Kinematic Evaluation of the Classical Ballet Step Â•Œpliã©Â•Š

Kinematic Evaluation of the Classical Ballet Step “Plié”

Kaanda Nabilla Souza Gontijo, P.T., M.S., Cláudia Tarragô Candotti, Ph.D., Grace dos Santos Feijó, Lais Paixão Ribeiro, and Jefferson Fagundes Loss, Ph.D.

Abstract 2 and 3, 18 dancers displayed pelvic between the lower-limb structures, Lack of alignment between the lower- instability tending toward retroversion such as the hips, knees, and longi- limb structures, such as the hips, knees, throughout execution of the plié; and tudinal arches of the feet, has been and longitudinal arches of the feet, has for criterion 4, 13 dancers presented with described as an important predispos- been described as an important predis- medial misalignment of the knees at all ing factor in musculoskeletal injuries phases of the plié. Using these criteria, it posing factor in musculoskeletal injury among classical ballet dancers.9-12 among classical ballet dancers. However, was possible to characterize the plié from a kinematic point of view. To the best of our knowledge, no studies were found that analyzed basic there are no studies in the literature ballet movements with quantification that specifically analyze the plié and of objective criteria of the movements. hile audiences may appre- The purposes of this study were: 1. to ciate the beauty and grace identify criteria considered essential establish a methodology to quantify, us- of ballet, the art form is for its proper execution. What has been ing kinematic evaluation, the technical Wphysically demanding and potentially recorded in the literature is that such criteria that guide the correct execution 1 criteria could avoid hyperpronation injurious for ballet dancers. Numer- 7,8,10-16 of all phases of the plié (simultaneous ous studies have reported injuries as- or pronation of the foot, main- flexion of the hips, knees, and ankle 2-4 tain knee aligned with the ipsilateral sociated with the practice of ballet. 7-9,12,14-16 joints); and 2. to explore whether experi- The lower extremities are the most foot, and avoid the adoption of enced ballet dancers respect those criteria angular misalignments that may cause commonly injured region of the body 7,8,14-17 when performing the plié. The technical 5 damage to the pelvis. Neverthe- criteria considered were the following: 1. in dancers. Specifically, the demi-plié (simultaneous flexion of the hips, less, no objective measurements associ- midfoot stability; 2. pelvic positioning in ated with these criteria were found. a neutral alignment; 3. pelvic stability, knees, and ankle joints without lifting represented by pelvic angle variation; the heels off the ground), repeated Thus, the purposes of this study and 4. vertical alignment of the knee innumerable times during a class or were, 1. to establish a methodology to joint with the second toe of the ipsilateral performance, may lead to problems quantify, using kinematic evaluation, foot. Twenty dancers from Porto Alegre, in the spine, knees, ankles, and feet.6 the technical criteria that guide correct Brazil, with 18 years of uninterrupted The prevention of injury is prefer- execution of all phases of the plié; and ballet training, were filmed while per- able to the management of injury.5 In 2. to explore whether experienced bal- forming plié using four synchronized order to prevent injury, it is necessary let dancers respect those criteria when cameras. The descriptive statistical to identify the main contributing performing the plié. Regarding this analysis involved calculating the median, quantification, the goal would be to minimum, and maximum of each of the factors. In classical ballet, ideal technical execution of the movements is establish classifiable parameters of plié technical criteria. Results showed that for implementation for each of the techni- criterion 1, the 20 dancers showed great thought to be essential to the preven- stabilization of the midfoot; for criteria tion of injury.5,7,8 Lack of alignment cal criteria, allowing for more precise kinematic profiles of this technical step. Kaanda Nabilla Souza Gontijo, P.T., M.S., Cláudia Tarragô Candotti, Ph.D., Materials and Methods Grace dos Santos Feijó, Lais Paixão Ribeiro, and Jefferson Fagundes Loss, Ph.D., Participants Federal University of Rio Grande do Sul, Brazil. Twenty dancers from three ballet Correspondence: Jefferson Fagundes Loss, Ph.D., Felizardo 750, LAPEX Building/Room schools in the city of Porto Alegre, 222, 90690-200 Porto Alegre – Rio Grande do Sul – Brazil; [email protected]. Brazil, 27 ± 8 years of age, with 18

