Written by Andrew Tilling
Table of Contents
- Blocks (Uke)
- Punches (Tsuki)
- Kicks (Geri)
- Basic Alignment
- Concepts in Power Generation
- Other Key Concepts
- Martial Arts Compared to other Striking Arts
- Final Thoughts
These Japanese characters refer to Go Ju Ryu, or hard soft style of Karate, this is the style referred to in this essay, although the information presented is not limited to this style exclusively and can be applied to other martial art disciplines.
It is important to keep in mind that this style of Karate in its name refers to the fact that some techniques are hard and some techniques are soft. A hard part of the body used as a weapon is not used on a hard part of the opponent; this leads to injury to both parties. Conversely a soft part of the body used as a weapon is not used on the soft part of the opponent, as this causes no damage. It should be kept in mind when reading the following that we are using this philosophy when discussing strikes to an opponent. That is, hard strike to soft target and soft strike to hard target. For example, a strike with a closed hand to the jaw will injure both
defender and attacker. If we use a palm heel strike to the jaw with the aspects of what is discussed as follows we will injure the attacker and not ourselves in the process.
It should also be noted that we discuss closed hand techniques in this essay, however, again the information presented is also applicable to open handed techniques. Both the circumstances and the skill level of the practitioner dictate the choice of either open or closed hand.
The punch, kick and block constitute the basic strike and blocks in the majority of martial arts. These techniques all require consistent training to become proficient in each. This essay will describe the basic movements and required alignments of each, then we will see how we can improve each of these with the help of physics.
In Karate we have blocks to defend against an opponents’ attack, it should be noted that the Japanese term for a block is Uke, the literal translation is to receive, and so we are receiving the strike of the opponent. It is then up to us what we do with this strike and the energy involved, the basic blocks are the upper block, inside, outside and lower parry. In Karate defences (Barai) can be also be an orthodox movement as well, this component does not form part of this essay.
First it should be noted that the overall mass of the arm and hand is much less than that of the leg – we will assume that the fist is 3% of the bodyweight – as such strikes with the hand can be performed extremely quickly. These strikes do not carry as much power as a kick, but still have considerable impact. There are 3 basic fist strikes in the Goju Ryu style; these are the upper, middle and lower punches. There are also other strikes with the fist that are used, these incude backfist and hammerfist for example.
There are only four types of kick in Karate, these are the front kick, of various heights and types and the crescent kick, sidekick and jumping kick. Each kick utilises a different part of the foot to strike the opponent. The mass of the leg is quite considerable when compared to the fist, it is approximately twenty five times the mass of the hand. This means that kicks when executed with correct form carry much more energy than a punch. It should also be noted that traditionally in karate we do not kick above the waist. The higher the strike with the foot the higher the centre of gravity becomes and the more unstable the person becomes, making it easier for an attacker to unbalance the defender.
For each of the blocks, punches and kicks there are some basic alignments that are required when executing each technique, these are essential to ensuring that each of the techniques is effective.
The basic blocks are listed in the table below.
The upper block is executed with the forearm and palm facing forward of the body, the fist must be both in front and above the head, the elbow must also come up when executing the block. This allows the arm to rotate in the last part of the block adding to the efficiency of the motion. The forearm should be angled down so that any attack blocked will tend to slide off the blocking arm and away from the centre.
The outside block sees the arm move from the inside to the outside of the body. The fist is level with the shoulder, with the elbow bent at around 100 degrees with the distance between the elbow and the body being one fist. The fist is rotated away from the body to ensure that the forearm muscles are tense when the technique is executed.
The inside block sees the arm move from the outside to the inside of the body, the alignments of this block are similar to the outside block. The arm moves across in front of the body to deflect the attack away from the centre, again with the elbow bent at around 100 degrees. The fist is also rotated away from the body as per the outside block to make the forearm muscles tense for the block.
The lower parry is different in that the alignment of this block relates directly to the thigh. The forearm should be parallel to the thigh, with the palm facing the leg. So depending upon the stance the height of the block will vary accordingly. The key to the effectiveness of this block is that the fist starts on the front of the opposite shoulder and the elbow joint then extended by leading the fist across and down the body. This block contains a large amount of energy because of the distance the forearm and hand travels.
These basic alignments ensure that the techniques are biomechanically sound and represent the most efficient and effective techniques.
The basic punches are listed in the table below.
