Background: In ballet dancing high demand is put on foot performance and ..... The mean contact area of three walks per foot didn't differ between the two ...
University of Potsdam University Outpatient Clinic Seminar: Applied Methods Wintersemester 2012/2013 Supervisor: Juliane Müller Student: Torsten Rackoll
Do trained female ballet dancers show differences in contact area and arch index compared to normal female in gait? Rackoll, T., Müller, J. University Outpatient Clinic, University of Potsdam
Abstract Background: In ballet dancing high demand is put on foot performance and several studies have examined foot related injuries but the effect of intensive ballet training on the foot geometry has not been examined so far. The goal of the study was to determine whether female ballet dancers show differences in contact area and arch index compared to normal females in gait. Methods: Four non-professional ballet dancers and four matched control subjects with an average age of 26 years (± 3.78 SD), a height of 1.68 m (± 7.04 SD) and a weight of 61 kg (± 6.87 SD) were assessed by gait evaluation system (emed-x, Novel), each side being measured three times. Outcome measures were the mean contact area of the entire foot of all walks and the computed arch index. Assessed anthropometric data was used to match the ballet dancers with the control subjects based on the Body Mass Index [BMI]. Statistical analysis was done descriptively. Results: Both groups showed similar results with an average arch index of 0.19 ± 0.07 SD for the ballet dancers and 0.20 ± 0.00 SD for the control subjects. The difference of contact areas between two groups (116.7 cm2 and 117.5 cm2) was small with 0.7 % difference. Discussion: The results showed no differences in the two groups. Still, in most outcomes the ballet dancers presented higher variations which could support the presumption that ballet training can change the foot geometry but due to the small sample size no final conclusion could be drawn. Keywords: plantar pressure distribution, training effect, intrinsic foot muscles
Introduction: In ballet, performance on muscular control in the feet is essential1,2 to fulfil jumping movements as in the Ballon, Chassé or Sissonne Fermée.3 Silent landing, a key technique in ballet dancing, involves trained foot muscles to decelerate the landing with an eccentric contraction of the plantar flexor muscles. During dynamic movement the intrinsic foot muscle are believed to play a bigger role although this has been examined only in gait.4,5 Furthermore a recent review4 underlines the role of intrinsic foot muscles(Fig. 1) on the foots geometry in the normal population. Strong muscles prevent a lowering of the longitudinal arch.5,6 Involved in the shape of the arch are the intrinsic plantar flexor.4 Those muscles seem to be diminished7 during evolution but are still involved in certain activities and therefore can be trained.
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Fig. 1: Intrinsic foot muscles They are organised in four overlapping layers and consist of the abductor halluces, the flexor digitorum brevis, the abductor digit minimi, quadratus plantae, the lumbricals, the adductor hallucis transverse, the adductor hallucis oblique, the flexor hallucis brevis, the flexor digit minimi brevis and on the deepest layer the three plantar interossei.
Several studies underline the impact on muscle activity in controlled landing8 and postural ability2,9 in ballet dancers. Kilby et al.9 compared postural stance under barefoot condition or wearing high heels between ballet dancers and recreational athletes. The standard deviation of the center of pressure in antero-posterior (p < .001) and medio-lateral direction (p < .001) were both significantly lower in ballet dancers in bipedal stance. Da Costa2 looked at the center of pressure displacement in different unipedal ballet positions under barefoot and shoe conditions but not in comparison to nonballet dancers. Further investigations in ballet dancers concentrated mostly on injury related issues such as ankle stability, and balance ability. Lin1 determined the maximal displacement of the center of pressure in the medial-lateral and the posterior-anterior directions on a force plate. The higher displacements of the center of pressure in non-dancers support the assumption that the muscle morphology and stability in ballet dancers differ from untrained people. Intrinsic foot muscles strength is stated to be measured either direct or indirect. Direct measurements are only assessable for toe flexor strength in which intrinsic and extrinsic muscles work together which underlines the overall problem of measuring intrinsic muscles. Plantar pressure distribution is a way to measure different values in a subject and can be applied during gait and stance alike. It may well be that one way of indicating indirectly the strength of the intrinsic muscles is by looking at the longitudinal arch which is hypothesised to have a higher potential of intrinsic muscle activation with almost no activation of the extensor muscles. Although there are direct measuring devices to assess the height of the arch10–13, those cannot be applied during gait. Another method to assess the geometry of the arch is to calculate the arch index after assessing the contact area during gait via a plantar pressure distribution measurement. The arch index has been validated as a method to give information on the longitudinal arch.14–16 Neither the effect of ballet training on foot geometry nor on intrinsic foot muscle strength has been examined until now. Therefore the study aims to examine whether a difference in the arch index and 2
in the contact area [CA] of female ballet dancers during gait is visual compared to matched female, untrained subjects. The authors hypothesize that a higher muscle capacity in the plantar muscles in ballet dancers elevates the arch and might minimize the contact area of the foot. The hypothesis relies on the presumption that the length of the plantar muscles shortens more actively in trained subjects during activity than in untrained ones leading to an elevated arch.
