I’ve been following Masumi for a while now on Youtube. He was recently playing in Japan with Mark Boals and Ronnnie Romero. He uploaded this cool POV footage of him playing Black Star and I thought I would share here.
In some of Masumi’s videos he appears to say that all of his fast picking comes from thumb and finger movement. The videos are in Japanese, but this is what the auto-translated subtitles are telling me 
He definitely uses it, but it’s mostly a combination effort like most things are and it’s visually obvious in that clip that he using various movements. Lots of wrist movement in there too particularly when he is playing the faster passages.
thanks for the reply.
You can see some finger / thumb movement here too - https://youtu.be/EKl_BNCebqw?t=65
I can’t help but think this would wreck your joints in the long run!