KINETICS AND ELECTROMYOGRAPHY OF THE MARTIAL ARTS HIGH FRONT KICK Henrik S0rensen 1 , Morten Zach0 2 , Erik B. Simonsen1 and Klaus Klausen 2 1lnst. of Medical Anatomy and 2August Krogh Inst., University of Copenhagen, DK INTRODUCTION It is generally accepted that fast unloaded movements (i.e. striking, throwing and kicking), in which the aim is maximal linear speed of the involved segmental chain's most distal end (i.e. hand or foot), are perlormed in a proximo-distal sequential order (e.g. 1985; Herring, 1992; Putnam, 1993). In kicking, for instance, it is observed that the total movement starts with forward angular acceleration of the thigh while the shank lags behind. Then the thigh decelerates while simultaneously the shank accelerates (Putnam, 1983, 1993). The simultaneous thigh deceleration and shank acceleration often lead to a situation in which the thigh is completely decelerated when the shimk reaches its maximal angular velocity. This seems disadvantageous from a kinematic point of view, considering that the resulting linear velocity of the foot relative to the ground equals the vector sum of the resulting linear velocities of the knee relative to the ground and the foot relative to the knee. Two conflicting kinetically based theories concerning this thigh deceleration have been proposed. One states that the thigh is actively decelerated by the hip extensor muscles and that thigh deceleration enhances shank acceleration in a whiplash like manner (Herring, 1992). The other theory states that the thigh is decelerated because of the shank's motion, i.e. when the shank rotates it exerts a decelerating force on the thigh (Putnam, 1983, 1993). In an attempt to clarify the contrasting theories mentioned above we decided to examine the high front kick used in most oriental martial arts. To the best of our knowledge this type of kick has not yet been analysed. Previous work has dealt with soccer or punt style kicking (e.g. Putnam, 1983; Robertson, 1985; Dunn, 1988). In these types of kicks the target is typically situated at ground level or slightly above, whereas the high front kick is aimed at chin level. Similar to previous studies of kicking the present study used kinematic measurements to derive the kinetics through inverse dynamics. Furthermore, in addition to methods used in previous kicking studies, we recorded the electrical activity of selected muscles in order to asses their temporal activation during the kick. In summary, the purpose of this study was to establish the kinetics of a typically proximo-distal sequential movement with special emphasis on the possible use of active deceleration of proximal segments to enhance acceleration of distal seg­ ments. METHODS Subjects. Seventeen skilIed taekwon-do practitioners ranging from club level to top European level gave informed consent to serve as subjects for this study. The kick. The basic high front kick is executed from normal standing position by flexing the hip and extending the knee (see the stick figure sequence in Fig. 2). The ankle joint of the kicking leg is kept fully plantarflexed and the toes fully dorsi­ flexed in order to hit the target with the ball of the foot. During the kick the sup­ 439 porting leg remains on the floor. Contradictory to most other taekwon-do kicking techniques the front kick is confined to the sagittal plane which makes it suited for two-dimensional analysis. After warming up each subject performed three kicks aiming at a tennis ball suspended from the ceiling and adjusted to chin level. The fastest kick from each subject was selected for further analysis. Cinematography. The subjects were filmed from their right side with a 16 mm high speed camera (Teledyne DBM 45) operating at a picture rate of 200 frames per second. Markers were placed on selected anatomical landmarks in order to identify body segments and joints. Position coordinates of the optical centres of the markers were automatically digitised (Peak Performance Technologies). Displacement data were lowpass filtered with a digital fourth order Butterworth lowpass filter with zero degrees phase lag (Winter, 1990). Optimal cut-off frequencies (6-10 Hz) were determined from residual analysis (Winter, 1990) and the Jackson Knee method (Jackson, 1979). From the filtered displacement data angular velocities and accelerations of the thigh and lower leg as weil as linear velocities and accelerations of the hip, knee and an1- ro,. ---&- ~ O-r+ 1 -200 I~ 200,Nm Shank moment -la ___ M. --g -<>- ro,. ---&- ~ ---*- aH o I ß ~-=l'~;;' 1.5~mv glut. max. 0.0 1.5~mv vast. lat. 0.0 15rV .1.,1.11I ",­ 0.0. ~AIJiVllW~\.... 1.5Jm~.. ~ll 0.0 ~~~ 1.5 Jmv_.