Methods: Subjects: Eight females were included in the study consisting of four non-professional ballet dancers and four control subjects (Table 1). The measurements were conducted in between January and March 2013. All volunteers signed the written informed consent. Table 1: Averaged anthropometric data and standard deviations (±SD) Age (years) Height (cm) Weight (kg) BMI Foot Length (cm) Foot width (cm)
Ballet dancers, n=4 24 (± 4.89) 166 (± 6.77) 59 (± 6.4) 21 (± 2) 25 (± 1.39) 9 (± 0.56)
Control group, n=4 29 (± 2.67) 169 (± 7.35) 63 (± 7.35) 22 (± 1.72) 25 (± 0.5) 9 (± 0.29)
The ballet dancers were recruited through direct contact to ballet schools, e-mail advertisements and university bulletins. Included in the study were female ballet dancers (age ≥ 18 years) with a training intensity of at least two training sessions a week over a period of minimum three consecutive years prior to the study. Subjects were not eligible if they suffered from a foot injury in the past (e.g. Hallux Valgus, ankle sprain). Exclusion criteria were controlled due to subjects selfreport on their health status and training intensity. Late in the study it was revealed that one of the ballet dancers had not trained within the last five years and within her reported twelve years (Table 2) her training intensity was only one to one and a half hours per week. Therefore her training exposure didn’t meet the inclusion criteria. However, the data remained in the study because it didn’t change the outcome but allowed to include more control subjects for comparisons. Table 2: Self-reported training exposure in years Subject 1: Subject 2: Subject 3: Subject 4:
Training years (Ballet dancers) 13 12 18 17
The control group was recruited through direct contact of co-workers in the outpatient clinic and email advertisements. It was looked for females being untrained or recreational athletes who didn’t follow an exercise plan with more than an hour of recreational sports per week and with a BMI of equal or less than 25. The control subjects were compared to the ballet dancers after the measurements based on their Body Mass Index [BMI] as an indicator of a weight to height relationship.
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Instrumentation: In a first statement body height and weight was self-reported to help identify possible control subjects. Simple anthropometric data was determined and plantar pressure distribution was recorded using the emedx platform17 (Novel AG, Munich, Germany) with a sampling rate of 100 Hz and a resolution of 4 sensors per cm2. The anthropometric data included height, weight, foot length and foot width. Height was measured with a metering ruler and weight with an electronic, calibrated scale. Foot length was measured from heel to the longest toe and foot width at the metatarsal heads. All measurements were done by one experienced examiner and one research assistant. Procedure: During one measurement session first the anthropometric data and then the plantar pressure distribution were assessed. The anthropometrics included height first, then weight followed by foot length and width. For the plantar pressure distribution measurements the subjects were given several familiarization trials to accustom to the laboratory situation18 and then six to ten walks were stopped until three valid measures for each foot were performed. The subjects were asked to self-select their gait velocity which was controlled visually by the examiner to differ between walking and running. Additionally the time between the first step and the last contact with the plantar pressure distribution platform was stopped to assure that the subjects kept their gait velocity. Furthermore the starting foot could be self-selected as well but the subjects were asked to start with the other foot as soon as three steps were recorded for one side. The subjects normal velocity was expected to be reached with the third step which has been validated elsewhere18. Therefore the subjects were asked to position themselves in an appropriate distance to reach the platform with their third step.
