Authors: Jessica Aquino, MS and Tal Amasay, PhD
Corresponding Author:
Tal Amasay, PhD, CSCS, EPC
11300 NE 2nd Ave
Miami Shores, FL 33161
[email protected]
305-899-4893
Jessica Aquino has a master degree in movement sciences, is a certified athletic trainer, and work with professional Ballet dancers. Tal Amasay is associate professor at Barry University and the head of the Motion Analysis Center. He is a certified exercise physiologist and certified strength and conditioning coach
Biomechanical Comparison of “Dead” and “New” Pointe Shoe in Female Professional Ballet Dancers
ABSTRACT
Purpose: Available research on pointe shoes often compare pointe shoes to other dance footwear, however there is a lack of studies comparing dancers’ biomechanics when using “new” pointe shoes and pointe shoes that have worn down, “dead”. The aim of this study was to examine the biomechanical differences exhibited by professional ballet dancers while performing relevé, sous-sus, and pirouette in “dead” and “new” pointe shoes. Methods: Thirteen female (20.9 ± 1.9 years old) professional ballet dancers were asked to perform three trials of relevé, sous-sus, and pirouette in “new” pointe shoes and “dead” pointe shoes. Center of pressure sway area and ground reaction forces in the anterior-posterior, medial-lateral, and vertical directions were recorded using one AMTI force plate. Results: The “dead” pointe shoe condition had significantly higher sway area during relevé, sous-sus, and single pirouette (103 ± 95 mm2; 256 ± 133 mm2; 178 ±129 mm2, respectively) than the “new” pointe shoe condition (50 ± 65 mm2; 110 ± 64 mm2; 77 ± 39 mm2, respectively), p-value < 0.05. In addition, peak ground reaction force in the anterior-posterior direction during relevé movement was higher in “new” pointe shoes (35 ± 6% body weight) than in “dead” pointe shoes (32 ± 6% body weight), p-value = 0.019. No significant differences were observed for the other dependent variables. Conclusions: A larger sway area was observed in “dead” pointe shoes compared to “new” pointe shoes, which indicates that the participant had less balance in the “dead” pointe shoes than in the “new” pointe shoes. The larger sway area in “dead” pointe shoes may indicate that decrease in shoe integrity may contribute to decrease in the support and stability of the pointe shoe while the ballet dancer maintains the ‘en pointe’ position. Moreover, higher forces in the anterior direction state that the dancers can apply more forces in the “new” point shoe. Application in sport: These results can educate ballet teachers and ballet dancers about the mechanical changes in “dead” pointe shoes, which may lead to injury and reduce in performance. Furthermore, these results may lead to enhancements in footwear design.
Keywords: Balance, Ground Reaction Forces, Ballet, Pointe Shoe
INTRODUCTION
The field of sports medicine and performing arts medicine continues to expand and recognize dancers as athletes and aesthetic performers who have their own set of difficulties and common injury trends similar to other traditional sports. Ballet dancers, both male and female, extensively train daily throughout their careers. Professional dancers train and perform in excess of 60 hours a week. Pre-professional dancers train and perform about 30 hours a week. Dancers that are still in primary or secondary dance schooling train and perform less than 30 hours a week (15). The prevalence of injuries in youth, amateur, and professional dancers was reported as 0.97 injuries per 1000 dance hours for males, and 1.24 injuries per 1000 dance hours for females (14). Gamboa, Roberts, Maring, & Fergus (4), reported a five year rate of injury of 0.77 injuries per 1000 dance hours (4). Smith et al. (14) and Steinberg et al. (15), identified chronic/overuse injuries as the most common injuries, where amateur ballet dancers showed a higher proportion of overuse injuries than professionals (14, 15). Male professional dancers showed a higher proportion of traumatic injuries than overuse injuries, when compared to female professional dancers (4, 14). Current research in dance and sports medicine points to the prevalence of lower extremity injuries in ballet dancers; however, research lacks evidence that identifies the possible underlying causes of these injuries (4, 5, 7, 13-15). It has been identified that up to 85% of dance-related injuries are in the lower extremities (4, 5, 8, 14, 15). The most common injuries in dancers include lateral ankle sprains, Achilles’ tendinopathy, Lisfranc injuries, flexor hallicus longus tendinopathies, anterior cruciate ligament injuries, meniscal injuries in the knee, snapping hip, labral tears in the hip joint, and low back pain (4, 5, 7, 8, 13-15).
