Kinematic Analysis of Sautés in Barefoot and Shod Conditions Alycia Fong Yan, Ph.D., Claire Hiller, M.App.Sc., Ph.D., Peter J. Sinclair, M.Ed., Ph.D., and Richard M. Smith, M.Sc., M.Ed., M.A., Ph.D.

Abstract Dancers are exposed to many landings from jumps during class and performance, and repetitive loading has been linked with an increased risk of injury. Little is known about the effect of different dance shoe types on jump landings, and with so many dance shoe designs available to choose from, a thorough exploration is warranted. Dance technique dictates that jump landings be “rolled through the foot,” with a toe strike followed by controlled lowering of the ball of the foot and heel. For this study, 3D motion analysis was used to capture the movement of 16 female dancers performing sautés in second position. Lower limb joint kinematics were examined during the landings, both barefoot and in different jazz shoe designs. The results showed that all dancers executed the technique of “rolling through the foot.” All jazz shoe designs increased knee and ankle sagittal ROM (p < 0.05) but reduced ankle frontal plane ROM and midfoot ROM in all three planes (p < 0.05). Chorus shoes increased maximum knee flexion by more than 5° during the plié. Jazz shoes restricted midfoot sagittal and transverse plane motion and MPJ sagittal motion compared to barefoot throughout stance phase (p < 0.05). These changes may translate

to a reduced capacity to absorb impact or decreased propulsion. Dance jump landings in the jazz shoe designs tested may appear to be heavier due to the greater reliance on knee flexion to absorb impact and less push-off for subsequent jumps.

M

otion analysis of dance movement and the effect of footwear on it is an area of dance science that is still evolving.1 An understanding of the kinematics of dance movement can provide insight into injury mechanisms and techniques for improving performance. Dance practice features highly repetitive loading throughout the kinetic chain; in particular, dancers can perform more than 200 jumps in a single class.2 The most basic dance jump is the sauté, taught from a very early age so that the foundations of correct jump landing and propulsion technique can be established. This technique forms the basis for many single-legged vertical, travelling, and turning jumps. Sauté landings have only been investigated in one prior study, which examined the effect of two different landing types in first position, en

Alycia Fong Yan, Ph.D., Claire Hiller, M.App.Sc., Ph.D., Peter J. Sinclair, M.Ed., Ph.D., and Richard M. Smith, M.Sc., M.Ed., M.A., Ph.D., Faculty of Health Sciences, The University of Sydney, Australia. Correspondence: Alycia Fong Yan, Ph.D., Faculty of Health Sciences, The University of Sydney, 75 East St, Lidcombe, NSW 2141, Australia; alycia. [email protected] Copyright © 2014 J. Michael Ryan Publishing, Inc. http://dx.doi.org/10.12678/1089-313X.18.4.149

pointe and rolling through the whole foot.3 In that study, the action of rolling through the foot was shown to be important for reducing the impact shock at initial ground contact; however, kinematic differences were not explored to validate this. Several studies have examined kinematics in dancers performing drop landings, comparing dancers to non-dancers,4 males to females,5 and measuring the effect of high heels on a raked stage.6 Asking non-dancers to take off with a plantar flexed ankle and foot and land toe-heel led to no difference in vertical ground reaction forces between dancers and non-dancers.4 Male and female professional dancers used similar landing strategies for drop landings, suggesting a dance training effect.5 Drop landings in high heels significantly increased ankle plantar flexion and inversion and changed joint moments compared to barefoot, contributing to an increased risk of ankle sprain injury.6 Conventional shoes have been found to alter lower limb kinematics compared to barefoot during weightbearing activities. 7-10 If the shoes dancers wear during class or rehearsal can significantly alter their landing kinematics, there is potential for changes in landing technique that may be missed by the teacher. Since poor technique has been suggested as a risk factor for injury,11 if poor technique continues unchecked, there could be an increase in injury risk. However, it 149

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Table 1 Description of Shoe Conditions Shoe Name

Make and Model

Description

Barefoot

N/A

Control condition

Chorus

Bloch Cabaret S0306

High heeled court shoe with ankle strap

Elastabootie

Bloch Elastabootie S0499L

Slim line leather jazz shoe with separate forefoot and rearfoot sections (split sole design): 3.5 mm forefoot, 13 mm rearfoot thickness

Evolution

Bloch Evolution Dance Sneaker S0510

Split sole design low profile jazz sneaker; multi-density rubber outsole and additional divisions in the forefoot section; air punched compressed EVA sock material: 14 mm forefoot and 20 mm rearfoot thickness

