- Email: [email protected]
Impact attenuation properties of jazz shoes alter lower limb joint stiffness during jump landings
Accepted Manuscript Title: Impact attenuation properties of jazz shoes alter lower limb joint stiffness during jump landings Author: Alycia Fong Yan Richard M. Smith Claire E. Hiller Peter J. Sinclair PII: DOI: Reference:
S1440-2440(16)30211-0 http://dx.doi.org/doi:10.1016/j.jsams.2016.09.011 JSAMS 1397
To appear in:
Journal of Science and Medicine in Sport
Received date: Revised date: Accepted date:
3-2-2016 30-5-2016 10-9-2016
Please cite this article as: {http://dx.doi.org/ This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Impact attenuation properties of jazz shoes alter lower limb joint stiffness during jump landings
Alycia Fong Yana, Richard M. Smitha, Claire E. Hillera and Peter J. Sinclaira
Address: The University of Sydney, Cumberland Campus, 75 East St Lidcombe, NSW, 2141, AUSTRALIA a
Faculty of Health Sciences, The University of Sydney, Sydney, Australia
Corresponding author: Alycia Fong Yan. Email: [email protected] Word Count: 2998 Number of Tables: 3 Number of Figures: 0 1
Abstract Objectives: To quantify the impact attenuation properties of the jazz shoes, and to investigate the in-vivo effect of four jazz shoe designs on lower limb joint stiffness during a dance-specific jump. Design: Repeated measures. Methods: A custom-built mechanical shoe tester similar to that used by athletic shoe companies was used to vertically impact the forefoot and heel region of four different jazz shoe designs. Additionally, dancers performed eight sautés in second position in bare feet and the shoe conditions. Force platforms and 3Dmotion capture were used to analyse the joint stiffness of the midfoot, ankle, knee and hip during the jump landings. Results: Mechanical testing of the jazz shoes revealed significant differences in impact attenuation characteristics among each of the jazz shoe designs. Gross knee and midfoot joint stiffness were significantly affected by the jazz shoe designs in the dancers’ jump landings. Conclusions: The tested jazz shoe designs altered the impact attenuating capacity of jump landing technique in dancers. The cushioned jazz shoes are recommended particularly for injured dancers to reduce impact on the lower limb. Jazz shoe design should consider the impact attenuation properties of the forefoot region, due to the toe-strike landing technique in dance movement.
Keywords: ankle joint; cushioning; forefoot; knee joint; dance; foot
2
Introduction Dance performance and practice features highly repetitive loading; however, the implementation of periodization throughout the calendar year or performance schedule to allow adequate recovery is rare.1, 2 Combining this with a culture that promotes “dancing through pain” to ensure opportunities are not lost 3 could potentially put dancers at a greater risk of injury. Proper selection of footwear has the potential to attenuate impact forces transmitted into the body4 Despite dancers performing approximately 200 jumps per 1.5 hour class each day5, the impact attenuation properties of dance shoes have not been quantified.6 The relationship between the impact attenuating characteristics of shoes and injury risk is not yet fully understood, however, several studies have shown that shoe design has the potential to reduce impactrelated injury risk.7-10 Athletic shoe testing has shown that various midsole materials are capable of attenuating impact.10 Investigations into the biomechanical characteristics of weight bearing activities when wearing shoes compared to bare feet yield a variety of results.4 It has been found that changes in muscle activation occur in response to shoe cushioning prior to ground contact11 due to the perception of the hardness of the landing surface,12 and greater impact force when landing on a soft surface during gait and drop landings was due to the body allowing the landing surface to dissipate the impact as opposed to requiring the body to absorb the impact.13 Furthermore, a study found reduced ankle joint mechanical demand with reduced hardness of dance floors 14 However, running shod compared to barefoot showed no difference in ankle or knee joint stiffness values.15 Joint stiffness can also be modified by changing foot strike pattern during gait.16 Inconsistent effects of cushioning could be a result of the variability of the human response to cushioning in conjunction with the mechanical properties of the shoe. In dance jump technique, the feet and ankles must be plantar flexed in the air with toe strike at initial ground contact, while between jumps the feet remain flat on the floor during the half-squat (demi-plié).17
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To maintain the requisite dance aesthetic of ease of movement and weightlessness, jump landings should be quiet and controlled. The common instruction to dancers is to “roll through the balls of the feet” and to “lift up when landing” to emphasise the control of the landing and appearance of weightlessness.18 Correct technique is thought to be facilitated through eccentric control of metatarsophalangeal and ankle dorsiflexion, and knee and hip flexion. There is a large variety of jazz shoe designs available and many long-held common beliefs on the contribution of dance shoe design to performance and injury risk.19, 20 Rather than simply applying the findings of athletic shoe research to dance footwear, investigations must be dance-specific in order to draw appropriate conclusions. Research exploring the effect of dance footwear is sparse in comparison to athletic footwear.6 The primary aim of this study was, therefore, to quantify the impact attenuation characteristics of different designs of jazz shoes using a mechanical testing rig. We hypothesised that the jazz sneaker, with more cushioning and a thicker outsole, would have greater impact attenuation capacity. A secondary aim of this study was to investigate the effect of different jazz shoe designs on dancers’ joint stiffness during a dance jump landing. Knee joint stiffness was hypothesised to be greater in the more cushioned shoes. Methods Four jazz shoe designs were selected for testing, three with a split-sole design (separate forefoot and rearfoot outsoles) and one high-heeled design (Table 1). Impact attenuation properties of the shoes were measured in a custom mechanical impact rig according to the design and protocol outlined in F1614.21 The impact rig dropped a flat bottomed mass of 8.5kg to land with a kinetic energy of 5J. New specimens of the Elastabootie, Evolution, Boost and Chorus jazz shoes (Bloch, Australia) were prepared and tested with five impacts at the forefoot region and five impacts at the heel region of each shoe. Following standard protocol, the variables analysed were: peak force, maximum displacement of the mass in the sole of the shoe, total impact time, hysteresis energy ratio (HER), and normalised average stiffness.21 The mean energy applied to the shoe specimens was 4.8±0.4 J, which conformed to standard protocol.21
