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Introduction

    The human body can execute several actions like rotate, bend, stretch, jump, turn, etc., varying them and using a different dynamics. Human beings can create a limitless number of body movements. Among this wide range of moves, each culture stresses certain characters according to their style if we take into account that anthropologists have classified over 1.000.000 postures, so it is impossible to imagine how many different moves we can make. According to Yamada (1997) “Aikido is good for health, among other things because its moves are natural, and favour calm and relaxation”.

    Aikido comprises two movement categories: stabilizing (Katame-Waza) throwing the opponent (Nage-Waza). There are over 700 moves that belong to these Waza techniques and almost all derived from the basic Kata: escapes (Te-Hodoki), throwing an opponent by pressuring the limbs (Rofiwe) and inmobilization by pressuring the the joints Kansetsu-Gaeshi). These three series are the basics of all the self-defence movements.

    An axiom of Aikido states that softness can control violence and strength by studying the technique. However, Aikido is more than simple physical ability. To coordinate with the movement and strength of the opponent it is necessary to get a flexible mind and body. In that regard Yamada affirmed “To lead another person we should first be able to lead ourselves. This kind of mind-body control can only happen when both individuals are relaxed and calmed” (Yamada, 1997). In other words, the mind needs to be on alert and become flexible in order to take advantage from the opponent’s move giving a step ahead. That is to say, the aikidoka must understand his opponent and his feelings; therefore the final goal is not to inflict an injury but to preserve harmony.

General considerations

    The technique is the means to reach an utmost effect with the lowest effort (Izquierdo, 2008; McGarry, O'Donoghue, Jaime, Sampaio & Sampaio, 2013). Essentially, this can be achieved by using the opponent’s strength in his disadvantage. Thus, when the Aikidoka is pushed, he moves the attack’s direction adding his own strength to the opponent’s. Likewise if he is pulled, he adds his own weight to the direction of the pull, then he applies the appropriate technique and takes control over the opponent. The Aikidoka wins, not as a result of a clash of strengths, but using his opponent’s. As Yamada said (1997) “…the movement of the body coordinates in such a way that his energy blends with the attacker’s, so that Aikidoka becomes the force that controls both energies. Thus the strength can be harmless and harmoniously redirected”.

    That is to say, the secret of being able to take advantage from the attacker’s physical strength lies on the principle of motion. According to the classifications made by Yamada (1997) “In Aikido there are to basic types of movements: One that follows a straight line called Irimi (entering) and another called Tenkan (turning using a pivoting motion). To execute the first we should take into account that the displacement is made sideways in order to get off the line of action of the attacker, in the case of the second, the movement of feet, trunk and arms describe a three-dimensional arch resembling a sphere or sometimes a spiral. The curve motion allows the aikidoka to add his weight and strength to the pushing or pulling movement of the opponent without fear to collide. In that regard Yamada says (1997) “…Most of the attacks come in a straight line, while most of the Aikido moves are circular. The direct attack is reduced in the centrifugal force of the circular movement and casted outward”.

    The curvilinear movement is not limited to an area; it may vary from the front to the back, from the right to the left, from top to bottom; a skilled Aikido practitioner must be able to apply it in any direction all along the surface of the sphere, if he wants to a null any kind of attack.

    The curvilinear movement originates from applying a centripetal force (aimed at the center of the curve), which alters the initial path of the body. This force collects things inside a swirl turning rectilinear movement into curvilinear; when action stops things are ejected out of the swirl recovering their rectilinear way.

    The analysis of a perfectly executed technique shows that Aikido makes use of such an important fact. “…thanks to the stress of the spherical movement, some techniques that were formerly rough have been refined and others whose execution required a wide space could be reduced to a small sphere…” (Ueshiba, 1983). In most of the cases where the opponent’s balance has been completely broken, it has been caused by a twist or turning move, not a direct attack. To consider this movement in details, let’s suppose the opponent strikes directly from the front, if one moves out of his way and without opposing the strike, just applying a light force from one side, the attack’s direction can be easily changed, the attacker’s balance is destroyed (since it is accelerated by changing the direction of his movement’s speed) and loses his strength.