Boost

Bloch Classic Boost S0538L

Split sole design jazz sneaker with thick multi-density rubber outsole and compressed EVA sock material: 24 mm forefoot and 38 mm rearfoot thickness

is unknown whether the results of this research apply to dance jump landings that are performed with well-executed dance technique. The design of jazz shoes varies a great deal, even within the same brand. Split sole designed jazz shoes and the amount of materials used in the shoe’s upper have been found to reduce observed midfoot plantar flexion angles but increase observed ankle plantar flexion compared to barefoot.12 Conventional shoes were also found to reduce motion in the midfoot with a compensatory increase in motion at the rearfoot.13 These factors, in conjunction with outsole thickness and heel height, could also contribute to changes in jump landings. The aims of this study were to describe the 3D kinematics of sauté landings and takeoffs in second position and to evaluate the effect of various jazz shoe designs on the 3D kinematics in these movements. It is hypothesised that jazz shoes will restrict foot and ankle motion, and knee flexion will, therefore, increase as a compensatory mechanism to reduce impact on the lower body.

Methods Sixteen female dancers (mean age: 25 ± 5.9 years; mean mass: 55.9 ± 7.4 kg), of Royal Academy of Dance (RAD) Intermediate standard or above, volunteered for the study. Dancers were excluded if they had a current injury that reduced their class or performance participation. All participants gave informed consent, and the investigators’ University Ethics Committee approved the study. The dancers were instructed to perform eight consecutive sautés in second position to RAD Grade 1 music (to control the tempo of the movement). Each foot was placed on a separate force plate (Model 9287BA, Kistler, Switzerland), and the dancers were told to keep their hands on their waist to minimize movement of the arms and torso. Five shoe conditions were presented, in a randomized order, with ample time for the dancers to familiarize themselves with each shoe condition. The control condition was barefoot, which was compared to four jazz shoe designs. The experimental jazz shoes included a chorus shoe and three split-

Figure 1 Jazz shoe designs (L-R): Chorus, Elastabootie, Evolution, and Boost.

sole design shoes of varying outsole thickness: a slip-on traditional jazz shoe (Elastabootie), a low profile jazz sneaker (Evolution), and the classic style jazz sneaker (Boost). The tested shoes are illustrated in Figure 1 and descriptions of the shoe conditions are presented in Table 1. Dance shoes were fitted by the lead investigator to ensure a firm fit, minimizing foot movement inside the shoe. A 14-camera motion analysis system (Eagle and Cortex 1.1.4.368, Motion Analysis Corporation, Santa Rosa, CA, USA) captured the movement of 35 retro-reflective markers placed on the pelvis and lower limbs (sacrum, left and right ASIS, greater trochanter, mid-thigh, medial and lateral femoral epicondyles, upper and lower tibia, lateral shank, medial and lateral malleoli, medial, lateral, and posterior calcaneus, navicular, head of the first and fifth metatarsals, and the distal end of the hallux).12 The markers were placed on the shoes in the corresponding position to palpated bony landmarks on the foot. Orthogonal axes were embedded in the pelvis, thigh, shank, rearfoot, and forefoot segments. Joint coordinate systems

Journal of Dance Medicine & Science • Volume 18, Number 4, 2014

were constructed for the toe, midfoot, and ankle according to the methods outlined by Chard and coworkers,14 and the knee and hip joint coordinate systems embedded according to the International Society of Biomechanics recommendations15 (Fig. 2). Data during stance phase were normalized to 100% from toe strike to toe off. Sagittal plane hip, knee, and metatarsophalangeal joint (MPJ) motion was computed while all three planes of motion were calculated for the ankle and midfoot. Peak measures of the following joints were identified during the initial 10% and final 10% of stance: extension of the hip and knee; plantar flexion of the ankle, midfoot, and MPJ; inversion and adduction of the ankle; and eversion and adduction of the midfoot. These phases of stance corresponded to the initial impact of landing and the final propulsion into the subsequent sauté. Hip, knee, ankle, midfoot, and MPJ angles were found at 50% of stance. Of the eight consecutive jumps, the first and last were excluded from analysis, as they were not rebounding jumps. The mean value from the remaining six trials was collected for statistical analysis. Repeated measures ANOVAs of the five shoe conditions during the six trials were used to determine within-subjects effects of the shoe conditions, with a simple comparison to the barefoot condition. Pair-wise comparisons revealed further significant differences between the other shoe conditions. The level of significance was set at p < 0.05.

