The Effects of Different on Plantar Forces in

Paul Trégouët, Ph.D., and François Merland, M.Sc.

Abstract n recent years, Irish dance has while muscles also modulate the trans- 10,11 The purpose of this study was to compare seen a surge in popularity, such mission of impact forces. the effects of different on plan- that there are now approximately Another feature of Irish dance is tar loading in Irish dance. Participants I4,000 teachers worldwide. There has the repetition of impact loading.12-14 were 12 open class dancers (the highest not been, however, a concomitant Irish dancing is made up of highly level) who were actively engaged in Irish evaluation of the scientific aspects of repetitive actions of the legs and feet. dance competition. Subjects had a mean this dance style, especially as it relates Furthermore, dancers work at high of 9.2 ± 2.1 years of experience in Irish to the health and wellbeing of the speeds, and in performance, they are dance. All dancers completed one bar dancers. One of the more prominent encouraged to increase their move- of a set phrase in each of three shoes: Irish dance soft , hard shoe, and a features of Irish dance is the footwear ment velocity to produce loud and dance trainer. The order in which the involved, notably hard shoe and soft explosive effects. Thus, the dancers en- shoes were tested was counterbalanced shoe. While there is substantial evi- dure large amounts of lower extremity with a Latin square design. The variables dence to implicate footwear in injury impact stress, which could be a factor compared were maximum force, maxi- incidence,1-3 the role these specific in the incidence of injury. mum pressure, and impulse. Data were styles of footwear play as contributors Footwear is the only interface collected at 100 Hz using a Pedar insole to injury remains equivocal.4,5 The between the foot and the ground, so pressure sensor system. Values were ana- foot is where the body and the ground it is a crucial factor in the interaction lyzed for the whole foot, forefoot, and interact, and the foot can be subjected between the dancer and the perfor- rearfoot. Significant differences between to high forces during gait and other mance surface. There are differences shoe types were observed in impulse (p < functional activities. The impact be- of opinion regarding the effect of foot- 0.01) and maximum pressure (p < 0.01), 15-18 with the trainer exhibiting lower values tween the foot and the ground during wear on that interface. On the one than the other shoes. Differences were a single running step has a maximal hand, there are indications that dance also found between shoes in loading on force magnitude in excess of two to footwear may predispose a dancer to 6 regions of the foot (p < 0.01), with fore- four times bodyweight. This impact injurious forces, especially with repeti- foot values highest in the soft shoe. The force is absorbed by the foot and tive loading.15 Repetitive impact has footwear choice had a significant effect transmitted up the kinetic chain in the been cited as a primary risk factor for on the measured kinetics of the dancers. form of a shock wave.6,7 These impact lower extremity overuse conditions The trainer displayed significantly lower and shock magnitudes are influenced such as stress fractures, shin splints, values for kinetics than did the soft shoe. by surface, footwear, body mass, and plantar fasciitis, and Achilles tendon Thus, it may be a safer (less injurious) acceleration properties. In order to strains.6,19 On the other hand, there choice for daily training. protect the body, the shock wave of is some evidence to support the idea impact is attenuated by anatomical that, with proper attention to detail, structures, for example the heel pad,8,9 footwear can reduce the forces experi- enced by dancers,20 thereby mitigating 21 Paul Trégouët, Ph.D., and François Merland, M.Sc., are at the Centre the risk of some overuse injuries. Audomarois de Recherche Biomécanique, Saint Omer, France. Irish dancers use two types of shoes for performance and competition, a Correspondence: Paul Trégouët, Ph.D., 22 rue des Epeers, 62500 Saint Omer, hard shoe and a soft shoe, neither of France; [email protected]. which, it has been said, is well designed