    In Aikido, the speed is a vital element. Essentially there are two uses of speed: the first is to follow the opponent’s motion (Omote variant of technique execution), the second is to be able to avoid an initial attack with a side motion (Tenkan), turning the opponent’s rectilinear movement into curvilinear.

    The speed cannot only be seen in relation to the skills of the opponent, and truly it is not only a matter of developing enough to defeat an average man –in Aikido one shall make constant efforts to develop an exceptional speed synchronizing one’s movement with the opponent’s –that’s the essence of Aikido-. This is a combination of speed and concentration of energy.

    Another vital aspect to guarantee the effectiveness of the Aikido techniques is the concentration of all the strength in a given moment and in a given point. Although the total concentration of the strength is not possible, with a regular training the practitioner can reach a considerable level.

    The effectiveness level of the technique depends of the extension in which the weight is properly used. There is a remarkable emphasis in the transfer of the gravity center to the basic movements, and the student should preserve it.

    Aikido is based in a movement that when it is watched, the spectator notices security, attitude, self-control, flexibility, coordination in the body language; and without saying a word it transmits gestures and expressions that show the practitioner not only knows what he is doing, but also teaches something with his body. For that reason Aikido is a synonym of mental, physical and emotional health, granting our body and our mind, the correct attitude of an emotionally calmed, but psychologically willing state.

    In the practice of Aikido, the individual not only executes movements, but actions in order to control and throwing an opponent. During these actions, the moves are not executed all the time or in all the joints; some parts of the body can move and if necessary the reciprocal disposition of some limbs can be preserved.

    For those reasons this paper transcends the limits of the physical elements of the motion actions present in the practice of Aikido. It also refers to the preservation and alteration of the postures of the body while practicing.

    Our research study consists of the mechanical causes for keeping and losing the body’s balance, the origin of moves and the peculiarities of their execution in every technique.

    The motion of the human body is a displacement in time and space where many joints are simultaneously and consecutively executed. Precisely Aikido is characterized by causing the loss of balance and throwing an opponent through the action on the joints.

    There is also a possibility of offering an energy description of the movements under analysis taking into account the work of the forces applied and the internal strength opposite the motion.

    In the mechanical analyses we said above that such work depended on strength and displacement, but in the case of the actions of the muscles it is necessary to have in mind that the muscle can even work whether its length varies or not (without relative displacement), that is called static work.

    In all the cases where the muscle works both static and dynamic (with relative displacement) the body will experience a loss of energy, since every system can only work according to its energy.

    In the case of the body, this energy emerges as a result of the synthesis of food through breathing the chemical energy way, which will successively transform in mechanic energy: kinetic (associated to the body motion) and potential (determined by the relative position of the bodies in a given system). Unlike other martial art practitioners, the aikidoka executes his techniques with a minimum amount of energy given the fact of executing them with a small amount of strength and a very effective use of breath.

    The mechanic approach of the study of the posture and the movements during the practice of this art will allow, out of all questions, to acquire an average knowledge about the physical essence of the mechanic moves of the body, leaving other considerations (as important) aside for those who search for a deeper biomechanical research.

    In order to provide an easier comprehension of the physical description we intend to explain in the present article about the techniques of Aikido, we think it is worth expressing first some physical definitions and later some main aspects of the mechanic features and moves of the human body.

The mechanical properties of the human motion

    When studying the mechanical properties of the human motion “…it is used to analyze the parts of the body with moving links as elements that form pairs and biokinematic chains. These links and joints are undergone to the action of the forces applied on them. These conditions set the peculiarities of the structure and the functions (mechanical properties) of the human body that make influence on the execution movements”. (Donskoi, 1971)

    The forces applied on a body can cause traction, pressure, inflection and torsion. The forces that cause inflection are frequently found when the bones that form the joints work as a lever. The torsion forces are mostly found in the rotation moves of the limb around an axis.

    We will make emphasis in these two forces given their frequency in the execution of the Aikido techniques.

    By the character of its action, forces can be static and dynamic. The former, have frequently a constant magnitude and it is regularly small. The second emerges during the moves; such force can surpass the security margin of a limb, which causes an injury in the locomotive apparatus.

