| Simple problem, simple exercise to give you the feeling of what should happen, what should not happen, and also exercise all i one.
Go to your kitchen and find 3 glasses, cups, etc. that have a different size base. When i do this i get an egg cup, a slim tall glass, and a broader based Brandy or whisky glass. Cups and mugs are good as well but you need the range of ever increasing base sizes in them.
Hold the smallest in your fingers and thumb without using your little finger, it should be free and low enough to move under the glass and touch the pad at the base of the thumb.
There you have the exercise. While holding the glass or cup move the little finger under it and touch the pad at the base of the thumb 10 times.
So what you're doing is giving the fingers something to do while you exercise one of them, not restricting the fingers, but give them something to do.
If you cannot touch the pad, just go as far as is comfortable for just now. As you use this exercise flexability will come and you will eventually touch the pad.
When you hold the smallest you need a natural curl in your fingers and the tips, not the flat, but the tips of the fingers to hold the it. If you use the flat (and try this) of the fingers to hold the smallest one, the ring finger joints flex in, collapes if you will. It is the design of the joints that stops hyper-extention of this joint that would surely occur if the movement was not checked by the design of the hand joints.
Now curl you fingers and use the tips. You most likely need to have the fingertips all close to level to achieve this( if this means using all four fingers to set the curl then do so and then slip down the hand to free the little finger) and repeat as before. This time you will notice the little finger works better and the joints do not collapes, actually the opposite if you will, they move and support the little finger allowing it to move with a certain freedom and no real strain on any of the joints of the fingers. Use the natural curl if you can it is the best way to utilize the design of the hand.
You can do it with 3 finger on the glass little finger moving under, two fingers on the glass, 2 fingers moving under, or 3 finger on the glass, 3 fingers moving under, the principal is the same, though the body mechanics slighty different.
So start with the smallest for a few weeks and and look to work up to the broadest glass. the obect is to exercise this particular plane of movement so no more than 10 mins a session and no more than 2 sessions a day.
It is to be used as part of practice not an extention. If you practise 20 mins a day then 10 mins of it will be this, not 10 mins and then 20 mins of practice.
As with any bass playing this should be pain free, so if you have any pain stop and rest. If pain continues in you playing, see a medical pro. Any questions post them or drop me a line. | 
