There has been no question that some strings do provide a better grip than others, but that did not guarantee that the ball will produce more spin. Furthermore, experiments with hand-held rackets has been needed to solve the difficult question of how players can tell the difference between different strings when laboratory tests indicate that they should play the same. The previous paper performed an experiment measuring the spin rates of used, notched strings in both their natural condition and after applying a lubricant to the string intersections to facilitate tangential string movement. It found that the lubricated notched strings produced 30% more spin than the unlubricated notched strings. The lubricant materials are effective to the notched strings, because the lateral movement and snap-back of the main strings increase spin. It also showed that the more spin results in the reduction of shock vibrations of the wrist joint during impact. Furthermore, it was shown that the used natural gut with notches decrease 70 % of spin rate compared to the new natural gut without notches in the another experiment, which has remained to be seen whether the same results will be obtained. Recently, International Tennis Federation researchers reported that the same movement that was observed with lubricated strings occurs with copoly as well. Copoly strings-slippery and stiff-generate more spin not because of more friction, but because of less. The old argument was that the better the grip between the strings and the ball, the more spin we would get, but that was not true. Many experiments has started to investigate whether the lateral movement and snap-back of the main strings increase spin, which strings generate the most spin, and so on. The research has also started to investigate the effect of a inter-string friction upon the main strings movement against the cross strings. Furthermore, the research has started to investigate how tennis strings "Go Dead". This paper clarified a mechanism for impact that is able to account for the wide range of string performance seen in the literature.

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Recently, International Tennis Federation researchers reported that the same movement that was observed by Kawazoe with lubricated strings occurs with copoly as well. Copoly strings-slippery and stiff-generate more spin not because of more friction, but because of less. The old argument was that the better the grip between the strings and the ball, the more spin we would get, but that was not true. Many experiments has started to investigate whether the lateral movement and snap-back of the main strings increase spin, which strings generate the most spin, and so on. The research has also started to investigate the effect of a inter-string friction upon the main strings movement against the cross strings. Furthermore, the research has started to investigate how tennis strings "Go Dead". This paper clarified a mechanism for impact that is able to account for the effects of various parameters on ball control, spin performance and string performance life, which is seen in the literature with laboratory experiment compared with experiment by the author who paid its attention to string-to-string friction in player tests.

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This paper deals with a comparison of three different types of double leg circles performed by three gymnasts. The differences were examined between circumferential speeds, between heights of gravitational center, and between angular velocities of hip rotational motion around the body axis. We obtained three-dimensional position data using 35 markers attached to the gymnast's body and detected muscle activities using 24 sensors. As a result we conclude as follows: (1) In order to perform graceful double leg circles, a quick hip rotational motion is required. (2) This performance is associated with the acceleration of hip rotational motion in advance of the start of the hip rotation. (3) It is suggested that the muscle activities not only of the upper part but also the lower part of the body are required for the performance.

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Circles on pommel horse are analysed from the dynamics and biomechanics point of view in this paper. There are few studies on the dynamics or the biomechanics of the circles on the pommel horses that gives insight to the training. We measure the circles on the pommel horse with an optical motion capture system, force plates and electromyograph. The whole-body detailed musculoskeletal model is applied for the kinematics and dynamics computation of the motion. The process of estimating the muscle tensions during the circles computes the kinematics and dynamics characteristics of the motion. We plot these characteristics and analyse them with comparing with the physiological data from the biomechanical point of view.

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We aim to clarify the mechanism of the wrist turn in a golf swing and the effect of the acceleration pattern of the down swing on the release point. First, the mathematical model of the golf swing is assumed to be a 2-dimensional double pendulum. By applying Lagrange's equation to this system, we derive the equations of motion of the golf swing. We consider that the wrist turn begins naturally under the centrifugal force of the swing and angular velocity of the arm at the release point is very important parameters from the view point of increasing head speed. When the angular velocity of the arm and the centrifugal force which applies to the club head become sufficiently large compared to the angular acceleration and tangential inertial force, the wrist of the player begins to turn naturally without driving torque being applied to the wrist. We derive simple theoretical equations which express when and where the wrist begins to turn and effect of acceleration pattern on release point. From the derived equation, it can be seen that the acceleration pattern has large effect on the release point and velocity of swing, and ratio of club length to arm length and the cock angle are also important parameters.

