Module: Biomechanics in Judo

Module 4: Biomechanics and Judo

BAMM!! Ughh! “Damn,” thought Sam as Ishizaki repeatedly threw him. How does he do that?  Resist harder this time.  I need to stop his right hand and 00oppsss, THUDD! got caught again.  okay, that’s enough, flying time for me.  Thanks for the work out.  By the way, how do you do your throw?  Can you show me?”  Ishizaki somewhat at a loss stops and in his broken English begins to explain, “You put this foot here, the other foot there, you twist your hips in front of your opponent.  Also don’t forget to pull your opponent forward.”  Okay I got it, but why doesn’t it happen when I do it”?


  1. What are the governing forces that allow one person to throw another person?  What different terminology is used in physics, in biomechanics?
  2. What are kuzushi, tsukuri, kake, and kime?
  3. How is a judo throw explained in terms of modern biomechanics?
  4. Does the cognitive understanding of how a throw is accomplished enable the practitioner to throw someone or is there a missing element?
  5. What importance can judo biomechanics play?

Physical Movement

In 1877, Jigoro Kano entered Tokyo University.  That same year he also began his study of jujitsu under Masanori Minamoto-no-Hachinosuke Fukuda.  His first attempts at grappling with the master met with disaster as he was repeatedly tossed aside with ease.  His query as to how this was accomplished was met first with silence then an admonition to keep quiet and just practice and he would eventually get it.  Kano’s young, educated, logical mind could not comprehend the magic that had repeatedly turned him upside down and onto his back.  After several years of intensive training, he was finally able to repeatedly throw another master.  The master was so perplexed by this turn of event that he asked Kano, “How did you do that”?  To which Kane replied, “Each time I saw you throw someone I noticed that you pulled them off balance first before entering into the throw.  I merely did likewise.”

Although more easily able to execute the throws now, he wondered if it was enough just to verbally explain a throw.  The question, would someone learn just from a verbal explanation?  How strange, to be able to finally answer your own original question and yet later question whether it was sufficient.

Listed below are a few of the words and concepts found in most basic physics textbooks.   We will have to become familiar with if we, like Kano, are to answer from a cognitive prospective how a throw works: matter, mass, base of support, gravity, line of gravity, center of gravity, friction, inertia, energy, opposite and equal reaction, velocity, force, force couple, centrifugal force, centripetal force, torque, and levers are but a few of the more relevant to judo.  These terms should give the coach an introductory means of understanding how movement occurs.  In this way, the coach may analyze his student’s techniques and make suggestions for improvement.  First, we will look at those items that deal with stability, and then proceed to the items dealing with mobility.



Essentially similar matter and mass are anything that takes up space and has quantifiable properties.  Matter/mass is usually divided into three categories of solids, liquids, and gases.  For our purposes, our bodies and our judo environment make up the mass we are concerned with.

If we look at the body as a human machine there are three areas of concern: biomechanics, musculoskeletal anatomy, and neuromuscular physiology.  In this section, we will be dealing mainly with the first two areas.  The third area will be covered in-depth in level “D” assistant judo instructor certification.

In judo, we like to think that with skill and technique a small man can defeat a slow un­skillful big man.  Sometimes this is the case however, at higher levels even the large man is quick and technically sound.

Up until the 1961 World Championships it was generally thought that body mass should have little bearing on the outcome of a match.  Moreover, even if one was smaller, if you were skillful and had heart, you could defeat a usually slower larger opponent.  With all due respect to skill and determination, this has not seemed to be the case since almost all of the world champions, both when there were no weight divisions and when the open weight category began in 1965.  No lightweight has ever won.  Thus mass does seem to matter.  All other things being equal the person with more mass has that advantage.

Nonetheless, the lighter divisions seem to demonstrate more speed, agility, and technical grace while the heavyweight divisions depend heavily on strength.  In addition to the physical skill, the physical environment includes the practice or competition surface tatami mats, and the judogi uniform.  Both of which may vary widely and therefore alter the outcome of a match.

