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.
In general, when Uke attacks 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/Tori. As Uke approaches the target he uses his front leg to slow his momentum, if required. On contact with Tori, Uke will experience a contact reaction 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 reaction force is zero. Uke resists the unbalancing effect of his momentum and the subsequent contact reaction 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 reaction force and therefore their balance. For example, if Tori does not resist Uke's attempt to pull or push, the contact reaction force for both of them becomes zero. Normally, a sudden change in the direction of the contact reaction force by the actions of either Uke or Tori would lead to them both becoming unbalanced. This is prevented by them instantly relaxing their muscle torques about the joints of the upper body to reduce the contact reaction force to zero allowing them the freedom of movement to reposition their lower body joints/muscle-torques before balancing the new direction of the contact reaction force.
This section summarises the three forms of Aiki and how these forms can been applied by Tori to unbalance Uke. Video footage has provided the source material for bio-mechanically analysing how Aiki has been applied. This has resulted in being able to identify a small number of fundamental methods by which Aiki can be implemented that is in keeping with the following definition of Aiki.
Aiki is a method of unbalancing Uke by reversing a muscular torque that Uke depends on to maintain his balance.
1. Aiki-Feint. As Uke advances towards Tori (zero contact force), Tori induces Uke, in the form of a Feint, to apply Aiki by reversing a muscle torque in Uke's front leg that he normally uses to slow down his momentum as he steps forward. This can take the form of either reversing the muscle torque about the ankle joint or about the foot joint. Consequently, Uke becomes unbalanced to his front. For example, this can be achieved by Tori offering a target and then withdrawing it 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. Aiki-Parry. As Uke applies force to the contact point, Tori applies Aiki by reversing a muscle torque so as to rotate the contact point about a joint/centre point in the form of a Parry. Consequently, Uke's force is no longer balanced in this circular direction and he becomes unbalanced. Tori stays balanced by balancing the radial component of Uke's force as he rotates the contact point about the joint/centre point. This action is only possible if Tori uses an unbendable arm/upper-body and uses his ground reaction force and weight to balance the radial component of Uke’s force. This form of Aiki appears to be used mainly in Aikido .
3. Aiki-Lead. Tori induces Uke to apply Aiki by reversing a muscular torque that Uke normally needs to maintain his balance. This is achieved using the following methods all of which make use of Tori employing an unbendable arm/upper-body and delivering force to the contact point from the ground reaction force and body weight/gravity:-
If the contact force is zero when Uke and Tori are in contact, Tori applies a force to the contact point which will elicit Uke’s natural response to relax his upper body, allowing him the freedom of movement to reorientate his lower body and apply force to increase his momentum, before applying force with his upper body to the contact point to counter Tori’s force/movement. Tori exploits Uke’s initial response of relaxing his upper body to induce Uke to apply Aiki by adjusting the orientation/position of the contact point or applying an atemi so as to lead Uke into reversing a muscle torque that he normally uses to stay balanced.
The duration that Uke relaxes his upper body increases with the size of Tori’s initial force. Large initial body movements by Tori that could unbalance Uke to the side will require Uke to take a longer time(upper body relaxation period) to reorientate his lower body than if Tori applied a smaller force directly at Uke. The former case is more common in Aikido whilst the latter is more commonly demonstrated in Aikijujutsu which uses small body movements which are rapid enough to fit within the smaller relaxation period.
If Uke is applying force to the contact point, Tori reduces the contact force to zero by going with Uke's force which induces Uke to relax his upper body so that he can reposition his lower body. The relaxation of the upper body can be exploited by Tori to induce Uke to apply Aiki by adjusting the orientation/position of the contact point or applying an atemi so as to lead Uke into reversing a muscle torque that he normally uses to stay balanced.
If Uke is applying force to the contact point, Tori resists Uke’s force and induces/leads Uke to apply Aiki by applying, for example, either an atemi or a force to a pressure point/locked joint or applying a pinch to the skin. Consequently, Uke reverses a muscular torque that he normally uses to balance the contact force and therefore becomes unbalanced. Tori retains his balance by stepping. These techniques are commonly used in Aikijujutsu.
Atemi A strike by Tori, using the arms or other parts of the body, with the intention of inducing Uke to respond in a desired way.
Muscle Torque An action produced by the muscles of the body that rotates the body or part of the body about a centre point or joint.
Tori refers to the person who is executing the technique and Uke the person who is receiving the technique and therefore Uke is the one who is restrained or thrown. Consequently, Uke is the person that is unbalanced by Tori.
Unbendable Arm/Upper-Body An action where the muscles that provide torque in opposite directions to move a part of the body about a joint are simultaneously actuated so as to produce no resultant torque/movement about that joint. This action allows the whole of the upper-body/arms to be locked to the lower body which can use the ground reaction force and gravity to control the movement of the whole body.
Feint A deceptive action by Tori to induce Uke to react in a way that is of benefit to Tori.
Parry An action by Tori to deflect Uke’s force.
Lead An action by Tori to guide Uke’s movements.