Dancing en pointe means the ballet dancer is dancing on her toes. Dancing in pointe shoes creates a magical illusion of the dancer floating while dancing. To meet the choreographic and aesthetic demands pointe shoes were developed. As pointe shoes gain more wear and tear, the normal biomechanics and ankle stability of professional ballet dancers performing simple ballet combinations may be negatively affected, possibly causing the ballet dancers to compensate and leading to a higher risk of acute and chronic lower extremity injuries, especially ankle injuries. Few studies exist that analyze the effects of different types of dance shoes on dancers (8, 13). Studies that focus on footwear in dance examine differences in types of shoes as well as differences in shapes of shoes (2, 16). Cox and Herzog (2) concluded that there are no significant differences in the proprioception of novice ballet dancers when balancing on pointe shoes with different toe box shapes (2). Walter et al. (16) found significantly larger forces on the lower extremities when landing in flat ballet technique shoes versus landing in pointe shoes, speculating that the increase in materials around the forefoot and toe box could account for the shock absorption noted in the study (16).
There is a lack of research which examines the effects of “wear and tear” or loss of shoe integrity on the biomechanics of ballet dancers. Training in “dead” pointe shoes (pointe shoes that are too old) may be similar to a track athlete or runner who trains in old athletic footwear for too long. In general, most experts will suggest to runners to change their running shoes every 300 to 400 miles (6, 10, 12). It is well established in running research that when the shoe support and cushioning decreases, runners modify their patterns to maintain comfortable and constant loading (6). The same can be assumed for all athletes and their specific footwear, specifically for ballet dancers when their pointe shoes begin to soften. Unlike running shoes, which provide both shock absorption and support, ballet pointe shoes have little support or shock absorption (7, 13). Primarily, they are designed to maintain the foot in the en pointe position, or fully plantarflexed (7, 13).
Despite the understanding that better cushioned shoes may help reduce ground reaction forces when landing and provide shock absorption to reduce the chance of lower extremity pathology, dancers are unlikely to change or modify the traditional footwear. The reason is associated to the specific “feel” of the floor by their feet which is essential to their success in performance and dancing (13).The integrity and strength of the shoe wear down with use, possibly causing the ballerina to compensate while training or performing, similar to runners who compensate due to improper footwear (10, 12). Improper technique has also been identified as a concern and risk for injuries in dancers, leading to overuse and acute injuries (7, 13). Ballet and dance are complex art forms that encompass many differing movements, techniques, levels, and genres. In this study, researchers examined footwear as one of the possible causes for overuse injuries in professional female ballet dancers. The purpose of this study was to examine ground reaction forces and center of pressure differences exhibited by female professional ballet dancers while performing sous-sus, relevé, and a single pirouette in “dead” and “new” pointe shoes.
METHODS
Participants
All participants recruited for the study were female, professional level ballet dancers currently hired by a professional ballet company. Thirteen professionally employed ballet dancers volunteered to participate in the study (Table 1). All participants gave informed consent prior to data collection and the study was approved by the university’s Institutional Review Board (IRB) committee. All ballet dancers had over 10 years of experience and training on pointe shoes. Inclusion criteria required participants to be injury free for at least three months prior to data collection, and were presently not experiencing any limiting pain that may hinder their performance. Each dancer brought two sets of their own pointe shoes; “dead” and “new”. Individual participants were allowed to determine which pointe shoe pair was considered “dead” to ensure levels of comfort when participating in the study. The manufacturers of the pointe shoes were not regulated between participants. Participants used pointe shoes that have already been trained in for at least 1 rehearsal or class but for no more than 3 months of rehearsals and classes.
