The EMF of a keyboard are caused by the short pulses of currents that go through its printed circuits whenever a key is pressed. Actually there are some EMF emissions even without pressing any key because the keyboard's circuit checks the status of the keys. All these emissions are very low in magnetic intensity and just some V/m of electric intensity, but they are of a middle to high frequency band so they cause some effects to the human body.
The EMF emitted from the computer keyboard usually is a problem because it radiates all around and reaches the user's head or chest. Such EMF may affect the also the user's hands and fingers, which are directly above the keys, at short distance from the printed circuits.
Of course we are considering only keyboards with wire, because wireless keyboards create a lot of EMF in addition to this, the problems of wireless devices have been described here .
Notice that some kind of stress of the hands and fingers is not due to electromagnetic emissions from the keyboard but it is caused by the pressure and hits of the fingers on the keys. The bones of human fingers may suffer of some stress when the fingers are used to type on a computer keyboard for long time, especially if this work is repeated every day. This mechanical stress would happen even with an old typewriter machine, without any electric circuit inside. The mechanical stress depends on the mechanical stiffness of the keys, the way that the user presses the keys and so on.
The keyboard emits always some EMF so the stress caused by EMF can be recognized and distinguished from the mechanical stress by holding the hand or fingers on the keyboard for some minutes without applying any forces with the fingers. If you feel some strange sensations or pain in your hands and fingers then the cause is very probably the pulsed electric field of the keyboard's circuits.
We have tried many types of keyboards and we have found that those very flat keyboards, with flat keys, have much more electromagnetic field around them.