Posted 24 April 2017 - 07:05 PM
(Obligatory "I am not a doctor"). What you're describing sounds like your muscles getting tired/overworked. That's good news, cause it's temporary! Tendonitis suuuuuuucks
Finger strength is very relevant!! When I was playing a lot of upright bass, I'd do exercises to strengthen the muscles that I needed. I had to, because those muscles aren't gonna get strong enough through every day use. Trills are great for finger strength. Actually, on electric bass, that's all I can imagine you'd need. Trill all your fingers: 1-2, 1-3, 1-4, 2-3, 2-4, 3-4. Slow trills are fine, especially to monitor technique while you do it (curved fingers, play with finger tips)
"Strengthening products" are dumb, though. I've never heard someone actually recommend one. Actually, I don't know if I've known anyone who uses one. Just get your strength from the thing you want to be stronger at (bass).
Tendonitis/carpal tunnel/etc are so complicated. Definitely avoid awkward positions, but also be relaxed. For example, I put my thumb on the pickup pretty often when I play, and it's never been the cause of tendonitis for me, but I wouldn't say it's "anchored" or sticking out at a weird angle, it's just a convenient place to rest my thumb. That said, I can't think of a downside to free-thumb, so I'd say get in that habit early. Also for trills, try to use your bigger muscles, ie, the ones that move your entire finger, not the ones that move just two knuckles. Keep your wrists pretty straight.
Warming up is good. And stretching when you are warmed up (that can be after warming up, or even after all your playing).
Oh, make sure your bass action is low. High action makes everything harder.