Copyright © 2013 J. Michael Ryan Publishing, Inc. 41 http://dx.doi.org/10.12678/1089-313X.17.1.41 42 Volume 17, Number 1, 2013 • Journal of Dance Medicine & Science for protection against impacts when est level) and were actively engaged in consisted of steps that are common to compared to the footwear used in Irish dance competition. Subjects had both hard shoe and soft shoe dancing. other sports.22 For Irish dance compe- a mean of 9.2 ± 2.1 years of experience PEDAR- X pressure measuring insoles tition, the rules limit variability in the in Irish dance. All were injury-free for (Novel GmbH, Munich, Germany) appearance of shoes, yet manufacturers at least 6 months prior to testing. were inserted into each of the three have found means to incorporate dif- There were three types of shoes types of shoes, and all subjects per- ferent materials into the design of the used in this project: Irish dance hard formed the sequence to the same mu- shoe. As competition takes up a much shoes, soft shoes, and a dance trainer. sic track and maintained the tempo smaller percentage of total time spent The dance trainer was a Bloch Boost once in each of the three shoe types. in dance than practice, in recent years DRT (Bloch Inc., Reno, Nevada, All data were collected in the same dancers have included a “dance train- USA). Examples of the different shoes session, and the presentation order of er” in their footwear choices, and this are shown in Figure 1 (it should be the shoes was counterbalanced using has been associated with a decreased noted that the soft shoe shown is a a Latin square design. There was a rest risk of injury at the ankle.4 However, man’s shoe, not the typical Ghillie period of approximately 3 minutes no published literature could be lo- that is worn by female dancers and between sequences, which allowed cated that compares the differences, has even less support and sole struc- for downloading the data from the if any, in the loads experienced at the ture than the shoe depicted). All Pedar-X, provided time for the dancer plantar surface of the foot in this type participants used their own hard and to change shoes, and also minimized of shoe versus the traditional soft shoe soft shoes for testing, and while all ap- the possible effect of fatigue. The or the hard shoe. peared to be in reasonably good state same floor was used for all trials. The There are significant differences of repair, it was not possible to control sequence was stopped and repeated in the construction of the two types for precision of fit or condition of the if mistakes occurred; thus, only trials of shoe. The standard soft shoe, also footwear. No inserts or other insoles that were free of error were saved. called a Ghillie, (which is comparable were used in the shoes. Data were recorded at 100 Hz to a ballet ) has a sole made up For the data collection, each dancer and saved to a laptop for analysis. In of 2 to 4 mm of leather. This is usually was taught a dance sequence and then addition to comparing data from the as pliable as possible, in order to pro- practiced it until he or she was capable entire plantar surface, the Pedar soft- vide optimal control of movement and of repeating it exactly. The sequence ware was used to divide the data into display the visual aspect of the foot. The hard shoe has a stiffer sole, with A the addition of a 3 to 4 cm fiberglass heel and a 1 to 2 cm fiberglass tap in the forefoot, located just anterior to the metatarsophalangeal joints. Thus, it is similar to a tap shoe in terms of its form and function. A newer design of dance trainer, such as that used in B this project, has a blown polyurethane outsole as well as an ethyl-vinyl- acetate insole, which provides shock absorption.23 With such greatly divergent mate- rial characteristics among footwear, dancers and teachers should be aware of the effects such differences can generate. Based on the available data and the necessity of choosing optimal footwear for dance training, the pur- C pose of this study was to compare the effects of different footwear on plantar loads to the foot in Irish dance. Methods Twelve trained dancers (11 females and 1 male) volunteered for this study and gave their informed consent to participate. All of the participants Figure 1 Examples of the three types of shoes used in this study: A, soft shoe; B, hard were “open class” dancers (the high- shoe; C, dance trainer. Journal of Dance Medicine & Science • Volume 17, Number 1, 2013 43 separate regions of the foot, thus al- in Ns. Although there was a signifi- Discussion lowing for specific comparisons of the cant difference between the regions The data indicated that the footwear rearfoot and forefoot, in addition to of the foot with regard to impulse studied could exert significantly dif- the whole plantar surface. It also was (p < 0.001), the repeated measures ferent forces on the dancers’ feet. For used to calculate force (N), maximal ANOVA of the impulse only showed example, there was an 18% increase pressure (force per unit area, or N/ a trend toward a difference between 2 in impulse with the soft shoe relative cm ), and impulse (the