Introduction

Ever since the winning two gold medals from Seoul Asian Games in 1986, unlike its male counterpart, international performance of Korean female artistic gymnastic has been stagnated for more than 30 years. However, talented young female gymnast Yeo Seojeong (YSJ) won a gold medal in the vault event from Jakarta Palembang Asian Games in 2018, which made her to be one of the major contender for a medal in 2020 Tokyo Olympics. Competing in her first adults competition at 2018 World Championships, YSJ scored average score of 14.233, which had put her 5th out of 8 finalists. This was rather disappointing; however, she has shown that she is not far from the winning the medal. Indeed, YSJ showed her potential by winning a gold medal from 2019 Korea Cup with introduction of new vault technique. This new technique is a variation of Yang Hak Seon vault (Federation of International Gymnastics, 2020), involving handspring forward and a salto forward with a 720-degree twist. International Gymnastics Federation (FIG) has officially authorized this new technique with difficulty score of 6.2 and named this as YEO Seojeong (Federation of International Gymnastics, 2020). Therefore, expectation for the medal from the Olympics is higher than ever before.

Other contenders for a medals are world class gymnasts such as Simone Biles, Shallon Olsen, and Alexa Moreno, who won medals from the event YSJ competed in 2018 (World Championships). In 2018, these gymnasts performed more difficult technique than YSJ, therefore in order for YSJ to win the medal, she needs to execute Yeo Seojeong vault perfectly since it has the second highest difficulty score (6.2) after the Biles with difficulty score of 6.4. Thus, providing technical support, which would aid her to perform successful Yeo Seojeong vault consistently, might be pivotal to secure a medal at the Olympics. However, currently we do not have much knowledge on this novel technique since we have only observed the performance at the 2019 Korea Cup. In order to enhance our knowledge regarding Yeo Seojeong vault, biomechanical analysis, especially comprehensive kinematic analysis, is a good option. Therefore, the purpose of this study was to explore key kinematic variables of Yeo Seojeong vault by analysing series of vaults performed by YSJ in order to provide technical support for YSJ and coaches to develop successful training strategy. This study also aimed to provide comprehensive understanding of kinematic variables associated with YEO Seojeong vault, facilitating its adoption by other gymnasts striving to achieve high scores in competition. Since Yeo Seojeong vault can only be performed by YSJ and considering its level of difficulty, our best option for the data were successful trial collected during Korea Cup 2019 and unsuccessful trials collected from the practices.

Methods

Data collection

Five Sony (JPN) NEX-FS700 digital video cameras were used for video recordings. Video recordings were done at 60 frames per second (120 Hz) with Full HD quality (1920 × 1080). Figure 1 shows the layout of video cameras setting as well as Visol's control points (1 cube = 1 x 1 x 1 meters) for creation of global coordinate system, which was identical to setting used by Song et al. (2018). Five video cameras filmed two control points at 120 Hz with shutter speed of 1/500 s and then 3D coordinates were set using direct linear transformation method (DLT, Abdel-Aziz & Karara, 1971) by merging five video recordings obtained from video cameras.

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Figure 1

Layout of cameras and control point installation 

Data analysis

Data of one successful vault and three unsuccessful vaults were collected and analysed. Unsuccessful vaults were determined by agreement between coach and research team, while successful vault was the one that YSJ performed during 2019 Korea Cup. Successful vault was named S1, while unsuccessful vaults with landing on the mat and backward step to collapse, landing and sitting down, and collapse with side turn were named F1, F2 and F3, respectively. In order to explore the kinematics of Yeo Seojeong vault, key events and phases were established as shown in Figure 2, based on the previous study by Song et al. (2018). Below are the definition of each events and phases.