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Repeated measures ANOVA’s were used to determine the effect of shoe condition on each variable with level for significance set at p<0.05. Sixteen female dancers (mean age: 25.0 ± 5.9 years, mean mass: 56.0 ± 7.4 kg) volunteered for the in vivo study. All participants were required to have attained a minimum of Royal Academy of Dance (RAD) Intermediate syllabus standard, or equivalent, to ensure consistent and proficient technique execution. Dancers were excluded if they had a current injury that reduced their class or performance participation. All participants gave informed consent; the study is in accordance with the National Statement on Ethical Conduct in Human Research (2007) issued by the National Health and Medical Research Council (NHMRC) in accordance with the NHMRC Act, 1992(Cth), and was approved by the institutional Human Research Ethics Committee (reference number: 2012/467). 3D motion analysis of 35 markers placed on bony landmarks on the pelvis and lower limbs was used to capture the dance movement.22 Marker locations were used to define the segments and calculate the joint centres.23 The task was performed in each of the five shoe conditions (barefoot, three split sole jazz shoe designs of increasing outsole thickness, and the high heeled shoe) in a randomised order (randomization.org), with time allowed for familiarisation with each shoe condition immediately prior to testing. For the shod conditions, markers were placed on the outside of the shoes in the corresponding positions to the bony landmarks. Dancers were instructed to perform eight sautés, bilateral vertical jumps, with the feet on two separate force plates (Model 9287BA, Kistler, Switzerland). Dancers performed sautés in time with RAD syllabus Grade 1 music. Consecutive jumps were performed in second position (feet slightly wider than shoulderwidth), with maximal external rotation of the hips, and weight evenly distributed between the feet. The principal investigator observed the force vectors during the trials to ascertain even weight distribution. A trial was deemed unsuccessful if the heels did not stay on the floor in the demi-plié during stance phase.24 Erect posture was maintained throughout the task and dancers were instructed to keep their hands on their
5
waist as they were not allowed to use their arms to assist the jump. Dancers were given ample time to warm up and practice the dance movement. Force values were normalised to body weight in each shoe condition.25 Pilot testing found no significant differences between legs so, for reasons of clarity, only data from the left leg has been presented. Continuous joint stiffness values were time normalised to 100% of stance, from toe strike to toe off. Gross joint stiffness values were calculated as the total change in moment over the total change in angle 16, from toe strike to peak joint angle. To determine the impact attenuation effect of the jazz shoe designs on the settings of discrete joint stiffness during the initial impact of the jump landing, maximum, minimum and mean joint stiffness values were also calculated during the first 40% of stance. The time of maximum knee flexion occurred at 50% of stance; although, the maximum angle of other joints and planes of motion may not have been reached at this point. The data close to 50% of stance was excluded to avoid mathematical results approaching infinity as the angles of the lower limbs reach their maximum and begin to move in the opposite direction. Repeated measures ANOVA was used to determine the effect of shoe condition on the gross and discrete joint stiffness values on the hip, knee, ankle and midfoot, with the level for significance set at p<0.05. Results Mechanical testing of the shoes showed significant differences (p<0.05) among all the jazz shoe designs at both the heel and forefoot test regions (Table 2). The Chorus shoe was the least impact attenuating shoe, while the Evolution shoe was the best impact attenuating shoe. At the forefoot, the Evolution performed the best in most variables to reduce the impact; although, the Boost had a greater peak displacement compared to all other shoe conditions (p<0.05). At the heel, the Evolution performed the best in all variables to reduce impact attenuation compared to all other shoe conditions (p<0.05). Compared to the other shoe conditions, the Evolution displayed the lowest peak force at both the forefoot (p<0.001) and rearfoot regions (p<0.001), had the greatest displacement at the forefoot (p<0.001) and rearfoot (p<0.05) regions, greatest impact time at the forefoot and rearfoot regions (p<0.05), greatest
6
hysteresis energy ratio at both the forefoot and rearfoot regions (p<0.001) and lowest mean stiffness at the forefoot (p<0.05) and rearfoot regions (p≤0.001). In contrast, the Chorus shoe displayed the greatest peak force and mean stiffness at both forefoot and rearfoot regions (p<0.001), the lowest peak displacement and hysteresis energy ratio at both rearfoot and forefoot regions (p<0.001). The Elastabootie displayed significantly less impact time compared to all other shoe conditions (p<0.001) (Table 2). Jazz shoe design had a significant effect on midfoot joint stiffness (p=0.005). Pairwise comparisons between the shoe conditions showed the midfoot joint stiffness in the Chorus shoe was significantly less than the Evolution (p=0.008), Elastabootie (p=0.012), and the Boost (p=0.013) (Table 3). Overall, the jazz shoes significantly affected knee joint stiffness (p <0.001). The barefoot condition had significantly greater knee joint stiffness compared to the Evolution (p=0.004), Chorus (p<0.001), and the Boost (p=0.029). The Chorus shoe also produced significantly lower knee joint stiffness compared to the Evolution (p<0.001) and the Elastabootie (p<0.001) (Table 3). Ankle and hip gross joint stiffness were not significantly affected by the jazz shoe designs, p=0.434 and p=0.384 respectively (Table 3). In addition, no significant effect of the jazz shoe designs on peak, minimum or mean joint stiffness values during the initial 40% of stance was found for any joint (p>0.05). Discussion and Conclusions The effect of the cushioning in the four jazz shoe designs investigated had several different outcomes. Mechanical testing showed a range of values for the mechanical parameters across the shoe specimens with a clear difference in impact attenuation capacity. However in vivo testing revealed the cushioning of the different shoe designs had little effect on lower limb joint stiffness, except at the knee. Mechanical testing of the jazz shoes revealed significant differences in the material characteristics among all four shoe designs. Contrary to our hypothesis, that the Boost sneaker would be the most impact attenuating design because of its thicker sole, the Evolution sneaker reduced peak force over a longer duration than the other shoe designs. Despite the thinner sole, the newer Evolution may incorporate improved material technology and construction that dissipates impact more efficiently than its
7