The alignment of each varies with the type of punch, as listed in the table above each has its own target area. Another key is the elbow position, when the arm is extended the elbow must still contain a slight bend this allows for the arm to absorb any energy returning to the body like a spring. This also means that the elbow will not be hyperextended. When striking, the arm that is chambered can gain additional speed by retracting the opposite arm as fast as possible, this also aids with the hip movement, to do this we must be as relaxed as possible. Another part of the alignment is that of the fist itself, the table above also shows the alignments for each punch. The rotation of the fist from chamber should occur as late as possible in the technique, this has the effect of increasing the penetration power of the strike. Finally when striking the knuckles used must be the forefinger and middle finger, the importance of this is discussed later in this paper.
The basic kicks are listed in the table below.
The alignments for the front kick are shown in the table above; with all the kicks we use the ball of the foot except for the groin kick, which uses the distal part of the foot. It should be noted that to use the ball of the foot the toes are pulled back, this has two important uses. The first is to ensure that the toes are not in a position to be damaged and the second is that this ensures that the ball of the foot is in its strongest position for the impact of the strike. With both the middle and upper kick the foot should rise upwards and into the body leading with the ball of the foot, another key is thing is the knee lift. The height of the knee is directly related to the height of the final kick. This relies on both strength and flexibility, which is discussed later in this paper.
Like a punch the foot must also be chambered prior to the technique, this allows for greater speed and power in the technique. The leg should always have a slight bend, again for the same reasons as when punching, to ensure that the knee is not hyperextended and that the leg can also act as a shock absorber for any returning energy. Depending upon the type of kick required it could be either a push or snap kick; this is discussed later in greater detail.
Concepts in Power Generation
Now that we have described the basic alignments and features of each technique we can now introduce the key concepts in the generation of power for striking, kicking and blocking. It should be noted that all of the following concepts should be incorporated into all of the aforementioned techniques. Incorporation of these concepts during practice will allow for a greater efficiency and power for each of the techniques.
Ground Reaction Force
According to Newtons third law “For every action there is an equal an opposite reaction”, how does this statement relate to martial arts and specifically Karate?
The above statement means that the power originates from the ground, not the muscles of the body, but how is this the case?
Newtons third law can be better understood when shown as a diagram as per below:
To produce power for a strike you must first push into the ground with the legs through the feet. The more that you can push into the ground the greater the ground pushes back with the same amount of force. This is called the “Ground Reaction Force”, GRF. It is this force from the ground that is the starting point for the power that is generated during the execution of a punch, or any other strike, including kicks. This force must then be transferred through the legs to the hips and ultimately the fist. The harder that you can press your rear foot into the floor the more force is available for the strike, the conversion of this power is the key and requires rigorous and consistent training to refine.
There is also another very important reason to understand this concept, and this is the reaction force from striking the target itself. This reaction force will travel back to the striking body as per Newton’s third law so that means energy goes back towards your body. To make sure that the force created in the strike is properly transferred, the heels of the feet must be down and the feet flat. Failure to do this means that the reaction force will come back into you, this essentially means that you are striking yourself.
Once this force has been exerted from the ground it must then be transferred through the lower body to the upper body and ultimately to the target. The hips represent a vital connection between the lower and upper body, and it is the hips that are used to transfer this energy.
As stated above, the hips are the key to power transfer, to transfer this force the hips need to rotate, this has the benefit of increasing the speed of the strike, via an increase in distance that the fist travels.
It should be noted that the degree of hip rotation to perform this transfer is small; we do not need to turn the hips beyond 20-30 degrees to achieve the transfer of power and other benefits. In turn this lends itself to the rotation of the hips being a rapid “snapping” movement rather than a slow “pushing” movement.
When we strike with a fist we chamber the fist before striking, this increases the distance the hand travels, this means that the fist has more time to accelerate. Hence the fist has a greater velocity than if it is not chambered and more energy is contained in the strike. It is the same with a kick, we must chamber (bend) the knee prior to the kick, the greater the degree of chambering, i.e. the greater flexion of the knee joint, the greater power that will be generated in the kick, for the reason stated above.
This can be expressed using the following equations:
According to the above if we increase the distance travelled over the same period of time then the speed also increases.
If we increase the acceleration portion of the equation, then we can see that the force also increases with the strike.