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Figure 2

Events and phases of the Yeo Seojeong vault 

Events;

• BTD (Board touchdown): the point when the feet touch down on the board

• BTO (Board take-off): the point of take-off from the board

• HTD (Horse touchdown): the point when the hands touch down on the horse

• HTO (Horse take-off): the point of take-off from the horse

• Peak: the highest point the CoM reaches during the in-flight move in the post-flight stage

• LD (Landing): the point when the feet touch down on the mat

Phases;

• BC (Board contact): board contact phase, when the feet touch the board

• PrF (Pre-flight): pre-flight phase, from the moment of take-off from the board to the moment left hand touches the horse

• HC (Horse contact): horse contact phase, when both hands touch the horse

• PoF (Post-flight): post-flight phase, from the moment the right hand takes-off from the horse to the moment the feet touch down on the mat

Based on the previous studies, key kinematic variables for vault were subject to analysis (Song & Park, 2016; Takeiet al., 1996, Takei, 2007). These variables were phase time, changes of body Centre of Mass (COM) according to phase, changes in the angles of body segments, and changes according to body rotation, and changes in angular velocity. Figure 3 illustrates the definition of the angles; (1) hip joint angle, (2) knee joint angle, (3) shoulder joint angle, (4) Board touchdown and take-off angles, (5) Horse touchdown and take-off angles, (6) Thigh rotation angle, (7) Trunk rotation angle, and (8) Trunk twist angle.

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Figure 3

Definition of angles 

Results

Discussion

The purpose of this study was to explore key kinematic variables of Yeo Seojeong vault by analysing series of vaults (1 successful and 3 unsuccessful) performed by YSJ in order to provide technical support for YSJ and coaches to develop successful training strategy as well as promote adoption of the technique among gymnasts. In this regards, we have selected the variables used by previous studies (Song and Park, 2016; Takei et al., 1996, Takei, 2007). Currently, there are not many studies, which explored vault performance of world level gymnasts. However, group of Korean researchers have been consistently publishing study findings regarding world level male vault performance since 2012 (Park and Song, 2012, 2015; Song and Park, 2016; Song, 2017, 2018, 2019). These studies have used similar criteria with our study for analysis of kinematics, therefore we might be able to compare successful vault (S1) of our study to these studies, especially study by Park & Song (2012) which explored kinematics of YANG Hak Seon vault of male competition, as Yeo Seo Jeong vault is variation of YANG Hak Seon vault. Also, considering the fact the studies are scarce in this field, we could compare the kinematic variables from other sports which have similar movement characteristics such as pole vault, diving, and etc.

S1 was the vault performed by YSJ in 2019 Korea Cup, which was almost perfect except for the landing where YSJ moved one step left. S1 had relatively longer and higher post-flight in terms of vertical and horizontal displacement (2.62 m and 2.91 m) and also had faster rotation and twist velocity (775 deg/s) during post-flight. These are not comparable with YANG Hak Seon vault performed by Yang Hak Seon (YHS) from previous study (Park & Song, 2012), which has vertical and horizontal displacement of 2.89 m and 3.67 m, and twisting velocity of 991 deg/s (Park & Song, 2012). However, when we consider that height of YSJ is 9 cm shorter than YHS which allows YSJ twisting more easily, and also that Yeo Sejeong vault requires one less twist during the post flight than YANG Hak Seon vault, we could assume that vertical and horizontal displacement and fast rotation/twist velocity of the post-flight are likely the key factors for successful performance. In fact, previous studies showed that higher body COM and larger horizontal displacement are positively correlated with the score of vault (Farana & Vaverka, 2012; Takeiet al., 2003; Takei, 2007). We believe that the successful post-flight motion came from fast horizontal velocity at horse touch down while generating fast trunk rotation at horse contact. These have resulted fast thigh rotation during horse take-off as well as take-off angle exceeding 90 degrees which secured height as well as horizontally oriented body COM displacement during post-flight. Landing angle and trunk twisting angle at the landing are other possible candidates for successful Yeo Seo Jeong vault. S1 showed relatively large landing angle (50 degrees) as well as trunk twist angle (681 degrees) at landing compared to all of unsuccessful trials from our study and these are comparable to Yang Hak Seon vault which had landing angle of 57 degrees and trunk twist angle of 679 degrees. Indeed, as Yeo Seo Jeong vault requires two and a half twist to fulfil requirement, ability to generate large trunk twist angle is prerequisite for successful performance. Also previous study found that the higher height of body COM at landing is correlated with higher score (Farana et al., 2013; Takei et al., 2007).

