hands palms-down and close together (as forced by a standard keyboard) ends up causing an unpleasant trade-off between (a) forearms rotated uncomfortably inward about their axis (“wrist pronation”), or (b) elbows swung forward or out to the side to reduce the amount of wrist pronation, in the process causing shoulder and back strain, or causing people to rest their elbows, forearms, wrists, or palms on some surface to compensate for the shoulder strain, and often in the process flexing or extending their wrists.
Wrist flexion or extension is really the worst for any kind of repetitive motion. It dramatically reduces strength and responsiveness of the joints, and can cause severe repetitive strain injuries. You will notice that typical human tools are designed very carefully so they can be used with a mostly straight wrist (or tool-user technique has developed to work around tools which don’t make this obvious).
Tilting the keyboard the right amount front-to-back to match the height of the keyboard relative to the torso can help quite a bit; most keyboards I observe in the wild are tilted incorrectly. You want the plane of the keyboard to be roughly parallel to the plane of the forearm. So on a tall desk, the keyboard should be tilted up at the back. On a low keyboard tray they keyboard should be flat or even tilted slightly down at the back.
Personally for myself, I found the best option to be MS Natural 4000's backward tilt (down at the back)—I'd like it to tilt even more at low desk heights but oh well. That way, the wrists stay straight while the fingers drop downwards a bit, in a feeble emulation of Kinesis Advantage's pits. I even use a wrist brace sometimes, which pads the wrist on the palm's side giving even more of a raise—feels really great for the fingers, but alas can't be reproduced in a stock wrist support because of the thumb getting in the way.
On a tall desk with a low chair, a “backward” tilt to the keyboard almost inevitably causes some uncomfortable hand/arm positioning.
If you have a high desk and low chair and you can’t change the furniture, you want the keyboard to be tilted (sometimes very aggressively) “forward” so that the plane of the keyboard is parallel to the plane of the forearms.
The keyboard should be kept close to the torso so that the upper arms can hang loosely down with relaxed shoulders. A bent elbow isn’t generally too big a problem.
You’ll notice that typewriters from the middle of the 20th century have a very aggressive tilt to them, to compensate for the standard high desks and low chairs (which were optimized for a middle-sized male to write with a pen, but made quite poor furniture for typically female typists.)
> wrists stay straight while the fingers drop downwards a bit
I have observed people doing this; usually their wrists are not actually straight, but are extended (i.e. bent downward), which is also a bad idea.
Resting palms, wrists, or forearms on a surface is generally undesirable during active typing in my opinion. It’s fine when the hands are not being used but are just sitting there.
Also, the fingers should not “drop downward”. The primary joint which flexes to type a key on a keyboard is the near knuckle where the finger attaches to the palm. You want these first knuckles to be straight when the finger is on the top of the unpressed key, and then flex downward to press the key; this is the strong and efficient part of that joint’s range of motion.
When reaching the near rows of keys, the further knuckles will be bent. When reaching the far rows of keys (which should ideally have tall keycaps) the further knuckles will straighten to reach the keytops.
My personal opinion is that the Kinesis Advantage is not the most biomechanically sound design; it was more or less a cheaper-to-produce ripoff of the Maltron, made by people who had done less thinking about typing than Malt. With careful attention it is IMO possible to significantly improve on both of those designs.
> My personal opinion is that the Kinesis Advantage is not the most biomechanically sound design
I agree. I use an Advantage, and while it's certainly better than traditional keyboards, I think it has a couple of flaws in terms of ergonomics:
• the keywells are not rotated inward and the distance between them is not adjustable. Therefore, unless your shoulder width is exactly right for this keyboard, you have to rotate your hands in- or outwards
• the thumb clusters are too far away. For me, if I relax my thumbs, they drop right in that little space between the space/backspace keys and the keywells.
• the thumb clusters are also too high, making it somewhat painful to rest the thumbs on the keys for longer periods of time
The thing that splitting the keyboard (while keeping it flat) helps with is reducing “ulnar deviation” of the wrist (outward rotation in the plane of the hands). It is also possible to avoid ulnar deviation on a standard keyboard by changing typing style a bit, which is why some people have less problem than others with a single-piece keyboard.
What helps most is not splitting the keyboard per se, but tilting the two sides upward toward the center (the ideal is probably something like a 45° tilt, but even a bit of “tenting” helps significantly).
Having the hands palms-down and close together (as forced by a standard keyboard) ends up causing an unpleasant trade-off between (a) forearms rotated uncomfortably inward about their axis (“wrist pronation”), or (b) elbows swung forward or out to the side to reduce the amount of wrist pronation, in the process causing shoulder and back strain, or causing people to rest their elbows, forearms, wrists, or palms on some surface to compensate for the shoulder strain, and often in the process flexing or extending their wrists.
Wrist flexion or extension is really the worst for any kind of repetitive motion. It dramatically reduces strength and responsiveness of the joints, and can cause severe repetitive strain injuries. You will notice that typical human tools are designed very carefully so they can be used with a mostly straight wrist (or tool-user technique has developed to work around tools which don’t make this obvious).
Tilting the keyboard the right amount front-to-back to match the height of the keyboard relative to the torso can help quite a bit; most keyboards I observe in the wild are tilted incorrectly. You want the plane of the keyboard to be roughly parallel to the plane of the forearm. So on a tall desk, the keyboard should be tilted up at the back. On a low keyboard tray they keyboard should be flat or even tilted slightly down at the back.