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Figure 2 Joint coordinate system (JCS) and segment definition. ZJCS = flexionextension or dorsiflexion-plantar flexion axis; YJCS = inversion-eversion axis; XJCS = abduction-adduction axis.

Table 2. All jazz shoe designs increased knee and ankle sagittal ROM (p < 0.05) but reduced ankle frontal plane ROM and midfoot ROM in all three planes (p < 0.05). Total ROM for each joint is presented in Table 3. Hip Flexion-Extension Angle In the barefoot condition, the hip displayed extension at toe strike, moved into flexion, and then during propulsion into extension. There was no significant effect of jazz shoe design on hip sagittal angle in the initial 10%

of stance (p = 0.254), at 50% of stance (p = 0.219), or in the final 10% of stance (p = 0.366). Total hip sagittal ROM was not significantly affected (p = 0.899) by the jazz shoes (Fig. 3). Knee Flexion-Extension Angle The knee was slightly flexed at toe strike in the barefoot condition and continued to flex, reaching peak knee flexion at 49% of stance phase, then at toe off the knee was extended. Jazz shoe design significantly affected knee sagittal angle at the initial 10%

Results Kinematic analysis of sautés in bare feet reveals the pattern of each joint movement. The timing of each joint reaching maximal ROM in the barefoot condition showed that the MPJ reached maximal dorsiflexion at 5% of stance, followed by hip flexion at 47%, knee flexion at 49%, ankle dorsiflexion at 50%, midfoot inversion at 51%, ankle eversion at 52%, midfoot abduction at 58%, then finally ankle abduction at 63%. The kinematics of sautés in bare feet and the different jazz shoe designs are presented in

Figure 3 Mean flexion-extension hip angles in all shoe conditions and 95% confidence interval for the barefoot condition.

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Table 2 Peak Angles in the Initial 10% of Stance, at Midstance, and in the Final 10% of Stance Angle

