problems with finger tension

[Mitch]

I've encountered this issue in some students in the past. One thing to re-evaluate is how you're holding the horn. If you're using your right hand to hold up the horn, the excessive tension in your thumb can be enough to throw your hand out of whack, and could be a contributing factor to wrist problems. The solution for my students was experimenting with horn placement/assistance, and for myself the solution was the invention of the TubaHook (tm), an attached device that allows me to support the horn over my left leg so that neither arm is all that involved in having to hold up the horn. It's solved many excessive-tension issues.

Short of that, I'd suggest working with a movement specialist, like someone trained in Alexander Technique or Rolfing (rolf.org), who might be able to watch you play and evaluate what you're doing (assuming the problem is based in you and not the horn). As Bloke mentioned, it could be in the valve springs. FWIW, I would suggest against playing with flat fingers, as this is counter to the "design" of the hand and how it works best. In a curved position, the finger functions with the strength of an arch, thereby best able to accomodate any task requiring strength and flexibility. Many musicians encounter physical difficulties because they are using their hands/arms/bodies in a manner counter to that in which the body ideally functions. tubamantk may have found refuge in straightening fingers, but it's likely that more people wouldn't than would. Seek professional assistance.

[TubaTodd]

I'm not sure what tuba you are playing on, but I had a tuba with the Meinl Weston "Big Valves" and those valves hurt my hand BAD!! Eventually, I had to decide whether I was going to deal with it or get a different tuba. I opted for a different tuba and it made all of the difference in the world.

If the design of the horn is not the problem, then examine if there is any presure on your right forearm while you are playing. If your right arm is doing more than moving the valves (like helping to prop up the tuba), then you may be giving your right arm too much work. You might look into getting a tuba stand.