Effects of Shoe Characteristics on Dynamic Stability When Walking on Even and Uneven Surfaces in Young and Older People

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ORIGINAL ARTICLE

Effects of Shoe Characteristics on Dynamic Stability When Walking on Even and Uneven Surfaces in Young and Older People Jasmine C. Menant, BSc, Stephen D. Perry, PhD, Julie R. Steele, PhD, Hylton B. Menz, PhD, Bridget J. Munro, PhD, Stephen R. Lord, PhD, DSc ABSTRACT. Menant JC, Perry SD, Steele JR, Menz HB, Munro BJ, Lord SR. Effects of shoe characteristics on dynamic stability when walking on even and uneven surfaces in young and older people. Arch Phys Med Rehabil 2008;89:1970-6. Objective: To systematically investigate the effects of various shoe features (sole hardness, heel height, heel collar height, tread pattern) on dynamic balance control and perceptions of comfort and stability in young and older people walking over even and uneven surfaces. Design: A mixed-design 3-way repeated measures with age as a between-subjects factor and surface and shoe conditions as within-subjects factors. Setting: Gait laboratory. Participants: Young adults (n⫽11) and community-dwelling older adults (n⫽15). Interventions: Not applicable. Main Outcome Measures: Center of mass (COM)⫺base of support (BOS) margins, vertical and braking loading rates, and subjective ratings of perceived shoe comfort and stability. Results: Overall, compared with the standard shoes, the soft sole shoes led to greater lateral COM-BOS margin (P⬍.001), whereas the elevated heel shoes caused reductions in posterior COM-BOS margin (P⫽.001) and in vertical and braking loading rates (both, P⬍.001). Subjects rated the elevated heel shoes as significantly less comfortable (P⬍.001) and less stable (P⬍.001) than the standard shoes. Only the young subjects perceived the soft-sole shoes to be less stable than the standard shoes (P⫽.003). Conclusions: Both young and older subjects adopted a conservative walking pattern in the elevated heel shoes and exhibited impaired mediolateral balance control in the soft-sole

From the Prince of Wales Medical Research Institute, Randwick, NSW, Australia (Menant, Lord); School of Public Health and Community Medicine, University of New South Wales, Sydney, NSW, Australia (Menant); Department of Kinesiology and Physical Education, University Wilfrid Laurier, Waterloo, ON, Canada (Perry); Biomechanics Research Laboratory, University of Wollongong, Wollongong, NSW, Australia (Steele, Munro); and Musculoskeletal Research Centre, La Trobe University, Bundoora, VIC, Australia (Menz). Presented to Australian Falls Prevention, November 5–7, 2006, Brisbane, Australia, and the International Society of Posture and Gait Research. July 14 –18, 2007, Burlington, VT. Supported by Prevention of Older People’s Injuries through the National Health and Medical Research Council (NHMRC) Health Research Partnership Scheme; (grant no. 209799), the Canadian Institutes of Health Research (operating grant no. MOP-77772); the Canadian Foundation for Innovation/Ontario Innovation Trust/ Wilfrid Laurier University (new opportunities equipment grant no. 5141); and an NHMRC Clinical Research Fellow (no. 433049). No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Reprint requests to Jasmine C. Menant, BSc, Prince of Wales Medical Research Institute, Barker St, Randwick, NSW 2031, Australia, e-mail: [email protected]. Published online August 29, 2008 at www.archives-pmr.org. 0003-9993/08/8910-00808$34.00/0 doi:10.1016/j.apmr.2008.02.031

