I wanted to post a brief addendum to my initial article to clarify and expand on a few points. Although I feel my original post is technically correct, some topics weren’t addressed with the detail I believe they deserve.
I stated previously that uke’s muscles and joint cartilage take the brunt of stress during a fall. Although correct, I would point out that the energy absorbed by uke’s body is in reality quite negligible. Otherwise, the integrity of uke’s body would be compromised by the throw itself. By the same token, the amount of energy lost to friction is likewise miniscule. For our purposes, therefore, the majority of energy expended by nage is transferred directly to uke, which must then be exchanged, ideally in a non-injurious way.
Perhaps a clearer way to visualize this transfer is that of a bowler (the sport, not the hat). The energy expended by nage forces uke to roll in much the same way as a bowling ball when released from a bowler’s hand. Uke assumes a rounded shape, like a ball, specifically to facilitate this transfer of energy in a safe manner. When uke stays rounded during the throw, force is turned into work, as opposed to shear stress. It is the difference between rolling a ball, and rolling a brick.
Which brings us to the crux of uke’s dilemma. Since energy can neither be created nor destroyed, only exchanged, the energy transferred to uke is still very much present. How uke deals with that energy determines whether the exchange is injurious or not. By naturally allowing the forward force (i.e. the throw) to create a “moment arm”, uke is caused to rotate, which allows him return to his feet. Should uke resist the throw, and therefore not rotate, the energy transferred becomes shear stress on uke’s body, as he is thrown headlong into the mat.