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OMFG METAL!!!
OK
Speaking as a personal trainer...
These things are GOLD.
You can work all 3 energy systems on these baby's!!!
They use a good deal of energy too, they will help you very nicely with reps, great wrist strengtheners.
The plastic Powerballs could gain upto 20kilos of force going through the common flexors and extendors.
Metal would work so well.