Keeping your shoulders in a neutral position has been overlooked by many keyboard manufacturers. Even most natural keyboards are not split widely enough for most people. You may develop chest, back, or shoulder pain if your keyboard is too narrow. Personally, I use a tethered Kinesis FreeStyle, placing one side of the keyboard on each side of my lap. It would be best to have an adjustable keyboard tray, but once again, you can take your lap wherever you go, which is not necessarily true for the keyboard tray.
In general, you want to keep your feet flat on the floor, as this keeps your weight centered over your body. Some people like to cross their legs in their chair or sit on their leg. Unfortunately, these positions will tweak your spine and are not ideal postures. If you are too short, you should buy a foot rest so that your feet can rest flat on it. When using a foot pedal, resting your feet on a flat surface can be more difficult. This is why I prefer Frag Petals, as they can be pushed with your big toe with relatively little force. You can also activate the button while keeping your foot mostly flat.
Keep your feet flat on the floor.
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