± 8 years of ballet training and a leg; 23., 24., and 25. reference frame. execution of the plié, each reflective weekly frequency of 4 ± 2 classes, The reference frame, which was used point was captured by at least two were recruited for participation in to define the global system coordinate, cameras. During the image acquisition the study. The exclusion criteria were consists of three wands set at a 90° the cameras were connected to each any musculoskeletal or sub-acute angle to each other. Each wand has other through a wireless network.20 injury within the prior 12 months one marker and represents one axis. Image reconstruction was accom- that interfered with lower extremity X was approximately positioned in plished by using the DLT method function and moving the right foot the lateral-lateral direction (taking (Direct Linear Transformation).21 sole on the ground during execution the dancer’s body as reference); Y was Positioning data were smoothed of the movements being tested. Par- positioned in the vertical direction; with a digital Butterworth filter, low- ticipants previously signed an autho- and the Z axis was approximately pass, fourth-order, with an average rized consent form, and the study was positioned in anterior-posterior di- cut-off frequency of 2.1 Hz from the approved by the Ethics Committee of rection (taking the dancer’s body as established residual criteria.22 These the Federal University of Rio Grande reference). After the placement of data provided the results linked to the do Sul (UFRGS). these markers, the dancers performed movement of the anatomical points individually, and the execution time marked on the dancer’s body and Data Collection was not controlled. The protocol was established midfoot stability values, Four video cameras were used (JVC performed in a specific location with pelvic stability and positioning, and GR-DVL 9800, JVC Company, boundaries marked on the ground, alignment between the knee and ip- Yokohama, Japan), along with a 3D establishing the position of the right silateral foot during the three phases calibrator (Peak Performance® - model foot during the entire collection. This of the step (extended knees, demi-plié, 5.3, Colorado, USA), 22 reflective foot remained in the same location as and grand plié). sphere-shaped markers 15 to 20 mm designated by these boundaries, while in diameter attached with double- the left foot was not tracked. Technical Criteria Description of sided tape to specific anatomical The assessment protocol consisted the Plié parts of the dancers’ bodies and 3 of performing the plié according to Criterion 1: “Midfoot Stability” technical markers. For scanning and the following sequence: two demi- The height variation of the marker 3D kinematic reconstruction of film- pliés followed by two grand pliés in positioned over the right navicular ing of the dancers executing a plié, a first position (external rotation of the bone in relation to the ground (in cen- special software was used (Dvideow, lower limbs, keeping heels together), timeters) was analyzed. The variation UNICAMP, Campinas, Brazil).18 followed immediately by the same of this height measure throughout the For analysis of metrics and angular movements in second position (ex- step phases indicates the stability or variations of anatomical landmarks, ternal rotation of the lower limbs, instability of the midfoot, assuming a mathematical software was used keeping heels apart), maintaining the three categories: 1. “excellent stabiliza- (MATLAB® 7.9, Natick, MA, USA). arms abducted on the shoulder line. In tion” (navicular height decrease, from The protocol for placement of the other words, the arms were held in the one stage of the step movement to reflective markers19 was conducted by classical ballet second position, and the next step, of less than 0.7 cm); 2. a team of four trained and experienced the foot movement solely and exclu- “stable” (decrease from 0.7 to 1.3 cm); evaluators. The anatomical points of sively involved the left foot, keeping and 3. “unstable” (decrease greater interest were chosen in order to evalu- the right one as still as possible (Fig. than 1.3 cm).23 ate exclusively the right lower limb: 1). Again, the execution time was not 1. greater trochanter; 2. posterior controlled. Criteria 2: “Pelvic Positioning” superior iliac spine; 3. right anterior From the drawn line connecting right superior iliac spine; 4. left anterior Kinematic Variables anterior superior iliac spine and right superior iliac spine; 5. pubic sym- The four cameras were arranged in a posterior superior iliac spine, the an- physis; 6. anterior tibial tuberosity; semi-circle in order to view the right gulations were analyzed in relation to 7. lateral femoral condyle; 8. medial side of the dancer. One camera was set a second line drawn in parallel to the femoral condyle; 9. second toe of the at a height of 2.0 m, with the optical ground, assuming three categories: 1. right foot; 10. navicular bone; 11. axis positioned posteriorly. The other “neutral” (pelvic angulations between medial region of the first metatarso- cameras were each set 0.5 m lower 12° and 15°); 2. “retroversion” (pelvic phalangeal joint; 12. medial region than the next, with the optical axis angulations less than 12°); 3. “antever- of the calcaneus bone; 13. medial forming an angle of approximately sion” (pelvic angulations greater than malleolus; 14. lateral malleolus; 15. 60° with the adjacent cameras, so that 15°).8,15,16,24-27 lateral region of the fifth metatarso- the fourth camera was positioned at a phalangeal joint; 16. lateral region of height of 0.5 m, with the optical axis Criteria 3: “Pelvic Stability” the calcaneus bone; 17., 18., and 19. positioned anteriorly in relation to the From the drawn line connecting technical marker of the thigh; 20., dancer. The test environment was ar- right anterior superior iliac spine and 21., and 22. technical marker of the ranged so that, throughout the entire right posterior superior iliac spine, 72 Volume 19, Number 2, 2015 • Journal of Dance Medicine & Science

