The effect of different unstable footwear constructions ... - Gait & Posture

Gait & Posture 40 (2014) 305–309

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The effect of different unstable footwear constructions on centre of pressure motion during standing W. Plom a, S.C. Strike b, M.J.D. Taylor a,* a b

Centre for Sport and Exercise Sciences, School of Biological Sciences, University of Essex, Essex, UK Department of Life Sciences, University of Roehampton, London, UK

A R T I C L E I N F O

A B S T R A C T

Article history: Received 31 May 2013 Received in revised form 26 March 2014 Accepted 14 April 2014

Background: The aim of this study was to test the effect different unstable footwear constructions have on centre of pressure motion when standing. Methods: Sixteen young female volunteers were tested in five conditions, three unstable footwear (Reebok Easy-ToneTM, FitFlopTM and Skechers Shape-UpsTM), a standard shoe and barefoot in a randomised order. Double and single leg balance on a force plate was assessed via centre of pressure excursions and displacements in each condition. Results: For double leg and single leg standing centre of pressure excursions in the anterior–posterior direction were significantly increased wearing Skechers Shape-UpsTM compared to barefoot and the standard shoe. For the Reebok Easy ToneTM during single leg standing excursions in the anterior– posterior direction were significantly greater compared to the barefoot condition. Cumulative displacement of the centre of pressure in medial-lateral direction increased significantly during single leg standing when wearing Skechers Shape-UpsTM compared to barefoot and standard shoe as well as for Reebok Easy ToneTM vs. barefoot. Discussion: It would appear from these quiet standing results that the manner of the construction of instability shoes effects the CoP movement which is associated with induced instability. Greater CoP excursion occurred in the A–P direction while the cumulative displacements were greater in the M–L direction for those shoes with the rounded sole and soft foam and those with airpods. The shoe construction with altered density foam did not induce any change in the CoP movement, during quite standing, which tends to suggest that it is not effective at inducing balance. Not all instability shoes are effective in altering the overall instability of the wearer. ß 2014 Elsevier B.V. All rights reserved.

Keywords: Balance Posture Shoes Instability

1. Introduction Unstable footwear has been available since the 1990s with the development of Masai Barefoot Technology (MBT) shoes. The concept behind instability footwear has been defined by Nigg et al. as: ‘unstable shoes are built to provide a training device that use instability as a mechanism to train the neuromuscular control and/ or strengthen muscles in the human locomotor system’ [1]. Due to the market success of the MBT there are currently more than 25 companies producing or selling unstable shoes [1] of various constructions. The MBT, which has by far the largest evidence base in the scientific literature, provides instability via a rounded sole in

* Corresponding author at: School of Biological Sciences, University of Essex, Colchester, Essex CO4 3SQ, UK. Tel.: +44 01206872818; fax: +44 01206872592. E-mail address: [email protected] (M.J.D. Taylor). http://dx.doi.org/10.1016/j.gaitpost.2014.04.189 0966-6362/ß 2014 Elsevier B.V. All rights reserved.

the anterior–posterior (A–P) plane and through a thick, soft pad under the heel. Far less attention in the literature has been given to other unstable footwear such as the Skechers Shape-UpsTM, Reebok Easy-ToneTM and FitFlop’sTM. The Skechers Shape-UpsTM (Table 1) consist of a rounded sole in the A-P plane which is made of a soft malleable foam material. Reebok Easy-ToneTM (Table 1) has two inter-connected bulbous air pods under the heel and forefoot. It is assumed that these pods move air back and forth as the wearer walks, forcing a constant readjust to maintain balance [2]. The FitFlop’sTM (Table 1) incorporate different foam densities in the heel (high-density), mid-foot (low-density) and toe (mid-density) sections of the shoe creating what FitFlopTM call a micro-wobble board. There have been very few empirical studies testing how or if these different constructions of inducing instability affect the standing balance of the wearer. The research that has been reported for these constructions of unstable shoe has focused on EMG analysis during gait [3–5].

