Here is a basic fast twitch response exercise, first off warm your hands up by rubbing them and gently stretching them for a few minutes.
No the object of this exercise is to increase the repetition...not go for longer.
The video below will give you some exercises for stretches and will show the some of the basic movements of the hands.
Turn your hand palm up, and support the forearm with the other hand.
Now open your fingers fully and stretch them as far apart and as straight as you can.
Now starting with the joints closest to the finger tips, curl all the fingers into the palm in a rolling motion, keep the thumb out of it, and then un-roll them from the palm to be back to being straight and spread out again, (see the video at 2.20 for the movement) that is one repetition.
Now time yourself and see how many of those repetitions you can do within 15 seconds, the faster you do the motion the more you will do.
Be honest with yourself and write down the number you done.
Rest the hands for a few minutes, rub them and stretch them out, then repeat and see if you can beat your original score.
You can repeat this every 5-10 mins or so for about 6 times and each time your score should get higher.
Now over a few weeks you hands, depending on how well, or now well developed they are, will easily double what you started with.
This is partly because our motion was faster, but also because it was sustainable.
You will as a rule start to slow down after 7-10 seconds so the last five seconds will be tired repetitions, as you develop not only do you get faster but you fatigue less and less into the 15 seconds and eventual are as fast at the end of 15 seconds as you were at the start.
Your hands recover quicker and are ready to go again, so rather than building stamina through power, you build stamina through faster recovery.
Playing bass, as a rule, is about moving your fingers, so the better the quality of movement the better the response and the better the recovery.
So you will seldom be asking your fingers to move at such speeds over a short period of time when playing, so the recover faster as the movement they are doing is not a full extension, opening the fingers, or a full flexion, closing the fingers...it is just a small part of that movement so uses less energy.
You can look at each song as like an exercise and each break between songs as recover time, so the set you play is in fact lots of short twitch responses with recoveries. Now you can take this exercise up to 20 seconds and no more...after this you are not developing fast twitch, you start to develop slow twitch...you start to develop stamina to sustain the movement....not the speed. Everything you need is in that first 15-20 seconds, when doing the exercise swap hands, alternate between plucking and fretting hand and develop both.
But without putting it on the bass the exercises are pointless, you brain need to work out what is doing what, and how is doing it to develop the co-ordination and dexterity to control it. I usually recommend exercise one hand, then the other, then back, and then back again, so 2 sets of repetitions on each hand then get playing. By all means use them as warm up, include them in your practice, because you need to "remind your hands" that it has this range of motion....think when was the last time your fingers were using in a task that required them to be fully open and stretched?
Modern life is about holding and griping...so you get plenty of grip exercise during the day...what you do not get it is the opposite motion..this exercise will address that..... because the fatigue that is slowing you down in the exercise is extensor fatigue (opening your hand, because if you cannot open your hand fully and fast enough to close it fully and fast.....your flexors are resisting extensors the need to open.
So maybe are you tiring because you cannot lift your fingers off the fret board.....not on to it.........so what use is a grip trainer going to do?
Remember all exercise should be pain free, you will get some burn, but not pain, the burn will subside fast as the hand recovers. if you get pain stop and consult a medical professional. Any questions this post raises, post them and i will try and answer them.