KE D1 KE KE G1 KE

DD1 AD1 DG1 AG1

KE D2 KE KE G2 KE

DD2 AD2 DG2 AG2

Figure 1 Sequence of the plié didactically divided into three phases: with knees extended (KE); during the demi-plié (D1 and D2); and during the grand plié (G1 and G2). The eight stages of motion: during the descent (DD1, DD2, DG1, and DG2); during the ascent (AD1, AD2, AG1, and AG2). The numbers indicate the position of the feet in each phase: first position (1) – when the heels are together; and second position (2) – when the heels are apart. the angular variations of this line knee (metric variation from -1 to second repetition of each demi-plié were analyzed in relation to a second 1 cm of the distance between the and each grand plié, in both first and line drawn in parallel to the ground, center of the knee projection on the second positions. assuming two categories: 1. “pelvic ground and the ipsilateral foot refer- instability” (angular variation greater ence line); 2. “medial misalignment” Results than 3° from one stage of motion (metric variation less than -1 cm); The results of each of the four criteria to another); and 2. “stable pelvis” and 3. “lateral misalignment” (metric were grouped into two categories: (angular variation of maximum 3° variation greater than 1 cm).8,15,16,25,26 static situations—data corresponding from one stage of movement to an- to the start or end position of each other).8,15,16,24-27 Statistical treatment phase of the movement (Fig. 1 KE, Due to the non-parametric distribu- D1, D2, G1, and G2); and dynamic Criterion 4: “Alignment between tion of the data, the statistical analy- situations—data corresponding to the Knee and Ipsilateral Foot” sis was descriptive and comprised the transition period from the start to the The distance between the projec- calculations of median, minimum, end of each phase of the movement tions of the center of the knee was and maximum of each of the four (Fig 1. DD1, AD1, DD2, AD2, DG1, calculated (midpoint of the femoral technical criteria in order to quantify AG1, DG2, and AG2). condyles) on the ground to a rep- them during the realization of all resentative foot line (midpoint of the phases of a plié movement. As a Criterion 1: Midfoot Stability malleoli to second toe), assuming standard, the kinematic values ana- The navicular bone height reached in the following categories: 1. “aligned” lyzed in this study always refer to the relation to the ground in all static situ- Journal of Dance Medicine & Science • Volume 19, Number 2, 2015 73

Figure 2 Median, minimum, and maximum navicular bone Figure 3 Median, minimum, and maximum change of the na- height in relation to the ground in static situations of the plié, vicular bone height in relation to the floor in dynamic stages of either in first or second position (N = 20). Labels: KE – With the plié, both in first and second position (N = 20). Positive values knees extended; D1 – Demi-plié final in first position; G1 – represent reduction of the navicular height and negative values Grand plié final in first position; D2 – Demi-plié final in second represent increase of the navicular height. Labels: DD1 – Descent position; G2 – Grand plié final in second position. to demi-plié in first position; AD1 – Ascent from demi-plié in first position; DG1 – Descent to grand plié in first position; AG1 – Ascent from grand plié in first position; DD2 – Descent to demi-plié in second position; AD2 – Ascent from demi-plié in second position; DG2 – Descent to grand plié in second position; AG2 – Ascent from grand plié in second position. ations of the plié (Fig. 2) demonstrates the descent and ascent of grand plié in Criteria 2 and 3: Pelvic Stability the increased height expected during first position (DG1 and AG1, Fig. 3). and Positioning maximum knee flexion (G1), that is, In both situations, the navicular drop The position of the pelvis shows that in grand plié in first position. values are higher due to the proper when the knees were flexed (D1, G1, During dynamic situations, the ballet technical requirement that em- D2, and G2), the dancers presented, results show that none of the danc- phasizes the heel rising from the floor, on average, with the pelvis in ret- ers obtained navicular drop values rather than indicating instability of roversion (Fig. 4). When the knees greater than 1.3 cm, except during the midfoot. were extended (KE), on average, the