predecessor, the Boost sneaker. As expected, minimal cushioning in the Elastabootie led to minimal displacement of the mass into the sole of the shoe. The Evolution sneaker produced a greater hysteresis energy ratio compared to the other shoe designs meaning the materials could return more energy to the foot. Whether this is advantageous or not is unknown at this stage. For rebounding movements like sautés, energy return would be beneficial to assist dancers to achieve adequate jump height in the subsequent jumps. This can only occur, however, if the timing of energy return and propulsion are simultaneous. In vivo, when sautés were performed to music, the stance time was 40ms. In contrast, the time of the energy return phase during mechanical testing of the shoes was only 25ms. The energy returned by the shoes to the lower limb is therefore occurring during the stance phase when the foot is stationary, thus transmitting the impact into the foot rather than assisting with propulsion. The knee joint was found to be stiffer during barefoot landings compared to the two jazz sneaker designs, while the Elastabootie produced similar knee joint stiffness to barefoot. No increase in gross joint stiffness was found in the other joints to compensate for the softer knee joint in the shod conditions. Moreover, gross stiffness values at the midfoot, ankle and hip were fundamentally zero with large variability in the results. Jump landing research has not previously investigated the effect of different footwear; therefore a direct comparison cannot be made. The results of this study counter previous research on running shoes, which found that running in shoes resulted in no change to either ankle or knee joint stiffness.15 Other than the considerable difference in landing technique, runners may alter their joint stiffness as they wish, according to external cues from the environment or footwear. As dancers approached toe-strike in maximal ankle plantar flexion, the ankle was required to move through a large range of motion to ensure the foot was flat on the floor and the demi-plié position achieved. This range must be completed by midstance, a period of approximately 20 ms depending on the tempo of the music. Allowing the ankle joint to be compliant and reducing joint stiffness close to zero is advantageous to completing the movement in the given amount of time. Similarly, stiffness values were extremely low at
8
the hip joint where the trunk must remain upright in conjunction with hip external rotation and flexion during the demi-plié movement. This suggests the knee joint is largely responsible for the propulsive action during rebounding jumps while the other joints in the lower limb may act to reduce impact. The plantar flexed angle of the ankle when wearing high-heeled Chorus shoes reduced the range of motion available for the ankle22 to absorb impact, thus requiring greater knee flexion22, hence a more compliant knee joint during the demi-plié movement to help dissipate impact. Previous research has shown that instruction to land softly reduced peak vertical ground reaction force, however the mechanism by which this was achieved is not clear.26 In this study, it appears that the low stiffness values at the hip, ankle, and midfoot may contribute to the soft landing. In addition, the tempo of the music used in this study may not have been fast enough to elicit a stiffer ankle joint to optimise the energy return during the rebounding jumps.27 In the barefoot condition, it is possible that the need to protect the knee joint from large flexion angles, by reducing ankle joint stiffness and allowing ankle joint motion, may override the requirement for a quiet landing, whereas the quiet landing becomes a higher priority with the hard outsoles of the jazz shoes. The negative gross joint stiffness of the midfoot in the Chorus shoes may represent the foot sliding within the shoe due to the high rake. The gross joint stiffness value was opposite to that displayed in the other shoe conditions and warrants further investigation as to the mechanism of the midfoot stiffness. The variability in the stiffness values could be indicative of how well the shoes fit, the variance in flexibility of the foot, or different strategies for impact attenuation. Future studies could explore methodologies to view the in-shoe motion of the foot and determine whether this motion is beneficial for impact attenuation or detrimental for foot health. Discrete joint stiffness values were hypothesised to be increased in the shoes with greater cushioning, in line with Robbins’ theory that the joints will be stiffer if the dancer perceived the shoe to be soft as opposed to hard.12 The lack of significant effect found in the discrete joint stiffness values could be attributed to the large variability found across the sixteen dancers. The landing strategies employed by
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each individual dancer were varied, with some displaying stiffness peaks early in the landing, corresponding to the timing of the ball of the foot contacting the floor, and others showing a more gradual increase in joint stiffness peaking closer to mid stance. Perhaps, due to the familiarisation and practice period, once data collection commenced the dancers had already adopted their preferred landing strategy accounting for the new shoe condition resulting in no difference between shoe conditions. There does not appear to be a defined relationship between the capacity of the dance shoes to attenuate impact during mechanical testing and the response of the dancers to the different dance shoe designs. Although the Evolution sneaker can be deemed the most impact attenuating following the mechanical testing, it did not elicit any difference in lower limb joint stiffness compared to the other dance shoe designs. Investigation of the interaction between the lower limb and footwear is needed to better understand the overall effect on impact attenuation for an individual. Low joint stiffness has purported to be associated with an increase in soft tissue injury risk 27 and the results of this study showed that regardless of shoe condition the ankle joint stiffness was close to zero; neither increasing or decreasing the possible risk of soft tissue injury at the ankle. The dance jump tested in this study was a bilateral jump take-off and landing, that did not travel across the room. The results of the current study therefore may not apply to other jump types, particularly higher intensity and single leg landings. All of the shoe conditions reduced knee joint stiffness compared to the barefoot condition and it has been suggested that increased joint stiffness values may increase the risk of bony injuries such as stress fractures27. However, the ideal range for joint stiffness values is unknown and recommendations as to footwear selection contributing to knee injury risk or prevention cannot be made. Conclusion This study has found that jazz shoe design can affect the biomechanical characteristics of jump landings and successfully quantified the impact attenuation characteristics of different jazz shoe designs. Dance technique requirements appear to strongly influence the mechanics of jump landings. Considering that a majority of dance movements have a toe-heel footfall sequence, equal emphasis should be placed on
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shock absorbing materials at the forefoot as at the heel. Future dance shoe design should be evidencebased, accounting for both function and aesthetic. Other dance shoe designs can be compared to the ones in this study and subsequent research can potentially find associations with injury risk. Practical Implications
Shock absorbing capacity of dance shoes cannot be judged based on the appearance of a thick sole
Shock absorbing properties of dance shoes should be considered for the forefoot region because dancers land toe-first from jumps
Jazz shoe design alters the shock absorbing capacity of jump landing technique in dancers
Jump landings when barefoot put the knee joint under greater stress than when wearing jazz shoes
None of the shoe conditions had an effect on ankle joint stiffness and so may not influence soft tissue injuries at the ankle
Acknowledgements The authors wish to thank Bloch Australia for kindly providing the shoes for testing, and Ray Patton for his assistance building the shoe tester.
