If you use a muscle to move something you are essentially using a lever. To pull something towards you, you will use your biceps to flex the lower arm around the elbow joint. The power derives from the muscle, the pivot or fulcrum is the elbow and the lever is the fore-arm. It is not that simple though. To apply this force to something heavy we need to create a rigid framework which connects the body to the ground, stabilises the body and permits the arm to pull the object in. Without this framework we could just as easily pull ourselves to the object as pull the object to us. This means that the body is hardwired to create a rigid structure whenever we seek to use muscular strength against a resistance. A corollary of this is that if you use strength you inevitably block the energy that you can create by moving the entire mass of your body.
Given the point of attachment of muscles, the distance of that attachment from the fulcrum (ie the elbow) and the length of the forearm there are calculations that show that if you are holding up a 7lb weight a biceps muscle must exert a force of nearly 50lbs. Trying to move a lightweight (100lb) person just with the biceps requires the muscle to generate a force of 700lbs or 350lbs each if you are using two arms. Clearly this doesn't happen in real life. Of course when exerting a force on another person that person is likely to resist. Using muscular strength against the point of contact is therefore highly inefficient and likely to fail against stronger opponents.
Aikido seeks to overcome these difficulties by avoiding the direct use of muscle against an opponent but rather using the mass and momentum of the whole body. In addition an aikidoka will move an opponent in a direction where they have least ability to resist. Given that humans only have two legs they will always be unstable in two directions, namely perpendicular to the line drawn between their feet. Of course any opponent will not remain static so one of the skills of aikido is to continuously change position to maintain the ability to disrupt an opponent's structure.
The disruption of an opponent's balance is fundamental to the successful application of aikido techniques. Indeed, in some respects the focus on teaching “techniques” in aikido, particularly through the use of compliant partners, can lead to the illusion that such techniques will work in a real situation. Try to apply kote gaeshi to a strong partner who is stable and you will find that you will not be able to turn their wrist over. Similarly try to apply shiho nage to a stable partner and they will easily resist. Not only that, a focus on applying the technique while an opponent is stable suggests that you are still within easy reach of an opponent which gives them plenty of opportunity to launch another attack
Disrupting an opponent's balance can involve gross movements or extremely subtle movements, depending on the circumstances at the time. Trying to pull an opponent into a position of instability using the muscles of the arm both telegraphs your attempt, creating an immediate counter-reaction and causes your body to generate a rigid structure which prevents free movement. On the other hand leaving your arm relaxed and moving the entire body in the appropriate direction will weirdly move the opponent's centre beyond his base and create instability. Interestingly not much movement is required to destabilise an opponent and this opens up a whole range of subtle ways of ensuring that techniques cannot be easily resisted.
Moving the body is not the only way we can disrupt an opponent's stability. Atemi or strikes can have a similar effect.
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