Take a cup, mug or glass and hold it in your hand as nomal with you little finger not on it, but underneath supporting.
This will give your other fingers something to do while you exercise the little finger.
While hold the object you are using curl the little finger under it toward the palm as far as you can go, relax and return it to its starting point, relax and then stretch it away from the palm as far as you can go, relax and return to the starting point.
The starting point is the natural curl you have in your fingers when relaxed, you have movement either side of that point which is what this exercise does.
By doing the little finger on its own you will be helping the ring finger as the funtion overlaps both of them.
Do them as two sets of ten, then build up, keep it slow and remember to work on the relax motion in between each movement, that is important.
The playing of the bass will give the correct twitch reponse for building in fluency of playing and strength with all the fingers as you develop, but sometimes individual fingers need a help.
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Try it at breakfast or in a bar or when you have a drink or can of juice in your hand during a break.