product of the shoe types (p = 0.08). These data are to the trainer, but only a 5% differ- force and time over which the force presented in Figure 4. It is likely that ence between the hard shoe and the was applied). A repeated measures the impulse measured in the forefoot trainer. Similar results were observed ANOVA was used to compare the accounts for this trend. for maximal pressures (force per unit footwear groups, with an a priori level of significance set at 0.05. Post-hoc pairwise testing was conducted using Bonferroni and Dunn corrections to maintain type 1 error probability at 5%. Statview 5.1 (SAS, Inc., Cary, North Carolina, USA) was used for statistical analysis. Results The results of the repeated measures ANOVA indicated that there were significant differences in measured vari- ables between shoe types. There were also differences noted in the regional loading of the foot, with an interaction Figure 2 Forces in relevant regions of the foot. between foot region and shoe type. Force Statistical analysis of results for maxi- mal force showed no significant whole-foot differences between shoes, but there was a difference (p = 0.03) between regions of the different types of shoe. These are shown in Figure 2. Post-hoc analysis revealed that the highest values recorded in the forefoot were in the soft shoe, while there was no difference between the hard shoe and trainer in the forefoot. There was no significant difference in force in the rearfoot region among shoe types. Figure 3 Maximum pressures recorded. Maximal Pressure Results for maximum pressure showed a significant difference (p < 0.01) among the types of shoe, with the forefoot registering higher forces than the rearfoot. There were no significant differences in maximal pressure in the rearfoot among shoe types, likely a result of the very high standard de- viation noted in the hard shoe. These results are shown in Figure 3. Impulse The impulse was calculated as the force-time integral and measured Figure 4 Impulse calculated under the whole foot and in specific regions of the foot. 44 Volume 17, Number 1, 2013 • Journal of Dance Medicine & Science area). These differences were most shoe-surface interaction is a factor in The data from the soft shoe consis- notable in the forefoot region where, performance and probably in the oc- tently registering higher values in the surprisingly, the soft shoe trials re- currence of injuries. It is possible that forefoot for maximal pressure, force, sulted in the highest forefoot values. by modifying one or another aspect and impulse is an area that deserves While this study revealed several of this triad the potentially injurious attention as to its potential to lead such differences, whether or not these forces to which dancers are exposed to injury. Repetitive stress can lead are clinically significant differences might be reduced. The current study to stress fractures, or the more com- remains an area to be explored. It has suggests that changes in footwear mon shin splints, although the exact not been established in the literature might achieve that purpose by altering etiology is unclear and variable.1,14,15 at what point differences in loading the effects of plantar loading. The role of footwear and repetitive may reach clinically injurious levels. The role of the shoe in affecting ki- loading may be the cause of forefoot Furthermore, while the individual nematics is another area that has been stress fractures that were recently re- loads may remain at a level below the previously examined.33 The study by ported.37 The use of shock absorbing threshold for acute injury, it is possible Bishop and colleagues observed that insoles is suggested as a means of pre- that injury can stem from repetitive running kinematics differed between venting these injuries,38,39 and some tasks that contribute to an overall barefoot and shod but did not signifi- manufacturers have introduced them structural fatigue.24,25 Cumulative cantly vary with changes in the mate- in their shoes. There are prospective trauma disorders have been identi- rial properties of the shoes studied.33 studies that have shown a reduction in fied in a wide range of activities,26,27 In the current study, the differences the incidence of some lower extremity and dance is no exception.28-30 The in plantar loading were seen in the injuries with the use of such insoles.3,40 combination of repetitiveness coupled soft shoe, whereas the hard shoe and Whether this would be feasible in Irish with other detrimental factors—e.g., trainer were not significantly different dance, in practice as well as competi- floor or footwear—may result in the with respect to plantar loading. While tion, remains