Ground reaction forces in ballet dancers landing in flat shoes versus pointe shoes.
Most dance studios have sprung wooden flooring, often covered with linoleum or Marley vinyl. A sprung floor is needed because it is resilient (5); the more resilient the floor, the more force it can absorb. (5) Some dance movements, most obviously landings from jumps, can expose the lower extremities to loads up to 14 times body weight. (7) These forces need to be dissipated to reduce the risk of injury. Thus, it is important that dancers perform on resilient floors.
Incorrect technique can also contribute to greater ground reaction force. Every day dancers participate in a conditioning program that includes elements of strength and flexibility training, muscular and cardiovascular endurance, neuromuscular coordination, and balance.
(8) The repetition of this program, when done incorrectly, can contribute to injuries. Dancers must always be aware of their bodies in space but especially so when jumping and landing. Much attention is generally given to the technique required to go into the air, but the landing is often overlooked. Three phases of landing should occur when a dancer completes a jump correctly. (9) In the first phase, the toes make initial contact with the ground. During the second phase, the ball of the foot contacts the ground and the foot begins to descend to a flat position. In the final phase, the heel has to contact the ground. A controlled landing occurs when the dancer completes all phases correctly and the heel stays in contact with the ground. (9) The dancer who does not understand correct technique for landing jumps is likely to suffer the effects of higher ground reaction forces.
An additional factor, type of instruction, may also affect ground reaction force. McNair and colleagues (6) demonstrated that when subjects were given instructions on how correctly to position their limbs during jumps, ground reaction force was reduced. Also, when the subjects were instructed to decrease the "sounds" they made on impact, ground reaction force was diminished. (6) This shows that when dancers concentrate on a given task they can affect maximal ground reaction force, either positively or negatively.
The last factor that may affect ground reaction force is type of footwear. To date this factor has received minimal attention. Traditionally, ballet dancers use two different shoes. The first is a flat technique shoe that is made out of canvas or soft leather and has no padding or arch support of any kind. The second is the classical pointe shoe, made of corset satin on the outside. The toe box of the pointe shoe consists of layers of burlap, cardboard, paper, or a combination of these materials. There is a shank in the sole of the shoe that is made of cardboard, leather, or a combination of the two. (9) Pointe shoe construction has improved dramatically over the years. It was not until the 1950s that the modern shoe was constructed. (10) When the dancer acquires new shoes, she will usually customize them to her own foot by hitting them with a hammer, slamming them in a door, or steaming them over boiling water. (10) This act of fitting the shoe to one's own foot quickly breaks down the integrity of the shoe. Many times pointe shoes last only one performance, despite the layers of leather and fabric. Dancers wear pointe shoes to achieve a variety of aesthetic and practical advantages. (10)
As increased ground reaction force may lead to increased injury rates in dancers, investigating the effect of different types of ballet shoes on these forces is a potentially valuable step in determining how injuries may be prevented. Therefore, the purpose of this study was to determine whether the type of dance shoe used affects maximal ground reaction force ([GRF.sub.max]) during a basic ballet jump.
Eighteen healthy female dancers enrolled in a rigorous bachelor of science degree program in ballet performance (19.94 [+ or -] 1.16 years of age, 169.12 [+ or -] 6.40 cm in height, 55.44 [+ or -] 5 .40 kg in weight) volunteered to participate in this study. All participants had similar activity levels (22.97 [+ or -] 8.41 hours of ballet training a week) and years of experience (14.17 [+ or -] 2.92 years). Prior to participating, subjects read and signed an informed consent form approved by the University Institutional Review Board for Protection of Human Subjects, which also approved the study.
Subjects were tested at the Athletic Training Research Laboratory on one occasion for approximately 30 minutes. They performed an assemble (Fig. 1) in both flat shoes (Fig. 2) and pointe shoes (Fig. 3). The shoe order was randomized, with half the dancers completing the trials in the flat shoe first and half in the pointe shoe first. Each dancer was allowed to practice the jump until she was comfortable with it.