 Base of Support

The base of support is the area the mass must rely on in order to maintain a degree of stability.  For humans it is our feet when in an upright position, hand or hands when inverted; our posterior side when in a supine position; our sides, when lying on our side, or on our hands and knees when in a crawling position. Basically any part of the body upon which the body may be supported becomes the base of support. Naturally the more surface area supporting the rest of the body the more stability there is.  Mobility, however, is compromised when there is more surface area contact. Humans in the biped position have better stability when their feet are approximately shoulder width apart rather than together in an “attention” position, (feet together).  If stability is sought in an anterior/posterior direction, then its one foot in front of the other.  There is however a point of diminishing return as the feet are spread ever wider, losing anatomical advantage and balance.  For the judoka, it is a constant struggle to simultaneously maintain stability for defense and optimize mobility for offense against an aggressive opponent hoping to do likewise.


Gravity is the perpendicular force exerted upon an object pulling it in toward the center of the Earth.  For falling objects this pull is at thirty-two feet per second squared.  Due to the pull of gravity we are able to estimate the weight of an object.  We may also make further estimations as to how much work it would take overcome inertia.  Gravity may also act upon a body that assumes a different position.  For example, were it upright, the famous Leaning Tower of Pisa would have less of a likelihood of falling over, but because of its leaning position gravity has a greater ability to pull on its now exposed side surface.

When objects are pulled or pushed out of their line of gravity they become subject to the pull of gravity.

Line of Gravity

The line of gravity is an imaginary line around which the body’s mass is evenly distributed when the body is perfectly perpendicular.  A plumb line, if you will.  For our segmented bodies the line of gravity may also be maintained if the ankles, knees, hips, arms, neck and head are bent as long as the distribution of body is evenly displaced over the imaginary line of gravity.  There are a myriad ways to compensate and to stay within the line of gravity.  In more sophisticated maneuvers a judoka may even counterbalance his weight against the pull of his opponent in order to maintain himself within the line of gravity.


Often in judo, we hear our instructors / coaches’ command, “PULL!”  This is usually in an effort to move the opponent out of his line of gravity and in a defensive mode, either in getting him off balance for an impending throw or continuing to finish a throw.  In judo terminology, this action of pulling the opponent out of his line of gravity is referred to as kuzushi.

The Planes Of The Body

The center of gravity can be located at the spot where the sagital, frontal, and transverse planes intersect.


Pulling or pushing the opponent out of the line of gravity just prior to throwing makes it easier since the pull of gravity now assists in getting the person down.


Center of Gravity

The center of gravity is an imaginary center around which the body is evenly distributed.  In men this is around 56% of their height and for women at around 55% of their height.  This center may also change if the arms or legs are altered, changing the body position.  Often we are told to bend our knees to avoid being thrown.  This action lowers the center of gravity and makes throwing more difficult to accomplish for tori.  On the other hand, Tori is often told to get his center of gravity below that of his opponents if he expects to accomplish a throw.

Friction is caused by two surfaces sliding over each other. The heavier the object the more surface area there is; the rougher the surface; the harder it is to move. In this illustration one can push a car on wheels  to realize what is meant by friction and gravity.



Friction occurs as the body of one surface moves over the surface of another.  The coefficient of which may be altered by the surface of the two bodies.  Therefore, a rough surface will resist the movement and a smooth surface allows movement.  For judoka, the surface is the bottom of the feet.  The other surface is the mat.  Unfortunately, not all mat surfaces are suitable for judo.  Although there are no set standards for mat surfaces, they should not be lumpy nor should they be rock hard.  They should not be excessively sticky nor should they be slippery.

The base of support for the person on the bottom is his back. With more surface area he has more friction and stability: The knee, lower leg and feet are the base of support for the person on top.