F1 was the unsuccessful vault which landed on the mat and took a back step and then collapsed. There was no difference between F1 and S1 in terms of knee and hip angles at the landing, however F1 had the smallest landing angle and trunk twisting angle at landing (38 degrees and 650 degrees). This means F1 did not have enough body rotation as well as twist to complete execution of Yeo Seojeong vault. We believe that these lack of rotation and twist are due to relatively shorter post-flight time that YSJ did not have enough time to complete required rotation as well as twist. F1 approached the board relatively slow and had the low body COM, small horizontal displacement of body COM, and slow vertical velocity at every preparation phase, which were reflected on rather small post-flight. This means F1 relied only on the movement which attempted to accelerate thigh and trunk rotation during preparation phases instead of attempting to generate forceful propulsion for the pre-flight using springboard. Indeed, previous study stressed that the initial pre-flight values must be increased or the losses during horse contact must be reduced for greater take-off (Hiley et al., 2015). And the studies on pole vault, which shares similarities with vault up to the flight-phase, also suggested that generating great speed as well as take-off velocity to bend the pole sufficiently is critical to achieve great height (Hanley et al., 2022). We assumed that relying only on the acceleration of rotation during springboard contact caused hyperextension of hip joint through the horse take-off which hindered the propulsion for post-flight, rotation of thigh and trunk, and the twist of trunk.

F2 was also the unsuccessful vault which landed in a sitting position. When landing, F2 had knee angle of 118 degrees, vertical body COM of 0.57 m, landing angle of 44 degrees, and twist angle of 668 degrees. This means that the body was quiet bent while failed to plant the feet on the ground resulting in sitting down. Furthermore, F2 had small landing angle as well as not enough twist of trunk. This is not only a form break, but also a failed landing that may not be recognized as difficulty level as Yeo Seojeong vault due to the lack of twisting. One of candidates for the unsuccessful landing might be the lack of rotation and twist velocity during descending phase of post-flight. F2 had faster thigh rotation and trunk twist at the ascending phase of post-flight, however there was deceleration in these rotation and twist during descending phase. We presume that when YSJ was performing F2, she kinaesthetically sensed slower horse take-off as well as improper trunk and thigh rotation, which forced her, as a compensation, to attempt to maintain the high speed of thigh rotation as well as increasing the speed of twisting in order to complete the execution of the vault. She did this by kicking the leg too early from the ascending phase of post-flight to retain speed of thigh rotation while wrapping the upper trunk with arms (narrowing shoulder joint) to increase speed of trunk twist. In other words, it looks like YSJ attempted to twist the trunk at the horse take-off, where her head was pointing downwards, which limits her ability to twist. Furthermore, even though YSJ wrapped the trunk with arms to speed up the twisting, she lost her momentum to twist after the Peak, where head was pointing upwards. Thus, we believe that the factors, which caused weak propulsion off the horse, are slow approaching speed to the board, touching down board with bent body posture, slower vertical velocity off the board due to not fully extending shoulder joints, failing to generate enough propulsion off the board, and improper trunk rotation at horse contact leading to inefficient blocking. As we have discussed earlier, fast horizontal velocity towards board is an important determinant for successful vaults, because energy built up during the running phase will be converted to linear and angular momentum at board touch down (Fernandes et al., 2016). As illustrated in the studies anlaysing diving performances, maintaining relatively straight trunk posture (high COM) during board touch down is critical because it increases rotation potential (Koschorreck & Mombaur, 2012; Mccormick, Subbaiah, & Arnold, 1982; cited in Kim et al., 2019) and also allows gymnast to quickly rebound off the board (Uzunov, 2011). Fully extended shoulder during board touch down is another important factor. A study by Hiley et al. (2015), which tested optimal touch down technique to maximise rotation potential and angular momentum for post-flight, illustrated that shoulder angles of 180 degrees had the greatest rotation potential as well as angular momentum for post-flight compared to angles of 137 degrees and 128 degrees.

