They are actually Yoga Therapy Balls. Jill Miller wrote a book (and has lots of free info on YouTube) called "The Roll Model." That particular placement of the balls is too intense for me since I have REALLY tight muscles between the shoulders and the neck. I used to do landscape work for several years, plus lot of other manual labor....and I am paying for it now.
You can use tennis balls but the rubber in the therapy balls has a pliability that is kinder to the body (and the therapy balls are solid unlike the tennis balls).
Thanks for stopping by and commenting @steemsilo!!