How do I "use my back" to help take pressure off my thumb?

[Earthday]

I've encountered some thumb pains, and realized I've been using my hands like vicegrips and getting a lot of pressure from my thumb. I've read I can help this by pulling with my back to apply some pressure and relive my thumb, but I cant figure out exactly how to do this. Anyone have any pointers, or any way I can find the muscle I'm suppoed to be using?

[Christopher]

It's probably too difficult to give you ergonomics pointers without seeing you play, but I'll try anyway.

The idea is to use the weight of your left arm rather than pressure from the thumb to hold down the string. It helps to wear your bass relatively high on the strap and at an approximate 45 degree angle to the floor. Form a C-shape with your left hand fingers and thumb, and finger a random string using the fingertips, rather than the pads of the fingers. Now take the thumb off the neck slightly and use just the weight of your left forearm to hold the string down. If you keep your left arm bent at an approximate 90 degree angle at the elbow, the force of gravity pulling on the left forearm should naturally push the string down without any pressure from the thumb, or any tension elsewhere in the arm.

[lemur821]

Right, your arm should just dangle from your fretting fingers. If you can't play well without thumb pressure then you've got a massive technique problem. The thumb is just a guide.