Avoiding injuries from kneading
I see two questions here that are connected.


The right height of a kneading surface.


As a rule of thumb, the surface for kneading should be about 10 cm / 4 inches lower than for chopping. You want to be able to ever so slightly lean on your dough, using your body weight to support your hands and arms. Too high means you will inadvertently pull up your shoulders and are more likely to put pressure on your hands and wrists at a bad angle.
I, being on the shorter side, have occasionally used a small step, especially when working with heavier or lots of dough.


Good technique to protect your joints.


If you knead with the part of your hands where the fingers meet the palm, you are putting a lot of pressure on a piece of your anatomy that is designed for finer tasks. Plus you are likely splaying out and bending your fingers more or less backwards, which you will also feel in your wrist.

Imagine yourself leaning on a table, for example during a conversation with a coworker who is just telling a captivating story. You will almost certainly use the base of your palm, near the wrist. And that's roughly what you should use for kneading as well. 

Apart from that, don't bend your wrists to far backwards. Even with a lower surface, I aim for 45 degrees (give and take) between my forearms and the counter and I keep my hands on the same axis as my forearms, never turning them in- or outwards. If I need to adjust, I will work nearer to my body or keep the dough a bit farther away to compensate for the height.

Keep your back mostly straight and your shoulders down - most of us sometimes go "hunchback-style" after a while, so check yourself. (You don't want to have an aching back instead next time!)