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References 1. Wyon M. Preparing to perform: periodization and dance. J Dance Med Sci. 2010; 14(2):67-72. 2. Wyon MA, Twitchett E, Angioi M, et al. Time motion and video analysis of classical ballet and contemporary dance performance. Int J Sports Med. 2011; 32(11):851-855. 3. Stretanski MF, Weber GJ. Medical and rehabilitation issues in classical ballet. Am J Phys Med Rehab. 2002; 81(5):383-391. 4. Fong Yan A, Sinclair PJ, Hiller C, et al. Impact attenuation during weight bearing activities in barefoot vs. shod conditions: A systematic review. Gait Posture. 2013; 38(2):175-186. 5. Liederbach M, Richardson M, Rodriguez M, et al. Jump exposures in the dance training environment: a measure of ergonomic demand. J Athl Training. 2006; 41(2 Suppl.):S85-86. 6. Fong Yan A, Hiller C, Smith R, et al. Effect of footwear on dancers: a systematic review. J Dance Med Sci. 2011; 15(2):86-92. 7. Grier TL, Knapik JJ, Swedler D, et al. Footwear in the United States Army Band: injury incidence and risk factors associated with foot pain. Foot. 2011; 21(2):60-65. 8. Hawke F, Burns J, Radford J, et al. Custom-made foot orthoses for the treatment of foot pain (Review). Cochrane DB Syst Rev. 2008(3):1-91. 9. Hoydicz J. Stress fractures in athletes. O&P Business News. Vol 112009:21-25. 10. Forner A, Garcia A, Alcantara E, et al. Properties of shoe insert materials related to shock wave transmission during gait. Foot Ankle Int. 1995; 16(12):778-786. 11. Nigg B, Stefanyshyn D, Cole G, et al. The effect of material characteristics of shoe soles on muscle activation and energy aspects during running. J Biomech. 2003; 36:569-575. 12. Robbins S. Hazard of deceptive advertising of athletic footwear. Brit J Sport Med. 1997; 31(4):299303. 13. Robbins S, Waked E. Balance and vertical impact in sports: role of shoe sole materials. Arch Phys Med Rehab. 1997; 78(5):463-467.
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14. Hopper LS, Alderson JA, Elliott BC et al. Dance floor force reduction influences ankle loads in dancers during drop landings. J Sci Med Sport. 2015; 18(4):480-485. 15. Hamill J, Russell EM, Gruber AH, et al. Impact characteristics in shod and barefoot running. Footwear Sci. 2011; 3(1):33-40. 16. Hamill J, Moses M, Seay J. Lower extremity joint stiffness in runners with low back pain. Res Sports Med. 2009; 17(4):260-273. 17. Abergel D. Plié in practice. Pointe. 2006; 7(2):72-73. 18. Franklin E. Floors and soft landings, in Dance imagery for technique and performance. Champaign, IL, Human Kinetics, 1996. 19. Stretanski MF, Weber GJ. Medical and rehabilitation issues in classical ballet. Am J Phys Med Rehabil. 2002; 81(5):383-391. 20. Weiss DS, Shah S, Burchetter RJ. A profile of the demographics and training characteristics of professional modern dancers. J Dance Med Sci. 2008; 12(2):41-46. 21. ASTM. Standard test method for shock attenuating properties of materials systems for athletic footwear. F 1614-99. West Conshohocken, Pennsylvania: ASTM International; 2006. 22. Fong Yan A, Hiller C, Sinclair PJ, et al. Kinematic analysis of sautés in barefoot and shod conditions. J Dance Med Sci. 2014; 18(4):149-158. 23. Wu G, Siegler S, Allard P, et al. ISB recommendation on definitions of the joint coordinate system of various joints for the reporting of human joint motion - part I: ankle, hip, and spine. J Biomech. 2002; 35(4):543-548. 24. Walter HL, Docherty CL, Schrader J. Ground reaction forces in ballet dancers landing in flat shoes versus pointe shoes. J Dance Med Sci. 2011; 15(2):61-64. 25. Mullineaux D, Milner CE, Davis IS, et al. Normalization of ground reaction forces. J Appl Biomech. 2006; 22(3):230-233. 26. Milner CE, Fairbrother JT, Srivatsan A, et al. Simple verbal instruction improves knee biomechanics during landing in female athletes. Knee. 2012; 19(4):399-403. 27. Butler RJ, Crowell Iii HP, Davis IM. Lower extremity stiffness: implications for performance and injury. Clin Biomech. 2003; 18(6):511-517.
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Table 1. Description of shoe conditions. Shoe Name
Brand/Name/Number
Description
Barefoot
N/A
Control condition
Elastabootie
Bloch
Elastabootie Slim line leather jazz shoe with separate
S0499L
forefoot and rearfoot sections (split sole design). 3.5mm forefoot 13mm rearfoot thickness
Evolution
Bloch
Evolution Split sole design low profile jazz sneaker.
Dance Sneaker S0510
Multi-density rubber outsole and additional divisions in the forefoot section. Air punched compressed EVA sock material. 14mm
forefoot
and
20mm
rearfoot
thickness. Boost
Bloch Classic Boost Split sole design jazz sneaker with thick S0538L
multi-density
rubber
outsole
and
compressed EVA sock material. 24mm forefoot and 38mm rearfoot thickness.
Chorus
Bloch Cabaret S0306
50mm high heel court shoe with ankle strap
14
Table 2. Impact attenuation characteristics of the jazz shoes, mean (SD). Variable
Region
Elastabootie
Evolution
Boost
Chorus
Peak Force (N)
Heel
1933(92)b
1068.45(28)b
1172(37)
5181(325)b
Forefoot
3983(321) b
1470.07(79) b
1590.50(73)
4197(359)b
Heel
5.35 (0.07)b
11.01 (0.07)a
10.62 (0.18)
4.14 (0.13)b
Forefoot
3.60 (0.09)b
8.03 (0.04)a
8.41 (0.12)
3.23 (0.24)b
Heel
16.48(0.18)b
35.08(0.23)a
32.60(1.17)
11.64(1.23)b
Forefoot
10.04(0.09)b
25.48(0.41)a
24.48(0.54)
10.32(0.09)b
Heel
0.077(0.002)b
0.286(0.007)b
0.245(0.009)
0.006(0.001)b
Forefoot
0.004(0.000)b
0.213(0.004)b
0.201(0.004)
0.002(0.001)b
Heel
360(21)a
97(2.2)b
110(5.3)
1,310(180)b
Forefoot
1,100(114)a
180(9.3)b
190(11)
1,260(123)b
Peak Displacement (mm)
Impact Time (ms)
HER (J)
Mean Stiffness (N/mm)
a
denotes significant difference to the Boost shoe (p<0.05), bdenotes significant difference to the Boost shoe (p<0.001).
15
Table 3. Gross joint stiffness values (N. mm/degree) for each joint in each shoe condition, mean (SD). Joint
Barefoot
Elastabootie
Hip
30.38(24.53)
103.89(243.11) 77.49(81.62)a 114.46(259.98) 60.64(183.67)
Knee
34.81(14.23)b 27.88(14.16)b
25.42(9.49)ab
16.58(21.17)a
15.27(7.56)a
Ankle
40.39(12.27)
39.55(12.34)
39.52(10.90)
40.41(9.52)
37.63(9.41)
2.68(31.41)b
4.51(26.89)b
5.21(28.94)b
-6.75(22.93)
Midfoot 5.32(29.89) a
Evolution
Boost
Chorus
denotes a significant difference to the barefoot condition, p<0.05. b denotes a significant difference
to the Chorus shoe.