    When the muscles, joint capsules, ligaments and cartilages that link the bones of the human skeleton deform, the action of the dynamic force is reduced. Muscles play an important part in this given their elasticity. Aikido bases most of its techniques in the incorporation to the attacker’s movement and its continued reorientation, which makes of the techniques really painful and dangerous actions for the physical integrity of the recipient.

Linkage of the elements

    The linkage of the elements of the biokinematic chains makes easier the multiple possibilities of movement. Their direction and range depend on the connection type and the participation of the muscles in motion.

    In Aikido, the Katame Waza or control techniques are executed by shutting down the biokinematic chains; it allows immobilizing the opponent’s body by grabbing a joint. Likewise, when executing a wrist-grabbing technique, the technical move of this joint will imply a technical response of the elbow and shoulder joints. When closing up a chain to execute a technique, the possibilities of motion are reduced and the direction of the moves is more precise in the open chains.

    The skeleton, made up by bones linked in a movable way, is the rigid base of the biokinematic chains.

Elements of the biokinematic chains

    The joints and bones that form these chains act as levers, with the difficulty of carrying weight, and while accelerating they make an inertial resistance that depends on its moments of inertial.

    The fixation (blocking) of the joints and their release (suspension of the links) favors the presence of a variable number of moving elements, which is caused by the redistribution of their speed.

    The length of the chain (during inflection and extension) varies the conditions of action of the forces in the levers (force arms) and the inertial resistance of the limbs (inertial moments).

    An important part of the linkage of the biokinematic chains are the pluriaxial mechanisms incoherent with plenty of motion possibilities, out of which depending on the forces applied there is only one trajectory, which guarantees the direction of the movement.

    The active limitations (muscles) determine the limit of movements.

    The practitioner executes actions in a field of variable forces (group of external and internal forces applied over him, which shift their value with the passing of time). For that reason, the effect of muscle traction in the joints does not remain constant. In addition there is also variation in the internal passive resistance: elastic, viscous, friction and support reactions, etc…

    Plenty of Aikido techniques like Irimi nage, Kote gaeshi and Kaiten nage require bone levers of the trunk, the arms, wrists, etc., in order to make forces that conveniently applied provides a big rotational effect with a subsequent throwing as a way to protect the compromised joint.

Dynamic peculiarities of the human motion

    According to Donskoi (1971) “the inertial characteristics of the body (mass and moment of inertia) in the subjects of diverse height, weight and body structure, are different”.

    The centre of gravity of the human body in the anatomic position is located in the hip area. Over it will be applied the result of the gravity force executed on the center of gravity of the limbs; in such position, these forces pass through the spinning axis of the joints and will not have rotational effects on them (Fig. 1) (For that reason this position is considered of relative rest).

    That resultant will act: on the support (at rest) as a weight and will be annulled by its very reaction force. The weight of the bodies cannot be changed, but it is possible to vary the moments of the gravity forces during the execution of the techniques.

    According to the aforementioned, we can point out the benefits of the practice of Aikido; by adopting the Sankakutai posture (Fig. 2) the practitioner will exercise his body by executing an isometric work (without relative displacement) in order to annul the rotational effect of the gravity force on the limbs during the position.

    When making external action on a body (or limb) it will make a similar action over the attacker, and it will be always aim at the opposite way of the acceleration experienced by the body (or limb) and applied over the point of contact.

    During his action, the aikido practitioner overcomes the forces of resistance through the muscular strength and makes an effort to: overcome all the resistance and communicate acceleration to their organs in motion and to his opponent’s, to whom the motion volume, the moment and the kinetic energy is transmitted. By making those moves the opponent’s motion way, the value of the force applied is considerably reduced and therefore forces of reaction.

    A fluent and continuous movement guarantees an execution with an approximately constant speed, which implies very small values of acceleration. The forces of muscular traction are the only internal sources of the man’s energy. Only with these forces the man can take advantage from the rest of the forces and direct the moves.

    The variation of the force arm of the muscle traction in regard to the rotation axis produces variations in the rotational effect of this force; this arm varies along with the variation of the joint angle. For example, with an elbow making a 90 degree angle, its flexors lose 10 times the strength.