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Several studies have investigated the effects of MBT shoes on standing stability via centre of pressure (CoP) measures [5,6]. MBT shoes cause a significant increase in the excursion of the CoP during two-legged stance in both the A–P and M–L directions compared to barefoot [6,7]. It was perhaps unsurprising that instability occurred in the A–P direction (due to the construction of the sole) but there was also instability in the M–L direction possibly due to the soft heel pad. For one-legged stance, however, there was no significant difference between MBT and barefoot [8]. There is a paucity of work investigating standing stability in other constructions of unstable footwear. Nigg et al. [1] stated that instability whilst standing would likely be assumed for all unstable shoes. Therefore, the aim of this study was to test this assumption on 3 different constructions of unstable shoes. It was hypothesised that, (1) CoP excursions and CoP cumulative displacement will be significantly increased in both the M–L and A–P directions while wearing unstable footwear compared to the standard shoe and barefoot during (a) double leg standing and (b) single leg standing; (2) CoP excursions and CoP cumulative displacement, will significantly differ between the unstable shoes during (a) double leg standing and (b) single leg standing. 2. Methods 2.1. Participants Sixteen females (aged 20 1.3 years, stature 166.4 5.5 cm, mass 65.0 9.9 kg) participated in this study and were selected

through a convenience sample of university students. All had UK size 5 or 6 feet. Participants by self-report had no musculoskeletal injury or vestibular or balance related conditions. The study was approved by the University’s ethics committee and written and informed consent was gained prior to data collection. 2.2. Protocol Each participant preformed single and double stance stability tests while in 5 different conditions (Table 1); (1) Barefoot (BF), (2) Standard shoe (Reebok – stable reference condition, ST), (3) FitFlopTM (FF) shoes, (4) Skechers Shape-UpsTM (SU), and (5) Reebok Easy-toneTM (ET). The order of each condition was randomised by participants selecting a condition from a black bag. Before each test condition was recorded each participant completed a minimum of 5 min familiarisation [7] where they walked until feeling comfortable in the shoe. None of the participants had previous experience with the tested unstable shoes. Participants stood for 10 s [9] on a Kistler force plate operating at 40 Hz [10]. The participant’s hands were placed on their hips throughout the procedure. Each participant completed two practice trials in each condition followed by 3 recorded trials [7]. For double leg standing participants placed their feet laterally 15 cm apart [6] and focused on a point 3 m in front of them at eye level. During single leg standing participants stood on the centre of the force plate and flexed their left knee (the free leg) to 908 in order to raise the shank parallel to the floor and again focused on a

Table 1 Footwear characteristics. Image

Shoe

Fitflop

Abbreviation

TM

shoe

FF

Skechers Shape-UpsTM

SU

Reebok Easy-toneTM

ET

Standard shoe (Reebok)

ST

[TD$INLE]

[TD$INLE]

[TD$INLE]

[TD$INLE]

a

Sole thickness measured using callipers from the thickest part of the shoe.

Description

Sole (mm)a

thickness

Size 5

Size 6

Shoe incorporates a high density foam heel, low density foam mid-sole and a medium density foam forefoot to create instability. Mass size 5–296 g Mass size 6–315 g Uses a rounded sole in A–P plane made from a soft foam material to induce instability. Mass size 5–475 g Mass size 5–493 g Uses two interconnected bulbous air-filled pods under the heel and forefoot to induce instability. Mass size 5–363 g Mass size 5–380 g

29

31

45

48

39

41

A typical mid-range running shoe. Mass size 5–257 g Mass size 5–278 g

31

35


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Table 2 Stability measure for five conditions during double leg standing (means SD). CoP measures

Excursion (M–L) Excursion (A–P) CD (M–L) CD (A–P)

Condition

F

BF

ST

FF

ET

SU

11.9 3.6 21.1 4.9 97.7 20.4 145.3 15.8

13.5 5.4 23.0 9.1 98.5 22.8 149.9 21.4

13.9 3.9 23.1 5.4 97.7 15.4 147.1 22.2

13.7 4.2 26.9 10.9 95.7 16.8 147.9 25.3

15.7 4.6a 33.8 9.9a,b,c 105.9 26.5 185.1 33.5a,b,c,d

2.455 8.851 1.209 18.413

BF, bare-foot; ST, standard shoe; FF, Fit-Flop; ET, Reebok Easy-Tone; SU, Skechers Shape-Ups; Excursion (mm); CD, cumulative displacement (mm); F, indicates the F statistic for the ANOVA. a Significant difference compared to BF. b Significant difference compared to ST. c Significant difference compared to FF d Significant difference compared to ET.