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shoes. In contrast, increased sole hardness (above that found in a standard shoe), a tread sole, and a raised collar height did not improve walking stability in either group. It is concluded that shoes with elevated heels or soft soles should not be recommended for older people and that a standard laced shoe with a low collar and a sole of standard hardness with or without a tread provides optimal dynamic stability when walking on even and uneven surfaces. Key Words: Aged; Balance; Gait; Rehabilitation. © 2008 by the American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation OOTWEAR, BEING AT THE interface between the body and the supporting surface, has the potential to affect balance F and, subsequently, the risk of falling. Indeed, inappropriate footwear has been reported to be involved in 45% of falls experienced by older people,1 and wearing slippers versus shoes is independently associated with greater risk of injurious falls.2 Wearing shoes with heels greater than 2.5cm nearly doubles the risk of falling in community-dwelling older people, whereas wearing shoes with a large sole or surface contact area significantly reduces the risk.3 Furthermore, a recent epidemiologic study4 revealed significant associations between shoes with a medium high heel or shoes with a narrow heel and increased risk of fractures in people aged 45 and older. Poor footwear has also been reported to be worn at the time of a fall by 75% of a sample of older people who suffered fall-related hip fractures.5 Walking barefoot does not appear to be a safe alternative because it has been found walking barefoot or in socks also increases the risk of falling.6-8 Despite the number of studies that have addressed the effects of footwear on balance,9,10 there are still no evidence-based guidelines to assist older people with regard to which specific shoe features are optimal for balance. For example, conflicting findings have been reported regarding shoe midsole hardness,11-14 and little is known about the effects of different sole tread patterns on balance. The available evidence pertaining to footwear and stability is limited in that studies have compared different shoe designs, such as sneakers versus shoes with stiletto heels,15 making it difficult to isolate the effects of specific shoe features. Additionally, some studies11,16 have evaluated balance by using

List of Abbreviations ANOVA AP BOS COM ML MPJ MCAR

analysis of variance anteroposterior base of support center of mass mediolateral metatarsophalangeal joint missing completely at random

SHOE FEATURES AND DYNAMIC STABILITY, Menant

routine clinical tests,11,16 which provide limited insights into stability under more challenging conditions. For example, although shoes with high heels have been shown to be detrimental to standing balance16 and increasing shoe collar height appears to improve leaning balance,11 it is not known how these shoe features influence gait stability. Because older people most often fall in response to unexpected perturbations when walking,17 it follows that the effects of footwear should be tested under such conditions.13 Maintenance of stability when walking requires people to control their total body COM within a constantly changing and moving BOS.18 Therefore, the primary purpose of this study was to systematically investigate the effects of various shoe features (an elevated heel, a high collar, a tread sole, a smooth soft sole, a smooth hard sole) on balance control when walking on even and uneven surfaces in young and older people. We hypothesized that (1) a tread shoe would assist balance, particularly on an uneven surface; (2) additional sensory cues provided to the foot and ankle (ie, by increasing sole hardness or collar height) would increase stability by improving an individual’s perception of the position of their COM relative to their stability limits; and (3) wearing shoes with an elevated heel or a soft sole would restrict the natural balance range and reduce the ability to appropriately respond to perturbations.12 METHODS Participants Eleven young adults (7 women, mean age ⫾ SD, 22.5⫾2.5y; height range, 157–183cm; mass range, 50 –93kg) and 15 older

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adults (7 women; mean age ⫾ SD, 73.7⫾4.2y; height range, 152–188cm; mass range, 53–98kg) volunteered to participate in this study. To be eligible to participate, the older adults were required to be community dwelling, aged over 65 years, and be able to walk 800m without assistance. Participants were excluded from the study if they suffered from any cognitive, neurologic, musculoskeletal, or balance impairment. The experimental protocol was approved by the university ethics review board, and all subjects gave written informed consent before data collection. Shoe Characteristics A standard Oxford-style laced shoea with a suede leather upper, ethylene vinyl acetate sole material of average hardness (shore A-40) and thickness (27mm and 13mm under the heel and the first MPJ, respectively), low heel collar, square heel, and standard shape smooth sole was used as the standard shoe. Five additional shoe designs were fabricated by an orthopedic boot maker by altering the standard with respect to 1 feature only, namely, sole hardness, heel height, heel collar height, or sole pattern, features posited as potentially affecting balance.9 The shapes and specifications of each shoe are shown in figure 1. The shoes were comfortable to wear and designed to fit both men and women in sizes ranging from size 5 to 10. All subjects wore socks and a 5-mm thick inner sole enabled half size adjustment if required. The principal investigator fitted subjects into their shoes by palpating the subject’s hallux while the subject was standing to ensure that there was approximately 1.5cm between the hallux and the shoe end.19 The principal investigator also laced the

Fig 1. The 6 shoe conditions: (1) the standard sole (shore A– 40 hardness), soft sole (shore A–25 hardness), and hard sole (shore A–58 hardness) shoes (all of identical shape); (2) the high collar shoe with an 11-cm high suede leather collar around the ankle; (3) the elevated heel shoe with a 4.5-cm high heel of same contact area as for the standard shoe; and (4) the tread sole shoe with an indented tread all over the outer sole.

