For you lap (or mountain) dulcimer players, I have some thoughts.

I play my dulcimer quite a bit, and lately my right thumb and elbow have been bothering me. (I fingerpick it, using a thumb pick for the melody, which means I make many, many sideways motions with my right thumb.)

I usually play at the table in our sunroom. This is one of those new "pub height" tables, 38 inches (91 cm) high. Recently I noticed that when I hold the d'er on my lap and sit on the matching chair with its deep seat, that my back doesn't get any support. (My legs are too short for my back to reach the back of the chair.) So I tried setting the dulcimer on the table and standing up to play.

The result is that my thumb and elbow no longer hurt. Also, my husband says that my motions look freer and more relaxed when I play standing.

I don't find it as comfortable to play on the dining room table, which is only 28 inches (71 cm) high, but a person with longer arms might like it. Of course, you can always put the d'er on something (stack of books) to bring it to just the right height for you.

It's necessary to put something under the d'er to keep it from sliding around. I use two small squares on non-slip rubber "shelfpaper".

So if play mountain and you are developing both a big repertoire and a case of tennis elbow, I suggest you try this.

Be sure you are not cocking your wrist when playing. Cocking the wrist can lead to carpal tunnel problems.
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This is an instrument which needs a new name. "Mountain" isn't descriptive enough - many people play it in areas with no mountains around. "Lap" is misleading; we don't have to play it on our laps.

How about "hourglass" dulcimer? It's a good approximation of the shape.
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My newest piece of Offenbach's Barcarolle.