Applying Aiki

To understand how to apply Aiki it is necessary to have some knowledge of the biomechanics involved in the movement and application of force by Uke and Tori.  The following paragraphs summarise this information which will help understand the analysis of how Aiki is applied in the YouTube videos given in the following pages.


Maintaining Balance


In general, when Uke attacks Tori he uses his rear leg to apply the force that is needed to increase his momentum to move his body towards the target/contact point. On contact with Tori, Uke will experience a contact force the direction of which will be determined by whether Uke is pushing or pulling with his arms. If Uke is simply holding Tori the contact force is zero. Uke resists the unbalancing effect of his momentum and the subsequent contact force by using the ground reaction force produced at the base of his feet and the gravitational force due to his weight.  The direction and magnitude of these two balancing forces are controlled by the position of Uke’s joints at the waist, knees, ankles and feet and the torque (rotary force) that the muscles exert about these joints.


As Uke makes contact with Tori, Tori will experience the same magnitude of contact force as Uke but in the opposite direction. The actions of both Uke and Tori determine the contact force and therefore their balance. For example, if Tori does not resist Uke's attempt to pull or push, the contact force for both of them becomes zero. Normally, a sudden change in the direction of the contact force by the actions of either Uke or Tori would lead to them both becoming unbalanced. This is prevented by them instantly relaxing their muscles to reduce the contact force to zero allowing them the freedom of movement to reposition their joints before balancing the new direction of the contact force.


Applying Aiki


Aiki is a method of unbalancing Uke by reversing a muscular torque about a joint that Uke depends on to maintain his balance.


Aiki can be applied in three basic forms:-


1. As Uke advances towards Tori (zero contact force), Tori induces Uke to apply Aiki by reversing a muscular torque in Uke's front leg that he normally uses to slow down his momentum as he steps forward. Consequently, Uke becomes unbalanced to his front. For example, this can be achieved by Tori withdrawing the target just before contact is made, applying a light-touch downward force or applying an atemi as Uke steps forward. Since there is no substantial contact between Tori and Uke, Tori’s balance is unaffected.


2. As Uke makes contact with Tori and applies force, Tori applies Aiki by reversing a muscular torque so as to rotate the contact point about a joint in the same general direction as Tori's contact force. Consequently, Uke's force is no longer balanced in this direction and he becomes unbalanced. Tori stays balanced by balancing the radial component of the contact force as he rotates the contact point about the joint.


3. After Uke makes contact with Tori, Tori can induce Uke to apply Aiki in a number of ways:-


(i) If Uke is just holding Tori (zero contact force), any movement by Tori will prompt Uke to momentarily relax his body so that he can realign his body/balance before applying force to resist further movement. To induce Uke to apply Aiki, Tori can use his initial actions to guide Uke, in his relaxed state, to reverse a muscular torque about a joint that Uke subsequently relies on to maintain his balance as he resists Tori's movements. Consequently, Uke becomes unbalanced when he applies force, whilst Tori retains his balance by stepping.


(ii) If Uke is applying force the zero contact force condition in (i) can be obtained by going with the force and then applying Aiki as in (i). 


(iii) If Uke is applying force Tori can resist Uke’s force and induce Uke to apply Aiki by applying, for example, an atemi or a force to a pressure point. As Uke becomes unbalanced, Tori retains his balance by stepping.


(iv) Tori can induce Uke to apply Aiki by making contact with Uke when Uke has a muscular torque in the reverse direction to that required to resist the unbalancing effect of the contact force.