Pain In Thumb. Wrong Technique?

I don't know if I've come to the right place. I've recently returned to piano playing. I have gone from playing nothing to playing a lot in a short time, and started experiencing pain in my left thumb (at the MCP joint). It looks like it's getting better, but I'm thinking of taking preventive measures. I studied the finger a bit and it looks like I have some issues with it. Comparing it to the right thumb I notice that the joint is often raised up (flexed). This is before pressing down the key, where it then looks the same as right thumb (most of the time, not always). Any idea how to approach this? It looks like the finger is not relaxing properly.

I attached a picture showing the problem more clearly. I don't know if it's possible to do something about this? Exercises or devices? What's even the cause? Muscle imbalances, improper movement pattern (muscles firing incorrectly), hyperextensibility, limited flexibility, ...? I have used the thumb quite a lot at the computer (think alt+tab).

You need to activate your left 2nd finger MCP joint bigtime. You need to start using the lumbrical & interosseous muscles for that finger way more than you are doing. This is the first step towards improving the situation. Hard to say more at this distance, but following this advice alone will get you started on the road to improvement. Thanks for posting! AFF