So chambering both the hand and foot for striking is essential to increasing the speed of the strike and thus the energy contained in the strike.
Mass behind the Strike
The last key concept is the incorporation of body weight into the strike. Body weight can only be incorporated into the strike by the body moving forwards towards the target. The technique for this is to time the strike so that the foot lands just before the strike hits the target. This allows the momentum of the bodyweight to be carried by the hand into the target. This technique can also be used for blocks as well, but not however for kicks.
Another important way we can increase our effective mass is by the use of momentum and impulse and use this to our advantage. A technique that immediately springs to mind is the pulling of the opponent towards the defender while simultaneously performing a striking technique. In physics this is basically a collision between two moving bodies, in this case we are trying to make this an inelastic collision, what does this mean?
An inelastic collision is one in which the kinetic energy or energy of movement is not conserved, and is converted into another form. In this case the kinetic energy is converted to an impact force in the form of a pressure wave into the body, along with heat and sound. This type of technique is much more effective than that of a simple strike into the body, why?
If a 75kg Karateka strikes the opponents body at 4m/s then the impact force is 75 x 0.05 x 4 = 15N, if we pull the attacker towards us at 1.5m/s we are effectively increasing the speed of the strike to 5.5m/s. Now using this greater speed we have a force of 75 x 0.05 x 5.5 = 21N, and increase of 38% over the punch alone. Converting this to pressure we can see that the first technique produces 60kN/m2 (9 psi) while the second technique produces a pressure of 84kN/m2 (12 psi).
By pulling the opponent towards us we have effectively increased our body weight by using the opponents’ own momentum, this is the key to the effectiveness of Tai Chi Chuan, it requires no great strength, but correct technique and execution based on the physics and the biomechanics of the human body.
The key to the aforementioned techniques is the understanding of the kinetic chains, so what is a kinetic chain?
A kinetic chain consists of a combination of joints in the body that have an effect each other, in the case of the arm this chain would be the shoulder, elbow and finally wrist. For the legs it would be the hip, knee then ankle. The greater the understanding of this concept, the greater the power that can be produced. It is this concept that we will use in conjunction with the previous concepts to execute blocks, punches and kicks.
Punching Kinetic Chain
The kinetic chain in the punching sequence without hip rotation means that the fist is pushed forwards away from the body using both the ability of the muscles in the shoulders and arm. However, the nature of this kind of kinetic chain is limited by the muscles ability to contract, and so is limited in its ability to produce a fast movement. The sequence for this movement is shoulders, elbow and finally hand. We have already seen that using Newtons second law that the force produced is directly proportional to the acceleration and that this power comes from the ground and is transferred by the hips.
Basically the kinetic chain sequence is: hips, shoulders, elbow and finally hand. Importantly we want the speed of each of the segments to increase speed through the kinetic chain. So the hips are slower than the shoulder, which is slower than the elbow, which is slower than the hand. This leads to a final velocity of the hand that is much higher than the initial hip velocity. As stated previously, a push like kinetic chain does not allow for inclusion of the hips, and is thus limited.
The change to a throw like kinetic chain starts with the hips. The hips are rotated backwards with the chambered hand; since the hips rotate around the spine this means that they have an angular velocity. In turn this gives the hips torque, to successfully explain this throw like kinetic chain we use this information and the equations below:
If we substitute for F we get:
So the torque of the hips is a function of the speed of the movement. The faster that we can rotate our hips the greater the amount of torque produced, how does this help with the strike?
Using the above we see that the angular acceleration produced by the hips is directly proportional to the torque produced.
The torque of the hips produces angular momentum, H, which is analogous to linear momentum. Linear momentum is the mass multiplied by the velocity, so in this case the angular momentum is:
Once this momentum has been generated we then stop the hips from moving, since momentum is conserved this momentum is then transferred to the shoulders. Again we stop the movement of the shoulders transferring the momentum to the arm. Once we have extended the arm, the momentum then transfers to the hand. Since angular momentum is conserved, and the mass of the hand is much less than that of the hips the velocity of the hand must increase, according to the equation below:
Since I has decreased from the start of the kinetic chain, then ω must increase.
The equation above is important in understanding how the hips impact the speed of the hand in the last part of the kinetic chain. The moment of inertia is directly proportional to the mass and the distance of this mass from the centre of rotation. Using this we can understand that the further away the segment of the body is the greater its moment of inertia, this means that the hand requires a large force for it to be deflected from the target, this is another useful consequence of rotating the hips when striking.