Figure 4 Median, minimum, and maximum pelvic tilt in static Figure 5 Median, minimum, and maximum change in pelvic phase of plié, both in first and second position (N = 18). Labels: angulations during dynamic phase of the plié, both in first and KE – With knees extended; D1 – Demi-plié final in first position; second position (N = 18). Labels: DD1 – Descent to demi-plié G1 – Grand plié final in first position; D2 – Demi-plié final in in first position; AD1 – Ascent from demi-plié in first position; second position; G2 – Grand plié final in second position. DG1 – Descent to grand plié in first position; AG1 – Ascent from grand plié in first position; DD2 – Descent to demi-plié in second position; AD2 – Ascent from demi-plié in second position; DG2 – Descent to grand plié in second position; AG2 – Ascent from grand plié in second position. 74 Volume 19, Number 2, 2015 • Journal of Dance Medicine & Science dancers presented with the pelvis in forming the ideal movement accord- the practice of classical ballet, stability neutral position. Regarding the pelvic ing to the classical ballet technique. of the midfoot or longitudinal arch of angle variation, the dancers had an the foot is maintained, many of the average pelvic instability during the Discussion common dance-related injuries can plié (Fig. 5). Criterion 1: Midfoot Stability be prevented. The dancers’ evaluations demon- Criterion 4: Alignment between strated great stability of the midfoot Criteria 2 and 3: Pelvic Stability the Knee and Ipsilateral Foot in all eight phases of plié movements and Positioning The results for alignment between the (Fig. 3). The literature highlights In general, when the dancers’ knees knee and the second toe of the ipsilat- the importance of keeping the foot were straight they kept their pelvis in eral foot demonstrate that when the continuously positioned with all its the neutral position (Fig. 4); however, knees were flexed (D1, G1, D2, and parts on the floor, maintaining the as soon as they performed the phases G2), on average, the dancers presented sustained arch, with the toes stretched of descent or ascent of plié, they with medial misalignment (Fig. 6). out and pressing down on the floor developed pelvic instability (Fig. 5). Only four dancers had the proper in the phases where heel elevation This instability was further reinforced knee alignment with the second toe, is not required.8,15,16,26 This posi- by the pelvic tilt displayed by the and four others were classified as tion prevents hallux strains, ensures dancers during demi-pliés and grand having lateral misalignment. Of the uniform body weight distribution pliés, that tended to move from the 20 dancers in the complete sample, between three points of support neutral position (ideal) to retroversion 7 were excluded from these results (the hallux, fifth toe, and heel),26,28 (a detrimental position for the lumbar because they moved their right foot and avoids foot pronation, which spine, for example). on the ground during data collection. can lead to potential strain of the The pelvis is one of the most impor- The distance variation between the medial collateral ligament (MCL) tant structures in the human body34; center of the knee projection and the and other injuries associated with the it equilibrates the spine, providing it foot line represented during dynamic knees, hips, and spine.29-32 When the with the stability necessary to support situations reinforces the results ob- dancer erroneously performs a foot the body and facilitates the transition tained in static situations, confirm- hyper-abduction and consequently of forces between the spine and the ing that there is on average a medial a collapse of the longitudinal arch lower extremities and vice versa. Thus, misalignment between the knee and occurs, damaging shear forces in the changes in pelvis positioning directly ipsilateral foot during execution of patellofemoral joint, femorotibial fric- affect the spinal alignment,35-41 and the plié (Fig. 7). Only one dancer tion, and detrimental foot pronation improper alignment can cause postur- achieved the value of -1.1 cm, per- take place.33 We believe that if, during al changes that increase the potential

Figure 6 Median, minimum, and maximum distance between Figure 7 Median, minimum, and maximum change in distance the projection of the center of the knee on the ground and the between the projection of the center of the knee on the ground foot reference line in static phase of the plié, both in first and and the foot reference line in dynamic phase of the plié, both in second position (N = 13). Aligned knees are represented by values first and second position (N = 13). Labels: DD1 – Descent to between -1 and 1 cm. Medial misalignments are represented by demi-plié in first position; AD1 – Ascent from demi-plié in first less than -1 cm values and lateral misalignments are represented position; DG1 – Descent to grand plié in first position; AG1 – by greater than 1 cm values. Labels: KE – With knees extended; Ascent from grand plié in first position; DD2 – Descent to demi- D1- Demi-plié final in first position; G1 – Grand plié final in first plié in second position; AD2 – Ascent from demi-plié in second position; D2 – Demi-plié final in second position; G2 – Grand position; DG2 – Descent to grand plié in second position; AG2 plié final in second position. Ascent from grand plié in second position. Journal of Dance Medicine & Science • Volume 19, Number 2, 2015 75 for lesions.10,42-47 Kendall and cowork- but this is strictly conjecture. Our re- Injury incidence in hip hop dance. ers48 also emphasized that the pelvis sults simply do not provide an answer Scand J Med Sci Sports. 2012 is the key to an overall favorable or to this question. Jun;22(3):347-55. defective postural alignment. Hence, The study had several limitations: 5. Russel JA. 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