Another Perspective on the Reaction Arm

I wrote about the reaction arm in fundamental techniques, particularly traditional punching, a few weeks ago. In that particular post I basically said that there are two main views regarding the function of the reaction arm. The one view sees it as critical for maintaining proper structural balance. The second view holds that these reaction arms act as pulling techniques. Therefore, an interpretation for a traditional punch would basically be that you pull your opponent towards you with the one hand (reaction hand) while pommeling him with the other. There is a third view which I did not mention in that post, partially because it is such a very traditional view that is dismissed by many people, and because it doesn't apply to most techniques—only a small group of techniques may actually benefit from this interpretation of the reaction arm.

This third view considers the spine of the body as one's central axis and your arms as extensions on opposite sides of each other that rotates around this axis. There are two objects that can help you envisage this view.

A Korean pellet drum,
which is called a do 도 in Korean.

The first object is a pellet drum, known in Korean as a do 도 / 鼗 or nodo 노도 / 路鼗. A pellet drum has two membranes on opposite ends of the barrel. On the sides of the barrel are two strings with pellets attached at their ends. A pole is attached through the drum (through the barrel, not the membranes).

A Japanese pellet drum toy,
known as a denden daiko でんでん太鼓.

One plays a pellet drum by swiftly rotating the pole in one direction and then in the other direction, which causes the pellets to swing because of the centrifugal force that occurs when the pole is rotated. The pellets hit the membranes and create sound.

Aeolipile, aka Hero's Steam Engine

The second object is an aeolipile, sometimes refered to as Hero's steam engine, after Hero of Alexandria that described its mechanics in the first century A.D. An aeilipile is basically a boiler that is so set-up that it can rotate around an axis. On opposite sides of the boiler are two curved nozzles pointing in opposite directions to each other and perpendicular to the axis of the boiler. When steam escapes from these nozzles, thrust is caused, based on the the rocket principle in accordance with Newton's Third Law of Motion. This causes the boiler to spin around its axis.

Both objects are somewhat flawed to describe what happens in the human body in the third view of the reaction arm. Nonetheless, they help to illustrate the underlying idea. Note that in both cases there is a pair of opposing elements: the two strings-and-pellets on the drum and the opposing curved nozzles on the aeolipile. In both cases the pair of opposing elements rotate around a central axis. The traditional martial art idea is that the one opposing element contributes to the force of the other opposing element. In other words, by forcefully swinging my arm in one direction, this adds to the force of the other arm moving in the opposite direction.

Me trying out Pak Hok Pai in Hong Kong

In some of the Chinese martial arts one sees this idea flamboyantly applied. Sometimes the arms are literally flung in opposite directions just like the strings-and-pellets on the pellet drum. A good example where this idea is applied is Pak Hok Pai, the (Tibetan) White Crane Kung Fu system.



There is definitely power in these techniques caused by the centrifugal force, but I would be hard pressed to say that the arms moving in opposite directions cause the noticeable force in each other. It is probably more correct to say that the arms swing because of the centrifugal force. It is not the arms in themselves that cause the force. It is easy to demonstrate that the arms do not directly influence each other's reaction to the centrifugal force. Stay on one spot and then start to spin around, keeping both arms relaxed so that they naturally swing away from your body because of the centrifugal force acting on them. Now bring one arm towards your body, keeping it tight against you. The fact that this arm is kept from acting on the centrifugal force, contrary to the other arm, does not influence the other arm from still swinging normally as before while you keep on spinning on the spot. The essence of the force, therefore, lies not in your arms working in opposite directions, but lies in the actual rotation of your body that is spinning around an axis. (What is properly influenced when you use only one arm during this spinning exercise is your balance. You have better equilibrium with both arms out.)

The important thing is therefore not the opposite directions that your arms are moving in; the important thing is the rotation of the body around its axis which causes centrifugal force. While I don't believe that the arms moving in opposite directions contribute to each other's force, that doesn't mean that they could not contribute in another way. If pulling back my arm helps me in rotating my body around it's axis faster, then it can actually influence the centrifugal force, which in turn will influence the other arm.

Sanko's Imaginary Jet-Propelled Rotating Glove-Weapon

To explain this, imagine a ridiculous weapon I invented for this post that works similar to an aeolipile. The weapon is made up of a structure with two beams attached at opposite ends of an axis. At the far end of one of the beams is the attacking tool—a boxer's glove—attached perpendicularly to the beam. Perpendicularly attached at the far end of the other beam is a rocket engine. The jet propulsion of the rocket, in accordance with Newton's Third Law of Motion, causes the structure to rotate around the axis, resulting in the glove hitting a person standing close by. We see here that the beam moving in one direction causes the motion of the beam moving in the opposite direction—the glove's force is equal and opposite to the force of the rocket.

Speaking of this principle of Reaction Force, i.e. Newton's Third Law of Motion, the ITF Encyclopaedia states that “A punch with the right fist is aided by pulling back the left fist to the hip” (Volume 2, p. 15). The encyclopaedia doesn't expand on this topic, so it is unclear how pulling back the left fist contributes to the punch with the right hand. The only way I see this to be feasible is if the reaction arm (the one that is pulled back) contributes to the acceleration of the body's rotation around its axis. This in turn will help to push the punching arm which is structurally on the opposite side of the body, just like the glove is opposite to the rocket as depicted in my imaginary weapon.

Many traditional martial arts are of the view that the reaction arm contributes to the force of the acting arm; that pulling back the non-punching arm aids the punching arm. It must be remembered, however, that the reaction arm is not directly affecting the acting arm; instead, it is possible for the reaction arm to contribute to the rotation force of the body. If the reaction arm is not positively affecting the rotation of the body around its axis, then it is not contributing to the force of the strike. On the other hand, if the reaction arm contributes to the rotation of my body around its axis, then it may very well contribute to the force of the punching arm.

It is important to remember, as I mentioned at the beginning of this post, that only some techniques—those that potentially benefit from centrifugal force—will benefit from the reaction arm in this way. For many techniques the reaction arm does not contribute to centrifugal force; instead it contributes to better structural balance and its associated benefits.

Read the previous post on "The Reaction Arm."