I recommend Okatsune pruners, it has the smoothest movements compared to Felco & ARS (I have all three). In fact you might find the ARS VS8Z challenging to lock and unlock (I don’t have arthritis and still find it difficult).
https://www.amazon.com/dp/B001Y524CM/?tag=thewire06-20&linkCode=xm2&ascsubtag=AwEAAAAAAAAAAMl2
I’ve never heard of anyone having trouble doing this and I’ve recommended it to many very happy advisees. What is nice is being able to lock it and open it with one hand.
This was a very good review of pruners and though the judges said the ARS cut better than any of the competition they selected a Felco. I forget the bogus reason and don’t feel like reading it again right now.
It cuts well, but one has to squeeze the handles hard for the lock to release. Not the case with Okatsune pruner. Perhaps my ARS is faulty or something. 
A few year back there was a problem with them in that they were manufactured in a way that made it hard to force the metal piece in to close them, and I suppose they were harder to open as well, but now they should only require the same pressure as it takes to make a cut to open the blade. The problem with this is that they can accidentally open in ones pocket, which I think is a flaw and if you put them there with the blades on top you might accidentally cut yourself badly reaching into your pocket. I prefer the closure system of the Felco, but once I got use to how the ARS cuts and feels in my hands it overrode any objections in the other aspects of its design. When I’m doing serious pruning I keep them in a holster, but when I’m not wearing a holster they are my version of a pocket knife. I make sure the blade is pointed down after once cutting myself.
Happened to me. Steroid shot helped
The problem is muscles in forearm getting tired and cranky. Lay your forearm on a table and get the opposite elbow on it to rub around and find the sore spots. A little dab or cooking oil will help slide around a bit, not too much. The muscles between thumb and forefinger will also be tired. If the base of the thumb aches, that is, the fleshy part of the heel of the hand, try working the brachialis in the upper arm. It’s shorter than Biceps, running from outer upper arm coming medial to the proximal ulna, that is the pinky side of the forearm just below the elbow. You want to start working it about halfway up the upper arm and down to the crease of the elbow. It’s your strongest elbow flexor, and apparently lays across the nerve to the thumb.
For any of you getting arthritis in hands, feet, knees or hips that don’t have a really good reason for it, esp. if you have a history of heart attacks, arthritis, or cancer in the men in your family - become a regular blood donor.