Those powerball gyroscope thingys?
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They are good, but not for that.
And possibly not good enough to justify spending the money.
The idea is that the gyroscope will try to stay still, or twist in a different direction, when you twist your wrist. This produces resistance that you can train against. You can get a similar effect by waving a bar with weight discs attached to one end only, or a traditional 'indian club' (these are what juggling clubs evolved from, except the traditional version are solid wood or metal).
You can also make a fist (traditionally around a small ball, which is the object of a game) and challenge a training partner to either open it (winning the ball to score points) or change its orientation (for example, so that your thumb touches a bottle on the table between you). In a more difficult version, you hold an egg or a small orange and lose the game if you break/burst it.
You can train 'against yourself' by interlacing your fingers so that each hand holds the other, and resisting with one hand as the other rolls around.
All of these exercises are designed to increase your gripping and manipulating power. They won't help with your own problem, which is caused by the high number and frequency of repetitive movements that you use to play an instrument.
You need stretches for the muscles on the inside of your forearm. The most useful will probably be the one where you hold both hands out in front of you, palms up, one on top of the other. The bottom hand's fingers hold on between the finger & thumb of the top hand. (The top hand is the one you will stretch out.) GENTLY pull the thumb side of your top hand downwards and towards you, while you push the heel of the hand up and away from you. Your top arm will straighten, exposing the veiny part of your forearm upwards. DON'T do this hard, or quickly.
The result is a bit like trying to unscrew your forearm from your upper arm; done gently and for a long time, it will relieve the tension produced by your hand actions while playing.
Another good stretch uses a tabletop or other flat surface. Put your hand flat on the surface and try to spread your fingers evenly as far apart as possible. Relax into the widest positiopn you can manage, for about a minute. It will help if you can have your forearm at a right angle to the surface, but without too much pressure in it (eg don't lean your body on it).
A gentle version of the wrist-rolling exercise (fingers interlaced, lightly rolling one hand around the other, with your wrists relaxed and arms soft), would be a useful limbering technique before playing.
The stretches, and the wrist rolling, should also be performed after (and before, if possible) any period of inactivity. (Before bed, and upon waking, works well as a routine; other times of the day can be added where appropriate. Warm water can make them even more effective; for example, in the bath or shower.
When they ache, wearing an elasticated support for an hour or so (no more than 2 hours continuously) can also help. Ice, or a chemical cold-pack, can be inserted in the support if the aching is severe (both need to be wrapped to avoid direct contact with your skin).
I have to disagree with the other answer. I'm a bass player and hand percussionist, and I haven't found anything like the Powerball to strengthen my wrists and forearms. Not saying that it's the best workout imaginable, but it's VERY convenient (I use it while sitting at my computer or watching movies) and kind of fun/addictive once you get the hang of it.
I would recommend getting the least expensive one possible with an RPM counter. And try not to buy through the distributor. I'm sure that there are less expensive ones elsewhere online.
It takes a bit of dedication to get used to it, so if you're not the type to endure a bit of awkwardness/pain, I wouldn't recommend picking one up. But it is the easiest and most effective way I know of to build up some strength. I usually spend 15-20 minutes a day alternating hands, and it has increased my stamina and speed on both the hand drums (rolls in particular) and the bass.
If at all possible, try to borrow one from somewhere. Try it before you buy it, because it's not for everyone.
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