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shoes and verified with the subject that both the left and right shoe laces were secured with a similar level of tension. Experimental Protocol Subjects were first assessed on a series of sensorimotor tests relating to balance control that included visual contrast sensitivity, hand reaction time, and knee extension strength. Descriptions of the apparatus and procedures for these tests are reported elsewhere.20 In addition, plantar tactile sensitivity was assessed at the heel, fifth MPJ, first MPJ, and hallux by using Semmes-Weinstein monofilaments and following a 2 alternative forced-choice protocol.21 The subjects then performed 3 walking trials on the level linoleum-covered floor of the laboratory followed by 12 trials over an uneven walkway in each of the 6 shoe conditions presented in a randomized manner. Between each shoe condition, the subjects were seated, allowing them to rest to prevent fatigue and for the principal investigator to assist in fitting the next pair of shoes. The uneven walkway consisted of 6 inclined wooden platforms (45.5⫻46.5⫻9.3cm; 10° angle) placed one after another so the foot contacted a different platform with each consecutive step.22,23 Subjects were allowed a practice trial over the uneven walkway. If they indicated that the step length was too long or too short, the placement of the platforms was adjusted accordingly. The orientation of the first and last 2 platforms was ML, whereas the orientation of the 2 middle platforms resting on the force platforms was randomly changed after each trial to create an unpredictable balance perturbation. Thus, the 2 middle platforms were placed with the incline in any of the 4 orientations: slanted toward the back, the front, the left, or the right. To prevent preplanning of foot placement, subjects were asked to look away from the walkway between each uneven surface trial. In addition, while walking over the uneven surface, the subjects were asked to look straight ahead fixating on an X marked on the opposite wall at eye level. Three-dimensional kinematic data were collected for 5 seconds at 100Hz by using 2 OptoTrak 3020 Position Sensors.b Twelve infrared light-emitting diodes were placed bilaterally on the third metatarsal, ankle, knee, hip, and acromion as well as the xyphoid process and forehead to track the motion of the whole body. The position of each subject’s COM was calculated by using a 7-segment model23 and anthropometric data compiled by Winter.24 Ground reaction force signals were collected for 5 seconds at 200Hz, in synchronization with the kinematic data, by using 2 force platformsc embedded flush in the walkway. Subjective ratings of perceived stability and comfort were assessed at the end of each shoe condition by using a 5-point scale (1 [most unstable] to 5 [most stable] and 1 [least comfortable] to 5 [most comfortable]). Data Analysis The ground reaction force signals and the kinematic data were low-pass filtered at 10Hz by using a fourth-order Butterworth dual-pass filter. The following gait variables were examined: walking velocity (in m/s), defined as the ratio of the AP distance traveled by the COM during total contact time with the force platforms divided by that time; step length (in millimeters), defined as the AP distance between the ankle infrared light-emitting diodes during 2 consecutive heel strikes; step width (in millimeters), defined as the ML distance between the ankle infrared light-emitting diodes during 2 consecutive heel strikes; and double-support phase, defined as the time between the initial contact onto the second force platform and Arch Phys Med Rehabil Vol 89, October 2008