    As long as the aikido practitioner improves his moves, he makes a better use of his muscle strength reflected in the execution of the technique and saves more energy. For that reason, it is affirmed that the systematic practice of Aikido contributes to the reinforcement of a correct posture, plus favouring a satisfactory development of the motor skills.

    The simplest movement of a limb in a joint is that around an axis and in a determined direction. The value of this movement will depend on the moment of the force applied on itself. The motor activity will always include groups of joint movements.

    The space elements of the motion system are a group of simultaneous joint moves that when being replaced by others, develop as consecutives series.

    The time elements of the motion system are the phases of the moves. The phase is a part of the motion system isolated in time. During its execution the character of the motion system has no essential variations. The moment when an essential variation begins, a phase ends and the next starts.

    Internal interactions favour the integrity of the system. The movements of the system are coordinated in space and time. Even when in the Omote variant of Aikido the different phases of movement become evident, the transition from one to the other should take place continuously, avoiding the typical accelerations of the sudden changes of speed, which proves the saving of forces achieved in the application of the techniques.

Structures of the motion system

    These structures can be: kinematic (regularities of the interaction among moves in space and time), and dynamic (regularities of the interaction of forces of the parts of the body and with the opponent).

    When there is an improvement of the kinematic structure coordination is noticed, a precise space and time harmony of the motion system.

    The improvement of the dynamic structure is reflected in actions of enough strength executed in the precise instant.

    These are attributes that characterize the execution of the techniques when the aikido practitioner masters the art: a perfect coordination among the moves of all the parts of the body, with a total mastery of his center during the entire execution and an utmost use of his relative postures and his opponent’s in order to take advantage of the balance achieved so as to reach his goal with a minimum use of strength.

    To activate the motion system its direction is indispensable, since it comprises, among other aspects, the actions and ways that lead to achieve the target.

    The actions of direction make an easier achievement of the goal through:

1. The tensions of the muscle work

2. The use, with its help, of the external and passive internal forces.

3. The direction moves of the farthest limbs.

    The direction program is the composition and succession of concrete moves that are indispensable for the solution of a given task.

    The main programs are made up by partial programs:

1. Preparation program: Both before the beginning and before each phase.

2. Election program: the election of a more useful variant during an ongoing action depending on the reaction of the opponent.

3. Follow-up program: Variations from the optimal program under given conditions.

4. Objective program: Mobilization of the subsystems that guarantee optimal execution in cases of interferences that demand restructuring the move.

5. Amplification program: Possibilities of reserve when the most suitable variant is not applied.

    Aikido provides the practitioner with skills that ease the execution of all these programs by developing a training system that favours an open, flexible mind able to suggest variants in different situations. The aikidoka begins his action from the opponent’s with the subsequent level of improvisation it demands, he manages to be one step ahead by studying the opponent’s posture, but he is also able to execute rapid and secure variants before every reaction of the attacker.

    Another distinctive trait of an Aikido practitioner is the coordination of his moves, which is very important to print beauty to the execution of a technique.

Coordination of movements

    Coordination is a process of sequential concordance of moves (Flanagan, 2014) that leads to fulfil a goal: there is nervous, muscle and motor coordination.

1. Nervous: Combination of nervous processes that lead to the solution of the motor activity. It is the decisive aspect of coordination. Hence it is stated that the aikido practitioner should have the mind open for the execution of the techniques, ignoring all kinds of conflict that might affect him psychologically.

2. Muscle: Combination of muscular tensions that make action on the parts of the body both according to the signals of the nervous system and the action of applied forces. Hence it is deduced that an open mind will correspond to a distended muscle system, without rigidness to achieve a fluent technique with less effort.

3. Motor: Combination of movements of the body parts in space and time where the motor activity executes the actions in concrete conditions.

    Coordination is the group of movements made by the agonist muscles (causes specific movement), antagonists (acts opposite the specific movement) and synergists (motion neutralizers). It is defined as the distribution of the individual muscle activation combined with joint moments (Romero & Tous, 2010).

    Therefore, biomechanical coordination is the quality in charge of regulating the action of all the muscle groups that participate in a movement, or the capacity of ordering different muscle groups studied from the physics applied to human motion.

