Effect of Toe Type on Static Balance in Ballet Dancers

Article Effect of Toe Type on Static Balance in Ballet Dancers Momoko Kizawa, MD; Toshito Yasuda, MD, PhD; Hiroaki Shima, MD, PhD; Katsunori Mori, MD; Seiya Tsujinaka, MD; and Masashi Neo, MD, PhD OBJECTIVES: Some forefoot shapes are ideal for pointe Most ballet movements require two specific relevé posi- work in ballet. Egyptian-type, with the hallux being tions of the feet, i.e., demi-pointe and en-pointe. In both posi- longest and the remaining toes decreasing in size, and tions, dancers are required to stand on their forefeet. The Greek-type, with the second toe longer than the hallux, metatarsophalangeal (MTP) joints of the first to fifth toes are considered less optimal for pointe work. Square-type, with the second toe the same length as the hallux, is con- are extended for demi-pointe (Fig. 1a) and minimally flexed sidered optimal. This study compared postural stability in for en-pointe (Fig. 1b). In demi-pointe, positioning of the feet the bipedal stance, demi pointe, and en pointe between on the metatarsals and phalanxes raises the center of grav- ballet dancers with the two toe types using a stabilome- ity and displaces the weight of the body forward on the METHODS: ter. This study included 25 Japanese ballet transversal arch,[3,4] so that the body weight rests on the academy dancers who had received ballet lessons for at [5] least 6 years. Toes were categorized into Egyptian-type metatarsals and toes. (n=14) and square-type (n=11). Bipedal stance, demi During en-pointe, dancers wear pointe shoes. The out- pointe, and en pointe were tested. Center of pressure side of pointe shoes surrounding the forefoot is called the (COP) parameters were calculated from ground-reaction toe box, which is made from tightly packed layers of paper forces using two force plates: total trajectory length and fabric glued together. The sole is constructed from (LNG), velocities of anterior-posterior (VAP) and medial- lateral directions (V ), and maximum range displace- leather, and the shank is comprised of leather, plastic, ML cardstock, or layers of glue-hardened burlap. The fabric ment in the anterior-posterior (MAXAP) and medial-lateral directions (MAXML). Mann-Whitney U-tests were used to ribbons and elastic bands secure the pointe shoes to the examine differences in COP parameters. RESULTS: There foot.[6] Inside a pointe shoe, the toes are squeezed together, were no differences in parameters during bipedal stance and the forefoot is supported by the interior sides of the or demi pointe. However, dancers with Egyptian-type toes had significantly greater LNG (p<0.01), V (p=0.01), toe box. Not all ground reaction forces transmit through ML the tips of the toes, but there is support from the forefoot MAXML (p<0.01), and MAXAP (p=0.03) during en pointe. CONCLUSIONS: Ballet dancers with Egyptian-type toes system as a whole. The phalanxes are placed almost verti- demonstrated greater displacement in the medial-lateral cally in continuation with the metatarsals, which reduces and anterior-posterior directions during en pointe. Ballet the area of floor support to the first three toes.[7] dancers should be aware of toe types and sway character to optimize ballet training and balance. Med Probl Per- In past literature examining pointe work from the point form Art 2020;35(1):35–41. of view of toe length, the ideal foot for dancing en pointe has been described as a square foot in which the hallux DANCERS are required to achieve a high level of balance and second toe are of the same length, because it more control.[1] Some reports suggest that ballet dancers have evenly distributes peak pressure between the tips of the better postural stability than other athletes and healthy toes and the interior part of the toe platform.[8–11] In con- controls.[2] trast, the Egyptian-type foot, where each of the lateral four toes is shorter than its medially adjacent neighbor, is con- From the Department of Orthopedic Surgery, Osaka Med- sidered less optimal for en-pointe because all floor contact ical College, Takatsuki, Osaka, Japan. occurs through the tip of the hallux and the load concentration becomes too large and thus painful.[8] The Greek- The authors declare no funding or conflicts of interest type foot, in which the second toe is longer than the related to this study. hallux, is also considered less optimal because it is not bio- Address for correspondence: Dr. Momoko Kizawa, Dep. of mechanically ideal for the pointe position.[9.10] Orthopedic Surgery, Osaka Medical College, 2-7 Daigaku- Postural stability is a whole body phenomenon. It is the machi, Takatsuki, Osaka, 569-8686, Japan. Tel +81-72-683- ability to maintain the center of gravity when the center of 1221, fax +81-72-683-6265. [email protected]. mass is horizontally within the base of support.