Re: Thick forearms
My sensei said before that his sensei i.e., Shioda Kancho has forearm/wrist that are disproportionately huge for his size. He said that it is probably from his Kendo days. I think it is true, ken suburi will strengten forearm and wrist.
Other activities that will make huge forearm...
1) Slapping the mat hard during ukemi
2) Grabbing tightly as uke
3) Ken/jo suburi
Of the above, IMO #3 is the greatest contributor.