Table 3 Stability measure for five conditions during single leg standing (means SD). CoP measures

Excursion (M–L) Excursion (A–P) CD (M–L) CD (A–P)

Condition

F

BF

ST

FF

ET

SU

29.4 4.9 34.3 5.2 283.4 61.4 273.1 62.1

29.9 3.6 37.7 6.2 290.7 48.1 271.6 46.2

30.8 6.0 39.3 9.6 301.1 65.7 284.9 64.5

32.5 4.7 41.4 7.2a 330.2 72.9a 292.1 58.6

33.5 4.6 51.4 2.8a,b 337.2 65.6a,b 307.0 59.7

2.635 13.06 7.095 3.571

BF, bare-foot; ST, standard shoe; FF, Fit-Flop; ET, Reebok Easy-Tone; SU, Skechers Shape-Ups; Excursion (mm); CD, cumulative displacement (mm); F, indicates the F statistic for the ANOVA. a Significant difference compared to BF. b Significant difference compared to ST.

point 3 m in front of them. The position of the feet for both double and single leg stance were marked on the force plate to ensure the same foot placement for all conditions. If the participant lost balance and their contralateral limb (single-leg stance protocol) touched the ground the trial was rejected and repeated. Trials were also rejected if the hands of the participant were removed from the hips. 2.3. Data analysis The CoP excursions [7] and cumulative displacement of the CoP were measured in the M–L and A–P planes. CoP excursions were defined as the movement of CoP from its two furthest opposing points (minimum and maximum) in each plane. The total path was the cumulative displacement of the CoP during 10 s in the M–L and A–P directions. It has been suggested that an increase in excursion represents gross changes in movement (possibly induced by proximal muscles with large cross-sectional area) [11] whereas the cumulative displacement represents corrective contractions resulting in small changes of movement. All data was collected in Bioware software version 3.12 and variables were processed using Microsoft Excel TM. The mean of the 3 trials were calculated for the selected variables. The variables were statistically assessed in isolation as each reflects a different aspect of stability. A repeated measures ANOVA was calculated to identify significant differences between conditions. In a post hoc analysis direct comparisons were conducted using paired sample t-tests with Bonferroni corrected threshold. The significance level was set at p < 0.05 and the statistical package for the Social Sciences for Windows v19 (SPSS Inc., Chicago, IL) was used for statistical analyses.

3. Results 3.1. Double leg standing A-P CoP excursions (Table 2) were significantly greater while wearing the SU compared to BF (p = 0.003), ST (p = 0.02) and FF (p = 0.016). In the M–L plane there was a significantly (p = 0.006) greater excursion in SU compared to BF. A–P cumulative displacement of the CoP (Table 2) was significantly greater for the SU compared to the four other conditions (BF p < 0.001; ST p < 0.001; FF p < 0.001, ET p = 0.005). There were no significant differences in the M–L direction for cumulative displacement (Table 2) for any of the conditions. 3.2. Single leg stance A–P CoP excursions were significantly greater while wearing the SU compared to BF (p < 0.001) and the ST (p = 0.001). A–P CoP excursions were also significantly greater (Table 3) when wearing the ET shoes compared to BF (p = 0.01). There was no significant difference between any of the conditions in the M–L plane excursions. M–L cumulative displacement of the CoP (Table 3) when wearing the SU was significantly greater compared to BF (p = 0.021) and the ST (p = 0.01) conditions and was also significantly greater when wearing the ET compared to BF (p = 0.031). There were no significant differences in cumulative displacement in A–P direction between any of the conditions.