    Without coordination movements deteriorate and become ineffective. It is clear that no matter the strength or speed we have; if it is not properly applied the motor performance will be lower.

    A distinctive feature of the aikido practitioner is the perfect movement coordination between the upper and the lower limbs, which gives an extraordinary beauty to this martial art.

    In one of his papers Kishomaro Ueshiba (1983) affirms …”in the practice of Aikido, the whole body, not only arms and legs, should move continuously in a coordinated way, and this should be done with speed, vigour and power and in order to act soft but rapidly it demands an extraordinary level of mental concentration and agility, balance and reflexes”.

    Coordination includes balance (Barth & Dreilich, 2010), which is a complementary quality; balance and coordination are regularly proportional. The Aikido practitioner should execute all his moves with total balance, using the lack of balance of the opponent for his benefit, which will guarantee efficiency in the execution of the technique with minimum strength.

    Elasticity is the ability of a muscle to regain its original shape and size after stretching (Clark, Lucett & Kirkendall, 2010). It gives a wide range of moves to the body. One of its consequences is relaxation, a variant of the abovementioned qualities but goes together with muscle coordination and is considered a psychosomatic element, since both mind and body act simultaneously. When relaxation is only muscular or neuromuscular, fluency (physical form of relaxation) comes to action. The aikido practitioner looks fluent; with an open and low posture since relaxation demands total body-mind coordination.

    A quality that not only implies flexibility but speed as well is agility, an indispensable element from most of the martial arts and sports, especially for those that require sense and direction changes as fast as possible (Brown, 2007; Gamble, 2013).

    Although the semantic definition is almost identical, speed and fastness are very different. Fastness is the speed of the move that is defined as the high and low contractile ability of the muscles or the high and low ability to make a move.

    Fastness includes no locomotion; on the contrary the term speed is a locomotive concept, since it is the ability to experience a given displacement in a determine period of time, seen from the physics point of view.

    The correct coordination of movements is a typical feature of the Aikido practitioner, since he executes complex moves that demand all of these qualities described above: the necessary flexibility to adequate his actions to the Uke postures, elasticity to achieve a convenient amplitude of his displacements, the necessary agility to shift from a technique to the other before sudden Uke responses, fastness to anticipate to one or several opponents’ moves and the typical speed, indispensable for Ura techniques.

    The effectiveness of a move is determined as long as the motor action has been solved and depending of the result of its fulfilment, taking into accounts its saving, stereotype and adaptation (Izquierdo, 2008).

    The moves of the practitioner are aimed at the variation of the opponent’s conditions, which comes from the conditions of the body to achieve the necessary interaction; therefore, its effectiveness lies in the price paid for varying the initial state of the opponent, having in mind that the evaluation of a specific action comprises a physical area (relative position achieved with action; a psychic area (effect of the action performed in the opponent’s psyche) and a tactical one (what has been achieved following the action against the direct opposition of the attacker)

    Another relevant aspect for the execution of Aikido techniques with elegance and poise is the preservation of balance.

Balance of the body

    When the human body thoroughly preserves the posture, the valid laws for the balance of the rigid body can be applied on it.

    The balance is kept given the situation of the different parts of the body thanks to the muscle, dynamic and static efforts, which makes a favourable influence in the development of the vestibular apparatus.

    To preserve the position of the body, the individual must be in balance. The position of the body is determined by its posture (disposition of the limbs that do not depend on the orientation of the body in space or with respect to support), by its orientation and situation in space, as well as for its correspondence with support. For that reason, to preserve the position of the body, the man should fix the posture and avoid that applied forces vary the posture. In addition he cannot allow a displacement of the body from a given place to any direction or a turn from his support, since in a motion state an acceleration of this movement makes the body lose balance.

    That explanation shows the advantages of the defence postures (Sankakutai) adopted by the Aikido practitioner; open postures with a great area of support, and a very low gravity center that guarantees stability.

    This posture is also useful from the therapeutical point of view given the isometric work executed by the muscle systems aimed at keeping the posture, as it has previously noted.

    With the motor mastery, the aikidoka makes an utmost use of the muscle strength being able to keep his position for long periods of time.