16
S1440-2440(16)30211-0 http://dx.doi.org/doi:10.1016/j.jsams.2016.09.011 JSAMS 1397
To appear in:
Journal of Science and Medicine in Sport
Received date: Revised date: Accepted date:
3-2-2016 30-5-2016 10-9-2016
Please cite this article as: {http://dx.doi.org/ This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Impact attenuation properties of jazz shoes alter lower limb joint stiffness during jump landings
Alycia Fong Yana, Richard M. Smitha, Claire E. Hillera and Peter J. Sinclaira
Address: The University of Sydney, Cumberland Campus, 75 East St Lidcombe, NSW, 2141, AUSTRALIA a
Faculty of Health Sciences, The University of Sydney, Sydney, Australia
Corresponding author: Alycia Fong Yan. Email: [email protected] Word Count: 2998 Number of Tables: 3 Number of Figures: 0 1
Abstract Objectives: To quantify the impact attenuation properties of the jazz shoes, and to investigate the in-vivo effect of four jazz shoe designs on lower limb joint stiffness during a dance-specific jump. Design: Repeated measures. Methods: A custom-built mechanical shoe tester similar to that used by athletic shoe companies was used to vertically impact the forefoot and heel region of four different jazz shoe designs. Additionally, dancers performed eight sautés in second position in bare feet and the shoe conditions. Force platforms and 3Dmotion capture were used to analyse the joint stiffness of the midfoot, ankle, knee and hip during the jump landings. Results: Mechanical testing of the jazz shoes revealed significant differences in impact attenuation characteristics among each of the jazz shoe designs. Gross knee and midfoot joint stiffness were significantly affected by the jazz shoe designs in the dancers’ jump landings. Conclusions: The tested jazz shoe designs altered the impact attenuating capacity of jump landing technique in dancers. The cushioned jazz shoes are recommended particularly for injured dancers to reduce impact on the lower limb. Jazz shoe design should consider the impact attenuation properties of the forefoot region, due to the toe-strike landing technique in dance movement.
Keywords: ankle joint; cushioning; forefoot; knee joint; dance; foot
2
Introduction Dance performance and practice features highly repetitive loading; however, the implementation of periodization throughout the calendar year or performance schedule to allow adequate recovery is rare.1, 2 Combining this with a culture that promotes “dancing through pain” to ensure opportunities are not lost 3 could potentially put dancers at a greater risk of injury. Proper selection of footwear has the potential to attenuate impact forces transmitted into the body4 Despite dancers performing approximately 200 jumps per 1.5 hour class each day5, the impact attenuation properties of dance shoes have not been quantified.6 The relationship between the impact attenuating characteristics of shoes and injury risk is not yet fully understood, however, several studies have shown that shoe design has the potential to reduce impactrelated injury risk.7-10 Athletic shoe testing has shown that various midsole materials are capable of attenuating impact.10 Investigations into the biomechanical characteristics of weight bearing activities when wearing shoes compared to bare feet yield a variety of results.4 It has been found that changes in muscle activation occur in response to shoe cushioning prior to ground contact11 due to the perception of the hardness of the landing surface,12 and greater impact force when landing on a soft surface during gait and drop landings was due to the body allowing the landing surface to dissipate the impact as opposed to requiring the body to absorb the impact.13 Furthermore, a study found reduced ankle joint mechanical demand with reduced hardness of dance floors 14 However, running shod compared to barefoot showed no difference in ankle or knee joint stiffness values.15 Joint stiffness can also be modified by changing foot strike pattern during gait.16 Inconsistent effects of cushioning could be a result of the variability of the human response to cushioning in conjunction with the mechanical properties of the shoe. In dance jump technique, the feet and ankles must be plantar flexed in the air with toe strike at initial ground contact, while between jumps the feet remain flat on the floor during the half-squat (demi-plié).17
3
To maintain the requisite dance aesthetic of ease of movement and weightlessness, jump landings should be quiet and controlled. The common instruction to dancers is to “roll through the balls of the feet” and to “lift up when landing” to emphasise the control of the landing and appearance of weightlessness.18 Correct technique is thought to be facilitated through eccentric control of metatarsophalangeal and ankle dorsiflexion, and knee and hip flexion. There is a large variety of jazz shoe designs available and many long-held common beliefs on the contribution of dance shoe design to performance and injury risk.19, 20 Rather than simply applying the findings of athletic shoe research to dance footwear, investigations must be dance-specific in order to draw appropriate conclusions. Research exploring the effect of dance footwear is sparse in comparison to athletic footwear.6 The primary aim of this study was, therefore, to quantify the impact attenuation characteristics of different designs of jazz shoes using a mechanical testing rig. We hypothesised that the jazz sneaker, with more cushioning and a thicker outsole, would have greater impact attenuation capacity. A secondary aim of this study was to investigate the effect of different jazz shoe designs on dancers’ joint stiffness during a dance jump landing. Knee joint stiffness was hypothesised to be greater in the more cushioned shoes. Methods Four jazz shoe designs were selected for testing, three with a split-sole design (separate forefoot and rearfoot outsoles) and one high-heeled design (Table 1). Impact attenuation properties of the shoes were measured in a custom mechanical impact rig according to the design and protocol outlined in F1614.21 The impact rig dropped a flat bottomed mass of 8.5kg to land with a kinetic energy of 5J. New specimens of the Elastabootie, Evolution, Boost and Chorus jazz shoes (Bloch, Australia) were prepared and tested with five impacts at the forefoot region and five impacts at the heel region of each shoe. Following standard protocol, the variables analysed were: peak force, maximum displacement of the mass in the sole of the shoe, total impact time, hysteresis energy ratio (HER), and normalised average stiffness.21 The mean energy applied to the shoe specimens was 4.8±0.4 J, which conformed to standard protocol.21
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Repeated measures ANOVA’s were used to determine the effect of shoe condition on each variable with level for significance set at p<0.05. Sixteen female dancers (mean age: 25.0 ± 5.9 years, mean mass: 56.0 ± 7.4 kg) volunteered for the in vivo study. All participants were required to have attained a minimum of Royal Academy of Dance (RAD) Intermediate syllabus standard, or equivalent, to ensure consistent and proficient technique execution. Dancers were excluded if they had a current injury that reduced their class or performance participation. All participants gave informed consent; the study is in accordance with the National Statement on Ethical Conduct in Human Research (2007) issued by the National Health and Medical Research Council (NHMRC) in accordance with the NHMRC Act, 1992(Cth), and was approved by the institutional Human Research Ethics Committee (reference number: 2012/467). 3D motion analysis of 35 markers placed on bony landmarks on the pelvis and lower limbs was used to capture the dance movement.22 Marker locations were used to define the segments and calculate the joint centres.23 The task was performed in each of the five shoe conditions (barefoot, three split sole jazz shoe designs of increasing outsole thickness, and the high heeled shoe) in a randomised order (randomization.org), with time allowed for familiarisation with each shoe condition immediately prior to testing. For the shod conditions, markers were placed on the outside of the shoes in the corresponding positions to the bony landmarks. Dancers were instructed to perform eight sautés, bilateral vertical jumps, with the feet on two separate force plates (Model 9287BA, Kistler, Switzerland). Dancers performed sautés in time with RAD syllabus Grade 1 music. Consecutive jumps were performed in second position (feet slightly wider than shoulderwidth), with maximal external rotation of the hips, and weight evenly distributed between the feet. The principal investigator observed the force vectors during the trials to ascertain even weight distribution. A trial was deemed unsuccessful if the heels did not stay on the floor in the demi-plié during stance phase.24 Erect posture was maintained throughout the task and dancers were instructed to keep their hands on their