Time Point

Barefoot

Evolution

Chorus

Elastabootie

Boost

Hip Flexion and Extension

Initial 10%

-10.7 ± 7.9°

-12.4 ± 9.5°†

-9.6 ± 10.46°

-11.7 ± 10.8°

-13.6 ± 11.0°†

Midstance

9.3 ± 9.8°

6.9 ± 10.2°

9.8 ± 11.1°

8.0 ± 12.1°

6.1 ± 13.2°

Final 10%

-19.9 ± 7.9°

-21.5 ± 10.5°

-19.6 ± 9.8°

-21.0 ± 10.3°

-22.8 ± 11.6°†

Knee Flexion and Extension

Initial 10%

-8.3 ± 6.1°†

-9.4 ± 6.9°†

-13.5 ± 5.6°*

-10.9 ± 8.3°

-10.1 ± 6.8°†

Midstance

64.1 ± 5.6°†

64.7 ± 5.2°†

71.0 ± 4.3°*†

66.0 ± 4.1*†

65.8 ± 4.7°*†

Final 10%

2.0 ± 6.1°†

2.8 ± 5.9°†

-2.3 ± 5.0°*

1.9 ± 5.6°†

3.3 ± 5.9°†

Ankle Dorsiflexion and Plantar Flexion

Initial 10%

-24.4 ± 10.1°

30.9 ± 10.5°†

19.5 ± 6.0°

26.4 ± 9.4°†

29.0 ± 6.8°†

Midstance

21.2 ± 6.2°†

28.0 ± 4.9°*†

32.7 ± 4.9°*

28.7 ± 6.4°*†

31.0 ± 2.9°*

Final 10%

31.7 ± 10.4°

41.1 ± 5.6°*†

28.8 ± 3.2°*

36.7 ± 6.7°*†

38.3 ± 4.8°*†

Ankle Inversion and Eversion

Initial 10%

-11.7 ± 3.9°†

-16.4 ± 4.0°*†

-9.6 ± 4.7°*

-12.0 ± 4.0°†

-14.1 ± 3.5°*†

Midstance

5.6 ± 5.6°†

-5.4 ± 4.7°*†

1.0 ± 4.6°*

-0.9 ± 4.3°*

-4.0 ± 3.5°*†

Final 10%

-13.6 ± 3.6°

-19.5 ± 3.2°*†

-14.6 ± 4.7°

-16.2 ± 3.6°*

-17.3 ± 3.8°*†

Ankle Abduction and Adduction

Initial 10%

-0.5 ± 4.0°

-0.8 ± 3.5°

0.4 ± 4.6°

2.3 ± 4.5°*

0.1 ± 2.9°

Midstance

7.2 ± 5.7°

9.6 ± 4.3°†

5.2 ± 5.7°

9.3 ± 4.9°†

9.4 ± 3.2°†

Final 10%

-3.9 ± 5.9°

-5.4 ± 4.6°

-5.3 ± 4.7°

-3.7 ± 5.0°

-4.7 ± 3.8°

Midfoot Dorsiflexion and Plantar Flexion

Initial 10%

-19.3 ± 9.8°†

-9.2 ± 4.1°*†

-5.54 ± 4.5°*

-8.6 ± 8.2°*

-9.6 ± 3.5°*†

Midstance

14.5 ± 7.4°†

5.7 ± 3.8°*

5.8 ± 3.2°*

5.8 ± 5.4°*

4.1 ± 2.8°*

Final 10%

-21.7 ± 9.2°†

-8.6 ± 4.4°*

-5.8 ± 4.4°*

-11.9 ± 5.2°*†

-9.5 ± 3.3°*†

Midfoot Inversion and Eversion

Initial 10%

2.0 ± 2.9°†

2.5 ± 1.6°†

-0.6 ± 2.2°*

3.2 ± 3.4°†

1.7 ± 2.3°†

Midstance

-6.6 ± 4.1°†

1.8 ± 1.9°*†

0.2 ± 2.9°*

2.6 ± 4.6°*†

-0.1 ± 1.7°*

Final 10%

-0.2 ± 2.7°

2.4 ± 1.5°†

-0.5 ± 2.2°

3.1 ± 4.1°*†

1.4 ± 2.0°†

Midfoot Abduction and Adduction

Initial 10%

-5.2 ± 3.1°†

-0.1 ± 1.4°*

-0.6 ± 1.7°*

-2.4 ± 1.7°*†

-1.1 ± 1.3°*

Midstance

4.7 ± 4.0°

2.9 ± 1.8°

2.7 ± 2.7°

2.2 ± 2.6°*

2.4 ± 1.7°*

Final 10%

-8.0 ± 4.2°†

0.0 ± 1.5°*†

-1.2 ± 2.0°*

-3.5 ± 2.7°*†

-1.2 ± 1.3°*

MPJ Flexion and Extension

Initial 10%

21.0 ± 7.0°†

14.1 ± 6.1°*

12.8 ± 4.0°*

10.2 ± 4.4°*

11.5 ± 5.3°*

Midstance

2.4 ± 4.4° †

-4.2 ± 4.6°*†

-6.4 ± 4.0°*

-2.00 ± 2.23°*†

-4.6 ± 3.6°*

Final 10%

17.4 ± 7.8°

11.7 ± 5.1°*

13.7 ± 5.2°

8.5 ± 4.7°*†

9.9 ± 3.8°*†

Flexion and dorsiflexion are positive, and extension and plantar flexion are negative. Inversion is positive, and eversion is negative. Abduction is positive, while adduction is negative. *Significant difference (p < 0.05) compared to barefoot; †significant difference (p < 0.05) compared to the chorus shoe.

of stance (p = 0.02), midstance (p < 0.001), and the final 10% of stance (p < 0.001). Knee flexion in the chorus shoe was significantly greater than barefoot throughout stance. Total knee ROM was significantly affected by the jazz shoes (p = 0.016), with all shoe conditions showing increased knee ROM compared to barefoot (66.15° ± 5.77°; Fig. 4). Ankle Angles The ankle was plantar flexed at toe strike, reached maximal dorsiflexion at 50% of stance, and then was plantar

flexed at toe off in the barefoot condition. Jazz shoes significantly affected ankle sagittal angle throughout stance (p < 0.001). Chorus shoes displayed less plantar flexion in the initial 10% of stance compared to the other jazz shoe designs but not the barefoot condition. At midstance the barefoot condition displayed significantly less dorsiflexion compared to all shoe conditions. In the final 10% of stance, the chorus shoe displayed significantly less plantar flexion compared to the other jazz shoes (p < 0.001). The barefoot condition was also significantly less

plantar flexed compared to the Evolution, Elastabootie, and Boost, but not the chorus shoe. Total sagittal plane ankle ROM was significantly affected by the jazz shoe designs (p < 0.001). Barefoot had the lowest ROM (53.62° ± 10.77°), which was significantly reduced by up to 7° compared to all of the jazz shoes (Fig. 5). The ankle was inverted at toe strike, reached maximal eversion at 52% of stance phase, and was inverted at toe off in the barefoot condition. Ankle inversion-eversion angles were significantly affected by