Procedures
All data were collected using an AMTI Force Plate (Watertown, MA) at a sampling rate of 960Hz. Participants were asked to arrive at the lab for a one-hour session at their assigned time. Upon arrival, participants filled out the pre-session questionnaire and signed a consent form approved by the university IRB committee. Participants were asked to wear common dance attire, which included tights that covered their toes, leotards, and a skirt, which was optional. Participants were asked to bring their own pointe shoes, a pair that is relatively “new” and a pair that is relatively “dead”, or old. Pointe shoes were not used if they were brand-new and straight out of the box, as the dancer has not broken them in which could cause injury and alter their biomechanics. “New” pointe shoes were defined in this study as the pointe shoes that the ballet dancers were currently training in. The shoes were not brand new, out of the box, unbroken in, or without having been trained in. “Dead” pointe shoes were defined in this study as pointe shoes that have been worn for many training hours or performances which cause them to wear down, at the shank and toe box specifically. The sole and toe box were examined by the researcher to make sure that both conditions were still safe for the dancers to use for this study to avoid potential injuries. Because there is still no hard definition for a “dead and “new” pointe shoes the participants subjectively determined if a pointe shoe was “dead” or “new”. Based on this “new” pointe shoes were within the range of 5 to 50 training hours and ”dead” pointe shoes were within the range of 51 to 400 training hours (Table 2).
Participants’ anthropometrics were taken before beginning the trial sequences. Sequence of pointe shoe conditions and ballet movement were randomized between participants. The participants were allowed to warm up as they would for class as well as practice before collecting the trials to ensure they are comfortable with the space, equipment, and flooring. The participants were asked to perform at least two proper trials of the following ballet moves: sous-sus, relevé in second, and a single pirouette. Participants were allowed to rest at least 30 seconds between trials and movements to ensure that fatigue is not a factor. If, for any reason, the dancer did not complete the movement with control or fell out of the data collecting volume, the participant was asked to repeat the trial.
During the sous-sus trials, the participant was asked to begin in fifth position with her right foot in front. The participant was instructed to sous-sus, in which she springs up from flat to en pointe in fifth position. Then the participant was instructed to maintain the en pointe position for at least a second and then come down into flat fifth again. During the relevé trials, the ballerina was instructed to start in second position with the left foot on the force plate. She was instructed to demi-plié first, to rise into relevé, maintain the en pointe position for at least a second and then come down into flat second position through demi-plié. During the single pirouette trials, the participant was asked to start in fourth position with her turning leg on the force plate. She was asked to perform a single pirouette and land back in fourth position.
Data Analysis
Data collected were processed using Nexus 2.2.3 (Vicon, Oxford, UK), and analyzed using Polygon 3.5.2 (Vicon, Oxford , UK) as well as Excel 2010 (Microsoft, WA, Redmond, USA). Data were filtered with a Butterworth Trajectory filter with a cutoff frequency of 20 Hz before the analysis. This study was a repeated measures design. Each of the participants completed each movement in both conditions, “new” pointe shoes and “dead” pointe shoes, and performed the three movements: sous-sus, relevé, and pirouette. The dependent variables analyzed were the sway area and peak ground reaction forces in all directions. The quiet stance of the movements was identified by examining the vertical ground reaction forces during the middle one second of the trial. Ground reaction forces in all directions were normalized to body weight for each participant. Sway area was estimated by an area of an ellipse. To find this, the ranges of the center of pressure (COP) in both anteroposterior and mediolateral directions were calculated then divided by two to find the radius in each direction (9).
Results
Using SPSS Statistics 19 Software (IBM, Armonk, NY), a two-way repeated measures MANOVA was used to statistically analyze the average of two trials of each movement in each shoe condition for each of the participants. Preliminary multivariate testing showed no significant interaction between condition and movement (F(14,78) = 0.784, p > 0.05); however, there were significant main effects between shoe conditions (F(7,78) = 3.615, p = 0.02). A Bonferroni post hoc test was performed to find significant differences between movements within each dependent variable. The sway area of oscillation was significantly higher in the “dead” shoe condition (Fig. 1) as compared to the “new” shoe condition (Fig. 2) in all three movements (p < 0.05). The peak force in the anterior-posterior direction was significantly higher in the “new” shoe condition as compared to the “dead” shoe condition during Relevé à la seconde condition (p < 0.05) (Table 3). There were no significant differences between shoe conditions for any of the other dependent variables (p>0.05) (Table 3).
