Hakkei – From potential energy to kinetic power


Movement produces kinetic energy, which can be converted into power. Delivering power into the body of an opponent is not an easy task, it needs a special skill called “hakkei”.
Hakkei (発勁), which literally means ‘release of power’, can generates power with minimal external body motion.
Before  we can use hakkei we need to accumulate pressure in the hara, this is created by using the muscles of the koshi, tanden and yōbu. Those muscles are full of potential elastic energy. See tenshikei and rendo.

Elastic potential energy is the energy stored in elastic materials as the result of their stretching or compressing. Elastic potential energy can be stored in rubber bands, bungee chords, trampolines, springs, an arrow drawn into a bow, etc. but also muscles and tendons.  The amount of elastic potential energy stored is related to the amount of stretching and releasing quality of the muscles and tendons.

Converting elastic potential energy

By releasing the pressure or tension, movement is created and “momentum” is born. Using momentum is a skill called ido-ryoku.
It is not always necessary to have physical distance between you and the opponent. It is possible to emit power into the opponent when you already are touching the body. momentum can travel in a straight line , but can also follow a circular line, called angular momentum.


Momentum is fairly easy to grasp, as we all have an intuitive sense of it. Momentum brings mass and speed together as a single meaningful quantity. If I say something is flying very fast toward you, you would want to also know whether its heavy or not! A light object with great velocity can have a similar momentum as a heavy object with a low speed. When you catch a ball, you absorb that ball’s momentum and transfer it to you, making you move backward. If you have a good posture and firm footing, you’ll be able to transfer your own momentum to the earth.

Angular Momentum

Momentum can also be considered in rotation, along an axis, which we call angular momentum. It gets a bit more complicated here because, as a body turns, its parts that are further away have more speed (if you have your arms extended while you rotate, the tip of your fingers goes at a much greater speed than your shoulders). This is also tied to inertia in rotation. So angular momentum is about mass and rotation, but also about how far the mass is from the axis of rotation.

Conservation of Angular Momentum

But here comes the fun part: Angular momentum is a quantity that is kept constant, conserved.

Let’s say you are sitting on an “office chair” that can rotate freely .

  1. Start by swinging your arms left and right, first extended, then close to your body. What happens? Your knees rotate as well, but the opposite way, and they rotate more when your arms are extended.
    You started with no angular momentum. As you created some angular momentum one way in your upper body, your lower body swiveled in the opposite direction, keeping the total angular momentum at zero.
  2. Now ask someone to give you a rotation speed, while sitting on the chair, with your arms extended, and bring your arms together.  Your rotation speed increases noticeably.
    This time, you started with a fixed angular momentum, but as you moved your arms inwards, you reduced the speed of your arms by bringing them in, and your overall speed increased as a result, keeping your angular momentum constant.

  A martial arts example : sayabiki


Sayabiki is the pulling of the scabbard (saya) when performing nukitsuke, a cutting motion as you swiftly draw the sword.
Doing sayabiki at the end of drawing the sword allows for greater cutting speed. Furthermore, as you reach the end of sayabiki and the scabbard slows and stops, you also help slowing the tip of the sword and allow for better control of the tip.

The cutting with the sword (nukitsuke) and the pulling of the scabbard is produced by using the muscles of koshi, tanden and yōbu as described earlier.

Budo Aikido example of angular momentum

We start with a big circle to a smaller circle. The initial circular action is building up pressure and tension in the hara, by releasing with the proper footwork, a sudden power or hakkei is produced. See 8 sotai dosa.

The example is a part of the 8 sotai dosa, but also a technique found in koryu no kata dai yon.

soatai dosa 2 A

Hakkei or fajin in Western literature

In martial arts literature (translated from Japanese), we cannot find a lot about hakkei. It has a rather obscure image although in some books about Aiki-Jutsu and Okinawa Kenpo or Karate we can find “how to do” information. The reason for lack on practical information can be found in the difficulties in the explanation of expressing in words, it is tacit knowledge.
Tacit knowledge (as opposed to formal, codified or explicit knowledge) is the kind of knowledge that is difficult to transfer to another person by means of writing it down or verbalizing it.

Hakkei fajin

In Chinese literature on martial arts, fajin is a more common item in training.

Hakkei is “sudden” power

Before you can use hakkei, you have to store potential elastic energy in the body, mostly in koshi, tanden and yōbu. You have to convert the potential energy into kinetic power by suddenly release the pressure or tension. Many so-called kokyu nage are a form of hakkei by using the transformed potential energy and breathing. This process of breathing is called “reverse breathing”.

Natural and Reverse Breathing

Not all the teachers of Japanese martial arts are promoting a special method of breathing. Building up pressure in the body can harm your health, especially people with high blood pressure, or people suffering from cardiac diseases. Before you start with a breathing program please consult your medical doctor.

(from Mike Sigman Blog)

There are two main types of breathing: natural-breathing and “reverse”-breathing. Natural breathing is the type of breathing where the inhale expands the abdomen, hopefully somewhat not only in the front, but in the kidney areas, also. “Reverse”-breathing refers to the idea that on the inhale the lower abdomen comes somewhat in and then goes somewhat out on the exhale. Because the lower-abdomen isn’t allowed to expand on a reverse breath, there is a slight pressure build-up in the abdominal area.

Reverse Breathing is the type of breathing practiced in the internal-arts proper, after real development and training begins. Reverse breathing does a number of things, but it does two things that are particularly important for someone who is learning to move the whole body as a connected unit :

  • reverse breathing controls the body-wide tensions it initiates and
  • reverse breathing helps control the pressures which are an intrinsic part of internal-arts that are controlled by the dantian.

Pressure basics

Within the body cavities, breath initiated tensions are used in conjunction with the increase in pressure to train and develop the connective tissues.

As a person inhales while either slightly pulling in the abdomen or at least holding it in stasis so that it isn’t allowed to bulge outward, the diaphragm comes down. It must come downward or air can’t be pulled into the lungs. As the diaphragm comes downward and the front of the abdomen is kept from expanding outward, pressure increases in the abdominal cavity and kidney areas.

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