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Table 3 Range of Motion (Mean and Standard Deviation) for Each Shoe Condition Barefoot

Evolution

Chorus

Elastabootie

Boost

Hip Sagittal

29.6 ± 6.8

28.9 ± 5.7

29.7 ± 5.6

29.7 ± 5.2

29.8 ± 6.4

Knee Sagittal

66.2 ± 5.8

68.0 ± 6.0*

69.1 ± 4.9*

68.4 ± 5.9*

69.4 ± 5.4*

Ankle Sagittal

53.6 ± 10.8

70.1 ± 6.2*

62.4 ± 4.1*

66.31 ± 6.2*

70.00 ± 5.4*

Ankle Frontal

19.9 ± 4.3

14.8 ± 4.9*

16.5 ± 5.5*

16.2 ± 4.1*

14.0 ± 4.6*

Ankle Transverse

13.3 ± 3.5

16.4 ± 3.1*

12.8 ± 4.2

14.9 ± 3.9*

15.6 ± 2.9*

Midfoot Sagittal

38.6 ± 8.8

16.6 ± 3.3*

12.8 ± 2.8*

19.7 ± 4.4*

15.2 ± 3.5*

Midfoot Frontal

10.0 ± 4.2

2.7 ± 1.0*

4.2 ± 1.4*

4.2 ± 1.7*

3.2 ± 1.1*

Midfoot Transverse

13.3 ± 5.0

3.6 ± 1.6*

5.0 ± 2.1*

6.3 ± 1.7*

4.3 ± 1.8*

MPJ Sagittal

21.4 ± 5.7

19.6 ± 6.2

22.5 ± 4.3

13.5 ± 4.4*

17.0 ± 5.6*

Sagittal plane motion = flexion and extension or dorsiflexion and plantar flexion; frontal plane motion = inversion and eversion; transverse plane motion = abduction and adduction. *Significantly different from barefoot, p < 0.05.

jazz shoe design throughout stance (p < 0.001), and each shoe condition presented significant differences from all others (p < 0.05). The Evolution

sneaker showed the greatest amount of inversion in the initial 10% of stance. At midstance, the ankle was everted in the barefoot condition,

Figure 4 Mean knee flexion-extension angles in all shoe conditions and 95% confidence interval for the barefoot condition.

Figure 5 Mean flexion-extension ankle angles in all shoe conditions and 95% confidence interval for the barefoot condition.

significantly greater than the chorus shoe, while the ankle remained inverted in the split sole design jazz shoes. In the final 10% of stance, the barefoot condition had the lowest degree of inversion compared to the Evolution, Elastabootie, and Boost. Ankle inversion-eversion ROM was significantly reduced by all jazz shoe designs compared to the barefoot condition (19.89° ± 4.32°; Fig. 6). In the barefoot condition, the ankle was neutral if not slightly adducted at toe strike, abducted until approximately 10% of stance phase, adducted toward a more neutral angle until approximately 15% of stance, then continued to abduct. The ankle reached maximal abduction at 63% of stance phase, and at toe off it was adducted. Jazz shoe design did not significantly affect ankle abductionadduction motion in the first 10% of stance (p = 0.068) or the final 10% of stance (p = 0.552) but had a significant effect at midstance (p = 0.001). The chorus shoe produced less abduction compared to the other jazz shoe designs but not the barefoot condition. Total abduction-adduction ankle ROM was significantly affected by the jazz shoes (p = 0.002), with the barefoot condition (13.30° ± 3.47°) producing statistically similar ankle abduction-adduction ROM to the chorus shoe (12.78° ± 4.23°), significantly lower than the Evolution (16.38° ± 3.09°), Elastabootie (14.91° ± 3.89°) and Boost (15.64° ± 2.91°; Fig. 7).

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Figure 6 Mean inversion-eversion ankle angles in all shoe conditions and 95% confidence interval for the barefoot condition.

Figure 7 Mean abduction-adduction ankle angles in all shoe conditions and 95% confidence interval for the barefoot condition.

Figure 8 Mean plantar flexion-dorsiflexion midfoot angles in all shoe conditions and 95% confidence interval for the barefoot condition.