Kicking Kinetic Chain
Like a punch there is a kinetic chain for the execution of a kick, again like a punch the generation of power begins from the ground, then the hip, the knee and finally the foot. It is critical to push into the ground prior with the non-kicking foot as this generates the power of the kick. Like a punch the hips are the connection point between this power transfer to the leg and ultimately the foot. The same rationale that applies for a punch also applies to a kick and a block. It is the understanding of this concept that is one of the keys to effective techniques. When this concept is first introduced students find it difficult to join all the links in this kind of kinetic chain, like all aspects of any sport or art continuous practice is the key. As the student practices the greater the force that can be generated using this technique, this is due to the physical movements that make up the kinetic chain become more fluid and integrated.
The above discussion in regards to the conservation of momentum is also applicable to kicks, with the foot being much lighter than the thigh it will have a higher speed, and as above will require more energy to be deflected once it is moving.
Other Key Concepts
Muscle Tension and Speed
Another key to striking is the tension of the muscles; muscles that are tense reduce the speed and flow of movement. To move we must contract muscle fibres, so to move muscles under tension they must be relaxed first, then contracted again. This adds extra time and slows the movement, be it a punch, kick or a block, or any other movement. Muscle tension should not be confused with tonus or muscle tone; tonus refers to the natural passive state of partial tension of a muscle. It is this that allows the body to keep its posture and stops the muscles from being overstretched. When a muscle is tense and not passive it means that there are a larger number of muscle fibres being contracted, so we need to be relaxed when executing any technique.
As earlier discussed briefly the elbow position is also a critical component to the techniques being discussed. Biomechanically if we move our elbow out to the side of the body it means that we reduce our effective strength. The elbow must be kept close to the side of the body and when striking the elbow must travel forwards in a straight line, this is because this movement represents the shortest distance and hence the fastest execution. It also allows for the most efficient transfer of power to the target, which means that strength is not required, both of which are essential.
We also need to consider the retracting arm, this is also important. Retracting the arm as fast as possible allows the speed of the hips to increase further, again adding to the overall energy of the strike via a higher acceleration of the hips.
This can be contrasted with a strike from boxing such as a hook where the strike forms the path of an arc; this puts the arm in a much weaker position so this type of strike relies heavily on strength for execution. This type of strike is a consequence of wearing gloves, instead of the bare hand.
There is another important aspect of the punch that needs to be considered, which is the maximum speed of the strike and the distance from the target. The start and end of the punch both have a speed of zero, but in between these two points there is a point at which the speed of the moving body is at a maximum, where is this point and what does this mean to a punch?
The graph below shows the speed of a punch from the hip, it can be seen that the maximum speed is somewhere around 75% of the arm extension. This means that to exert the maximum speed and thus force into the target the strike must be at a distance of 75% of the arm. This is why we train to drive through the target not into the target. As from previous equations we can see that the higher the velocity, the greater force is carried by the fist.
Speed, Impact Time and Penetration
Why is speed important when striking? Apart from the obvious benefit of being able to strike before the opponent is able to react there is another important benefit to speed. Using Newtons second law, F = ma, we can better understand this concept.
According to the Newtons second law the force that is generated in a strike is directly proportional to the mass of the object and inversely proportional to the speed of the object. So to increase the force of a strike we can:
(i). Increase the mass of the object,
(ii). Increase the speed of the strike.
The mass of a 70kg adult arm and fist is approximately 3.5kg (~5% of bodyweight), how do we increase the mass of the object? We can’t obviously increase the weight of the hand, and to increase the mass of the arm we need to increase the overall body mass. There is a physical limit to body mass whereby over a certain point the speed of the body will begin to slow down. However, we can put more mass behind our strike if use biomechanics. In light of this, the easiest way to increase the force is to increase the speed of the strike and impart more mass behind the strike.
We need to make some assumptions so that we have a meaningful comparison, for this example we will assume that the striking hand moves over a distance of 1.2m from the hip to the target.
