I’m not sure what I did but differently I’m getting cramping on my fretting hand. I’m also fumbling and having buzzing especially on strings that I’m fretting with my ring and pinky. I’ve been working on learning Money, and I’m wondering if I’m overworking those two fingers. Any thoughts?
Anyone have this happen to them and what did you do?
Do you do any warmups or stretches?
Could just be a magnesium deficiency so try a multivitamin and some bananas so you don’t go on a wild goose chase trying to find some technique issues etc if it’s that
Yeah, I do go through a warm up.
Had cramps usually from extra effort on fretting and insufficient fingers strength. Best to first work on your fingers strength and flexibility.
Also eating a banana can help with cramps
Forgot this earlier, if you keep getting cramps after making sure you don’t have eg a magnesium deficency or does some weird things with your fingers. Try to find what muscle you actually get cramps in since that’s a big clue on what it could be if it’s not the common issues.
It happens to me usually when I’m too sedentary with my wrist or if my hands are absolutely wrecked from work already. I would try keeping it moving if possible when you get a break or at least put your wrist down below your heart level for some quick relief. I have had spasms below the thumb in my hand and it doesn’t feel good. Usually means I’m in a bad position on the neck or pressing too hard. I’ve set my basses lower action and it helps a lot.
You’re trying too hard which used to happen to me but not much anymore. This advice might not be for everybody, but it snapped me out of it, learning to relax is the key.
It used to happen to me when I played during the day, but not at night after dinner time which usually included a glass of wine, or two. That got me thinking it induced relaxation, and I could play relaxed w/o any cramping at all, and the playing would be smoother with almost no fret noise. Since that revelation, I don’t have that problem anymore, even during the day, learning to relax, but the fact remains, I still play better after dinner.
Can verify, having had this before - I cramp up in my calves and forearms when I need more magnesium.
I used to get insane, debilitating hand cramps. I could only play one song on the guitar before my hands would spasm and cramp.
Found out it’s an uncommon side-effect of having undiagnosed celiac disease. I stopped eating gluten, and I now play bass with no issues.
Aha!! The real reason for Josh’s tattoo has been revealed! Take that, Velvet Underground!
Another athlete/ construction worker trick is downing some pickle juice. Something in the brine blocks the response that causes the spasm or cramp. I’ve done it. I’ve had bad ones in the truck driving home after sweating out in the heat all day doing heavy work.
Very good advices in the other comments 
Could it be that maybe you are gripping the neck of the bass too hard? Many people do that on the beginning. In some videos Josh talks about that. The thumb should rest comfortably and lightly on the back of the neck.
I notice that when I’m playing a faster song, or when I’m trying something new, I might tense my hands more, without noticing it. When I started playing bass, I often had to use some pain relief gel on my hands at night before going to sleep. It helped.
Magnesium tablets are very helpful too in these cases, as other people pointed out.
I’m definitely going too hard. It mainly seems to occur when I’m working on something technically difficult or really new. I tend to grip harder when I’m tense. When I’m playing relatively easy, I don’t grip as hard, and I don’t cramp.
Making sure to take my multivitamin every day which seems to help.
I’ll going to try having a drink before playing and see if that helps 
Subliminal message of the tattoo
*Someone beat me to it . Damn it
I’m definitely gripping the neck wrong and too tightly when I’m getting some cramps, especially when I cramp between my index finger and my thumb. I noticed that my thumb slides from the back of the neck to the top, even having my thumb overhang on the neck.
Any advice on keeping my thumb centered on the back of the neck?
I’d focus on relaxing the hand more than where your thumb is sitting. I normally play with my thumb almost over the top. It’s just where it sits in a way that I can let it relax so I don’t squeeze too tight. Is it perfect form, nope. But that’s better than gripping the neck. In my very humble opinion, relaxing your fretting hand will lead to immediate improvement in your ability to play.
I noticed my hands tense more when I’m learning a new song, so all my attention goes into playing the right notes, and there isn’t much attention left to monitor all the other things that our brains will do automatically, like what the right hand is doing, or what the left thumb is doing, or how hard we are gripping.
So to work on those things, it is best to practice them while playing something easy or something you already play well. So you can focus more on relaxing the hand, and keep the thumb centered. If we practice enough, it might become automatic. But probably if you feel tense while learning something new, difficult or fast, you might tense the hand and the thumb might slide upwards. But when you have learned the song, it will be easier to relax again.
But if it doesn’t feel bad that the thumb is not centered in the back of the neck, and if it isn’t causing problems, then it is probably OK. People who play with picks use the thumb to silence the E string, so many keep the thumb a little higher up.
A useful exercise is to spend a few minutes trying to see how lightly you can fret and still get a clean note. A properly set up bass should take very little pressure to fret cleanly.
Next, once you have dialed this in, attempt to play for a few minutes without touching the back of the neck at all, just using your forearm on the bass body to counter the fretting pressure.
It will surprise you. It’s entirely possible to do and should drive home how little a need there is for tight fretting grip.