Tai-otoshi

Tai-otoshi is a hand technique in which tori must break uke’s balance in order to rotate uke 270 degrees, resulting in ippon or an instant win.
Photo credit: Zhaofan Chen, Judokas: Matt Gnolfo and Jason Dwyer

Tori pulls on the sleeve of uke’s gi to break the balance of uke.

 

When the balance of uke is broken, uke’s base of support becomes narrower, going from their flat feet to their toes. This shift causes the center of mass of uke to move from over their feet to over the outside of the body. When the base of support narrows, it is more difficult for uke to keep their center of mass over their base of support, making it more difficult for uke to remain balanced.

 

Tori applies an upward force to uke at the collar and wrist of the gi.  

Photo credit: Zhaofan Chen, Judokas: Matt Gnolfo and Jason Dwyer

Tori stands on the side of uke’s body and places their ankle in front of uke’s ankle. Tori needs to make sure that their body doesn't block uke’s fall. While lowering the body, to stabilize themself, tori pull the arm of uke out to further break the balance of uke.

The purpose of tsukuri is to move uke’s center of mass further from uke’s base of support. By blocking uke’s leg, uke will not have the opportunity to move their legs and adjust their center of mass to balance themself. Through our experiment, the moment of inertia of a sample subject with an extended arm is 15.91 kgm^2. This moment of inertia dictates uke’s ability to rotate based on the force applied by tori. Tori can stabilize their body by lowering their center of mass.

 

A normal force exists between the foot of uke and the foot of tori during tsukuri. There is also the force of tori pulling on the arm of uke parallel to the ground, which causes a torque resulting in a 270º rotation of uke about their center of mass. The force of static friction is what causes tori’s foot to stay planted on the ground when uke’s pushes against it. Tori’s leg moves to block uke’s leg to prevent uke from moving their leg and increasing their base of support to rebalance. This will help break the balance of uke and get uke ready for kake.

Photo credit: Zhaofan Chen, Judokas: Matt Gnolfo and Jason Dwyer

Tori uses their arm to pull uke’s hand forward and down to rotate the body of uke. Tori needs to make sure uke’s body is away from their body so uke’s rotation isn’t blocked by tori.

 

Tori applies multiple forces to the arm of the gi and the collar of the gi which results in a torque applied to uke, causing uke to rotate. Uke’s body continues to rotate 270º about their center of mass while being pulled down by gravity. Uke transitions from a standing position to a lying position after the rotation process. The force applied to uke by tori in kake is a continuation of the forces applied in tsukuri. According to an outsider observer, these forces change direction, instead of going forward, they are now going downward. However, for uke, the forces do not change direction and remain perpendicular to uke’s body.

Photo credit: Zhaofan Chen, Judokas: Matt Gnolfo and Jason Dwyer

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