Are there any good hand exercises that can help me to build my fretting-hand strength while I sit at my desk at work? I have a problem with endurance, my fretting hand frequently cramps up during technical songs where my hand moves around/stretches a lot. It's getting better the more I play, but I'd like to accelerate the building of muscles in my hand while I'm doing things other than playing bass (I have a work and social life lol). I perform various stretching exercises at my desk, but am unaware of any strength building exercises I can do while sitting down. Any suggestions?