there was a study using Kuroda sensei as the subject.
Kuroda sensei used small amount of biceps and mostly triceps and grand dorsal for up and down movement compared to normal folks. same with the horizontal cut movement. it's not that he didn't use the other muscles normal folks used to move their arms, but he used much less of them.
i suspected that Horikawa sensei used similar muscle groups.
now it would be even more interesting if they have done the full body monitoring. would be interesting to find out what his lower body doing at the same time.