A few points being made here are wrong, and so are the theories being used to support them.
Strength is a word used to describe all the elements that muscles of the body use, so strong coffee is a flavour, and super strength beer is an alcohol content, bit we all just of what is meant.
So splitting hairs over the semantics of language does nothing but confuse the issue.
If you have the strength to fret a note....etc is the same as if you can take a step you can hike for miles......no you cannot because this does not adress the application on use, it just shows a single motion and does mot adress the issues.
So what is the Issue?
The issue is muscle coordination and the strength of that coordination,
players with a strong coordination connection will fair better than those with a weak one.
Within the coordination is dexterity, and within the dexterity comes stamina and with in the stamina comes fuel (food for energy)
Bass playing is what your brain defines the use as, to your body it is movement and as such the laws of anatomy will always out weigh what you think is happening.
What is happening is happening in your hand use?
The exact same as in a job where you use your fingers in manual labour, or are playing a sport that requires motion for example. The motion all works in the same way, but how you see it is different even though it is not.
Muscle tissue contains these four main properties;
Excitability, the ability to respond to stimuli.
Contractibility, ability to contract/change form
Extensibility, ability to be stretched/change form without tearing.
Elasticity, ability to return to its original form.
Depending on the functional characteristics, muscle tissue is classified into three types: cardiac, smooth and skeletal.
They have two functions, voluntary and in-voluntary.
And they have two tissue compositions, striated and non-striated .
So those combinations can have various results depending on many variables.
Muscle use has these two issues to facilitate, the length of time used and the intensity of use....playing bass also has these two issues to facilitate for the player.
So a high intensity use will fatigue faster over a low intensity use, as seen in runners that cannot pace themselves to ensure that the muscle use has a constant fuel source over the duration of their race.
But a sprinter can go for it, expel all the energy they have in a high intensity use, because the body muscles have neither the time or duration to fully fatigue.
Since our hands are governed by the exact same factors then they can be conditioned to work better, and that means if the use is to play bass they will play bass better by better coping with the demands involved.
Playing bass is not the answer to development, playing bass is the problem, we are not designed to play bass.....the bass was designed within in our capabilities to cope.
This is a broad capability, and again it will involves what the person wants to happen rather than what they can do. They will makes decision based on what others can do rather than what they can do, they will find ways to justify what works for others, and the correct way and will work for them and produce the same results.
So to cut a very long story short, you can indeed improve you bass playing with exercise away from the instrument because you improve your ability to cope with the demands.
Most important points are,
Warm ups and Stretches.
Diet, you need fuel and water for muscles to work best.
Rest, you need to mix high intensity use songs with low intensity use songs....pace your set, or put more of the earlier points in place so your body can better cope and recover.
Keep hydrated, muscles are approx. 70% water content, so to keep them working we need to take in water we lose those perspiration under lights, dehydration through alcohol (we all like a beer) and of course over use of our bladders.
Energy is best in small portions and frequent portions, hi energy foods and drinks are good in the run up to shows or practice....do not over eat or the blood supply your muscle need will go to your digestive system.
After playing replace what you have lost, eat and drink to give you back the energy you have lost. But good carbohydrates, and protein foods foods that have a slow release of energy over fast release foods.
Final point, hand exercises that squeeze are no good for playing bass unless you want a grip.
The most effective exercises are the exact opposite of squeezing it is opening the hand fully and exercising the extensors....not the flexors.
Extensors open and extend the fingers, this movement is very under used compared to the Flexors which close that hand. We are always fully closing the hand when we grip or hold daily things, but seldom do we fully open the hands so the extensor muscles never get fully used as much as the flexors.
When you hold something you hold it tight, so the flexors are being fully used, but when you let it go you just release it with enough pressure to allow what you were holding's own mass help......you never fully open you hands with your fingers fully extended we you put things down or let them go.
So rather than tap a table, put the hand flat on it with the fingers fully spread open and lift them off the table one at a time. The object of the exercise is to be able to lift them higher off the table.
For a few more basic ideas follow the link, and pay attention to the finger spreads, again another under used movement of the hand, but having a strong spread is a stable spread, so again you increase your chances of coping with playing bass.
Any questions this post raises, please post and I will answer them if i can.