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waist as they were not allowed to use their arms to assist the jump. Dancers were given ample time to warm up and practice the dance movement. Force values were normalised to body weight in each shoe condition.25 Pilot testing found no significant differences between legs so, for reasons of clarity, only data from the left leg has been presented. Continuous joint stiffness values were time normalised to 100% of stance, from toe strike to toe off. Gross joint stiffness values were calculated as the total change in moment over the total change in angle 16, from toe strike to peak joint angle. To determine the impact attenuation effect of the jazz shoe designs on the settings of discrete joint stiffness during the initial impact of the jump landing, maximum, minimum and mean joint stiffness values were also calculated during the first 40% of stance. The time of maximum knee flexion occurred at 50% of stance; although, the maximum angle of other joints and planes of motion may not have been reached at this point. The data close to 50% of stance was excluded to avoid mathematical results approaching infinity as the angles of the lower limbs reach their maximum and begin to move in the opposite direction. Repeated measures ANOVA was used to determine the effect of shoe condition on the gross and discrete joint stiffness values on the hip, knee, ankle and midfoot, with the level for significance set at p<0.05. Results Mechanical testing of the shoes showed significant differences (p<0.05) among all the jazz shoe designs at both the heel and forefoot test regions (Table 2). The Chorus shoe was the least impact attenuating shoe, while the Evolution shoe was the best impact attenuating shoe. At the forefoot, the Evolution performed the best in most variables to reduce the impact; although, the Boost had a greater peak displacement compared to all other shoe conditions (p<0.05). At the heel, the Evolution performed the best in all variables to reduce impact attenuation compared to all other shoe conditions (p<0.05). Compared to the other shoe conditions, the Evolution displayed the lowest peak force at both the forefoot (p<0.001) and rearfoot regions (p<0.001), had the greatest displacement at the forefoot (p<0.001) and rearfoot (p<0.05) regions, greatest impact time at the forefoot and rearfoot regions (p<0.05), greatest
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hysteresis energy ratio at both the forefoot and rearfoot regions (p<0.001) and lowest mean stiffness at the forefoot (p<0.05) and rearfoot regions (p≤0.001). In contrast, the Chorus shoe displayed the greatest peak force and mean stiffness at both forefoot and rearfoot regions (p<0.001), the lowest peak displacement and hysteresis energy ratio at both rearfoot and forefoot regions (p<0.001). The Elastabootie displayed significantly less impact time compared to all other shoe conditions (p<0.001) (Table 2). Jazz shoe design had a significant effect on midfoot joint stiffness (p=0.005). Pairwise comparisons between the shoe conditions showed the midfoot joint stiffness in the Chorus shoe was significantly less than the Evolution (p=0.008), Elastabootie (p=0.012), and the Boost (p=0.013) (Table 3). Overall, the jazz shoes significantly affected knee joint stiffness (p <0.001). The barefoot condition had significantly greater knee joint stiffness compared to the Evolution (p=0.004), Chorus (p<0.001), and the Boost (p=0.029). The Chorus shoe also produced significantly lower knee joint stiffness compared to the Evolution (p<0.001) and the Elastabootie (p<0.001) (Table 3). Ankle and hip gross joint stiffness were not significantly affected by the jazz shoe designs, p=0.434 and p=0.384 respectively (Table 3). In addition, no significant effect of the jazz shoe designs on peak, minimum or mean joint stiffness values during the initial 40% of stance was found for any joint (p>0.05). Discussion and Conclusions The effect of the cushioning in the four jazz shoe designs investigated had several different outcomes. Mechanical testing showed a range of values for the mechanical parameters across the shoe specimens with a clear difference in impact attenuation capacity. However in vivo testing revealed the cushioning of the different shoe designs had little effect on lower limb joint stiffness, except at the knee. Mechanical testing of the jazz shoes revealed significant differences in the material characteristics among all four shoe designs. Contrary to our hypothesis, that the Boost sneaker would be the most impact attenuating design because of its thicker sole, the Evolution sneaker reduced peak force over a longer duration than the other shoe designs. Despite the thinner sole, the newer Evolution may incorporate improved material technology and construction that dissipates impact more efficiently than its
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predecessor, the Boost sneaker. As expected, minimal cushioning in the Elastabootie led to minimal displacement of the mass into the sole of the shoe. The Evolution sneaker produced a greater hysteresis energy ratio compared to the other shoe designs meaning the materials could return more energy to the foot. Whether this is advantageous or not is unknown at this stage. For rebounding movements like sautés, energy return would be beneficial to assist dancers to achieve adequate jump height in the subsequent jumps. This can only occur, however, if the timing of energy return and propulsion are simultaneous. In vivo, when sautés were performed to music, the stance time was 40ms. In contrast, the time of the energy return phase during mechanical testing of the shoes was only 25ms. The energy returned by the shoes to the lower limb is therefore occurring during the stance phase when the foot is stationary, thus transmitting the impact into the foot rather than assisting with propulsion. The knee joint was found to be stiffer during barefoot landings compared to the two jazz sneaker designs, while the Elastabootie produced similar knee joint stiffness to barefoot. No increase in gross joint stiffness was found in the other joints to compensate for the softer knee joint in the shod conditions. Moreover, gross stiffness values at the midfoot, ankle and hip were fundamentally zero with large variability in the results. Jump landing research has not previously investigated the effect of different footwear; therefore a direct comparison cannot be made. The results of this study counter previous research on running shoes, which found that running in shoes resulted in no change to either ankle or knee joint stiffness.15 Other than the considerable difference in landing technique, runners may alter their joint stiffness as they wish, according to external cues from the environment or footwear. As dancers approached toe-strike in maximal ankle plantar flexion, the ankle was required to move through a large range of motion to ensure the foot was flat on the floor and the demi-plié position achieved. This range must be completed by midstance, a period of approximately 20 ms depending on the tempo of the music. Allowing the ankle joint to be compliant and reducing joint stiffness close to zero is advantageous to completing the movement in the given amount of time. Similarly, stiffness values were extremely low at