in question. However, high injury rate previously recorded in the physical properties were not mea- with forefoot loading being as high dancers.31,32 Thus, even the seemingly sured, as Bishop and colleagues had as six times body weight,41 it seems small percentage increases in force done, we do know that the material that any means of ameliorating the noted in this study may be a contrib- properties differed, but this difference repetitive loading would be beneficial uting factor to overuse injuries. was not sufficient to alter the partici- in reducing the risk of injury. While the present study did not pants’ kinetics, as was the case in the One limitation of this study is that directly examine injuries, it is possible Bishop study. the foot structure of the participants that the differences in loading noted While plantar sensation seems to was not assessed; thus it is possible here is a factor in the distribution of have an important influence on lower that the variations seen in some values, injuries. Interestingly, it was reported extremity kinetics and kinematics, it is notably in the rearfoot standard devia- in a recent study that the use of split not clear how the participants in this tions for mean and maximal pressure, sole trainers corresponded to sig- study adapted to the changes in shoe, could be attributed to differences in nificantly fewer ankle injuries in Irish as it has been demonstrated that adap- foot structure. Previous investiga- dancers.4 tation to different dance surfaces can tors have noted that the interaction With regard to dance trainers, it is have a confounding effect on the val- between foot structure and footwear the design that may allow for a differ- ues measured here.34 On a more basic can significantly affect plantar pres- ence in the distribution of forces. As level, it has been reported that humans sures,42,43 as well as the types of injuries opposed to the soft shoe, which has maintain a systemic leg-surface-shoe suffered.44 Another limitation has to minimal material between the foot relationship that will alter leg stiffness do with the footwear in which the and the floor, the trainer has a larger to accommodate to a change in surface subjects were tested. While the dance and more stable surface contacting stiffness.35 It is likely that humans trainers used were new, all participants the ground, which will distribute adapt to the surface, or shoe-surface wore their own footwear for the hard the forces over a larger area, hence interface, via some combination of shoe and soft shoe trials, which intro- the lower pressures. McGuiness and knee and ankle adaptations to alter duces the potential for variability into Doody theorized that the design of leg stiffness.36 It is still unclear whether the measurements. Furthermore, no the split sole trainer increases stabil- the differences in material properties differentiation was made in the final ity by having a larger contact area between the hard shoe and trainer analysis between the woman’s shoe, and a polyurethane outsole that may would be sufficient to cause an altera- i.e., the Ghillies, and the man’s shoe. offer greater traction on the floor, tion in the kinematics of Irish dance The differences in structure could af- in addition to increased support of steps. This seems unlikely among our fect plantar forces, but a larger sample the foot.4 The overall combination participants, as open class dancers of male dancers would be needed to of these various factors is likely the have to perform within very strict test this hypothesis. reason their study observed a lower in- movement guidelines if they are to be This study shows a significant dif- cidence of ankle injuries. The dancer- successful in competition. ference in force between regions of Journal of Dance Medicine & Science • Volume 17, Number 1, 2013 45 the foot and especially at the forefoot. 5. Lamb KL, McCauley A. An assess- A. [Sports medicine and rehabilita- Linking the levels of force to the lack ment of the prevalence and correlates tion. Surface and footwear]. Or- of cushioning available in the various of injuries in Irish dancers.Presented thopade. 1997 Nov;26(11):993-8. types of footwear used in Irish dance at the Annual Conference of the Brit- [Article in German] may very well explain in part the in- ish Association of Sport and Exercise 19. Schwellnus MP, Jordan G, Noakes Sciences, September 3-7, 2003, Shef- TD. Prevention of common overuse juries that McGuiness and coworkers 4 field, England. injuries by the use of shock absorb- have reported. Due to the lack of 6. Shorten MR. Running shoe design: ing soles: a prospective study. 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