Static and dynamic balance in ballet dancers : a literature review

Endereço para correspondência: Lilian Ramiro Felicio – Avenida Paris, 34 – CEP: 21041-021 – Rio de Janeiro (RJ), Brasil – E-mail: lilianrf@uol.com.br Apresentação: out. 2012 – Aceito para publicação: ago. 2013 – Fonte de financiamento: nenhuma – Conflito de interesses: nada a declarar – Parecer de aprovação no Comitê de Ética no 032/11. Resumo | A dança envolve integração de movimento, equilíbrio postural e aspectos relacionados ao controle postural. Informações sobre o equilíbrio em bailarinos são de grande importância, pois eles são considerados modelos de controle postural. O objetivo foi revisar estudos sobre equilíbrio postural estático e dinâmico em bailarinos, caracterizando o controle e a dependência visual desses atletas para a manutenção do equilíbrio. Para isso, foi realizada uma revisão nas bases de dados PubMed, SciELO, Lilacs e Science Direct, considerando o período entre 1997 a 2013, utilizando os descritores equilíbrio, controle postural, plataforma de forças, ballet, bailarinos clássicos e aferência visual. Foram selecionados 18 artigos capazes de fornecer dados quantitativos para avaliação do equilíbrio nesses atletas classificados pelo nível de evidência científica Oxford. A literatura revisada mostra completa concordância quanto ao efeito da retirada da informação visual sobre a estabilidade postural de bailarinos considerados como executantes altamente treinados. Estudos mostrando a comparação do equilíbrio de bailarinos com outras técnicas desportivas confirmaram um padrão específico de equilíbrio nesses indivíduos. Entretanto, associando-se à restrição visual, bailarinos apresentaram maior deslocamento do centro de pressão comparado a outras modalidades desportivas, sugerindo maior dependência visual para a manutenção do equilíbrio. Bailarinos apresentam menor oscilação postural em relação a indivíduos não treinados e indivíduos treinados em outras práticas desportivas, com maior dependência visual para manutenção do equilíbrio.


INTRODUCTION
Ballet is a physical activity that requires musculoskeletal conditioning by the use of complex high impact movements, with major joint amplitude, which works on the development of coordination, balance and laterality associated with efficient strength and flexibility in technical execution 1 .
Postural control is important to understand the capacity of the human being to perform their activities and maintain body balance, providing stability and orientation during motor tasks 2 .Such control depends on sensory information of the vestibular, visual and somatosensory systems, so that motor actions can be triggered based on experiences and skills [3][4][5] .One determining factor for balance control is the size of the support base, and the movements executed by the ballet dancer are usually performed on a small base, such as the en pointe balance 6 (Figure 1A).This movement involves major weight discharge at the tip of the foot, besides being more unstable and depending on ligaments and muscles for stability and support 6 .
Balance training begins early for dancers, around the age of five, and becomes more complex when they perform en pointe 6 , however, balance is rarely analyzed within the context of dance 7 .Training is conducted in the vertical position, with reduced support bases and the use of mirrors, and the direction of the eyes is important to keep control during spins, which implies visual dependency in order to maintain balance when compared to athletes of other sporting activities 8 .Some studies used stabilometry to quantify the dislocation of the center of pressure (CoP) during orthostatic posture 9 , and they demonstrated that visual restriction resumen | La danza envuelve integración de movimiento, equilibrio postural y aspectos relacionados al control postural.Informaciones sobre el equilibrio en bailarines son de gran importancia, pues ellos son considerados modelos de control postural.El objetivo fue revisar estudios sobre equilibrio postural estático y dinámico en bailarines, caracterizando el control y la dependencia visual de esos atletas para la manutención del equilibrio.Para eso, fue realizada una revisión en las bases de datos PubMed, SciELO, Lilacs y Science Direct, considerando el período entre 1997 y 2013, utilizando los descriptores equilibrio, control postural, plataforma de fuerzas, ballet, bailarines clásicos y aferencia visual.Fueron seleccionados 18 artículos capaces de proveer datos cuantitativos para evaluación del equilibrio en esos atletas clasificados por el nivel de evidencia científica Oxford.La literatura revisada muestra completa concordancia en cuanto al efecto de la retirada de la información visual sobre la estabilidad postural de bailarines considerados como ejecutantes altamente entrenados.Estudios mostrando la comparación del equilibrio de bailarines con otras técnicas deportivas confirmaron un estándar específico de equilibrio en esos individuos.Entre tanto, asociándose a la restricción visual, bailarines presentaron mayor desplazamiento del centro de presión comparado a otras modalidades deportivas, sugiriendo mayor dependencia visual para la manutención del equilibrio.Bailarines presentan menor oscilación postural en relación a individuos no entrenados e individuos entrenados en otras prácticas deportivas, con mayor dependencia visual para manutención del equilibrio.
Descritores | equilibrio postural; danza; terapia por la danza.A B limits postural control patterns, thus increasing postural oscillations 10,11 .The common interpretation of the stabilometry suggests that postural tasks with higher values of parameters deriving from stabilometric signals are related to postural instabilities 12 .Another important aspect in stabilometric tests is the possibility to identify the visual dependency of the individual, characterized by higher values of these parameters when vision is restricted during a motor task, such as more oscillation speed and longer distance crossed by the CoP 12 .
The dynamic balance of ballet dancers was assessed by a few authors, and the execution of spins (pirouette) was considered as a complex task, involving a strategy of head movement, the "marking the head", which dissociates the rotation of trunk and head -while the body spins, the eyes stare at an established point, and when the maximal cervical rotation is reached, the head performs a fast rotation towards the same direction of the movement, and then the eyes stare at the same point again; therefore, it is possible to observe the importance of visual information for balance and quality of the motor performance during the spin, correlated to less postural oscillation 8,[13][14][15][16][17] .
Considering that the analysis of static and dynamic balance of ballet dancers, in the main moves, is very important for their performance, besides collaborating with the elaboration of training and rehabilitation programs for these athletes, the objective of this study was to review scientific literature concerning balance in ballet dancers, by observing if specific training would lead to less postural oscillation and to more visual dependency in order to control balance.

