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Discussion Results suggest an association between gymnastics performance and the ACTN3 (R allele), PPARGC1A (482Ser allele), Β3-AR (Arg allele) and possibly the ACE (D allele). It is important to be aware of how authors statistically analyse gymnasts, as a specific genotypic association cannot be attributed to gymnasts unless they are the only athletes in a cohort. Results may otherwise be skewed by the demands of other sports, as phenotypic heterogeneity is introduced. Also, nationality and ethnicity may play a role in genotype and allele distribution amongst a population. This is particularly relevant for the distribution of the ACE gene polymorphisms.(14) Athletic Performance ACE Gene The majority of studies in the review investigated the ACE gene. Two casecontrol studies, Kim and colleagues 2011 and Taylor and colleagues 1999,(15, 16) discovered no association between gymnasts and ACE gene polymorphisms. Small numbers of gymnasts were included (13 and 5 respectively) and both studies employed gymnasts into mixed athletic cohorts. The lack of an association could be due to the broad athletic cohorts, as it has been suggested that well defined athletic cohorts are needed to find associations with the ACE gene.(3) Myerson and colleagues 1999 placed gymnasts in a cohort with divers, totalling a small sample of 23 athletes. They also discovered no difference in allele distribution compared to controls.(17) Boraita and colleagues 2010 performed a case study and investigated 12 gymnasts. Placing athletes in mixed cohorts using the Mitchell classification,(18) presented no association between ACE gene polymorphisms and the different sporting categories. Athletes were also grouped as power (gymnastics, jumping, sprinting) and endurance athletes (middle/long distance running, triathlon) whereby a significant association was discovered for the power group with the DD genotype. When gymnasts were compared individually, the highest frequency of the DD genotype was observed out of the 12 sports (DD 66.6%, DI and II 16.6 %).(19) Oh and colleagues 2007 investigated a small sample of 12 gymnasts. The authors present results on a bar graph for individual sports. Gymnasts had the lowest frequency of the I allele and II genotype compared to controls and other athletes. However, no numerical values or p-values are provided. Massidda and colleagues 2011 investigated 33 elite gymnasts and although this is the largest sample of gymnasts for the ACE gene, statistical power is still insufficient. No significant differences were found between gymnasts and controls. Gymnasts had a lower frequency of the I allele compared to endurance athletes from previous studies; rowers (Gayagay et al 1998 (5) P=0.007), swimmers (Tsianos et al 2004(20) P=0.02) and long-distance runners (Myerson et al 1999(17) P=0.006), however the Bonferroni-adjusted pvalue was not significant. The association of the D allele and gymnastics performance is not consistent between studies. The ACE gene is involved in a number of physiological functions.(21) The D allele is associated with higher ACE activity in serum and tissue compared to the I allele.(22) The consequence of this and the mechanism behind the ACE gene’s effect on athletic performance is not entirely understood.(3) The II genotype is believed to correlate with a higher frequency of type I slow twitch muscle fibres and the DD genotype with type II fast-twitch fibres.(10) The majority of evidence regarding other athletes suggests an association between the D allele and power performance.(20, 23-25) The ability to generate power is a major component in gymnastics, therefore it seems plausible that the D allele may be associated with gymnastics performance. Massidda and colleagues 2011 were given a quality assessment score of eight, compared to the other studies which scored 3-4. Therefore more credence could be given to Massidda and colleagues 2011 results, where a lack of association was observed with ACE gene polymorphisms. Nevertheless, there is insufficient evidence to definitively conclude whether there is an association between ACE gene polymorphisms and gymnastics performance. PPARGC1A gene Two authors investigated the Gly482Ser polymorphism of the PPARGC1A gene. Maciejewska and colleagues 2012, completed an initial and replication study. The replication study investigated 56 gymnasts alongside 27 other sporting disciplines. A lower frequency of the 482Ser allele was discovered in athletes compared to controls (30.3% vs. 34.5%, P=0.002). In contrast, when the authors compared sports individually, gymnasts had a higher frequency of the 482Ser allele compared to controls (43.6% vs. 34.5%, P=0.045); the highest frequency out of all the sports.