Midfoot Angles In the barefoot condition, the midfoot was plantar flexed at toe strike,

then dorsiflexed, reaching maximum dorsiflexion at 58% of stance, and at toe off it was plantar flexed. Midfoot

plantar flexion-dorsiflexion motion was significantly reduced in the jazz shoes compared to barefoot throughout stance (p < 0.001). The barefoot condition produced significantly greater plantar flexion in the initial 10% of stance, more dorsiflexion at midstance, and greater plantar flexion in the final 10% of stance compared to all of the shoe conditions. Also, total plantar flexion-dorsiflexion midfoot ROM was significantly greater in the barefoot condition (38.60° ± 8.79°) compared to all shoe conditions (Fig. 8). For the barefoot condition, the midfoot was everted at toe strike, then reached maximum inversion at 51% of stance phase, and at toe off it was inverted. Midfoot inversion-eversion motion was significantly affected by the jazz shoes in the initial 10% of stance (p = 0.002), at midstance (p < 0.001), and in the final 10% of stance (p = 0.002). In the initial 10% of stance, the chorus shoe displayed midfoot inversion, while all other shoe conditions displayed eversion. At midstance the barefoot condition had a significantly greater angle of inversion than the Boost sneaker, while the other shoe conditions produced eversion. In the final 10% of stance, the chorus shoe displayed midfoot inversion, as did the barefoot condition, while the Evolution, Elastabootie, and Boost exhibited eversion. Inversioneversion midfoot ROM was reduced by up to 6° in all jazz shoes compared to barefoot (9.97° ± 4.23°, p < 0.001; Fig. 9). In the barefoot condition at toe strike, the midfoot was adducted, approached a neutral position at 15% of stance phase, then abducted, reaching maximal abduction at 58% of stance, and then at toe off was adducted. The jazz shoe designs had a significant effect on abduction-adduction midfoot motion in the initial 10% of stance (p < 0.001), at midstance (p = 0.015), and in the final 10% of stance (p < 0.001). In the initial 10% of stance, the barefoot condition was significantly more adducted than all of the jazz shoe designs. At midstance, the barefoot condition was significantly more abducted than the Elastabootie

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condition (13.26° ± 5.02°) compared to any of the jazz shoe designs (Fig. 10).

Figure 9 Mean inversion-eversion midfoot angles in all shoe conditions and 95% confidence interval for the barefoot condition.

Figure 10 Mean abduction-adduction midfoot angles in all shoe conditions and 95% confidence interval for the barefoot condition.

First MPJ Angle In the barefoot condition, the MPJ was dorsiflexed at toe strike, reached maximal dorsiflexion at 5% of stance phase, and approached a more neutral angle at 23% of stance phase. Increased MPJ dorsiflexion occurred at 95% stance phase and then was dorsiflexed at toe off. The jazz shoes significantly affected MPJ motion throughout stance (p < 0.001). In the initial 10% of stance, peak dorsiflexion was greater in the barefoot condition compared to all shoe conditions. At midstance, the barefoot condition displayed 2.4° ± 4.40° of dorsiflexion, while all shoe conditions displayed the opposite motion of plantar flexion. In the final 10% of stance, the peak dorsiflexion angle was greater in the barefoot condition than the Evolution, Elastabootie, and Boost, but not the chorus shoe. Total MPJ plantar flexion-dorsiflexion ROM was greatest in the chorus shoe (22.46° ± 4.30°), with similar ROM in the barefoot condition (21.43° ± 5.72°). The chorus shoe had significantly greater ROM than the Evolution (19.62° ± 6.15°), Elastabootie (13.53° ± 4.42°), and Boost (16.96° ± 5.63°; Fig. 11).

Discussion

Figure 11 Mean dorsiflexion-plantar flexion MPJ angles in all shoe conditions and 95% confidence interval for the barefoot condition.

and Boost. In the final 10% of stance, the barefoot was significantly more adducted than the Chorus, Elastabootie, and Boost. In contrast to all other shoe

conditions, the Evolution produced abduction in the final 10% of stance. The total abduction-adduction midfoot ROM was greater in the barefoot

Jazz shoes restricted midfoot plantar flexion-dorsiflexion, inversion-eversion, and abduction-adduction ROM compared to barefoot. In contrast, all jazz shoes increased knee ROM during the demi-plié compared to barefoot, with the chorus shoe producing significantly greater peak knee flexion than all other shoe conditions. Sequential timing of each joint reaching its maximal ROM during the landing identified coordination of the joint motion as flexion-extension, followed by inversion-eversion, then abduction-adduction. It can be hypothesized that the body allocates impact attenuation mostly to joint flexion and extension, while the inversion-eversion and abductionadduction motions that occurred after 50% of stance may act to dissipate the

