there was an article on Kuroda sensei. it's in french. they hooked emg to Kuroda sensei upper body and measured his muscle activities during various cutting movements. they compared that to regular people muscle activities doing the same movements. Kuroda sensei muscle usage was completely different than normal folks. it was an interesting study.
i had a conversation with Ikeda sensei one time and he said he had never seen anyone move so fast like Kuroda sensei.
There was one on one of the Chen;s on aikiweb maybe 5-7 years ago too, by someone from Stanford, I think. I remember the authors being puzzled how the guy was generating forces 14x his body weight.