(26) Ahmetov et al 2009 focused on endurance athletic status and genetic polymorphisms. They grouped athletes into mixed cohorts (55 gymnasts were placed in the power group). The authors indentified 10 genetic polymorphism associated with endurance status, which included the Gly482 allele of the PPARGC1A gene, and grouped them together as ‘endurance alleles’. The authors then measured the number of ‘endurance alleles’ the athletic cohorts possessed. Unsurprisingly, the number of endurance alleles was not significantly different in the power group from controls.(27) The study provides little evidence for the PPARGC1A gene as no independent numerical values are provided for the gene. Quality assessment revealed Maciejewska and colleagues 2012 study to be superior to that of Ahmetov et al 2009 (scores of 6 and 4 respectively), endorsing an association between gymnastics performance and the 482Ser allele. The PPARGC1A gene is a coactivator of the subset genes that control oxidative phosphorylation and is abundantly expressed in skeletal muscle modulating muscle oxidative capacity.(10) The Gly482 allele is believed to benefit endurance athletic performance and positively influence VO2max.(28-32) Therefore research should establish whether the 482Ser allele benefits anaerobic or power performance. As the anaerobic system is responsible for supplying the energetic demands of gymnastics performance, this could be the reason why 482Ser allele is associated with gymnasts. Β3-AR gene Kim and colleagues 2010 investigated the Β3-AR gene in a small sample of 8 gymnasts (quality assessment score of five). Of all the sports, gymnasts were found to have the highest frequency of the Trp/Arg genotype (5 gymnasts 62.5%) and Arg allele (31.3%) and lowest frequency of Trp allele (68.7%). However a contradiction made by the authors, casts doubt on the reliability of the entire article. Athletes with the Trp/Trp genotype are described as having a significantly lower HDL-C (p=0.023) and higher plasma glucose (p=0.017), however later in the discussion the authors state that both HDL-C levels and plasma glucose levels were lower in athletes with the Arg allele.(33) The Β3-AR gene is believed to play a role in energy metabolism (lipolysis, thermogenesis) and cardiovascular function.(34, 35) There seems to be an association with the Arg allele and gymnastics performance. This is in contrast to a study by Santiago and colleagues 2009 who discovered the Arg allele to be associated with endurance performance,(34) as gymnastics is not an endurance sport. It is possible that Kim and colleagues 2010 results were unique for their subjects or the consequence of a small sample size. Alternatively, the Arg allele may have another undiscovered advantage, related to another physiological component relevant to gymnastics performance. ACTN3 Massidda and colleagues 2009 investigated the ACTN3 gene in 35 elite gymnasts. Compared to controls, gymnasts had a lower frequency of the X allele (P=0.039) and XX genotype (P<0.03). It was discovered that this significance was due to an association with male gymnastic performance. Male gymnasts had a significantly higher frequency of the RR genotype and a lower frequency of the XX genotype compared to controls and male endurance athletes, but not sprint athletes. No significant differences were discovered for female gymnasts compared to endurance or sprint athletes. It is worth noting that only one female gymnast carried the XX genotype. Thus it is possible the lack of significance may change with a larger sample size. As their study received a high quality assessment score of eight, these results strongly influence opinion on the ACTN3 gene and gymnastics performance. The ACTN3 gene encodes for α-actinin-3, a structural protein in skeletal muscle and is believed to influence muscle function.(10) The XX genotype encodes a stop codon and results in deficiency of α-actinin-3.(11) Little is known of the function of the ACTN3 gene on athletic performance.(3) It has been suggested that the R allele is associated with type II muscle fibres. This may be the reason for its association with power performance, and subsequently gymnastics performance.(36) With regards to gender differences, this was the only study in the review to find a significant difference in allele distribution between male and female athletes. The authors hypothesise that the α-actinin-3 protein may benefit male gymnastics performance more than female gymnastics performance because of an increased demand for muscular strength in male gymnastics.