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the hip joint where the trunk must remain upright in conjunction with hip external rotation and flexion during the demi-plié movement. This suggests the knee joint is largely responsible for the propulsive action during rebounding jumps while the other joints in the lower limb may act to reduce impact. The plantar flexed angle of the ankle when wearing high-heeled Chorus shoes reduced the range of motion available for the ankle22 to absorb impact, thus requiring greater knee flexion22, hence a more compliant knee joint during the demi-plié movement to help dissipate impact. Previous research has shown that instruction to land softly reduced peak vertical ground reaction force, however the mechanism by which this was achieved is not clear.26 In this study, it appears that the low stiffness values at the hip, ankle, and midfoot may contribute to the soft landing. In addition, the tempo of the music used in this study may not have been fast enough to elicit a stiffer ankle joint to optimise the energy return during the rebounding jumps.27 In the barefoot condition, it is possible that the need to protect the knee joint from large flexion angles, by reducing ankle joint stiffness and allowing ankle joint motion, may override the requirement for a quiet landing, whereas the quiet landing becomes a higher priority with the hard outsoles of the jazz shoes. The negative gross joint stiffness of the midfoot in the Chorus shoes may represent the foot sliding within the shoe due to the high rake. The gross joint stiffness value was opposite to that displayed in the other shoe conditions and warrants further investigation as to the mechanism of the midfoot stiffness. The variability in the stiffness values could be indicative of how well the shoes fit, the variance in flexibility of the foot, or different strategies for impact attenuation. Future studies could explore methodologies to view the in-shoe motion of the foot and determine whether this motion is beneficial for impact attenuation or detrimental for foot health. Discrete joint stiffness values were hypothesised to be increased in the shoes with greater cushioning, in line with Robbins’ theory that the joints will be stiffer if the dancer perceived the shoe to be soft as opposed to hard.12 The lack of significant effect found in the discrete joint stiffness values could be attributed to the large variability found across the sixteen dancers. The landing strategies employed by
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each individual dancer were varied, with some displaying stiffness peaks early in the landing, corresponding to the timing of the ball of the foot contacting the floor, and others showing a more gradual increase in joint stiffness peaking closer to mid stance. Perhaps, due to the familiarisation and practice period, once data collection commenced the dancers had already adopted their preferred landing strategy accounting for the new shoe condition resulting in no difference between shoe conditions. There does not appear to be a defined relationship between the capacity of the dance shoes to attenuate impact during mechanical testing and the response of the dancers to the different dance shoe designs. Although the Evolution sneaker can be deemed the most impact attenuating following the mechanical testing, it did not elicit any difference in lower limb joint stiffness compared to the other dance shoe designs. Investigation of the interaction between the lower limb and footwear is needed to better understand the overall effect on impact attenuation for an individual. Low joint stiffness has purported to be associated with an increase in soft tissue injury risk 27 and the results of this study showed that regardless of shoe condition the ankle joint stiffness was close to zero; neither increasing or decreasing the possible risk of soft tissue injury at the ankle. The dance jump tested in this study was a bilateral jump take-off and landing, that did not travel across the room. The results of the current study therefore may not apply to other jump types, particularly higher intensity and single leg landings. All of the shoe conditions reduced knee joint stiffness compared to the barefoot condition and it has been suggested that increased joint stiffness values may increase the risk of bony injuries such as stress fractures27. However, the ideal range for joint stiffness values is unknown and recommendations as to footwear selection contributing to knee injury risk or prevention cannot be made. Conclusion This study has found that jazz shoe design can affect the biomechanical characteristics of jump landings and successfully quantified the impact attenuation characteristics of different jazz shoe designs. Dance technique requirements appear to strongly influence the mechanics of jump landings. Considering that a majority of dance movements have a toe-heel footfall sequence, equal emphasis should be placed on
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shock absorbing materials at the forefoot as at the heel. Future dance shoe design should be evidencebased, accounting for both function and aesthetic. Other dance shoe designs can be compared to the ones in this study and subsequent research can potentially find associations with injury risk. Practical Implications
Shock absorbing capacity of dance shoes cannot be judged based on the appearance of a thick sole
Shock absorbing properties of dance shoes should be considered for the forefoot region because dancers land toe-first from jumps
Jazz shoe design alters the shock absorbing capacity of jump landing technique in dancers
Jump landings when barefoot put the knee joint under greater stress than when wearing jazz shoes
None of the shoe conditions had an effect on ankle joint stiffness and so may not influence soft tissue injuries at the ankle
Acknowledgements The authors wish to thank Bloch Australia for kindly providing the shoes for testing, and Ray Patton for his assistance building the shoe tester.
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References 1. Wyon M. Preparing to perform: periodization and dance. J Dance Med Sci. 2010; 14(2):67-72. 2. Wyon MA, Twitchett E, Angioi M, et al. Time motion and video analysis of classical ballet and contemporary dance performance. Int J Sports Med. 2011; 32(11):851-855. 3. Stretanski MF, Weber GJ. Medical and rehabilitation issues in classical ballet. Am J Phys Med Rehab. 2002; 81(5):383-391. 4. Fong Yan A, Sinclair PJ, Hiller C, et al. Impact attenuation during weight bearing activities in barefoot vs. shod conditions: A systematic review. Gait Posture. 2013; 38(2):175-186. 5. Liederbach M, Richardson M, Rodriguez M, et al. Jump exposures in the dance training environment: a measure of ergonomic demand. J Athl Training. 2006; 41(2 Suppl.):S85-86. 6. Fong Yan A, Hiller C, Smith R, et al. Effect of footwear on dancers: a systematic review. J Dance Med Sci. 2011; 15(2):86-92. 7. Grier TL, Knapik JJ, Swedler D, et al. Footwear in the United States Army Band: injury incidence and risk factors associated with foot pain. Foot. 2011; 21(2):60-65. 8. Hawke F, Burns J, Radford J, et al. Custom-made foot orthoses for the treatment of foot pain (Review). Cochrane DB Syst Rev. 2008(3):1-91. 9. Hoydicz J. Stress fractures in athletes. O&P Business News. Vol 112009:21-25. 10. Forner A, Garcia A, Alcantara E, et al. Properties of shoe insert materials related to shock wave transmission during gait. Foot Ankle Int. 1995; 16(12):778-786. 11. Nigg B, Stefanyshyn D, Cole G, et al. The effect of material characteristics of shoe soles on muscle activation and energy aspects during running. J Biomech. 2003; 36:569-575. 12. Robbins S. Hazard of deceptive advertising of athletic footwear. Brit J Sport Med. 1997; 31(4):299303. 13. Robbins S, Waked E. Balance and vertical impact in sports: role of shoe sole materials. Arch Phys Med Rehab. 1997; 78(5):463-467.