Newton’s Laws of Motion

First Law: (Inertia)

An object at rest will remain at rest until some external force acts upon it, and an object in motion will remain in motion until an external force acts upon it.  When we think of a ball we usually associate it with rolling, yet no rolling occurs without some outside force making it move: if on a slanted surface, it will roll downward due to gravity if kicked, it is propelled by muscular action.  The ball will also continue to move in the direction the force has propelled it until something stops it, like a wall, or an opposing force such as gravity.

In judo, inertia is overcome by muscular action.  Conceivably, in a world without gravity, when a throw is executed the opponent could he hurled into space and continue to travel until some other force acted upon it.

Second Law: (Acceleration)

If a force acts on a body, that force will produce an acceleration of the body in the direction of the force.  The magnitude of the acceleration is directly proportional to the magnitude of the force and in inverse proportion to the mass of the body.

Most judo throws are examples of how force (tori) accelerates a body (uke) as described above.  The force exerted by tori and the mass of uke will define the magnitude of acceleration.  A smaller tori must apply more force or combine his force with a similarly directed force from uke to provide adequate acceleration to effectively throw a larger uke.

Third Law: (Reaction)

For every action there is an equal and opposite reaction force.  Newton’s third law relates to two bodies; where a force is exerted by body A to body B and a reverse reaction force is exerted on body A by body B.

For judo throws this is important since anatomical positioning is crucial to the ability to properly throw.  In order to throw an opponent there is a need to lift the person off the mat surface.  To accomplish this, a downward force must be sufficiently applied to the mat surface to enable a lifting action of the opponent in the opposite direction of the downward force.  The angle at which the throw is executed is also important.  In most throws that throw the opponent to the front side, tori must step in deeper than the line of gravity with at least one of his feet.  In this way, he will be able to push off to the front.  In the following photograph you will notice that tori has stepped in deep enough so that when he turns he is leaning in front of his line of gravity.  Now as he pushes and turns, he can propel his opponent in the opposite direction.  This is called opposite and equal reaction.

Notice that the thrower is pushing against the mat surface with his feet.  As much force as he is able to drive down into the mat that is how much force he will be able to generate in the opposite direction.  This is the law of opposite and equal reaction.


Simply defined energy is the ability to do work. This work may be reflected in multiple ways: kinetic, electric, chemical, magnetic, nuclear, mass energy, muscular, etc.  In judo we mainly use muscular energy and mechanical advantage.

Speed & Velocity

Speed is a measure of how fast something is moving and is measured in terms of distance  traveled in a given time interval

Velocity on the other hand specifies not only the distance traveled within a given time but also the direction in which the object is going. Speed is one of’ the more important items for a judoka. The faster one can move into position the less time the opponent has to react and to resist a technique. Speed also adds to the force of impact.

Inertia is where an object will remain at rest or in motion unless acted upon by an outside force. Here the force is another person acting upon a person at rest.


Acceleration is the measure of the change in velocity.  For the judo student he is adjusting to the accelerated movements of his opponent.  Most of judo is dynamic.  There are very few moments where no action is occurring.  Movement in judo is always in a state of flux, sometimes moving slow, sometimes speeding up.  In most cases, we are trying to speed up to overcome a target which is moving in an unpredictable manner.  Because of this fact, the judoka must have the ability to adjust his speed, especially for entries into throws.

Acceleration is a change in velocity.  One may be going at a certain speed, get hit by a greater force and change in velocity.  The change in velocity occurs as a result of being hit by the player who is going faster.


Force is simply a push or a pull and can be generated by electricity, magnetism, gravity, nuclear or just plain muscle.  Force can be used to overcome inertia or accelerate an object.   Judo is full of examples of force being applied.  Most of the force generated in judo is initiated by muscular contraction but may also involve other contributing forces.  For example once pulled out of the line of gravity the off balanced body has more surface upon which gravity’s downward pull may act.

Force Couple

A force couple is a pair of opposed forces traveling in opposite directions, acting upon an object where the outmost end is being accelerated around a focal point.  An example of this would be a revolving door being pushed by two people in opposite directions.  An example of this in judo would be an osotogari (outside major reap) where the opponent is being pushed and pulled at the same time.  In this case uke is being pushed to his back side by tori’s hands at the top end of the body and pulled at the lower end in the opposite direction by the reaping action of tori’s leg. Yet another example is okuri-ashi-barai (chasing foot sweep), where the legs are swept in one direction while the body is pulled in the opposite direction.