    The angle of stability serves as a dynamic index of the body’s stability. It is made up by the line of action of the gravity force and the straight line that joins the gravity center with the corresponding limit of the supporting area. When stability is determined in the human body, it is necessary to take into account the limits of the supporting area, the security of the preservation of the posture and even the limit position of the body and the real line of the turn.

    In the struggle to preserve the position it is used the weakening of the action of the interference forces (which thwarts the endurance of the posture) and the strengthening of the action of the balance forces (favour the recovery of balance).

    During the recovery movements, the aikido practitioner makes use of the external forces without which the gravity center of the body cannot return to the preservation area of the position. Such forces may be caused by accelerated moves of the limbs. For that reason a right-handed practitioner accompanies his displacements stressed by his upper limbs.

    In the prologue of the book La Dinámica Marcial del Arte de la Paz (The Martial Dynamics of the Art of Peace), Miguel Morales Bermúdez expresses that “The living truth of the martial art collected in the relative principles to posture, position, displacement, breath, extension, connection and move completion never ends” (Yamada 1997)

    Given the characteristics of this martial art it is very important to have a clear idea of motion in place.

Motion in place

    Movements in place characterized by the existence of an invariable support and by the preservation of the balance conditions.

    During any movement the limbs displace one into the other, the gravity centres displace as well and most of the times the gravity and mass center of the body.

    If the external forces are balanced, the main vector of the force (resultant force that acts on the body) equals zero and the speed of the mass center of the system is constant.

    The external forces of reaction (support reaction, normal); friction forces (during the moves in place) balance the actions of the movable parts over the supporting ones. These forces vary themselves according to the acceleration of the movable parts and the bodies displaced.

    The movements in place are executed with invariable support, and with the preservation of balance, the gravity center of the body remains in the recovery zone of the position. This movement is typical of the Ura variant, on which a lower limb is fixed incorporating to the opponent’s move and with a little lateral force compelled him to turn along with us around that limb, which functions as a turing axis. This way who executes the technique remains perfectly balanced by becoming a twisting point experiencing no acceleration, while the one receiving the action experiences a huge acceleration made up by the variation of the values of speed due to the tangential component of the force applied and a centripetal acceleration given the change of direction of the speed.

Motor and resistant moves

    During the movement of the movable parts and the displaced bodies, accelerations emerge and they prove their refusal to vary from their original state.

    For example, the motion of limbs normally alters the reaction of the support, which is translated into the manifestation of a dynamic component of reaction of the support (plus the static component).

    If the acceleration of the limb is not normal friction forces emerge.

    In the motor movements the traction of the muscles is aimed the movement way and in the resistant moves it goes inverse. In the motor moves the muscles reduce by surpassing the resistance force applied to the limb.

    During the execution of a technique we count of the adversary’s resistance; in this case the containment forces are the weight and the muscle strength of the opponent. In the fast moves accelerations are big. In this case the muscles contract (by their elastic force) and can stop the motion and cause an inverse movement.

    “Given the character of the movements in place the phases of impulse and containment are distinguished, sometimes a phase of a relative uniform movement is noticed in between” (Donskoi, 1971). In the rapid move, the phase of constant speed does not exist; the impulse is rapidly replaced by conteinment. During the movement, the impulse can be continued until fulfilling the goal in order to gain a precise completion of the move. When a continuous precision of the movement is necessary, the impulse can be rapidly replaced by a phase of a relatively uniform movement. In Aikido, the Omote moves (following a rectilinear trajectory) present this last characteristic while the Ura moves distinguish by the fact that the impulse continues until fulfilling the goal and the impulse gets rapidly replaced by containment.

    That is the reason why for the achievement of an efficient technique, the Aikido practitioner should take into account the determination of: the initial and final position and the kinematic and dynamic features (both his and the opponent’s), the phases of the movements and the balance conditions.

    During the active movements of the human body there is not an only force that moves all his parts as body systems. For example, when displacing the opponent by applying a pushing technique, the result is the tension of the muscles. The reactions of the accelerated parts emerge simultaneously. As a result the reaction of support increases and it counteracts with the weight of the movable parts.

    To complete an exhaustive analysis of the displacements as a fundamental action in the execution of an Aikido technique, it is necessary to know the peculiarities of the step.

