Judo is a sport involving sparring in which the athlete is required to act instantaneously, reflecting the opponent’s move and responding to their technique. In judo, various techniques, such as nage-waza performed in a standing position and ne-waza performed in a supine position, are applied according to the given position. Between the two techniques, the athlete performs nage-waza by lowering or tilting the opponent’s center of mass (COM) using his trunk as a lever to carry their opponent on the back or by turning him over while using the force as efficiently as possible. It is an important technique that accounts for 60% of all attack moves in judo. Among the nage-waza techniques performed in a standing position, seoi-nage is the most well-known and frequently used, which includes morote-seoi-nage (using both arms) and ippon-seoi-nage (using one arm), as well as kata-eri-seoi-nage and seoi-otoshi, while morote-seoi-nage is generally referred to as seoi-nage (Kim, 2000; Kim et al., 2002).

Performance of seoi-nage is broadly divided into three phases. The first is kuzashi—a move used to destabilize the opponent’s balance through pulling or pushing; the second is tsukuri, a move to make the opponent lose their balance or increase the COM to make it easier to throw him subsequently; the last is kake in connection with tsukuri (Blais et al., 2007). Here, the most important phase to ensure the successful performance of seoi-nage is kuzashi because this move allows effective implementation of the subsequent move presuming the balance of the opponent has been adequately lost (Kim & Yoon, 2003).

Previous studies on successful kuzashi during seoi-nage were as follows: Yilmaz (2015) measured the force of pulling by both hands during seoi-nage in 40 elite judo athletes and showed that the athletes applied a greater force on the hand pulling the sleeve than the one pulling the collar, irrespective of the dominant hand in performing morote-seoi-nage. Nowoisky (2008) also analyzed the force of pulling during seoi-nage in elite judo athletes and reported that the force increased as the time up to the point of maximum force decreased in performing kuzashi. These results were indicative of the greatest emphasis placed on the move to lift the opponent’s arm in kuzashi, while the time at which the force is applied is as important as the simple magnitude of force. In addition, Hassmann et al. (2010) made a direct measurement of the pulling force in judo athletes by attaching a load cell to their garment, and the results indicated an increase in the maximum pulling force with a decrease in the overall time of the move, which lent support to the findings of previous studies.

As previously mentioned, the seoi-nage technique is complete as kuzashi is performed, immediately followed by tsukuri and kake, for which the rotation of the trunk, the force of the feet pressing against the ground and the perfect balance of the movements of all segments are the prerequisites. In a diversity of previous studies, therefore, kinematic devices (infrared camera, force plate, etc.) were used to analyse the force applied by the athlete on the ground during tsukuri and kake and the kinematic variables of the trunk and lower limbs (Kim, 2011; Kim & Kim, 1991; Kim et al., 2002), while most studies divided the seoi-nage technique into the rotation phase (kuzashi-tsukuri) and the nage-waza phase (completed through kake-nage-waza) to analyze the time taken by each phase as well as the rate related to the COM (Ishii et al., 2018). The results of these studies showed that the ratio for the time of the rotation and nage-waza phases was 50:50, while the kuzashi and tsukuri phase was reported to be as important as the nage-waza phase, and that the change of COM was generally faster on the left-right plane than the front-back plane so that the importance of the front to back and top to bottom movements was highlighted.

As can be seen, continuous efforts have been made to improve the seoi-nage technique in judo, and previous studies have concurrently suggested the need for an in-depth discussion on the pulling force in terms of both direction and magnitude during kuzashi for an athlete to accomplish a successful performance of seoi-nage to the highest level. The previous studies have analyzed the force or the rate on its own without considering the force direction in kuzashi and neglecting to evaluate the force magnitude while performing that technique. Therefore, a study examining the direction of tilting or pulling of the opponent by judo athletes and the force magnitude upon such moves in varying conditions (morote-seoi-nage and ippon-seoi-nage, with a change of performance height) is needed in the actual performance of seoi-nage for the improvement of this technique.

Thus, the purpose of this study was to conduct an in-depth analysis of the magnitude and direction of the force applied at the kuzashi and tsukuri-kake phases in seoi-nage using a vector measuring device developed for nage-waza in Kil et al. (2018). Identifying the differences in the force characteristics between morote-seoi-nage and ippon-seoi-nage could provide basic data for improving future training and feedback.

In this study, the differences in performing seoi-nage by the national team elite judo athletes were determined according to the performance height. We analyzed the variables (time, force magnitude, force rate, and the upper and lower angles and left and right angles of the force vector) according to performance height (80cm, 60cm) of the two most widely used techniques in judo morote-seoi-nage and ippon-seoi-nage.

Phase Time in morote-seoi-nage and ippon-seoi-nage according to the Performance Height

This study examined each phase time in morote-seoi-nage and ippon-seoi-nage according to performance height, and no significant variation was found across all phases (Table 1, p>.05).