toe off the first force platform and expressed as a percentage of stance time. COM-BOS relationships in the AP and ML directions (in millimeters) were calculated during the single-support phase of stance only by using the kinematic data according to the method devised by Perry.12,23,25 That is, the posterior COMBOS margin was calculated as the minimum distance between the COM AP position and the posterior border of the BOS and normalized to step length. The lateral COM-BOS margin was calculated as the minimum distance between the COM ML position and the lateral border of the BOS and normalized to step width. The average braking and vertical loading rates were calculated from the AP and vertical forces recorded over the first 100ms of each foot contact with the force platforms and normalized to subjects’ body mass. The vertical and the braking loading rates were further normalized to walking velocity to ensure that any significant differences between shoe conditions in the kinetic variables were not solely caused by variations in walking velocity. Normalization to walking velocity involved dividing each subject’s mean vertical and braking loading rates in each shoe and surface condition by their mean walking velocity in the same condition and then multiplying the result by the mean walking velocity of the full study sample across all conditions. The braking and vertical loading rates were therefore expressed in N·kg⫺1·s⫺1. Statistical Analysis Initially, the data were inspected, and outliers resulting from measurement error (forceplate contact missed, hidden marker) were excluded from the analysis. Because of positive skewing, the hand reaction time data were log10 transformed, and because of negative skewing, the toe and first MPJ tactile sensitivity data were square root transformed.26 Two subjects were not able to complete the walking trials in their last shoe condition (tread sole and elevated heel shoe, respectively) because of fatigue. To avoid the loss of these cases, the missing data were estimated. First to test the null hypothesis that the missing data were MCAR, the Little and Rubin’s MCAR test27 was conducted. Then, missing values were input by using the expectation maximization algorithm, which estimates missing values by an iterative process.28 A multivariate ANOVA was conducted on all the demographic and sensorimotor-dependent variables to identify significant between-group differences. A mixed design 3-way repeated-measures ANOVA was then conducted to determine between-group effects of age and within-subjects effects of surface and shoe conditions on the temporospatial, COM-BOS, and loading rate variables. Where main effects and interactions effects of age and surface conditions were identified, simple contrast analyses to the standard shoe were used to determine significant differences between the standard shoe and the modified shoe conditions. When significant contrasts resulted from an interaction of the independent variables, separate 1-way repeated-measures ANOVA with simple contrasts to the standard shoe were performed. Similarly, a mixed-design 2-way repeated-measures ANOVA was conducted to determine between-group effects of age and within-subjects effects of shoe conditions on subjective ratings of perceived stability and comfort. Despite the multiple comparisons made, P values were not adjusted to Bonferroni in this exploratory study because such adjustments may increase type II errors, especially in studies with a small sample size.29 All significance levels were set at P less than .05. Effect sizes were computed as partial ␩2 values. All statistical analyses were performed by using SPSS.d

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SHOE FEATURES AND DYNAMIC STABILITY, Menant Table 1: Values for the Sensorimotor Tests in Young (nⴝ11) and Older (nⴝ15) Subjects Variables

Young Group (n⫽11)

Older Group (n⫽15)

Visual contrast sensitivity (dB) Hand reaction time (ms) Knee extension strength (kg) Heel tactile sensitivity* First MPJ tactile sensitivity* Fifth MPJ tactile sensitivity* Hallux tactile sensitivity*

24.0⫾0.0 205.0⫾26.0 52.0⫾18.0 3.7⫾0.9 3.5⫾1.0 3.8⫾0.9 3.6⫾0.8

21.0⫾2.0† 231.0⫾40.0 33.0⫾13.0† 4.2⫾1.0 4.2⫾0.6† 4.4⫾1.1 4.0⫾0.8

NOTE. Values are mean ⫾ SD. *Recorded in evaluator size of the filaments; the filaments used in this study were 1.65, 2.36, 2.83, 3.61, 4.31, 4.56, 5.07, and 6.65 in evaluator size, which results in 0.008, 0.02, 0.07, 0.4, 2, 4, 10, and 300g of force when the filament is applied perpendicular to the skin and is bent to half of its length. † Significantly different from the young group (P⬍.05).

RESULTS Age-Related Effects on the Sensorimotor Tests Mean data characterizing the sensorimotor ability of the young and older subjects are presented in table 1. The older subjects performed poorer than their younger counterparts in the tests of visual contrast sensitivity (F1,24⫽26.36, P⬍.001; partial ␩2⫽.523), knee-extension strength (F1,24⫽8.96, P⫽.006; partial ␩2⫽.272), and plantar tactile sensitivity at the first MPJ (F1,24⫽6.48, P⫽.018; partial ␩2⫽.213). Temporospatial, COM-BOS, Loading Rate, and Perception Variables Age-related effects. Table 2 displays the mean values obtained for the temporospatial, COM-BOS, and loading rate variables in the young and older groups when the data were pooled across surface and shoe conditions. The older subjects displayed significantly shorter step length (F1,24⫽7.70, P⫽.011; partial ␩2⫽.243), a greater percentage of stance time spent in double support (F1,24⫽21.56, P⬍.001; partial ␩2⫽.473), and a slower walking velocity (F1,24⫽14.17, P⫽.001; partial ␩2⫽.371) relative to the young subjects. A significantly smaller posterior COM-BOS margin was also noted in the older group compared with the younger group (F1,24⫽8.19, P⫽.009; partial ␩2⫽.254). However, there were no significant age-related differences in vertical or braking loading rates.