__ . O.O~~~ I I Ims o 200 400 Figura 2. Moments aeting on the thigh and shank with simultaneous EMG reeordings. Uppermost end of the hip segment determines the temporal alignment 01 the stiek ligure. See text lor explanation to legends. shank. This enables expressions of the equations of motion for the thigh and shank in which the influence from the segments angular acceleration and velocity on intersegmental forces is readily obtainable. See for instance Fig. 1 where the influence from thigh angular acceleration and velocity on shank angular acceleration is illustrated. RESULTS Fig. 2 shows the time dependant moments together with the rectified EMG recordings from one representative subject. Positive moments accelerate the segments in counter-c1ockwise (kicking) direction, while negative moments accelerate the segments in clockwise direction (i.e. decelerate the segments). The curves labelIed 10. (moment of inertia times angular acceleration) show the resulting moments acting on the respective segments. A segment's resulting moment is the sum of all moment components acting on the segment. For the thigh these moment components are the hip muscle moment (MH), a component due to gravity (g) and 5 motion dependant components due to thigh angular velocity and acceleration (ffir and o.T), shank angular velocity and acceleration (Ws and o.s), and hip linear acceleration (aH)' Identical nomenclature is used for the shank, MK is knee muscle moment, though. The curves stop when the shank resulting moment (la) becomes negative, i.e. the shank has reached maximum angular velocity and starts decelerating. The thigh moment curves show that the resulting moment (la) is positive (the thigh is accelerating) except fer the last 75 ms, where the curve is negative (the thigh is decelerating). The componemt primarily responsible for the positive resulting moment is the hip muscle moment (MH) which especially during the last 100 ms reaches a very high magnitude. EMG recordings suggest that this hip flexor moment is generated by rectus femoris. lIio­ 441 psoas, which we did not record, is presumably also active. Note that no support for active hip extensor muscles can be observed, neither in the moment computations nor in the EMG recordings from gluteus maximus andhamstrings. During the last 200 ms the positive (flexing) hip muscle moment is counteracted by the moment component from shank angular acceleration (ad and during the last 100 ms also by the moment component from shank angular velocity (0\..). These negative moments, and especially the lalter, are of such magnitude that they force the resulting moment negative despite the large positive hip muscle moment. The shank moment curves show that the resulting moment (la) has a negative phase of app. 75 ms where the shank is accelerating backwards into knee flexion. This is followed by a 175 ms positive phase of considerable magnitude where the backward swing is stopped and the shank is forcefully accelerated forwards. The components primarily responsible for the positive resulting moment are the knee muscle moment (MK), the presence of which is consistent with the EMG recordings trom vastus lateralis and rectus femoris, and the component trom thigh angular velocity (ror). The decline of both of these components causes the decline of the resulting moment and eventually its shift from positive to negative. We suggest that the knee muscle moment component (MK) declines as a consequence of the knee extensor muscles high shortening velocity and that the component from thigh angular velocity declines as a consequence of the thigh's deceleration. CONCLUSION These data suggest that thigh deceleration is caused by motion dependant moments due to shank motion, primarily shank angular velocity. No support for active deceleration by hip extensor muscles was found. Shank acceleration is caused by a knee muscle extensor moment and a motion dependant moment due to thigh angular velocity. Thigh deceleration is therefore considered unwanted but unavoidable due to the intersegmental forces arising when the shank is accelerated. REFERENCES Dunn, E.G. and Putnam, CA (1988). The influence of lower leg motion on the thigh. In Biomechanics XI-B (edited by G. de Groot, A.P. Hollander, PA Huijing and G.J. van Ingen Schenau), pp. 787-790. Amsterdam: Free University Press. Herring, R.M. and Chapman, A.E. (1992). Effects of changes in segmental values and tim­ ing of both torque and torque reversal in simulated throws. Journal of Biomechanics, 25, 1173-1184. Jackson, K.M. (1979). Fitting of mathematical functions to biomechanical data. IEEE Transactions on Biomedical Engineering BME-26, 2, 122-124. MarshalI, R.N., Jensen, R.K. and Wood, GA (1985). 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