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remainder of the impact. It was also clearly shown that the instruction to the dancers to “roll through the feet” when landing was executed well, with the delay of the ankle reaching maximum ROM following maximal dorsiflexion of the MPJ and dorsiflexion of the midfoot. This landing technique not only fulfils the requirements of dance examinations but also reduces impact on the body by increasing the number of passive structures through which the impact must transmit.16 The results of this study suggest that, with respect to each shoe condition, the landing strategy that occurs predominantly in the distal segments changes to minimize impact. There were large effects of the jazz shoes on the observed midfoot and ankle motion, but only the high-heeled chorus shoe produced significantly greater knee flexion, and no significant effect of the jazz shoes was found on hip motion. It is interesting to note that although the Elastabootie has a minimal structure (with thin leather upper, thin rubber outsole at the forefoot, a heel outsole that only measures 1 cm thick, and no other materials to provide shock absorption), it produced significant kinematic changes during sauté landings that were similar to the changes seen in the more substantial shoe designs. All jazz shoe designs increased peak knee flexion angles compared to the barefoot condition, which can amplify the risk of injury. An increase of knee flexion will enlarge the patello-femoral contact area, which has been associated with an increased incidence of patello-femoral pain.17 In addition, to control the eccentric phase of landing and stabilize the knee joint at such a large angle of flexion, greater use of the lower limb muscles may be required. Enhanced recruitment of the musculature may add to the risk of repetitive strain injuries. An alternate view could be that an increase in knee flexion angle can be advantageous. Previous research has shown that decreased knee ROM during running was found in runners with low back pain, suggesting that a more compliant knee joint with greater knee flex-

ion during stance phase may reduce the risk of low back pain.18 Further exploration into the kinetics of the sauté landings will provide information about lower limb joint stiffness and torques. Contrary to our hypothesis, only the high-heeled chorus shoes further increased knee flexion angle to 70.95° ± 4.26°, a result previously seen in grand jeté landings in chorus shoes.19 The increase in knee flexion angle in the chorus shoes may be due to the plantar flexed ankle position when the shoe is flat on the floor. Additional knee flexion could also serve to reduce impact transmission to the lower back due to a perception that the chorus shoes decrease the available dorsiflexion range and do not have much intrinsic shock absorbing capacity. Hence, increased knee flexion may protect the lower back from injury at the expense of greater loading at the foot and ankle. At the ankle, jazz shoes showed increased dorsiflexion-plantar flexion ROM compared to barefoot but reduced inversion-eversion ROM. The increased dorsiflexion-plantar flexion ROM correlates with previous work investigating maximal observed active plantar flexion.12 Ankle inversion at toe strike can increase the risk of ankle sprains,20 and the ankle inversion during the initial 10% of stance was greatest in the Evolution sneaker. If the Evolution sneaker can further contribute to ankle sprain risk, then caution must be taken. It has been suggested in a previous study that Irish dancers who wore dance sneakers had a decreased incidence of ankle sprains, due to the thick outsoles reducing inversion-eversion.21 However, this conclusion by the investigators was only conjectural. A study of basketball players found greater inversion angles in the higher ankle support sneakers than the lower ankle support shoe. The investigators suggested that there may have been “forced contact with the ground due to the rigidity of the shoe.”22 Perhaps the construction of the Evolution outsole provided greater leverage at toe strike, causing the shoe to display an increased ankle inver-