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During a golf swing, a bending and torsion occur on the shaft of a golf club, and it changes a head speed and a head angle etc. The behavior of such a golf club mainly originates in the input from a swing of a golfer. In recent years, many researches and development pay their attention to more upstream stage of the swing motion. Moreover, to understand the mechanism which connects a swing and a golf club behavior, many researches in consideration of the oscillation characteristic of the golf club and of a simulation by Multi Body Dynamics are being studied. This paper, which studies the highly accurate analysis technology in consideration of those all, shows the technology called Swing Simulation. It can predict a golf club behavior during a swing, formed by Multi Body Dynamics constructed with three-body golf club model with oscillation characteristic and by a three-dimensional displacement data reproducing swing characteristics. Compared a simulation result with actual golf club behavior, such as a position, a head speed and a bending, a simulation accuracy is confirmed.

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A model that calculates the deflections of a golf shaft during the swing was developed. Applying static deflection to a shaft which is flexible and has varying bending stiffness, a differential equation for moment and curvature which characterize its behavior over the duration of the swing were generated. Incorporated into concentrated moment are moment of forces acting at the tip and grip end of the shaft. These loads were calculated using a inverse dynamics analysis of the motion of the golf club during the swing. A motion capture system was utilized to collect the necessary data. Solutions to the equations for the shaft deflection were calculated numerically. The results are presented as deflections along the shaft at each time. Bending deflections are determined in two-dimensional direction.

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The purpose of this study was to quantify the dynamical roles of racket shaft elasticity during badminton smash motion. The racket shaft was divided into a set of rigid segments connecting to its adjacent segments via virtual joints with rotational springs. The deformation of racket shaft was approximated by using a 2nd order polynomial function from measured data during swing motion. The contribution of the joint torques to the racket head speed was obtained from the equation of motion for the system. Furthermore, with use of the equation, a recurrent equation with respect to the expanded generalized velocity vector, which consists of generalized acceleration and velocity vectors and the time integration of the velocity vector, was derived to calculate the contribution of the joint torque terms, external joint force term, and gravity term to the generation of the expanded generalized velocity vector. In order to realize stable numerical calculation for stiff system, the Newmark-beta method was used at the computation of recurrent equation. From the results, motion dependent term (M.D.T.), which plays significant roles for the generation of racket-face speed, was generated by shaft restoring torque at the middle phase of swing, and by joint torque term toward the impact. The shaft deformation was caused by wrist radial-ulna and shoulder internal-external rotation joint torques. These results indicate that racket face was accelerated by wrist and shoulder joint torques which generated M.D.T. by bowing racket shaft.

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This paper proposes the estimation method of spring and attenuation constants of skier. Skier conducts ski turns three-dimentionaly, and the motion is complexity. The joints of skier are used such as spring and damper in turns. Therefore, it is necessary to claryfy the effect of spring and attenuation by the joints of skier gliding on the actual snow field. However, the spring and attenuation constants change during turns. In this study, we propose the estimation method of spring and attenuation method using the joint internal force, relative displacement and relative velocity. The Extended Kalman filter is used as the estimation algorithm. We applied the estimation method to the motion information of skier gliding on the actual snow field, and we estimated the spring and attenuation constants. These results indicated the major features of skier's motion.

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Until today since ski was born, the sliding mechanism of the ski board still has not been clarified completely. On our previous studies, the mechanical dynamic vibration generated on the ski board was thought to be the key to clarity the ski sliding mechanism. To purposes of this study are to develop a simulator generating a ski board vibration in actual skiing and to perform the basic construction for clarifying the ski sliding mechanism, which still has not been clarified completely and to compare the coefficient of kinetic friction with the sliding velocity between dry ice and ice with increasing in the mechanical dynamic vibrations on the normal temperature. This study were led us to following conclusions. From experiment, the dry ice mass velocity was increasing in accordance with increasing of the vibration frequency, but ice mass velocity showed unexpected behavior. From above results, ice was found to be susceptible to water of the contact surface and other parameter.

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Control of body position is important in skiing. During turn, novice skiers often lean back and lose their control. Leaning back is natural reaction for people. They are afraid of the slope or speed. We develop a device to provide realtime sonification feedback of the center of pressure of the skier. The device guides the position of skier. A preliminary experiment shows possibility of improvements that the user become to be able to control their position immediately and even to overcome the afraid of slope and speed.

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