Drinking water helps alot. It also sounds as if you are pushing your thumb too hard on the back of the neck, hence your sore thumb and lack of power/endurance. IMO, you need to develop a lighter touch. There are heaps of threads about this, just do a search and type in "lighter touch". if you warm up alot, you wont have to put as much effort into moving your fingers as much and you wont have to push as hard on the back of the neck, so warm up alot (smartest thing ive said all day

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Good Luck and keep us posted