4. Discussion The aim of this study was to test the assumption that different constructions of unstable shoes create instability during standing [1]. Therefore we tested 3 different constructions of unstable footwear (a shoe with a rounded sole with a thick, soft pad under the heel (SU); a shoe with different density foams (FF); and a shoe with bulbous air pods (ET) and compared them to each other and barefoot and standard shoe conditions. The construction of the shoe and thus the method of inducing instability on the wearer differed between the shoes but there was little difference on the

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induced instability, evidenced by only the excursions and cumulative displacement in the A–P direction during double leg standing were significantly greater in the SU compared to the ET and FF. For the remaining variables there were no significant differences between the unstable footwear types suggesting that in this study only the rocker heel design significantly altered CoP excursions. The only significant increase, compared to BF, in CoP excursions and cumulative displacement in the A–P direction during double leg stance was for the SU. This construction method is similar to MBT in that instability is induced by a rounded sole in the A–P direction. Previous work has shown that MBT shoes increase the excursion (therefore suggesting greater instability) of the CoP in the A–P direction compared to BF during double-leg standing [7]. The CoP excursions in the A–P direction for the SU (33.8 9.9 mm) were comparable to the participants wearing MBT’s in Nigg et al. [7] (27.2 9.2 mm) but markedly less than those reported for the participants wearing MBT’s in Landry et al. [6] (51.6 17.7 mm). When wearing the SU shoe, CoP excursions in the M–L direction were significantly different compared to BF only. This may be because of the soft malleable heel creating instability in this plane compared to other footwear. When wearing MBT’s, excursions in the M–L direction have also been reported to be significantly greater compared to barefoot and the control shoe. [5,7] The FF and ET means of inducing instability differ to the SU, with the ET using the movement of air between pods on the sole and the FF uses different density foam. During double leg standing the CoP excursions were not significantly increased in the M–L or A–P directions for these shoes which suggest that these footwear constructions were not effective in inducing instability and that not all unstable shoes create instability during double leg standing. Single leg standing is a more challenging, postural position compared to double leg standing, due to the smaller base of support, thus CoP excursions and displacements were greater than double leg standing. A–P excursions were again significantly greater for SU compared to BF and the ST. The increase in excursions for SU during single leg standing was in disagreement with Romkes [8] who tested MBT’s and reported no significant increases in CoP excursion compared to BF. Even though these two shoes are broadly similar construction they are not identical and are likely to be made of different materials possibly causing a difference in CoP response. However these differences could also be attributed to different standing postures. For example, during this study the shank was raised 908 parallel with the floor whereas Romkes’s [8] participants lifted the foot just above the ground. The cumulative displacement of the CoP for the SU during single leg standing was significantly greater in the M–L plane compared to BF and ST yet the M–L excursions were not significantly different. This suggests that both these CoP measures are quantifying different aspects of the response to the shoe conditions. Unlike double leg standing, there was a significant increase in A–P CoP excursions for the ET compared to BF during single leg standing. This suggests that ET challenge stability during more demanding postural tasks but only in the A–P direction. The airpods on the sole of the ET shoe did not cause a significant increase in CoP excursion compared to BF in the M–L direction but there were significant differences for the cumulative displacement of the CoP in this direction. This suggests corrective contractions in response to the footwear condition were from smaller muscles rather than the larger proximal muscles which may result in larger excursions of the CoP [11]. The predominant pattern of increased CoP excursion and cumulative displacement when wearing the SU was in the A–P direction during double leg standing. This was not the case for single leg standing. Instead M–L CoP cumulative displacement was significantly different compared to BF for the SU and ET and