METHODOLOGY
A literature review was performed with the following databases: Lilacs, Medline, PubMed, SciELO and Science Direct, with the descriptors: ballet dancers, postural control, balance, force platform, classic ballet and visual afference, combined in groups of three.The pre-selection inclusion criteria of the articles were: date of publication (1997-2013), language (English or Portuguese) and the relation of title and abstract with the subject of interest (ballet and balance).The articles identified in the pre-selection search were assessed according to the following inclusion criteria: (1) population (ballet dancers) and ( 2) intervention (assessment of static or dynamic balance).
Review was carried out in three stages.In the first one, there was a general selection of publications on the subject, resulting in 57 studies.In the second stage, those which were duplicated and did not belong to the theme were excluded -the 18 remaining articles were included in the review and grouped together according to the year of publication, being selected based on the abstract.In the third stage, these articles were classified according to the recommendation criteria and evidence-based classification from the Oxford Centre for Evidence-Based Medicine (OCEBM) 18,19 .Due to the low classification of the selected articles, the level of classification was not used as a selection criterion.The OCEBM levels consider the strength of evidence for therapeutic effect and damage, evidence of prevalence, accuracy of diagnostic tests, prognosis, therapeutic effects, rare damage, common damage, and are used as screening in review studies 18 .

RESULTS
The 18 articles selected for this study are presented in Table 1, describing sample, objective, tools used to assess balance, synthesis of results and Oxford classification 18,19 .

DISCUSSION
The studies included in this review mostly presented (83%) sample size between 8 and 45 dancers, and 11% of the articles ranged from 4 to 8 dancers.With that, it is possible to observe the difficulty to perform studies with larger samples, probably due to the level of expertise and training of these athletes.Because of the number of articles discussing specific movements of professional dancers, especially those related to dynamic balance, these articles were included in this review despite the low sample, level of evidence 3-4 and recommendation B (little satisfactory).
Stabilometry was the most used method to assess static balance (55.5%).However, a major variation was observed as to positions and time of permanence on the force platform for the static balance evaluation according to CoP variables and anterior-posterior and medial-lateral dislocation areas, thus characterizing the balance of dancers by means of these variables.
The difficulty of the balance task increases with unipodal support, and this position is common for the ballet practice 20 , considered to be of easy execution with good reliability 21,22 .Therefore, considering specific positions of classic ballet, 27% of the articles assessed the static balance in specific unipodal postures 6,10,11,20,[23][24][25][26][27][28] .Lobo da Costa et al. 29 assessed ballet dancers in several  1B), wearing ballet slippers and barefoot, being the use of ballet slippers the cause for more dislocations of the CoP in all of the positions.Bruyneel et al. 10 also found a much smaller dislocation area in similar conditions when the free limb was turned backwards (attitude derriére), in conditions with vision 29,30 .Barcellos and Imbiriba 23 verified, by means of kinetics, the angular variations of pelvis, hip, knee and ankle joints, and, together with the mean speed of CoP oscillation, they observed a smaller dislocation area in the en pointe position than in a position with total plantar support.The oscillating platform and the accelerometer (33% of the articles) were used to compare dynamic balance in relation to different visual conditions and different positions, among individuals grouped as to age and sex 8,15,17,21,31 .It was observed that male professional dancers (>18 years old) with more practice time presented more visual dependency on a bipodal support over a mobile platform in comparison to younger male dancers (<18 years old) 15 , as well as women (>18 years old), because under the same conditions they presented smaller CoP area than the men 31 ; according to the authors, the relationship between balance and gender would be related to the different movements performed by the dancers during a technical execution, in which men perform rougher and more explosive movements, while women perform more contained and soft moves.