Data process: Outcome variable was the mean contact area of three steps averaged by the Novell software of the entire foot as well as for the forefoot, the midfoot and the heel17 and the arch index. The contact area for the forefoot, the midfoot and the heel were measured using the predefined automask of Novel-win software package (Novel Projects®, Munich, Germany). Based on the values for contact area the arch index was calculated in Microsoft Excel (Microsoft Office 2010, Redmond, USA) using the formula by Cavanagh19 which is validated20 and used in various methods.10,21
Cavanagh defines high and flat arches with an arch index [AI] using the following thresholds: High arch Normal arch Flat arch
AI ≤ 0.21 0.21 < AI < 0.26 AI ≥ 0.26
Additionally the mean contact area was computed for both sides including the toes which were not included in the calculation by Cavanagh. Statistical analysis was done descriptively with mean and 4
standard deviation [SD] for all parameters. For comparison of the arch index the two groups were matched using the BMI of each ballet dancer and compare it with the control subject whose BMI is closest.
Results: The eight subjects showed the anthropometrics as shown before (Table 1). Of the two groups, the ballet dancers (age 24 ± 4.89 years, height 166 ± 6.77 cm, weight 59 ± 6.4 kg) and the control group (age 29 ± 2.67 years, height 169 ± 7.35 cm, weight 63 ± 7.35 kg), the control group revealed a higher age, greater height and higher weight. Foot sizes were consistent. The mean self-reported training exposure of the ballet dancers was 15 (± 2.55 SD) years.
BMI and age showed were lower in the ballet dancer group. The mean arch index differed with 0.01 but the standard deviation present a higher variation in the ballet dancers (Fig.2). All four subjects in the control group had an arch index between 0.2 to 0.21 while the ballet dancers ranged from 0.09 to 0.26.
Mean arch index of ballet dancers and control subjects 0.30
Arch Index
0.25 0.20 0.15 0.10 0.05 0.00 Arch Index
1
2
0.19
0.20
Fig. 1: index 2: Averaged Mean archarch index (± SD) of balled dancers and control subjects 1: Ballet dancers 2: Control Group
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Arch Index 0.30
Arch Index
0.25 0.20 0.15 Ballet dancers
0.10
Control Group
0.05 0.00
1
2
3
4
Ballet dancers
0.18
0.23
0.09
0.26
Control Group
0.21
0.20
0.20
0.21
Fig. 3: Individual arch index
The arch index of two dancers (Fig 3: No. 1 and 3) revealed a high arch and the other two indicated a normal arch. One dancer of those with a normal arch was in the threshold in between a normal arch and a flat arch. Of the two dancers with a high arch the values were low (Fig. 2). The Contact area ranged from 93.9 cm2 to 134.9 cm2 in which the control group showed less deviation with three subjects ranging from 121.3 cm2 to 125.6 cm2 (Fig. 3). The mean CA of the ballet dancers was only 0.7 % lower than the one of the control group.
