Ballet is an art which expresses ideas and emotions with the movement of the body. Choreography of ballet requires high-intensity movement such as quick moving, jump, rotation and leap [32]. Especially, advanced movements and expressions of ballet require the deep and wide movement of arms and legs and their artistic expressions are using hyperextension and excessive range of motion of the body. As a result, flexibility of ballet dancers is a primary requirement among all the other physical-requirement. Also performing ballet requires high-intensity physical activities and thus uses much energy and therefore the body of ballet dancers should be harmoniously composed with proper elements [3,13,22,28]. As a result, ballet dancers need harmonious body composition along with beautiful physical condition in order to perform artistic expressions with the movement of the body. Therefore, the dancer needs to have a developed physical capacity, especially flexibility and harmonious body composition along with beautiful physical condition, to successfully perform ballet [3,13,22,28]. To perform ballet successfully, the dancer needs both of the beautifully developed body and developed physical capacity and such are determined by environmental factors and genetic factors. Environmental factors include experience of participating in the practice and performance of ballet, dietary habits and physical activities and genetic factors can be determined by the genes received from the parent. The ratio between environmental factors and genetic factors to determine the performance of ballet has not been known yet but if we consider the fact that 35~85% of physical elements, which determine performance of athletes, are determined by genetic factors, ballet performance would be determined by genetic factors by 35% or higher [10,23,24,26,27]. Therefore, the ballet performance requires genetic factors to certain degree. For example, body composition, physical condition and especially flexibility, which are considered as important ones for ballet, are determined by genetic factors [27,28]. There is not enough reports on the genetic factors which can predict the ballet performance. But considering the fact that ballet is an artistic expression requiring great physical capacity and beautifully developed body, physical capacity that is required for ballet including, especially, flexibility-related genes, muscle strength and power-related genes, endurance-related genes and injury-related genes are actively studied with the recent advancement of science and biotechnology. And applying these physical capacity and injury-related genes to ballet, which expresses art with the body, can offer the information which can estimate and improve the ballet performance and prevent injuries. Physical capacity of individuals are determined by phenotype and such phenotype is considered as a result of interaction between genotype and the environment [11]. Therefore, the interpretation of genotype information of individuals is a proper method of finding out the potential of physical performance of the body and it will be an important information for assessing physical capacity and thus provides individualized training program or performance program for ballet dancers. Among currently known genes, a total of 120 candidate genes, which are related to physical capacity (cardiorespiratory endurance, muscle strength, muscle endurance, flexibility, body composition), are introduced [6,14]. However, most of them have low predictability regarding physical capacity and ACE ID polymorphism, ACTN3 RX polymorphism and COL5A1 polymorphism are strongly suggested as physical fitness-related genes [15]. Especially ACE ID polymorphism and ACTN3 RX polymorphism are reported to have a characteristic of being able to predict body composition along with aerobic endurance and anaerobic muscle strength / power which are contradictory phenotype [2,6,14,17,19] and also COL5A1 polymorphism is reported to be related to the prediction of joint injury along with flexibility by predicting ROM (range of motion) and endurance-based physical performance [1,4,7,8]. Genes related to physical capacity such as ACE ID, ACTN3 RX, COL5A1 polymorphism genes have a potential to be related to the genetic physical capacity and injury of the dancer. Also, when 3 genes are working as genes related to physical capacity and injury, there is a possibility of a correlation between ballet dancers and ballet performance.

But genetic relation on ballet dancers are not reported yet. Especially it is expected that finding and revealing the proper genes for ballet performance of ballerinas can bring the effect of predicting suitability for ballet, physical capacity and injury risk level but there is no study for them yet. Therefore we conducted our study to find out physical fitness-related genes related to ballerinas and to reveal the injury.

