Kongoken

The Gojuryu Karatedo Yoyogi Ryushinkan Dojo is equipped with a kongoken, a metal bar formed into an extended oval that is used to condition the arms and legs, and strengthen the wrists and core.  

It is also used to develop a powerful and explosive thrust of the hips, especially when dropping from hachijidachi into shikodachi while lifting the tool above one's head, and then pulling back up into the hachijidachi stance.

The kongoken differs from other equipment used in the hojo undō  training of traditional Okinawan karate, as it was imported not from China but from Hawaii.  Chojun Miyagi, the founder of Okinawan Gojuryu, visited the islands in 1934 and observed the local wrestlers working with an enormous, heavy iron ring.  He was so impressed by its use that he brought it back to Japan in early 1935, and introduced it into the hojo undō repertoire of his students. 

As with all the lifting tools in the hojo undō, including the chiishi, ishi sashi, and nigiri game, constant use of the kongoken will also greatly strengthen the practitioner's grip.