F3 was the unsuccessful vault, which YSJ kept twisting during landing and then collapsed on the left side. Landing angle for F3 was 48 degrees while twist angle was 680 degrees that almost fulfilled twisting requirements for Yeo Seojeong vault, however YSJ could not control moment of inertia leading to loss of balance. This is because YSJ failed to make body upright at the horse take-off with short horizontal displacement and vertically oriented post-flight, which had negative effect for the trunk rotation at descending phase. Takei et al. (2007), in thier study, have shown that higher score vault had slightly arched body in line with shoulder and hand at the horse take-off while lower score vault had flexed body only half-way to the handstand, where F3 illustrates characteristic of lower score vault. Although achieving great height is crucial in post-flight, height alone might not ensure adequate time for rotation and twist. Horizonatal displacement also holds significant importance (Lee et al., 2012; Park, 2020; Takei et al., 2003; Takei, 2007), however F3 failed to generate longer horizontal displacement. F3 showed fast horizontal velocity at board take-off, small rotation angle of hip and trunk, and lower body COM at the horse touch down, and small horse touch down angle, which made powerful blocking at the horse contact leading to strong propulsion off the horse. Although, this allowed YSJ to have relatively longer flight time, she could not obtain greater angular distance of trunk off the horse, which hindered rotation during the post-flight.

Apart from whether vaults were successful or not, we should pay attention to neck angle at the moment of Peak. Because, in order to increase the twisting velocity, the chin must be pulled in to make a body forming one axis (vertical) from the feet to the head to reduce the moment of inertia to speed up the angular velocity. In other words, the moment of inertia can be minimised when the neck joint angle is close to 180 degrees. However, neck joint angles for all trials, including S1, were less than 160 degrees. This means that YSJ could not pull the chin in, leading to have larger moment of inertia, which is considered to have an adverse effect on twisting.

After careful appraisal of our results authors and coaches agreed that YSJ needs to improve core strength which will help her to perform Yeo Seojeong vault consistently with benefit of injury prevention. Core strengthening exercise would benefit YSJ in terms of building up internal muscle, improving strength as well as body balance (Brill & Couzen, 2008; Willdson et al., 2005). Core exercise program should involve dynamic resistance movement, for example, sled drag and farmer’s carry, which will allow YSJ to maintain stiffness against sudden force exerted on her body.

A limitation of our study is that we only analysed four vaults (1 successful, 3 unsuccessful) without the statistical analysis. However, this was the only option we had since Yeo Seojeong vault can only be performed by YSJ. Future research should collect more successful Yeo Seojeong vault, which will allow us to run statistical analysis to explore factors associated with successful vault in more details.

Conclusion

In conclusion, the purpose of this study was to explore key kinematic variables of Yeo Seojeong vault by analysing series of vaults (1 successful and 3 unsuccessful) performed by YSJ in order to provide technical support for YSJ and coaches to develop successful training strategy as well as promote adoption of the technique among gymnasts. To sum up our findings of kinematic analysis, successful Yeo Seojeong vault was possible when YSJ had fast horizontal velocity at horse touch down as well as fast trunk rotation during horse contact, these allowed her to have faster thigh rotation and take-off angle exceeding 90 degrees at horse take-off resulting to reach maximum height and the length during post-flight. Together with higher and longer post-flight, faster rotation and twisting warranted the successful completion of Yeo Seojeong vault. From the unsuccessful vaults, we found that slow horizontal velocity and bent body posture at board contact as well as low body COM, small horizontal displacement of body COM, and slow vertical velocity at every preparation phase had adverse effect on the horse take-off, i.e. improper blocking, weak propulsion, and etc. These resulted rather insufficient post-flight with deceleration of rotation and twisting in descending phase of post-flight leading to insecure landing. Neck angle not reaching 180 degrees is another factor hindering successful performance. Prescription of core exercise program, which involve dynamic resistance movement will aid YSJ to have consistent performance while preventing her from injuries.