demi-point positions (Figure
Besides, male ballet dancers seem to be more dependent on their vision than female dancers 6,15,21,31,32 , due to differences related to maturity and the development of the vestibular system, which takes place at the age of 9 and 10 years old among women, and 13 and 14 years old among men 33 .
Kiefer et al. 34 also assessed female ballet dancers during a static task, with the objective of identifying differences in postural coordination between professional dancers and controls, who were not trained during a dynamic visual follow-up task by means of a electrogoniometer, thus verifying the ankle and hip angular oscillations while the athlete searches for the target 34 .Since dancers are trained to look for reference points during gesture movements, they presented more control, as well as more stable hips and ankles when compared to controls 34 .
The balance of ballet dancers in comparison to athletes of other sporting activities, such as judo wrestlers, runners, acrobats and soccer players, was discussed in 28% of the selected articles, and the dancers' high balance pattern without visual restriction was consensual, probably related to the specificity of the training for these athletes 10,11,20,21,26,27,31 .However, 61% of the articles did not control the position of the upper limbs of the athletes 6,7,10,11,15,17,24,27,29,32,34 ; therefore, dancers could have taken the first rank for the upper limb, thus improving balance in the different assessed positions.
However, when assessing balance in association with visual restriction, ballet dancers presented a larger CoP dislocation in comparison to judo wrestlers, acrobats and non-athletes 11,31 , thus pointing out to the major Rein et al. 32 30 male ballet dancers To compare postural control between professional ballet dancers, amateurs and controls

Force scillating platform
Professional ballet dancers presente better postural control B2 Cheng et al. 36 34 , the difficulty to compare dancers with different groups of athletes is due to the fact that there are diversities not only concerning the different types of bodies, but also distinct training.
Despite the increased postural oscillation, other parameters should be taken into account to determine the efficacy of postural control, such as kinematics and kinetics.In relation to dynamic control, no studies comparing the specific activities of ballet with other sport modalities were found, nor those analyzing different types of dancers.Besides, assessing the visual dependency for balance during gestures of other modalities is important to understand the specificity of training for balance.
According to the selected articles, it is possible to observe moderate or unsatisfactory level of recommendation, thus, studies using different control groups, which can monitor the positions lower limbs, different support bases and different gestural movements, can contribute with the evaluation both of the static and dynamic balance of these athletes.
Based on the used articles, we understand that ballet provides better balance in relation to non-trained groups and in relation to other sport modalities, which leads to better body stability 34 , however, the visual dependency to maintain balance seems to be more present among dancers.Besides, it is possible to observe a relationship between sports training and postural oscillation, therefore, inserting balance exercises in the training of ballet dancers without emphasizing visual fixation could increase the postural control of these athletes.
Data in this study point out the increased oscillations among ballet dancers with visual deprivation in relation to other populations of athletes.Given the importance of balance for the performance of movement in classic ballet, it is important to explore different situations in dancing gestures, exploring its specificities.When postural control and balance is improved, producing more coordinated and coherent movement patterns, the probabilities of lesions could be reduced due to the presence of strategies of preprogrammed movements.
So, in order to better understand balance, it is important to emphasize aspects of the ballet dancer's gestures, and also work with a more homogeneous sample.

CONCLUSION
In literature, it was consensual that ballet dancers presented better static balance in relation to non-trained individuals and athletes of different sport modalities, however, dancers presented more visual dependency in order to maintain static balance due to the specificity of their training.

Table 1 .
17ticles distributed considering sample size and characteristics, objective, used tools, results and Oxford classification17 Continue....

Table 1 .
17commendation A -Consistent, controlled and homogeneous study ande; B -Controlled study with less quality; C -lower quality; poor reference pattern and D -Inconsistent or inconclusive.Level: 1 -Clinical, controlled and randomized studies; systematic homogeneous reviews; 2 -systematic review of cohort studies, cohort studies (including a randomized clinical trial with less quality); 3 -systematic review of case-control studies and case-control study; 4 -case report; and 5 -experts' opinion, explicit critical evaluation and translational research17Continuation visual dependency of ballet dancers to maintain balance.On the other hand, Schmitt, Kuni and Sabo 26 indicated that ballet dancers presented less postural oscillation in relation to athleticism.However, they assessed nonhomogeneous groups as to age, time of profession, gender, gestural and analyzed position.According to Kiefer et al.