[12] In static https://doi.org/10.21091/mppa.2020.1005 balance, the base of support remains fixed while the center © 2020 by the Author(s). Open Access: Licensed under CC of gravity moves. In this case, the sense of balance main- BY-NC-ND 4.0 Int. tains the center of gravity within the base of support March 2020 35 A B length, due to concentration of notably high pressure on the tips and the hallux. METHODS Participants A total of 63 Japanese ballet academy students admitted between April 2012 and April 2015 to the ballet academy at Kyoto were enrolled in this cross-sectional study. All ballet dancers answered a questionnaire about their previous injuries and ballet training experience. Seventeen dancers who reported pain at the time of responding to the questionnaire or who had experienced injuries of the lower extremities in the past were excluded (Fig. 2). All participants’ footprints were obtained from plantar images in the bipedal stance (BS) with the Foot Look (FOOTLOOK Inc., Fukuoka, Japan), which is a device for plantar image measurement that has a scanner capable of [21] FIGURE 1. A, In demi pointe, the metatarsophalangeal high-speed scanning of the plantar surface of the foot. (MTP) joints of the first to fifth toes are extended. B, In en Footprints were measured and evaluated, and 9 dancers pointe, the MTP joints of the first to fifth toes are flexed with hallux valgus angle >15°,[21] 5 flat-foot dancers with a minimally. Staheli Arch Index (SAI) >1.0, 5 cavus-foot dancers with a SAI of 0,[22] and 2 dancers with Greek-type foot were excluded. Finally, 25 participants who had ballet training defined by the feet, whereas in dynamic balance, both for at least 6 years were included in this study (Fig. 2). center of gravity and base of support are in constant in Based on Beighton score criteria of 4/9 or higher, no motion, and the center of gravity are never aligns itself to dancers were considered to be hypermobile.[23,24] All partic- the base of support during the stance phase of the move- ipants provided informed consent, and this study was ment.[13] These proprioceptive tasks of static (e.g., single or approved by our Ethics Committee of Osaka Medical Col- bipedal relevé) and dynamic balance and turning (e.g., lege (approval no. 1816). pirouette) are required in functional dance activities and improve the dancer’s proprioception and perform- Measurements ance.[14,15] Professional ballet dancers especially have more accurate proprioceptive feedback about lower limb posi- Measurements of differences between the end of the hallux tion in space as well as center of gravity in relation to base and second toe were taken from the footprints positioned of support.[16] so that the second toe and calcaneum were in a straight Multisensory information and motor mechanisms are involved in maintaining balance. It can be studied by recording the movement of the center of pressure (COP).[2] Consequently, the byproduct of these subsystems working together is expressed by the moving COP. The foot plays a role not only in supporting the body and contacting the ground surface, but it also has a large number of sensitive receptors on the sole that are directly involved in the man- agement of posture and movement.[14] Several studies have focused on the plantar receptors of the somatosensory system and balance and postural control.[12,17–20] To the best of our knowledge, the relationship between postural stability of ballet dancers and their toe types has not been quantitatively investigated. Therefore, this study aimed to investigate the postural stability of ballet dancers according to the dancers’ toe length using a stabilometer. Our hypothesis was that the Egyptian-type foot, in which each of the lateral four toes is shorter than its medially adjacent neighbor, would demonstrate larger moving area of the COP on static balance compared to the square-type foot, where the hallux and second toe are of the same FIGURE 2. Study enrollment subject accountability tree. 36 Medical Problems of Performing Artists line (Fig. 3). Line a connected the two points (the posterior A B C edge of the calcaneus and anterior edge of the second toe). The distance between the two intersecting lines (b, c) drawn perpendicularly from each ends of the hallux (line b) and the second toe (line c) to the first line (line a) was measured[25] (Fig. 3). In this study, the types of toes were classified into three groups based on whether they had a shape ideal for pointe work according to previous research.[8–11] We defined Egyptian-type foot when the hallux is longest and the distance between the two lines of the hallux (line b) and second toe (line c) is >2 mm; Greek-type when the second toe is >2 mm longer than the hallux; and square-type when the distance between the two lines of the hallux (line b) and second toe (line c) is 2 mm or less.

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