It's not a question of strength, it's endurance. Plus, the typical stroke used in kendo isn't a matter of strength, it's quickness. If you can put your shinai on your opponent quicker than your opponent can put his/her shinai on you, that's the idea. Suburi involves the wrists much more than the deltoids. You're just raising your arms with your deltoids, but your wrists are what give the shinai the necessary speed. The power for the strike comes from your legs as you drive into your opponent with the shinai. I am an experienced mukyu sensei, so you can believe everything I say! [haha]