I play the notes marked with a red circle with my thumb. After practising the passage for some time, I discovered some kind of pain in the first joint of my thumb (for clarification, I mean the joint which is the first if you begin counting from the tip of the thumb).
Of course I stopped playing that passage, and now I'm trying to experiment with my hand positioning in order to reduce the stress on my thumb in this particular passage. What I observed (and what I believe to be the reason for the sensation I felt) is that when playing with the thumb, the upward reaction force the piano-key acts on the thumb when pressing down the key is not oriented in the plane in which the thumb can bend:
Arrow 1 shows the direction of the piano-keys force. This force is almost perpendicular to the direction in which the tip of the thumb can move, which means that the thumb can't really give in to the force, which is good. It however also means that there are no muscles that could act against this force, because the main part of the muscles only acts in the directions of movement of the tip (I indicated that direction by arrow 2). This means that the reaction force of the key when pressing down the key is, with no backing or protection by muscles, absorbed by the joint of the thumb.
So I was wondering: Is this joint build to withstand such (abusive) force? I'm looking for well-founded information on the subject, if possible statements from human-medical science or comparable research.
If not, what would be the best hand position to minimize the stress on this joint? Is this a recurring topic in the piano-playing world?
EDIT: I don't remember experiencing a sensation like that before, when I played all the 32nd notes with my left hand. I assumed that my hand position was slightly different in that case, putting less stress to the thumb joint in question. The picture indeed doesn't show my hand-position while playing octaves, it just was an example pic to show two directions that forces can act on a thumb joint.