If we take the speed of the strike to be distance / time = (1.2 – 0) / 0.2 = 6 meters per second, and the mass of the object to be 5% of the bodyweight of a 70kg person, the impact time of the punch is 10ms, using Newtons second law we get:
To demonstrate the importance of speed we will now decrease the speed to 4 meters per second, and put this into the equation we now get:
We can see that by reducing the speed by a third we have reduced the force of the strike by a third. However, there is more to this than meets the eye. The above equation shows the importance of speed, but there is another aspect of this equation that is not so obvious, this is the time of the impact and the force produced over that time. Rearranging this same equation in terms of momentum, we can now see how this relates to the impact time and the peak forces produced.
Rearranging we get:
In terms of the change in momentum or impulse:
What is this equation really telling us, and how does this relate to strikes in martial arts?
We can use this equation to plot the change in momentum or impulse, which is equal to the force over a given time. Again how do we use this information to help us improve our striking in martial arts?
The graph below shows that if we make the impact time very short then the peak force of the impact is high, and it is this force that will imparted to the target. This graph is telling us that it is more efficient to strike a hard object quickly as this minimises the contact time with the hard target, thus reducing the possibility of injury. Conversely, it also shows that striking a soft target should have a longer contact time to ensure that the energy is transferred to the target.
It should be noted that the area under each curve represents the change in momentum, and is equal, the difference between the curves is the impact time of the strike, and the red curve represents a short time and the blue curve a longer time.
Speed is also a factor in kicks and it is this speed that lets us execute a front kick or a push kick, a front kick being much faster carries more force than a push kick, corresponding to the red line above. The time over which the energy is released is very short. However, a push kick is just that, a push and this would correspond to the blue line in the chart above. The contact time is much longer and this is what produces the movement of the opponent, the energy is spread over a longer time frame.
The story doesn’t stop here, there is even more going on, now we will talk about the penetration power of the strike. The equation that relates force to pressure is:
We need to make an assumption as to the area of the impact; this now brings us to the importance of using the correct part of the hand and aligning the knuckles appropriately. The knuckles that we use to punch with are the index and middle finger; below we can see the area that should be used when striking.
The area represents around 0.4 square inch, or approximately 2.5cm2. It sounds very small in comparison to the size of the targets that such a strike would be aiming for and it is! We will use this as our area in the above equation to calculate the pressure that is produced for both strikes that are in the graph.
Using our contact time of 0.02 seconds and the resultant force of 1,050N, we then have:
Using our other example with a contact time of 0.01 seconds and a resultant force of 2,100N, we then have:
It can be easily seen that if we increase the area that we use when striking with the fist then the pressure produced is reduced, when striking a hard target, it is the pressure produced that causes damage and thus is critical. Again this demonstrates the importance of using the correct part of the fist when striking.
It is also essential that muscles be trained for strength; we don’t particularly want muscle mass for martial arts. A heavier frame means that the practitioner will begin to slow down, however there is a weight which each person will have that represents the balance between speed and strength, thanks to nature the human body can be developed for strength without size, lucky! But why are strong muscles so important? Well, many scientific studies have shown that strong muscles have the ability to:
(i). Contract a higher number of muscle fibres than untrained muscles, this increases the energy contained in the strike.
(ii). Contract at a much higher speed, producing a faster strike.
(iii). Have greater muscle coordination when contracted, producing a more accurate strike.
All of these benefits mean that the muscle works more efficiently than an untrained muscle, with all of the benefits listed above. Having a good strength base will allow other skills to be complimented and built upon this foundation.
Flexibility of the muscles is sometimes overlooked, however the benefit of flexibility not only lends itself to high kicks for example. Greater flexibility reduces muscle injury and allows for ease of physical movement, which, in turn creates greater movement efficiency.
Other benefits of stretching include:
- Improved body alignment and posture
- Efficient muscular movements
- Greater ability to exert maximum force
- Reduced muscle soreness, aches, and pains
Centre of Gravity and Stances
The centre of gravity is a critical component of striking. Looking at the diagrams below it can be seen that the centre of gravity for a strike when standing is forward of the body. When the centre of gravity is forward of the body there is a tendency for the body fall forwards, and hence be unbalanced more easily. This is a distinct disadvantage when performing a technique unless this is specifically part of the technique itself. The second diagram shows a strike with the knees bent, this has the effect of lowering the centre of gravity. It also moves the centre of gravity towards the natural centre of gravity, what is called the “Dan Tien” in internal martial arts.
This serves two purposes, the first is the increase in mass of the strike by moving the mass of the body forward and the second is an increase in stability when striking.