To standardize test procedures all dancers started at the same place on a runway and landed on an X in the center of a force plate (AMTI Accugait System Model ACG, Watertown, Massachusetts). They were instructed to use maximal effort (amplitude) during the test trials. To evaluate this, jump height was assessed during each jump. All dancers wore an elastic band around their waist and performance was recorded with a standard video camera (Panasonic PV-GS500). Using a postural grid the resulting video footage was analyzed for jump height.
[FIGURE 1 OMITTED]
To perform the assemble, one foot goes into the air as the dancer pushes off the floor with the supporting leg and extends the toes. Both legs then land simultaneously in fifth position--i.e., with the feet completely crossed so that the heel of the front foot touches the toe of the back foot, and vice versa. The dancers were instructed to land in a proper fifth position, hold for two seconds, and then step off the platform. The trial was repeated if deemed unacceptable. An unacceptable trial was defined as one in which the dancer did not land in fifth position, lost her balance, landed with a double heel strike, or did not give maximal effort. Six acceptable trials were completed in each shoe condition. [GRF.sub.max] data were collected at the point when dancers landed from the jump.
[GRF.sub.max] (Newtons) and maximal jump height (centimeters) were assessed for each jump trial. The mean [GRF.sub.max] and mean vertical jump height for each shoe condition was used for statistical analysis. Two dependent t-tests were conducted to determine differences between the shoe types, one for [GRF.sub.max] and one for jump height. Alpha level was set at p < .05.
Means and standard deviations for each condition are displayed in Table 1. We found a significant difference in [GRF.sub.max] between the two shoe conditions (t = 3.53, p = .003). Specifically, the [GRF.sub.max] was higher for landings in flat shoes (1742.9 [+ or -] 252.6 N) than in pointe shoes (1612.7 [+ or -] 261.5 N). There was no significant difference in jump height between the two shoe conditions (t = 1.22, p = .24, Effect size = .08, Power = .21).
[FIGURE 2 OMITTED]
The primary focus of this investigation was to determine if a [GRF.sub.max] difference existed in ballet dancers landing an assemble jump in flat shoes versus pointe shoes. We were interested in [GRF.sub.max] because it may be linked to an increase in injury rates. (11,12) As previously stated, several factors may affect [GRF.sub.max], including landing surface, improper technique, incorrect landing, quality of instruction, and footwear. Minimal attention has been given to researching the footwear used in ballet dancing.
We found a significant increase in [GRF.sub.max] when dancers wore flat shoes. For this particular task, jump height may also affect [GRF.sub.max]. Therefore, we also evaluated whether there was a difference in jump height in the two shoe types. No significant difference was observed in jump height between the flat and pointe shoe trials. This finding indicates that any difference in [GRF.sub.max] is primarily due to the type of shoe used, or more precisely the mechanics involved when landing jumps in the different types of shoes.
[FIGURE 3 OMITTED]
We believe increased [GRF.sub.max] in the flat shoe could have occurred for several reasons. First, the pointe shoe may have absorbed more forces because of the layers of material that make up its sole. Conversely, the lack of any type of padding in the flat shoe allowed most of the forces to be absorbed by the body. Miller and associates (13) attempted to modify the sole of a flat shoe by using PPT[R], Spenco, and Sorbothane materials, all of which are typically used in making orthotics. The materials were applied in different ways to produce 11 modifications of a flat shoe. Each modification allowed plantar pressure to be distributed more evenly across the foot; thus, it was found that adding light layers to a typical flat shoe helped decrease the [GRF.sub.max]. This finding leads us to believe that extra material in the shoes does indeed help to absorb some of the ground reaction forces. However, while the results of Miller and coworkers (13) were interesting, it is unlikely that many dancers would be willing to pad their flat shoes. Dancers are concerned with being able to "feel the floor" and do not want anything extra in their shoes. Therefore, this laboratory finding may not successfully translate to actual dance practice.
Another factor that possibly decreased [GRF.sub.max] is the type of toe padding used in our subjects' pointe shoes. Each dancer has a preference for materials to use in covering her toes to prevent blisters, ranging from paper towels, to lambswool, to gel-filled sleeves. This material may well have helped to disperse the forces more evenly when compared to the flat shoes, but it is a variable that is extremely difficult to control for.
