[HeyLaughingBoy]

Try to relax just before impact.

I use a maul (about 5lb head) to split wood (mostly elm since a neighbor cut down dozens of elm trees in the attempt to stop Dutch Elm disease from propagating) and the best motion I know of is to lift it straight overhead with hands spread wide. Then accelerate it downward while sliding the hand closer to the blade down to the base. Focus on coming down in a line through the center of your body. At the moment of impact, both hands are close together and your grip is just tight enough to hold onto the maul, with arms and shoulders relaxed.

It substantially reduces stress on your body and you still maintain good control. Feels like a Kendo "shomen uchi" strike.