Re: Wrist Exercises
The powerball - I've finally seen one of these. They seem pretty good and can train some coordination, too.
I've answered similar threads with this in the past. It's a REALLY old fashioned "western" exercise: wrist roll-ups. A stick, a bit of rope, and about 5 - 10 lb (2-5 kg, approx). Tie the rope to the weight, drill a hole through the stick (heavy dowel, bit of broken ice-hockey stick, or the handle end of a brokken bokken 8-), for example), pass the rope through the stick and tie it off so it can't fall back through. Make the rope long enough to reach the ground from your shoulder or chest height.
Roll the stick with your hands out in front of you until the weight is at the stick, roll it all the way back down and then up the other way. Repeat as long as you can. It's not jo suburi or bokken suburi but it will make your forearm muscles and associated connective tissue stronger.
Then STRETCH. Wait a while Stretch again.
Stretch on the days off between, too.
Don't worry about losing flexibility when you get bigger muscles - keep working on flexibility... Male olympic gymnastics competitors are very muscular and also very flexible - you need to work on the flexibility just as much on the strength.
Try it - you'll hate it.