In this study, the change of the force characteristics during seoi-nage performed by elite male judo athletes of the national team was observed while changing the performance height. The analysis of the results provided basic data for future training and feedback. In judo, seoi-nage is one of the most widely used techniques accounting for up to 60% of all attack moves, whereby the athlete makes the opponent lose their balance or tilts the COM of the opponent using his trunk as the lever to carry him on the back or turn him over with the flow of his movement. Hence, it is essential to determine the force’s magnitude and direction characteristics according to the opponent’s position (morote-seoi-nage and ippon-seoi-nage, with a change of performance height).

Our results showed that, while no significant variation in time was found across all phases of the morote-seoi-nage and ippon-seoi-nage techniques (Table 1, p>.05), an overall shorter period of time was observed at the 80 cm height position than at 60 cm (at the time points of kuzashi and tsukuri-kake) and the force value of morote-seoi-nage was lower during kuzashi at 80 cm than at 60 cm (Table 2, p<.05). This result coincided with the general characteristics of the seoi-nage technique as in an actual game, it is easier to turn over the opponent if his COM is high and reduce the relative effort on kuzashi, creating conditions for performing tsukuri-kake in the lower direction. The effort on kuzashi increases if the opponent’s COM is low, thus making it more difficult to lose; hence, driving the athlete to focus more on kuzashi than tsukuri-kake. A previous study reported that, at the moment of kuzashi in seoi-nage, it is important to align one’s COM with the opponent’s COM. At that time the athlete should lower his body as much as possible, trying to achieve a lower COM than that of the opponent, which is essential for tsukuri-kake (Kim, 2001; Kim et al., 2002). Our study results demonstrated that the technique is performed with less force at the height of 80 cm than at 60 cm, which is regarded as a relatively low COM height compared to their opponent (Table 2, p>.05). This could be considered quantitative evidence for the practical application of lower posture in an actual game as the athlete would use efficient force to perform the seoi-nage technique. For ippon-seoi-nage, likewise, no significant variation was found, but based on the trend of decreased force in kuzashi at 80 cm compared with 60 cm, the ability to control one’s COM in line with the opponent’s COM is crucial irrespective of which seoi-nage technique is being performed.

Furthermore, regarding the upper and lower angles of force vector in morote-seoi-nage and ippon-seoi-nage, significant differences were observed for tsukuri-kake in morote-seoi-nage and for both kuzashi and tsukuri-kake in ippon-seoi-nage (Table 3, p<.05). In a previous study, it was highlighted that the possibility of successful performance increased as the vertical displacement (upper to lower movement at kuzashi) in seoi-nage increased (Ishii et al., 2017). In this study, likewise, the upper angle was higher at 60 cm than at 80 cm, as the onset of the technique is in a high position at 80 cm, and the resistance of the rubber band is low; hence, a high level of performance is not required. However, at 60 cm, the onset of the technique is relatively low, so the upper angle should be increasingly demand a higher level of effort during the seoi-nage performance. Lastly, the left and right angles of the force vector displayed no significant variation in morote-seoi-nage and ippon-seoi-nage (Table 3, p>.05), which might be due to the experience of the participants, being able to adequately match and maintain their COM in line with the opponent’s COM. Therefore, the left and right angles of the force vector should be a target variable in the training of judo athletes outside the national team, whose techniques are yet to reach completion. In that scenario, the applied strategy would notably enhance athletes’ performance.

In conclusion, our study identified how the characteristics of the morote-seoi-nage and ippon-seoi-nage techniques change according to performance height changes. Our results might be seemed that morote-seoi-nage and ippon-seoi-nage performance at 80 cm height is more efficient and effective than at 60 cm height. However, the time and force were higher at 60 cm than at 80 cm, and the upper and lower angles of the force vector in kuzashi were higher; hence, the training seoi-nage could be repeatedly practiced on an opponent with a lower COM and at 60 cm height. This type of repeated practice is anticipated to assist athletes in performing seoi-nage successfully by exerting a strong instantaneous force and increasing the angles in kuzashi against an opponent with a lower COM that otherwise poses difficulties in performing the technique.

This study aimed to identify the force characteristics during seoi-nage in elite male judo athletes of the national team and conduct an in-depth analysis to provide the basic data for future training and feedback. The study results showed that, in both morote-seoi-nage and ippon-seoi-nage, the 80 cm height condition led to lower levels of time and force for performing efficient moves. The ultimate goal, however, is for the athletes to execute a high level of performance as fluently as possible in an actual training setting. It has, thus, been determined, based on the findings of this study, that the repeated practice of seoi-nage at the height of 60 cm or below during training would be a suitable method to improve the athletes’ abilities against an opponent with a lower COM that poses difficulties in performing the technique. It is also suggested that future studies investigate the kinematic factors that reflect the athlete performing the technique in consideration of such variables as actual body angles and muscle activities while varying the conditions of the opponent to take into account that judo is a sport involving sparring.