Surface-related effects. The average values for the temporospatial, COM-BOS, and loading rate variables in the even and uneven surface conditions when the data were pooled across subject groups are shown in table 2. Although subjects displayed a similar step length when walking over the uneven surface (F1,24⫽.01, P⬎.05), they adopted wider steps (F1,24⫽ 69.64, P⬍.001; partial ␩2⫽.744), walked more slowly (F1,24⫽20.92, P⬍.001; partial ␩2⫽.466), and spent less time in double support (F1,24⫽16.33, P⬍.001; partial ␩2⫽.405) compared with walking over the even surface. Smaller lateral and posterior COM-BOS margins were recorded in the subjects walking on the uneven surface compared with the even surface (F1,24⫽78.25, P⬍.001; partial ␩2⫽.765; F1,24⫽487.89, P⬍.001; partial ␩2⫽.953, respectively). Both the vertical and braking loading rates were significantly lower when subjects walked over the uneven surface compared with the even surface condition (F1,24⫽5.39, P⫽.029; partial ␩2⫽.183; F1,24⫽19.48, P⬍.001; partial ␩2⫽.448, respectively). Shoe condition effects. Table 3 displays the mean values obtained for the temporospatial, COM-BOS, and loading rate variables during the 6 shoe conditions when the data were pooled across subject groups and surface conditions. Overall, subjects walked faster in the tread sole and in the soft sole shoes than in the standard shoes (F1,24⫽5.81, P⫽.024; partial ␩2⫽.195; F1,24⫽4.40, P⫽.047; partial ␩2⫽.155, respectively). There was also a significant surface-by-shoe interaction for step length between the tread sole shoes and the standard shoes (F1,24⫽9.77, P⫽.005; partial ␩2⫽.289) whereby subjects recorded a greater step length when walking in the tread sole shoes on the even surface than in the standard shoes (755⫾69mm vs 739⫾62mm, respectively; F1,24⫽7.65, P⫽.011; partial ␩2⫽.242). Subjects also spent more time in double support when wearing both the elevated heel and the hard sole shoes than the standard shoes (F1,24⫽26.76, P⬍.001; partial ␩2⫽.527; F1,24⫽5.07, P⫽.034; partial ␩2⫽.174, respectively) and recorded a smaller step width in the hard sole shoes compared with the standard shoes (F1,24⫽5.14, P⫽.033; partial ␩2⫽.176). In addition, there was a significant surface by shoe interaction for step width between the elevated heel shoes and the standard shoes conditions (F1,24⫽4.76, P⫽.039; partial ␩2⫽.165) whereby the elevated heel shoes led to a significantly greater step width on the uneven surface than the standard shoes (185⫾32mm vs 177⫾35mm; F1,24⫽5.29, P⫽.030; partial ␩2⫽.181). Both the soft sole and the high collar shoes elicited a greater lateral COM-BOS margin than the standard shoes (F1,24⫽20.70,

Table 2: Values for Temporospatial, COM-BOS, and Loading Rate Variables Between Age-Group and Surface Conditions Age Conditions

Surface Conditions

Variables

Young Group (n⫽11)

Older Group (n⫽15)

Even Surface (N⫽26)

Uneven Surface (N⫽26)

Step length (mm) Step width (mm) Double-support time (% stance time) Walking velocity (m/s) Lateral COM-BOS margin (mm) Posterior COM-BOS margin (mm) Vertical loading rate (N·kg⫺1·s⫺1) Braking loading rate (N·kg⫺1·s⫺1)

770.0⫾44.0 160.0⫾39.0 32.7⫾2.0 1.46⫾0.12 63.0⫾22.0 52.0⫾35.0 86.0⫾8.0 21.0⫾3.0

718.0⫾61.0* 162.0⫾38.0 36.6⫾3.2* 1.22⫾0.20* 68.0⫾29.0 28.0⫾32.0* 88.0⫾10.0 20.0⫾4.0

740.0⫾78.0 144.0⫾35.0 35.5⫾3.5 1.35⫾0.22 85.0⫾24.0 62.0⫾26.0 89.0⫾8.0 21.0⫾3.0

741.0⫾33.0 178.0⫾33.0† 34.4⫾3.1† 1.29⫾0.20† 47.0⫾10.0† 14.0⫾26.0† 85.0⫾10.0† 20.0⫾4.0†

NOTE. Values are mean ⫾ SD. *Significantly different from the young group (P⬍.05). † Significantly different from the even surface (P⬍.05).

