If you read my article on boxer Piston Horiguchi and Graham Noble's article about karateka Choki Motobu at JCbtSport at http://ejmas.com , I think you will get some insight into how kendo has affected karate.
If truly interested, there is also information on the culture of Imperial Japanese boxing in my article on Korean boxers 1926-1945 published by the Korean American Historical Society in "Occasional Papers." You can order copies through their website http://www.kahs.org . I mention this because after Piston Horiguchi got punchy from taking too many blows to the head, rather than retire him his trainers sent him to a kendo dojo to recover his Yamato damashii. Once his fighting spirit was discovered to lost, of course his handlers simply discarded him.