I've noticed that particularly in the past four months or so, my left hand has gotten considerably stronger. I basically went from noodling around once in a while, to playing at least two hours a day and playing in a few different bands on top of that, and the difference is pretty significant. However, it seems like there are certain repetitive bass lines that always give me a huge cramp in my hand. I start out strong and then I get tired after a verse. "Billie Jean" is one. "I Travel" by Simple Minds is another...I was trying it tonight and it was just like, argh! No matter what I do to try and give my hand a break, I cramp up either in my forearm or at this weird spot on my palm that's sort of like my thumb muscle...it's hard to explain. My wrist has finally become strong and flexible enough that I no longer feel any ache there even after playing for 3-5 hours. But this thing with my thumb is kind of a new one. I've tried different fingering, I've tried holding my thumb different ways against the neck (including not at all), and for whatever reason, certain lines just totally kill my left hand in that one spot. I guess my question is two-fold: what are some other ways to work around fatigue like this? And also, is this an issue of just not having the stamina yet, or could it happen to anyone at any point in their bass-playing career/existence?