Ballet uses developed flexibility and slim appearance to express high-intensity activities when using the body to express art. Therefore, dancers who are suitable for ballet need to have developed flexibility with the demand for little fat and FFM with strong muscle strength and muscle endurance. Environmental and genetic factors affect such characteristics of ballet dancers. This study was designed to find genes related to the characteristics of ballet. Also we tried to find the gene which is related to frequent joint injuries due to too much pressure on the joint during the high-intensity ballet performance with a slim appearance. For the first step, ACE ID polymorphism, ACTN3 RX polymorphism and COL5A1 polymorphism were selected as candidate genes related to ballet. And we hypothesized that ACE ID polymorphism, ACTN3 RX polymorphism and COL5A1 polymorphism were the related genes for successful ballet performance. Also these genes were hypothesized as the genes related to injury. And the basis behind that is that these 3 genes were strongly suggested as candidate genes that could related to muscle strength, body composition and injury [1,4,6,14,15,21]. We tried to find out whether these physical capacity and injury-related genes are related to ballet dancers. As a primary study for that, the case group vs. the control group study was done. The case group consisted of ballerinas and the control group consisted of normal female adults of the same age. There was no difference of genetic distribution between the case group and the control group from the result of comparing distribution of each gene by the study design. From the result of comparing the difference of flexibility, body composition and physical structure of each genotype of ballet dancers, ACE DD genotype ballerinas had genetic relation of having higher PBF and FM from ACE gene. From previous studies, a mechanism could be explained that when the activity of ACE protein is high, fat accumulation is increased and it becomes a risk factor for obesity [18,31]. High ACE activity brings the change of angiotensin I and angiotensin II on RAS (renin-angiotensin system) and thus maintains high level of activity in the blood and tissue [34]. Especially, the level of angiotensin II, which is not only circulatory RAS but also topical tissue which is high in fat tissue, increases fat accumulation and mobilization of blood flow and thus it was reported as a risk factor to increase blood pressure [5,29,30]. From recent studies on human beings and animals, high ACE level was highly related to BMI and increased ACE activity of fat tissue of animals had induced the accumulation of subcutaneous fat and visceral fat [5,9,29,30,34]. Therefore, ACE DD genotype in this study can be related to the mechanism inducing high PBF and FM by having increased angiotensin II, due to 2~3 times high activity of ACE, on fat tissue. Therefore, it can be explained that ballet dancers with ACE DD genotype can easily have higher PBF and FM. Also, based on the report that when ACE protein has high activity it increases fat accumulation and subsequent diabetes, ballet dancers prefer slim looks with little FM and FFM and in that case, DD genotype ballet dancers could have genetic difficulty in terms of having lesser FM [18,20,31]. And considering such result, it is estimated that a better strategy will be necessary for ballet dancer with ACE DD genotype to prevent fat accumulation. Especially, our study can suggest that additional aerobic exercise other than dance is necessary to promote the oxidation of fat to effectively reduce fat accumulation. On the other hand, body composition, flexibility and risk injury per ACTN3 polymorphism from ballet dancers were related. XX genotype among ACTN3 genotypes (RR, RX, XX) had low weight and it occurred because FFM was significantly lower from fat and FFM that compose weight. And XX genotyped ballerinas with low FFM had relatively low flexibility and high ankle injury risk. ACTN3 gene is a gene expressing α-actinin-3, which consists of myomere of rapid muscle and RR and RX genotype from ACTN3 polymorphism are expressing α-actinin-3 but XX genotype is a genotype which stops expression [17,20,25,]. α-actinin-3 has a characteristic of supporting myomere for the function of strong muscle contraction in rapid muscle. If α-actinin-3 is not expressed, then α-actinin-2 is expressed instead and rapid muscle becomes to possess the characteristic of slow muscle [16,17,20,25,].

Therefore, high power from genetic capacity could be gained based on ACTN3 genotype along with endurance and in some studies, FFM level came out different due to the adjusted muscle shape [17,20,25,31]. Also this study showed that XX genotype in ballet dancers, who tend to have lower fat and FFM compared to normal adults, had much lower FFM with lower flexibility and thus higher risk of injury was there. When ACTN3 was initially discovered, XX genotype was considered as a gene related to hereditary disease of muscle since there was no expression of a-actinin-3 [20,25], but from the studies afterwards revealed that even there was no expression of a-actinin-3 in rapid muscle, a-actinin-2 would be expressed instead in rapid muscle and there was no correlation with muscle disease [20,25,31]. But when a-actnin-2 was expressed instead of a-actinin-3 in rapid muscle, the contractility and the size of muscle fiber was decreased and also cell metabolism, which has advantage in endurance, was developed [15,16,17]. In other words, aerobic metabolism related substances were increased and also showed a mechanism of protein-increase related to aerobic metabolism. Such mechanism showed that the metabolism of rapid muscle became developed closer to slow muscle and also rapid muscle's size got decreased. Such mechanism is related to a mechanism which can bring the decrease of FFM and also possibly bring excessive burden on rapid muscle easily with physical activities that require powerful action in a quick moment such as in sports game or ballet and thus as a result, muscle strength for the wider range of joint movement is decreased and therefore it is potentially limiting to flexibility's development. Also we think that such limitation of decreased muscle strength and restricted flexibility-development of rapid muscle bring more burden during quick jump, rotation and the change of action and therefore contribute to the increase of injury of skeletal muscle. Therefore more efforts and strategy will be necessary for ballet dancers with ACTN3 genotype, which cannot express a-actinin-3 in rapid muscle, to develop flexibility and also injury risk should be prevented along with the improvement of ballet performance with muscle strength-increasing program. Considering that ballet requires slim looks with strong physical capacity to perform high-intensity performance, ballet dancers with XX genotype should be more careful. In other words, ballet dancers with XX genotype would need additional muscle strengthening exercise and with such, the improvement of ballet performance and injury prevention can be achieved. On the other hand, the hypothesis of this study was that COL5A1 polymorphism is a gene related to ballet and it can predict high flexibility and injury. But the study result showed that there was no difference of genetic distribution between ballet dancers and the control group and also genetic relation could not be found when flexibility and the injury level per genotype from ballet dancers were analyzed. Such results indicate that COL5A1 gene is not applicable for Koreans since the previous studies were done on Caucasian population. One of the reasons of such conclusion is that genetic distribution of COL5A1 of ballet dancers or normal adults (the control group) of this study showed different genetic distribution compared to genetic distribution of COL5A1 in Caucasian population. In other words, COL5A1's distribution was generated based on the characteristics of Koreans and it would be difficult to apply the result from Caucasian population to Koreans. To sum it up, COL5A1 genetic polymorphism came out with no relation to flexibility and ankle injury in Koreans contrary to the studies on Caucasian population. The relation of flexibility, body composition, injury risk based on genotype of ACTN3 gene should be studied in detail for future studies with intervention-programs and prospective studies.

From the result of this study, ACE DD genotype from ACE ID polymorphism was related to higher PBF for ballet dancers and ACTN3 XX genotype from ACTN3 RX polymorphism was related to low FFM and flexibility for ballet dancers. ACTN3 gene came out as a gene related to injury when checking out the genes potentially related to injury and people with ACTN3 XX genotype has 4 times higher risk of ankle injury during ballet compared to the people with different genotypes.