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SHOE FEATURES AND DYNAMIC STABILITY, Menant Table 3: Values for Temporospatial, COM-BOS, and Loading Rate Variables Between Shoe Conditions Shoe Conditions Variables

Step length (mm) Step width (mm) Double support (% stance time) Walking velocity (m/s) Lateral COM-BOS margin (mm) Posterior COM-BOS margin (mm) Vertical loading rate (N·kg⫺1·s⫺1) Braking loading rate (N·kg⫺1·s⫺1)

Standard (N⫽26)

Elevated Heel (N⫽26)

Soft Sole (N⫽26)

Hard Sole (N⫽26)

High Collar (N⫽26)

Tread Sole (N⫽26)

738.0⫾56.0 162.0⫾36.0 34.3⫾3.1 1.31⫾0.21 64.0⫾25.0 41.0⫾36.0 88.0⫾9.0 21.0⫾5.0

742.0⫾64.0 166.0⫾40.0† 36.3⫾3.9* 1.32⫾0.20 64.0⫾25.0 34.0⫾35.0* 80.0⫾10.0* 19.0⫾5.0†

744.0⫾60.0 160.0⫾37.0 34.7⫾3.1 1.33⫾0.22* 70.0⫾27.0* 39.0⫾36.0 89.0⫾8.0 21.0⫾5.0

735.0⫾64.0 157.0⫾39.0* 35.0⫾3.3* 1.31⫾0.21 65.0⫾29.0 35.0⫾39.0* 88.0⫾9.0 21.0⫾5.0

739.0⫾60.0 163.0⫾41.0 34.9⫾3.3 1.32⫾0.21 68.0⫾28.0* 38.0⫾34.0‡ 87.0⫾8.0 21.0⫾5.0

745.0⫾59.0† 158.0⫾37.0 34.3⫾3.2 1.33⫾0.22* 65.0⫾26.0 42.0⫾34.0† 88.0⫾9.0 21.0⫾5.0

NOTE. Values are mean ⫾ SD. *Significant contrast to the standard shoe (P⬍.05). Significant surface by shoe interaction contrast to the standard shoe (P⬍.05). ‡ Significant group by shoe interaction contrast to the standard shoe (P⬍.05). †

P⬍.001; partial ␩2⫽.463; F1,24⫽6.28, P⫽.019; partial ␩2⫽.207, respectively). In addition, the hard sole and the elevated heel shoes led to a significant reduction in the posterior COM-BOS margin compared with the standard shoe (F1,24⫽4.34, P⫽.048; partial ␩2⫽.153; F1,24⫽14.50, P⫽.001; partial ␩2⫽.377, respectively). However, there was a significant group by shoe interaction between the high collar and the standard shoes (F1,24⫽8.54, P⫽.007; partial ␩2⫽.262) in that the high collar shoes caused a reduced posterior COM-BOS margin compared with the standard shoes in the older group (25⫾31mm vs 32⫾34mm; F1,14⫽20.30, P⫽.011; partial ␩2⫽.592). Another significant surface by shoe interaction between the high collar and the standard shoes (F1,24⫽4.71, P⫽.04; partial ␩2⫽.164) showed that when walking on the even surface, the high collar shoes caused a reduced posterior COM-BOS margin compared with the standard shoes (63⫾20mm vs 68⫾21mm; F1,24⫽9.00, P⫽.006; partial ␩2⫽.273). Despite a significant surface by shoe interaction found for the posterior COM-BOS margin between the tread sole shoe and the standard shoes (F1,24⫽7.76, P⫽.01; partial ␩2⫽.244), further analyses did not reveal additional significant findings. Subjects experienced significantly lower vertical and braking loading rates walking in the elevated heel shoes compared with the standard shoes (F1,24⫽35.54, P⬍.001; partial ␩2⫽.597; F1,24⫽23.14, P⬍.001; partial ␩2⫽.491, respectively). A significant surface by shoe interaction for the braking loading rate between the elevated heel shoes and the standard shoes (F1,24⫽4.71, P⫽.04; partial ␩2⫽.164) showed that the ele-

