is this ‘getting arthritis’ your own diagnosis? Is it another way of saying that your right shoulder is beginning to hurt more and more often? You are a trainer; so I won’t bore you with the ‘get the doctor to check it out’. You know that yourself.
What I am wondering is whether you are starting to feel the onset of a rotator cuff problem. Having had this myself, I know that it initially is more like a bother and then turns into a low-level ache that does not want to go away.
I would put myself on a regimen of rotator cuff and shoulder girdle strengthening. Since it is often difficult so assess ourselves, you may also check with a good physical therapist or a chiropractor for an independent evaluation.
And if the arthritis assessment is correct, it is still my recommendation because you need to keep the foundation strong to protect the joints as much as possible.
Hope you’ll find a way to throw all those balls with the little guys.