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14. Hopper LS, Alderson JA, Elliott BC et al. Dance floor force reduction influences ankle loads in dancers during drop landings. J Sci Med Sport. 2015; 18(4):480-485. 15. Hamill J, Russell EM, Gruber AH, et al. Impact characteristics in shod and barefoot running. Footwear Sci. 2011; 3(1):33-40. 16. Hamill J, Moses M, Seay J. Lower extremity joint stiffness in runners with low back pain. Res Sports Med. 2009; 17(4):260-273. 17. Abergel D. Plié in practice. Pointe. 2006; 7(2):72-73. 18. Franklin E. Floors and soft landings, in Dance imagery for technique and performance. Champaign, IL, Human Kinetics, 1996. 19. Stretanski MF, Weber GJ. Medical and rehabilitation issues in classical ballet. Am J Phys Med Rehabil. 2002; 81(5):383-391. 20. Weiss DS, Shah S, Burchetter RJ. A profile of the demographics and training characteristics of professional modern dancers. J Dance Med Sci. 2008; 12(2):41-46. 21. ASTM. Standard test method for shock attenuating properties of materials systems for athletic footwear. F 1614-99. West Conshohocken, Pennsylvania: ASTM International; 2006. 22. Fong Yan A, Hiller C, Sinclair PJ, et al. Kinematic analysis of sautés in barefoot and shod conditions. J Dance Med Sci. 2014; 18(4):149-158. 23. Wu G, Siegler S, Allard P, et al. ISB recommendation on definitions of the joint coordinate system of various joints for the reporting of human joint motion - part I: ankle, hip, and spine. J Biomech. 2002; 35(4):543-548. 24. Walter HL, Docherty CL, Schrader J. Ground reaction forces in ballet dancers landing in flat shoes versus pointe shoes. J Dance Med Sci. 2011; 15(2):61-64. 25. Mullineaux D, Milner CE, Davis IS, et al. Normalization of ground reaction forces. J Appl Biomech. 2006; 22(3):230-233. 26. Milner CE, Fairbrother JT, Srivatsan A, et al. Simple verbal instruction improves knee biomechanics during landing in female athletes. Knee. 2012; 19(4):399-403. 27. Butler RJ, Crowell Iii HP, Davis IM. Lower extremity stiffness: implications for performance and injury. Clin Biomech. 2003; 18(6):511-517.
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Table 1. Description of shoe conditions. Shoe Name
Brand/Name/Number
Description
Barefoot
N/A
Control condition
Elastabootie
Bloch
Elastabootie Slim line leather jazz shoe with separate
S0499L
forefoot and rearfoot sections (split sole design). 3.5mm forefoot 13mm rearfoot thickness
Evolution
Bloch
Evolution Split sole design low profile jazz sneaker.
Dance Sneaker S0510
Multi-density rubber outsole and additional divisions in the forefoot section. Air punched compressed EVA sock material. 14mm
forefoot
and
20mm
rearfoot
thickness. Boost
Bloch Classic Boost Split sole design jazz sneaker with thick S0538L
multi-density
rubber
outsole
and
compressed EVA sock material. 24mm forefoot and 38mm rearfoot thickness.
Chorus
Bloch Cabaret S0306
50mm high heel court shoe with ankle strap
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Table 2. Impact attenuation characteristics of the jazz shoes, mean (SD). Variable
Region
Elastabootie
Evolution
Boost
Chorus
Peak Force (N)
Heel
1933(92)b
1068.45(28)b
1172(37)
5181(325)b
Forefoot
3983(321) b
1470.07(79) b
1590.50(73)
4197(359)b
Heel
5.35 (0.07)b
11.01 (0.07)a
10.62 (0.18)
4.14 (0.13)b
Forefoot
3.60 (0.09)b
8.03 (0.04)a
8.41 (0.12)
3.23 (0.24)b
Heel
16.48(0.18)b
35.08(0.23)a
32.60(1.17)
11.64(1.23)b
Forefoot
10.04(0.09)b
25.48(0.41)a
24.48(0.54)
10.32(0.09)b
Heel
0.077(0.002)b
0.286(0.007)b
0.245(0.009)
0.006(0.001)b
Forefoot
0.004(0.000)b
0.213(0.004)b
0.201(0.004)
0.002(0.001)b
Heel
360(21)a
97(2.2)b
110(5.3)
1,310(180)b
Forefoot
1,100(114)a
180(9.3)b
190(11)
1,260(123)b
Peak Displacement (mm)
Impact Time (ms)
HER (J)
Mean Stiffness (N/mm)
a
denotes significant difference to the Boost shoe (p<0.05), bdenotes significant difference to the Boost shoe (p<0.001).
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Table 3. Gross joint stiffness values (N. mm/degree) for each joint in each shoe condition, mean (SD). Joint
Barefoot
Elastabootie
Hip
30.38(24.53)
103.89(243.11) 77.49(81.62)a 114.46(259.98) 60.64(183.67)
Knee
34.81(14.23)b 27.88(14.16)b
25.42(9.49)ab
16.58(21.17)a
15.27(7.56)a
Ankle
40.39(12.27)
39.55(12.34)
39.52(10.90)
40.41(9.52)
37.63(9.41)
2.68(31.41)b
4.51(26.89)b
5.21(28.94)b
-6.75(22.93)
Midfoot 5.32(29.89) a
Evolution
Boost
Chorus
denotes a significant difference to the barefoot condition, p<0.05. b denotes a significant difference
to the Chorus shoe.
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