compared to ST for the SU only. This suggests that stability is being challenged in the M–L plane in these shoes during single leg support. The majority of past work on unstable footwear has focused on testing one type of unstable shoe. This present study attempted to test different designs/constructions of unstable shoes and to assess their ability to cause instability. Germano et al. [12] tested 4 unstable shoes, (3 shoes provided instability via a rounded sole construction [similar to the MBT and SU]), but these shoes were not named thus it is unclear how these shoes were constructed There were no significant differences in M–L or A–P CoP excursion between these constructions of shoe during single leg standing, but there were significant increases in excursions when standing BF compared to unstable shoes – suggesting that instability was greater when in bare feet. Our results do not agree with Germano et al. [12], who used pressure innersoles to recoded CoP excursions, where we showed that CoP excursions (A–P) were significantly greater when wearing SU and ET compared to BF and ST suggesting that these shoes did create instability during single leg standing. It is clear that there were marked differences in shoe construction between the footwear. The SU had the greatest mid-sole thickness (45–48 mm) made of a fairly soft foam material (we did not measure the Shore scale on our footwear). Robbins et al. [13] reported a change in mid-sole thickness from thin (13 mm thick at heel) to thick (27 mm thick at the heel) resulted in an increased balance failure of 54%. Balance failure was defined as the participant’s inability to traverse a beam with a cross-section outer diameter width of 7.8 cm, a height of 3.9 cm, and a length of 9 m. A change in firmness from hard (Shore A50) to soft (Shore A15) also resulted in an increased balance failure of 77%. This is the premise of the FF where a change in sole firmness will induce instability. By combining these two (thickness and softness vs. thinness and hardness) balance failure increased by 217%. [13] Contrary to Robbins et al. [13], Lord et al. [14] reported no association between firmness (hard-soled; Shore A58 or softsoled; Shore A42) of shoe sole and balance ability. However, a direct comparison between both studies is not possible because of different mid-sole densities and methods to evaluate balance. It is possible that the extra thickness (compared to the other shoes tested) and softness of the mid-sole of the SU resulted in sensory insulation [13] by reducing afferent input to the brain regarding foot position. The FF was the thinnest of the shoe constructions and this may help explain in part why there were no significant differences in stability when wearing this shoe even though it specifically has a low-density foam mid-foot portion. The relevance of the results of the current study is limited to the participant population (young, healthy and recreationally active females). We focused our study on this demographic because they are the target group for these footwear. We also didn’t include males because of the differences between genders seen in postural stability [15] and the kinematics and kinetics during walking [16,17]. Postural stability deteriorates from the age of 40-years-old and deterioration accelerates markedly after the age of 60-yearsold [15]. Therefore it is likely that older subjects will respond differently to the unstable shoes compared to young adults used in this present work. During walking Buchecker et al. [18] showed that joint kinetics were significantly reduced when wearing MBT’s for older adults (60.1 3.5 years) compared to young adults (27.2 5.4 years). To date, there appear to be no studies reporting older adults wearing the instability footwear (as used in this present work) during static balance or during locomotion. It is possible, in particular for the FF that the mechanism of inducing instability was not large enough in this young population but may have potential use for a less active older population who may benefit from the subtler instability mechanism.


This study only looked at the acute effects of each footwear condition as a short familiarisation period was used. Sto¨ggl et al. [19] compared short and long term adaptations to variability during walking using MBT shoes. They found that after 4 h of daily use over 10 weeks gait variability reduced to levels seen for the conventional shoe. This was seen as a positive adaptation in that the wearers get the benefit of wearing MBT’s but without regular exposure to higher risk of falling brought about if gait variability continued/increased after the training period. To date, long term effects of wearing the footwear tested in this present work have not been reported. Due to the nature of the testing it was not possible to blind the participants to the conditions. Therefore individuals may make conscious/subconscious changes to stability based on the knowledge of the shoe rather than because of any biomechanical response [3]. 5. Conclusion Not all instability shoes result in increased CoP movement, which is associated with induced instability. The construction of the shoe has an effect and the shoe with both rounded heels and soft foam were the most effective in altering the CoP excursion in both the A–P and M–L directions. The shoe which attempted to induce instability via air-pods induced instability compared to barefoot only during single leg stance. The shoe which attempted to induce instability through altered foam densities along the length of the foot was not successful in altering the CoP movement suggesting that not all unstable shoes induce instability. Clearly the design of the shoe is important and the most effective design is one with a small base of support when standing (due to the rounded sole) and made from soft foam throughout the shoe. Conflict of interest None. References [1] Nigg B, Federolf PA, von Tscharner V, Nigg S. Unstable shoes: functional concepts and scientific evidence. Footwear Sci 2012;4(2):73–82.