sion. In addition, the findings that the tested shoes increased sagittal plane ROM and concomitant reduced frontal plane ROM suggest greater concentric crural (muscle) recruitment due to an alteration of the efficacy of eccentric plantar flexor recruitment. Future electromyographical analysis will further inform on the effects of shoe design. At the midfoot, jazz shoes displayed reduced motion in all three planes. This adds further information to a previous study that examined a static position of maximal plantar flexion and found that the jazz shoe designs reduced midfoot motion despite the negligible sagittal plane bending stiffness values.12 It is interesting to note the considerable difference in shape of the time series curves in the shod conditions compared to the barefoot. Reduction in forefoot mobility during landing could reduce the capacity for the foot to absorb impact and adapt to any irregularities in floor surface.23 Decreased plantar flexion at the propulsion phase suggests a reduced propulsive capacity at the midfoot, which must be compensated through the increase found in ankle plantar flexion. Reduced midfoot motion in the shod conditions found in this study corresponds well with previous research in other footwear types.7,24-26 Stiffness in the shoe outsole reduced torsion movements in the midfoot and has the potential to reduce the risk of injury.27 Jazz shoes reduced the degree of MPJ dorsiflexion at initial impact and at propulsion, which could play a part in reducing impact attenuation and propulsive capacity. Increasing dorsiflexion of the MPJ during the initial 10% of stance will increase the amount of plantar flexion the MPJ must pass through to reach foot flat. With a greater change in angle, the initial impact can be distributed over a longer period of time. A decreased loading rate has been identified as a key variable in assessing impact attenuation.28 In the final 10% of stance, first MPJ dorsiflexion increases the tensile force of the plantar aponeurosis in conjunction with contraction of the

Journal of Dance Medicine & Science • Volume 18, Number 4, 2014

gastrocnemius and soleus, creating the windlass effect, which assists with propulsion.29-31 Whether the changes in kinematics due to the use of jazz shoes are detectable by an audience, dance-trained or not, is yet to be explored. If the differences can be discerned by a dance teacher, then corrections in dance technique can be made; however, if a non-dancer audience can see the differences in angle, this could be detrimental to performance. With regard to the overall dance aesthetic, jump landings in the jazz shoe designs tested may appear to be heavier due to the greater reliance on knee flexion to absorb impact and reduced push-off for subsequent jumps. Future studies should compare the motion capture data to subjective ratings to determine the acceptable change in angle before a reduction in dance performance is perceived. It is possible that our instruction to the dancers to have well-executed dance technique, specifically quiet landings, altered the landing kinematics so that the ankle was more plantar flexed and the knee more flexed at initial ground contact. Technical instruction to increase ankle plantar flexion and knee flexion was given to subjects in a study by McNair and colleagues, which successfully softened drop landings, and it was also found that the auditory cue of the sound from previous landings could soften subsequent landings.32 The instruction to land softly has also been found to increase knee flexion angle.33 Since the chorus shoes have a hard heel, after initial practice jumps or simply walking to the testing area, the dancers may have already used this auditory cue to increase knee flexion in order to attain a quiet landing. Exploration of the effect of shoe design on proprioception and the resultant effect on dance performance is required. This study used externally mounted markers on the shoe and skin mounted markers on the ankle, leg, and pelvis. It has been found that markers mounted on the shoe can overestimate skeletal motion.34 For future research investigating the effect

of shoe design on foot and ankle motion, holes could be cut into the shoes to allow markers to be placed directly on the bony landmarks. However, many dance shoe designs are quite minimal in the structure of the shoe’s upper. The materials used may lose their integrity and potentially their true effect on foot and ankle motion if they have been heavily modified. Also, the aesthetic nature of dance performance lends itself to having motion of the shoe represent the motion of the foot and ankle. It is important to note that the axes of motion for the MPJ and midfoot may not align with the axes for sagittal plane bending in the shoe design. This could explain the restriction in foot motion seen in this study. Future shoe design should consider the anatomical requirements of dance movement. For dance teachers, when giving technique corrections, there should be an awareness that shoe motion may not equate to foot and ankle motion. If there is pronation of the foot inside the shoe, it is not seen by the teacher, which could lead to ongoing propagation of bad technique. Perhaps an advantage for performance is that the audience will see a more stable foot upon landing, which could give the illusion of a safe and controlled landing. Kinematic analysis of the lower limb during a sauté landing confirmed that the pre-professional and professional dancers in this study employed correct execution of dance technique. The results of this study suggest that the instruction given by dance teachers, to “roll through the foot” and to strongly “push off with the toes,” has scientific basis for impact attenuation and propulsion and can be implemented effectively. Despite the observable structural differences among the three split-sole designed jazz shoes, no significant differences in lower limb kinematics were found. Increased knee flexion angles during sauté landings in the jazz shoe conditions are of concern as they may be a contributing factor to injury. When selecting dance shoes, it is important to consider the kinematic changes

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that may negatively affect dance performance and dancer safety. Acknowledgment The authors wish to thank Bloch Australia for providing the tested shoes.

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Kinematic analysis of sautés in barefoot and shod conditions.

Dancers are exposed to many landings from jumps during class and performance, and repetitive loading has been linked with an increased risk of injury...
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