With the blocks the centre of gravity will be higher since the legs are not as bent as in the above example. However depending upon the technique we may end up in Shiko Dachi, which in itself is a stable stance and will lower the centre of gravity adding to the stability of the practitioner. This is why traditionally we do not kick above the waist in Karate, the resulting position of the body is relatively unstable and also does not allow for a quick transition between techniques. This is why we should always have our knees slightly bent when striking to achieve a lower more stable centre of gravity.
Martial Arts Compared to other Striking Arts
The striking and blocking techniques in martial arts has been developed over long period of time based on using the bare hands, feet and body as weapons. This is in stark contrast to something like boxing and MMA, both of which use gloves of various types depending upon the sporting body that each belongs to.
Boxing is unique in that the sport has transformed over time from a bare-knuckle sport into one that uses gloves of various weights, this was to stop the fatalities that were occurring in the sport. This means that the strikes resemble more of a push than a snapping motion, relatively speaking. The contact area of the glove is large in comparison to that of the Karate or other martial art practitioner. This means that the pressure exerted by the glove is low in comparison to martial arts, however the contact time is much longer. To produce the greatest benefit from wearing gloves the athlete has to develop technique, power and mass behind the strike. This is usually performed by increasing the distance the hand travels, so the most powerful strikes take the form of hooks and uppercuts, those strikes that travel a longer distance and have a follow through. In martial arts we tend not to follow through when striking, as this will reduce the impact of the strike. In boxing the heel is not kept down and this represents the body moving forwards into the target, to increase the mass in the strike. However unlike martial arts this means that the boxer is committed and if they miss the target they will move forward and put themselves at risk of a counter strike. Finally most boxers are very powerful for relative to their size; again this is because of the nature of the glove itself reducing the impact on the target. This is in contrast to Karate which contains strikes that travel short distances in straight lines creating high speed, high impact forces.
The techniques described above would at first sight appear simple and relatively straightforward when executed; however this is not the case. The concepts described above allow the Kareteka to produce strikes and blocks that are extremely energetic in nature; these concepts are all based on physics and the biomechanics of the body. The hard work has been done for us with the techniques being innately handed down from master to student, well before the modern emergence of physics and biomechanics, as we know it
It should be noted that Karate originated from the Chinese martial arts and it can be seen that the concepts contained within the internal martial arts are present in Karate. The word soft should not be confused with weak or ineffective technique; it simply refers to the style of martial art. There is a saying, “Hard on the outside, soft on the inside, soft on the outside, hard on the inside”. This refers to the fact that the external arts such as Karate favour the development of technique based on muscular strength and power, whereas internal martial arts are developed using the intrinsic nature of the body and relaxation. They are indeed Yin and Yang, yet this within itself represents a complimentary relationship between both internal and external martial arts, they are ultimately one and the same.
The practice of martial arts is a life long journey that encompasses both physical and mental skill; it also encompasses those around you and ultimately oneself. This essay is only the beginning of the journey, and we need to understand and contemplate the techniques involved. Under the guidance of a good teacher the student is able to realise his/her potential with hard work and dedication.
Appendix 1: Newton’s Laws
(i). Every object in a state of uniform motion tends to remain in that state of motion unless an external force is applied to it.
(ii). Force = mass x acceleration.
(iii). For every action there is an equal and opposite reaction.
Appendix 2: Bibliography
Arus, E, (2012), Biomechanics of Human Motion Applications in the Martial Arts, CRC Press.
Blazevich, A, (2007), Sports Biomechanics, The Basics: Optimising Human Performance. A & C Black Publishers Ltd, United Kingdom.
Funakoshi, G, (2012), Karate-Do Kyohan The master text, Kodansha USA, United States
Giancoli, D C, (1985), Physics: Principles with applications 2nd edition, Prentice Hall, United States
Harrop, G, (2011), Parting the clouds: the science of martial arts. A fighter’s guide to the physics of punching and kicking for Karate, Taekwondo, Kung Fu and the Mixed Martial Arts. Dark Matter Books, United States
Kim, S H, (2009), Complete Kicking, The Ultimate Guide to Kicks for Martial Arts Self-defense & Combat Sports, Turtle Press, United States
Nakamura T, (2001), Karate Technique and Spirit, Tuttle Publishing, Singapore
Starr P, (2008), Martial Mechanics, Blue Snake Books, United States