(11) Other genes The study by Ahmetov 2009 compared nine other genes (in addition to the PPARGC1A gene) with gymnasts; the PPARA, PPARD, PPP3R1, UCP2, UCP, VEGFA, NFATC4, PPARGC1B and TFAM genes. As they were analysed collectively the study provides little evidence on the individual genes. Summary Overall the results suggest that there are associations between gymnastics performance and the ACTN3 (R allele), PPARGC1A (482Ser allele), Β3-AR (Arg allele) and possibly the ACE (D allele). Current evidence is limited and therefore these conclusions can only be deemed as preliminary. For confirmation, future studies should employ large samples of well-defined cohorts of gymnasts and control for competitive level, gender and ethnicity. Future research More genetic polymorphisms need to be examined in gymnasts, as only four out of the hundreds that are associated with athletic performance have been investigated. Furthermore, investigations should explore whether genetics plays a role in why some gymnasts excel on one apparatus. The separate apparatus require a different combination of skills and hence, it would be interesting to explore the possibility of different genotypes benefiting certain apparatus performance. Genetics is an extremely complex area and research has only just begun to understand its vast role in athletic performance. Many human phenotypes are polygenic and it is unlikely gymnastics performance is any different. Research is yet to unveil the effect different genetic interactions have on athletic performance. It is possible particular combinations of genetic polymorphisms may be associated with gymnastics performance. Injury After searching the databases, no relevant articles could be retrieved on genetic polymorphisms associated with gymnastics injuries. It seems that thus far no studies have investigated this topic, clearly highlighting a large gap in the field. Studies should be conducted which correlate injury occurrence data and genetic testing of gymnasts genotypes, to identify specific alleles which predispose to injury. Future implications In the future genetic testing could be used as part of the selection process for National Squads. Whether it is ethical to use information on a gymnast’s genetic makeup to help decide whether to accept or reject them, is debateable. Additionally, if certain genotypes were discovered to be advantageous to apparatus performance, genetic testing could be used to guide a gymnast into specialising on a particular apparatus, thus helping them reach their potential. An example of genetic polymorphisms associated with injury is the FokI and BsmI polymorphisms of the VDR gene; suggested to increase the risk of stress fractures.(37) Future research may identify gymnasts with these polymorphisms as having a greater risk of developing stress fractures and hence, genetic testing of young gymnasts could be instigated. If a gymnast was found to have the FokI or BsmI polymorphism, coaches could concentrate on reducing high impact loading though the increased use of protective matting or soft landing surfaces. Limitations The primary limitation of the studies, are the small sample sizes of gymnasts. This may conceal a true association of a genetic polymorphism with gymnastics performance or give a false indication of an association. One limitation of this systematic review is the possibility of publication bias, as unpublished studies from conference proceedings are excluded. Another limitation is the inclusion of articles in the English Language only. An article in Korean by Lim et al 2006 which reports on the Β3-AR gene,(38) is known to have investigated gymnasts. Quality assessment was conducted using the Newcastle-Ottawa scale.(13) Content validity and inter-rater reliability have been established and criterion validity and intra-reliability are still under assessment.(13) As the NewcastleOttawa scale relates to cases with diseases, whereas in this instance cases were gymnasts, the scale needed to be adapted slightly which may have affected validity. The Pedro scale is a more reliable and valid tool of quality assessment, however the studies in the review are not randomised control trials and so this method could not be applied.(39) Conclusion Artistic gymnasts are required to implement a range of physical and technical skills into their performance. Gymnastics performance and injury are influenced by environmental and genetic factors. The discovery of genetic influences may provide new ways of approaching how gymnasts train and perform. The results of this systematic review identified only four genetic attributes associated with gymnastics performance: ACTN3 (R allele), PPARGC1A (482Ser allele), Β3-AR (Arg allele) and possibly the ACE (D allele). Physiological effects of the genes differ, but a pattern emerges of gymnastic performance relating to those alleles associated with power performance and alleles unrelated to endurance performance. Results should be classed as preliminary as studies are limited by small sample sizes and occasionally mixed cohorts of athletes. The role of ethnicity and gender on allele distribution needs to be further investigated and controlled in future studies. Moreover, it is evident from this review that there is a large gap in the field and subsequent studies should explore genetic variants that may predispose to injury in gymnasts. This could lead to the implementation of preventative measures thus benefiting those gymnasts susceptible to certain injuries. It is evident that more studies are needed on gymnasts. This review has revealed that for artistic gymnastics, the field of genetics is currently a long way from influencing practice and benefiting those who participate in the sport.