I havnt been playing bass long, but having read a fair few articles and books the stress the importance of the left hand being relaxed and comfortable... and the thumb being a support and not putting pressure on it... But I physicaly cant play without putting a lot of pressure on my thumb - If I play for a while I actualy get an ache in my thumb joint. My question really is - what is this down to and how do I stop it! Is it down to tecnique and the way I position my thumb - or is it down to strength in my fingers (or a combination of the both). I try to keep my thumb in a natural curved and comfortable shape, but as soon as I start playing I end up bending my thumb and putting pressure on it. One of my books tells me to make sure that when I fret a note to make sure all the fingers behind the fretting finger is pressed against the string - I have a feeling this is a big contributor to the extra pressure im putting on my thumb, but at the same time it feels natural to do (otherwise my fingers are miles away from the fretboard and not ready to fret subsequent notes). It a geus I need to work on strengthening my left hand so I can fret notes accurately without needing to put some much pressure on my thumb - but I cant see what muscles do this in the first place - is the idea to press the fretboard into my left hand by supporting it with my right hand / strap?