Yet another hypothesized reason for the decreased [GRF.sub.max] in the pointe shoes was related to dance technique. As the dancers in this study were quite experienced, they landed correctly. A correct landing means that the three phases of landing were achieved, with the tip of the pointe shoe touching first and the rest of the foot following sequentially. This technique of rolling through the foot allows the pointe shoe to absorb more of the force and the dancer's lower extremity less. In flat shoes, on the other hand, the foot contacts the floor in demi-pointe, and this initial impact on the metatarsal heads may increase [GRF.sub.max]. Chockley (14) looked at the time required to progress through the three phases of landing, and how long the dancer was in each of these positions. She found that in flat shoes initial toe contact had a mean duration of 0.02 seconds, the ball of the foot 0.037 seconds, and the heel contacted the ground for 0.053 seconds. (14) It would be interesting to compare these times when the same jump is landed in flat shoes and pointe shoes. Also, these data would in all likelihood change dramatically with the dancer's experience level. The experienced dancer would be expected to control her landing much better than the inexperienced one.
It is important to note that in this study [GRF.sub.max] force was measured only during one ballet jump. This is, of course, just one component of a ballet class or performance. The stresses imposed on the body by pointe work in general is a different issue that needs to be investigated further. It is possible that pointe shoes might benefit dancers by decreasing [GRF.sub.max] if used for dancing at all times, even when achieving full pointe is not required. There are, in fact, many dancers who wear their old, broken pointe shoes during technique classes where full pointe is not used. They do this to help perfect working through the foot and strengthening its muscles. These shoes may provide the added benefit of decreasing [GRF.sub.max]. While the reduction of [GRF.sub.max] may not be as pronounced with a broken pair of pointe shoes, our study suggests that the additional material in the shoe will still help disperse forces away from the body better than a flat shoe.
Suggestions for Future Research
This study raises several questions that might be explored further to the advantage of ballet dancers, instructors, and researchers. First, how does the age of the pointe shoe affect the transmission of [GRF.sub.max]? Does [GRF.sub.max] increase or decrease as the pointe shoe is broken in or overused? Second, as there are many manufacturers of pointe shoes and all use slightly different components and procedures in constructing their shoes, to what extent do the materials used affect [GRF.sub.max]? Third, in this study we examined the effects of only one basic ballet jump (assemble). Future studies might look at any one of the many other jumps used in ballet to see how the resultant [GRF.sub.max] compares to that in the present investigation.
Results of this study demonstrate a significant decrease in [GRF.sub.max] when the participants landed a jump in pointe shoes relative to landing the same jump in flat shoes. After analyzing the two types of shoes used in ballet, it is understandable that the pointe shoe would create a smaller [GRF.sub.max]. The multiple layers of the pointe shoe help to absorb some of the forces and disperse them away from the dancer's body. Based on these findings, we conclude that it may be advantageous to have dancers take technique class in pointe shoes. As they will not be required to achieve full pointe during this class, they may benefit from the decreased [GRF.sub.max] by avoiding additional stress on the intrinsic muscles of the foot.
Caption: Figure 1 Assemble jump.
Caption: Figure 2 Fifth position in flat shoes.
Caption: Figure 3 Fifth position in pointe shoes.
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Heather L. Walter, M.S., L.A.T., A.T.C., Carrie L. Docherty, Ph.D., L.A.T., A.T.C., and John Schrader, H.S.D., L.A.T., A.T.C., are in the Department of Kinesiology, Indiana University, Bloomington, Bloomington, Indiana.
Correspondence: Carrie L. Docherty, Ph.D., .L.A.T., A.T.C., 2805 East 10th Street, Bloomington, Indiana 47408; email@example.com.
Table 1 Means and Standard Deviations for Each Shoe Condition Maximal Ground Reaction Vertical Jump Height Force (Newtons) (centimeters) Flat Shoe 1742.9 [+ or -] 252.6 33.1 [+ or -] 7.7 Pointe Shoe 1612.7 [+ or -] 261.5 32.4 [+ or -] 7.2
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|Title Annotation:||Original Article|
|Author:||Walter, Heather L.; Docherty, Carrie L.; Schrader, John|
|Publication:||Journal of Dance Medicine & Science|
|Article Type:||Clinical report|
|Date:||Apr 1, 2011|
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