[cm²]
Mean contact area (± SD) 160.0 140.0 120.0 100.0 80.0 60.0 40.0 20.0 0.0
1
Contact Area 134.9
2
3
4
5
6
7
8
116.6
93.9
121.3
121.3
124.5
125.6
98.4
1-4: Ballet dancers; 5-8: Control Group Fig. 4: Mean contact areas (± SD) of Ballet dancers (subjects 1 – 4) and control group (subjects 5 – 8) 2 Mean CA Ballet dancers: 116.7 cm ± 3.7 SD 2 Mean CA Control group: 117.5 cm ± 2.4 SD
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a
b
Fig. 5. (a) ballet dancer with flat arch and 17 years of training. (b) ballet dancer with high arch and 18 years of training
Discussion: The study aimed to examine whether a difference in the arch index and in the CA of female ballet dancers during gait is visual compared to matched female, untrained subjects. The arch index from two groups in this study didn’t differ to support the hypothesis of a higher arch in the ballet dancers group. The control group showed less deviation in the physical attributes. Compared to the ballet dancers they were slightly thinner but not to a degree that it should influence the pressure distribution measurement. Although subjects were matched, literature is not consistent on the exact impact of height and weight. The BMI as a weight to height relationship seemed to be rational indicator. However, it doesn’t take into account the foot sizes of the subjects. Figure 3 shows clearly that in comparison the arch indices of the four ballet dancers varied more than those in the control group. In contrast to the control group two subjects from the ballet dancer group are defined as having a normal arch with the threshold being determined by Carvanagh19 at AI ≤ 0.21 for a high arch. One of the ballet dancers was within the threshold of a flat arch with an AI at 0.26. A close relationship between the arch and the training exposure seems difficult to argue as the subject with the highest arch index reported a training time of 17 years with an intense training during the last year of five training sessions with 90 min each. The subject with the lowest arch index reported 18 years of training with at least three times per week which was only disrupted by three pregnancies. The sample size was too small to differ deviation deriving from average human variations19 to effects from ballet training. Cavanagh19 used in his study a sample of 107 subjects to determine the thresholds for the arch index. The thresholds were set to the first and third quartile of the sample. Compared to his sample all the subjects from my study, ballet dancers and control subjects alike, show a higher arch than the average. In another study by Bosch et al.22 a sample of 26 adults showed an arch index of 0.19 ± 0.05 SD. Butler et al.11 used an arch index measurement system device to 7
establish population normative values in recreational runners. Their sample of 100 recreational runners showed an average arch index of 0.34 ± 0.03 with similar indices between the genders. MeiDan et al.16 who were looking on the relationship of ankle sprains to the medial longitudinal arch, used the Chippaux-Smirak index to determine the arch height in 83 female infantry recruits. Comparisons of those measurements to the equation by Cavanagh19 could not be found in the literature. The mean contact area of three walks per foot didn’t differ between the two groups but showed higher variations between the subjects of the ballet dancer group. The values in the control group were consistent with only the last subject having a deviation of 19.1 cm2 to the mean of the control group. In general contact area and arch index show similar behaviour between the two groups as was expected. The contact area is used to calculate the arch index with only the toes not being considered for the arch index but for the total contact area. This study is a first pilot investigation to relate intensive foot strength training like ballet dancing to foot geometry. Although the hypothesis could not be supported by the findings the small sample indicates a difference between ballet dancers and non-active controls. Zifchock et al.23 were able to relate the dominant foot to an higher arch with p = 0.00. In our study the subjects were not asked for their dominant foot. In a future study the dominant foot should be included in the report. Study Limitations: Several limitations to this investigation exist. As the study design was kept small it couldn’t accomplish statistical significance with a sample size of four subjects per group. A statement on the overall population is therefore not possible. In addition any reported difference between the two groups can therefore not be traced back to the training effects. Likewise the training exposure was self-reported. The amount of training years is an unreliable measure in comparison to training hours24 but it seemed as if a retrospective description of precise training exposure in hours per week during each year of the averaged 15 years were not reliable as well. Therefore the amount of training hours and their direct influences could not be further discussed. The 3-step method was used to measure the subjects in mid-gait. It is reasonable that mid-gait is achieved after three steps17 but other procedures are used as well. Comparison of data should therefore be handled with caution. No visual information on the foot being pronated or supinated was collected. Regarding the hypothesis a subject with a high arch should have an otherwise normal rotation of the foot while a subject with a cavus foot over-supinates.10
Conclusion: Based on the small sample size ballet dancer did not show a significant difference in the mean arch index during gait evaluation but did show higher deviations towards the mean than the matched control group. The contact area showed no difference between the two groups. Therefore the hypothesis that a higher muscle capacity in the plantar muscles in ballet dancers elevates the arch didn’t find any proof in this study. 8
Acknowledgements: The author would like to thank Michael Rector for assessing the anthropometric data. References: 1.
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