DISCUSSION
Statistically significant differences during all three movements occurred between the shoe conditions, “new” and “dead”, when measuring the sway area of oscillation during quiet stance. Higher sway areas were observed in the “dead” pointe shoe condition than in the “new” pointe shoe condition. The mean sway area in “dead” pointe shoes during relevé, sous-sus, and pirouette were higher by 53 mm2, 146 mm2, and 101 mm2, respectively. Having a significantly larger area of sway in “dead” pointe shoes suggests that the “dead” pointe shoe condition may lack support and contribute to less stability in the foot and ankle joint for ballet dancers when up on their toes. As the muscles of the foot activate to correct balance during quiet stance (en pointe phase), a significantly smaller area generally represents greater proprioception, keeping the center of pressure under better control (2). The larger sway area in “dead” pointe shoes can indicate a lack of support and stability from the shoe to help the ballerina maintain her ‘en pointe’ position.
A shift of center of pressure is an accepted, indirect measure of postural sway and also a measure of a participant’s ability to maintain balance during tasks (3, 9, 11). Previous research has linked changes in postural sway with ankle physiology and possible injury risks at all lower body joints, not just the ankle (1, 3, 11). For ballet dancers, an increased sway area indicates a higher risk of instability and possibly a higher demand for compensation techniques in order to maintain balance en pointe. Compensation can lead to injuries, commonly overuse, in the foot and ankle joints. If the stability provided by the pointe shoe begins to fail or decrease, the ballerina may compensate in order to maintain the en pointe position. If she cannot compensate to hold the position she may fall out of the position, which may lead to a traumatic injury or the inability to complete the task or movement on hand.
Significant difference was observed in the anterior-posterior direction ground reaction force during relevé; “new” shoes had higher peak force than “dead” shoe. Although not statistically significant, there were noticeable differences in ground reaction forces in the vertical direction and shoe conditions. The average ground reaction forces were larger in the “new” pointe shoe condition versus the “dead” pointe shoe condition. The averaged difference in peak ground reaction forces in the vertical direction during relevé, sous-sus, and pirouette were 1% BW, 5% BW, and 9% BW, respectively. The “new” condition of the pointe shoe is stiffer, so the ballet dancers have the need to push harder to overcome the friction on the floor as well as more trust that the shoe will support them during the movements. The “dead” shoes may be less supportive to handle quick and forceful movements during a full performance weekend, and may influence the dancer’s decision making. However, the lack of knowledge about the time a pointe shoe becomes “dead” may expose the ballerina to higher risk for injury.
The manufacturers of each pointe shoe used in this study were not controlled for due to the differences in how the shoes are made, including the width of the toe box and the length of the shank. The participants were also the ones who determined whether the shoe was “new” or “dead” according to their personal standards and levels of comfort. This could influence the results of the study; however, the comparison was made within participants. The lower number of participants may have influence the level of statistical significance; however, power analysis in this study produced a power of 0.998. Moreover, the the use of professional dancers can improve the understanding researchers can have about the exposure these types of performers have during training and performing.
CONCLUSIONS
This study identifies a possible contributing factor to overuse injuries and performance inhibition in professional female dancers, which had not been formally researched previously. The study suggests that the condition and integrity of pointe shoes worn by professional ballet dancers may contribute to overuse and acute injuries as well as decrease in performance. This related to the higher sway area while performing with “dead” pointe shoes versus “new” pointe shoes. Moreover, it is important to identify at what point a “dead” pointe shoe becomes dangerous for the ballet dancers to train and perform in, as this study has identified higher forces exerted while using the “new” pointe shoe.
APPLICATIONS IN SPORT
These results can be used to further research on the pointe shoes of ballet dancers as well as lead to enhancements in ballet footwear design and development. The identification of the criterion at which the pointe shoe becomes “dead” is important. This may assist in minimizing the prevalence of injuries among ballet dancers, as a result of decrease in pointe shoe integrity. Moreover, the ability to correctly identify “dead” pointe shoes may help ballet dancers to keep their performing level at its best. Professional practitioners should inquire their ballet dancer clients as to the condition of their pointe shoes, just as they would a runner and their sneakers.
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