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[2] Horsak B, Baca A. Effects of toning shoes on lower extremity gait biomechanics. Clin Biomech (Bristol Avon) 2013;28(March (3)):344–9. [3] Burgess KE, Swinton PA. Do Fitflops increase lower limb muscle activity? Clin Biomech (Bristol Avon) 2012;27(December (10)):1078–82. [4] Elkjær EF, Kromann A, Larsen B, Andresen EL, Jensen MK, Veng PJ, et al.;1; EMG analysis of level and incline walking in Reebok EasyTone ET Calibrator. Paper presented at: 15th Nordic-Baltic Conference on Biomedical Engineering and Medical Physics (NBC 2011) 2011. [5] Porcari JP, Greany J, Tepper S, Edmonson B, Foster C. The physiologic and electromyographic responses to walking in regular athletic shoes versus toning shoes. Gundersen Lutheran 2011;3:3–7. [6] Landry SC, Nigg BM, Tecante KE. Standing in an unstable shoe increases postural sway and muscle activity of selected smaller extrinsic foot muscles. Gait Posture 2010;32(June (2)):215–9. [7] Nigg B, Hintzen S, Ferber R. Effect of an unstable shoe construction on lower extremity gait characteristics. Clin Biomech 2006;21(January (1)):82–8. [8] Romkes J, fu¨r Bewegungsuntersuchungen L, beider Basel U-K. Statische Gleichgewichtskontrolle mit dem MBT-Schuh. Schweizerische Zeitschrift fur Sportmedizin und Sporttraumatologie 2008;56(2):61. [9] Emery CA. Is there a clinical standing balance measurement appropriate for use in sports medicine? A review of the literature. J Sci Med Sport 2003;6(December (4)):492–504. [10] Granacher U, Roth R, Muehlbauer T, Kressig RW, Laser T, Steinbrueck K. Effects of a new unstable sandal construction on measures of postural control and muscle activity in women. Swiss Med Wkly 2011;141:w13182. [11] Gribble PA, Hertel J. Effect of lower-extremity muscle fatigue on postural control. Arch Phys Med Rehabil 2004;85(April (4)):589–92. [12] Germano AM, Schlee G, Milani TL. Balance control and muscle activity in various unstable shoes compared to barefoot during one-leg standing. Footwear Sci 2012;4(2):145–51. [13] Robbins S, Waked E, Gouw GJ, McClaran J. Athletic footwear affects balance in men. Br J Sports Med 1994;28(June (2)):117–22. [14] Lord SR, Bashford GM, Howland A, Munroe BJ. Effects of shoe collar height and sole hardness on balance in older women. J Am Geriatr Soc 1999;47(June (6)):681–4. [15] Era P, Sainio P, Koskinen S, Haavisto P, Vaara M, Aromaa A. Postural balance in a random sample of 7,979 subjects aged 30 years and over. Gerontology 2006;52(4):204–13. [16] Nigg BM, Tecante KE, Federolf P, Landry SC. Gender differences in lower extremity gait biomechanics during walking using an unstable shoe. Clin Biomech 2010;25(December (10)):1047–52. [17] Kerrigan DC, Todd MK, Croce UD. Gender differences in joint biomechanics during walking normative study in young adults. Am J Phys Med Rehabil 1998;77(1):2–7. [18] Buchecker M, Lindinger S, Pfusterschmied J, Mu¨ller E. Effects of age on lower extremity joint kinematics and kinetics during level walking with Masai barefoot technology shoes. Eur J Phys Rehabil Med 2013;49(5): 675–86. [19] Sto¨ggl T, Haudum A, Birklbauer J, Murrer M, Mu¨ller E. Short and long term adaptation of variability during walking using unstable (Mbt) shoes. Clin Biomech 2010;25(8):816–22.