Step phases

    In this movement there are two periods: of support and displacement (forward and backwards with respect to the hip).

    The support located in front of the gravity center of the body will always restrain its forward displacement if it is place behind, then it will guarantee the forward motion of the body.

    The support period divides in the suspension and pushing phases:

1. Suspension: From the instant the leg falls and until the movement of the gravity center of the body is suspended. First occurs the external contact (kinematic), then the dynamic interaction followed by the muscular one. It is considered that this phases ends when the motion of the gravity center ceases, although the displacement of the soft and liquid tissues and internal organs ends after the tissues of the rigid parts of the skeleton. The final phase takes the utmost pressure on the support or maximum flexion of the knee joint.

2. The pushing phase with the leg begins with its extension in the knee joint and ends when the leg of support is lowered.

    The displacement period of the leg divides into four phases: rise, impulse, containment and descends of the leg to the support.

    During the displacement the trunk drags the leg to the front beating the opposition of its parts to move. The leg is dragged into the trunk and raised.

    In the cyclic moves of the step, the change of the way of the oscillatory moves of the leg is produced in the highest instants of the gravity center behind and in front of the hip. In these points emerge the utmost acceleration as a consequence of the application of maximum efforts.

    Here, there is a constant transformation from potential energy to kinetics and from kinetics to potential. During the contention leg’s pendulum, and during the flexion of the support suspension, the kinetic energy becomes potential energy of elastic deformation of the muscles. During the impulse of the leg and when pushing the very support, the potential elastic energy becomes kinetic. To keep a constant regime of oscillations implies a constant dissipation of energy.

    The moves of legs during the step are synchronically related to the moves of the trunk and the arms.

    These considerations on the step justify the indications given to the Aikido practitioners aimed at keeping a relatively low position of the gravity center during displacements in order to avoid elevating the hip, maintaining an open posture with the gravity center near the middle point of distance between both legs in order to ease the displacement, as well as moving in a same level without wavings in the vertical area to perform minimum energy transformations and its consequent dissipation.

    Last, it is worth highlighting that the movements in a same direction increase the speed of the last element of the chain, movements in opposite ways (relapse moves) reduce the speed of the final element and the pauses in the moves of the elements of the biokinematic chains also reduce the speed. Hence the convenience of executing the techniques in a continuous and fluent way.

    In an exhibition made on the occasion of Kishomaro Ueshi’s visit to Uruguay, the director of the military academy said “In the fluid moves of Aikido there is always a steady center. A sense of balance penetrates each move of the hand and the foot and they softly slide as a dance because the movement of a whole body is just the movement of the center”. (Ueshiba, 1983)

Conclusions

    The present paper is not a treaty about Aikido, neither it intends to characterize thoroughly from biomechanical considerations, an art with an immense technical range. It is just an intention to offer an analysis that, at least provides enthusiasts and practitioners with a deeper and more complete knowledge about the extraordinary use this martial art makes of physical laws, which ease the execution of its techniques by making a comprehension from the scientific point of view to favour a better understanding in praxis.

    A group of extremely beautiful and sutil movements, an amazing space-time conception of its practitioners, and saving of efforts and energy in the execution of its techniques, make of Aikido one of the greatest of all martial arts.

Glossary

• Absorb: To take in the movement and make it own.

• Centrifugal: Directed outward from the center.

• Centripetal: Moving or directed toward a center or axis.

• Dynamics: Refers to the effects of forces on the motions of bodies.

• Main longitudinal axis: An imaginary line running down the body between the center of the head and the end of the pelvis.

• Link: Limitation of a limb to move.

• Phases: Diverse components of a technique with different actions.

• Iriminage: Entering throw.

• Kotegaeshi: Forearm return.

• Kokyunage: Breath throws.

• Omote: technique applied with motion to the front of Uke.

• Ripost: Action in response to the attacker’s action.

• Sankakutai: Defence position of the Aikido practitioner.

• Torsion: the act of twisting an element of the chain in a perpendicular plane to its longitudinal axis.

• Traction: the act of drawing or pulling.

• Udekiminage: Arm pin throw.

• Uke: The receiver of the throw.

• Ura: A rear technique.

Bibliography of references and consultation

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