In force couple, one force is going in one direction while the other force is moving in the opposite direction.  See the arrows.

 Centripetal Force

Centripetal force is the force exerted on a rotating body which keeps it in rotation about an axis. The force is generated inward by a connecting mechanism between the rotating body and the axis.  The result of this force keeps the rotating body moving in a circular path.  As a judo example; tori acts as an axis and exerts centripetal force via his arms onto uke when executing Hiza-Guruma.

Centrifugal Force

Centrifugal force is a reactive force exerted seemingly by a rotating body.  The force is equal to and opposite of the centripetal force exerted on the rotating body.  Centrifugal force is exerted by the same binding mechanism as centripetal force.  As an example, tori acts as the axis when spinning and pulling uke in a circular motion, such as Hiza-Guruma.  Tori has the sensation that uke has a force pulling him away.  In reality tori is feeling the reactionary force generated by tori’s arms in reaction to the centripetal force tori’s arms are exerting on uke.


Torque is the rotational effect of a force being exerted off-center and at a right angle to the axis.  A simple example of torque would be a wrench turning a bolt.  Also, the angular rotation of the axis is proportionate to the net applied torque which can be altered not only by the force applied but also where the force is applied along the lever arm.  Much of the torque produced in judo occurs as one is gripping the judogi in various positions, as in sleeve twists, strangulations, and throws where the judogi is round tightly around the hand to cinch up any slack.


A lever is a basic machine that produces work more efficiently.  Its component parts consist of an axis, a rigid object, and a resistance.  There are three classes of levers; first, second and third class levers.  The first class lever has a force on one side and a resistance on the other side of a rigid object and the axis or fulcrum placed somewhere between.  An example of this type of lever would be a teeter-totter, or a crowbar.  With a little imagination, if we think of the opponent as an upright bar and our hip as the focal point, then we can see that we have a first class lever complete with a force arm and a resistance arm on either side of the fulcrum.

The second-class lever is where the fulcrum or axis is at one end and the force at the other end.  The resistance would be in the middle as in a wheel barrel, or a door.  An example of this type of lever in judo would be the Sasae-tsurikomi-ashi with the force at one end and the fulcrum at the other.

The third class lever consists of an axis at one end and the resistance at the other.  The force is applied to the middle of the rigid object.  Examples of this would be shoveling dirt or the action of a catapult.  An example of a third class lever in judo would be a Hiki-komi-gaeshi (drag-down rollover) where the thrower reaches over the back grabs the belt, and while laying down backward placing a foot on the inside thigh of the opponent and kicking up-ward while pulling the opponent overhead.  Here the pull is to the mid part of the opponent’s body and the focal point is the opponents head while the lower part of the body is the resistance.




Module Review IV 

  1. Study figure “A” and discuss what forces of gravity are at work and how the throw might be improved.
  2. After looking at Figure “B”, explain what type of lever is at work.
  3. Discuss what forces, lever, and speed factors are concerned in photograph “C”.
  4. Discuss what class of lever an arm-bar is and why?
  5. Discuss the contributing factor of speed in the photograph and what other factors it may be compensating for.
  6. Explain the advantages and disadvantages of mass.  How can mass be used to one’s advantage?  What factors may overcome mass?  Are there psychological considerations that should be addressed whether coaching for or against a large person?
  7. How would you prepare an endomorph for competition?  How would you prepare an ectomorph who is about to compete against an endomorph?
  8. Identify and discuss the composite forces that make up the throw in this photograph.
  9. Discuss Newton’s Law in relation to the photograph below.
  10. From the photograph below, discuss what factors are at work and what improvements can be made for the thrower.
  11. Discuss what types of levers are being employed and what improvements can be made.