vated heel shoes caused greater reductions in braking loading rate on the even surface compared with the standard shoes (19⫾3N·kg⫺1·s⫺1 vs 22⫾3N·kg⫺1·s⫺1; F1,24⫽20.22, P⬍.001; partial ␩2⫽.457). Despite a significant surface by group by shoe interaction found for the braking loading rate between the hard sole shoes and the standard shoes (F1,24⫽6.63, P⫽.017; partial ␩2⫽.217), further analyses did not reveal additional significant findings. There was no significant sex by shoe interaction for any of the temporospatial, COM-BOS, and loading rate variables. Perceived shoe comfort and stability. There were no significant age-related differences in perceived shoe comfort or shoe stability. As depicted in figure 2, the subjects rated the elevated heel shoes as significantly less comfortable (F1,24⫽25.51, P⬍.001; partial ␩2⫽.515) and less stable (F1,24⫽17.86, P⬍.001; partial ␩2⫽.427) than the standard shoes. A significant group by shoe interaction for perceived stability ratings between the soft sole and standard shoes (F1,24⫽8.98, P⫽.006; partial ␩2⫽.272) revealed that only the young subjects perceived the soft sole shoes to be less stable than the standard shoes (F1,10⫽14.82, P⫽.003; partial ␩2⫽.597) (fig 3). There was no significant sex by shoe interaction for the perceived shoe comfort and stability variables.

Fig 2. Mean ratings ⴞ SE of perceived comfort and stability across the shoe conditions when pooled across subject groups. *P<.05, significantly different from the standard shoes.

Fig 3. Mean ratings ⴞ SE of perceived stability across the shoe conditions between the young (nⴝ11) and the older (nⴝ15) subjects. *P<.05, significantly different from the standard shoes.

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DISCUSSION This is the first study to systematically investigate the effects of salient shoe features (an elevated heel, a high collar, a tread

SHOE FEATURES AND DYNAMIC STABILITY, Menant

sole, a soft sole, a hard sole) on walking stability. We found that in addition to age-related differences, specific shoe features significantly affected walking stability in both young and older people. Walking on the uneven surface increased the proximity of the COM to the edge of the BOS in both young and older subjects, which indicates that the uneven surface imposed a balance control challenge.22 As a consequence, subjects reduced their walking velocity and took wider steps. Subjects also experienced significantly lower braking and vertical loading rates when walking on the uneven surface, which could be explained by changes in lower-limb kinematics. In fact, similar reductions in braking loading rates have been observed in young people walking on a slippery surface, which were attributed to a smaller sagittal foot angle at heel strike, which acted to reduce the braking impulse and the speed of vertical impact.30 It is possible that in the present study when walking on the uneven surface the subjects reacted in a comparable manner, adopting a flatter foot strike to optimize stability. The reduction in posterior COM-BOS margin noted in the elevated heel shoes is in contrast with a previous report of an anterior shift of the COM in female habitual high heel shoe wearers.31 The fact that our testing sample included young and older adults, of both sexes, who were not all used to wearing elevated heel shoes may explain the discrepancy in the findings. Aware of the smaller critical tipping angle of the elevated heel shoe,3 subjects might have required more time to efficiently control ML stability and, consequently, increased the time spent in double support. Again, confronted with a potentially unstable situation, subjects might have adopted a more probing walking pattern when wearing the elevated heel shoes, aiming to increase the contact area at foot-ground contact by striking the ground more flat footed, explaining the significant reduction in vertical and braking loading rates induced by the elevated heel.30 We hypothesized that the extra sensory input provided by a high collar, similarly to an ankle pressure device, may facilitate joint position sense32 and, in turn, improve ML balance. In fact, a tactile stimulus applied to the leg of both younger, older, and neuropathic subjects has been found to reduce body sway during standing.33 Although a greater lateral COM-BOS margin was noted in the high collar shoes, the clinical significance of this finding is questionable considering its small effect size (partial ␩2⬍0.2). The soft sole shoes led to a greater lateral COM-BOS margin than the standard shoes, although no concurrent increase in step width was observed. This suggests that subjects restricted the ML excursion of their COM rather than proactively adjusting their foot placement to maintain frontal plane stability. Similarly, a recent study12 has found a significant reduction in the ML range of COM displacement in young adults wearing soft midsole versus hard midsole shoes during an unexpected stopping task. This adaptation may in part be explained by the poor mechanical support of the soft sole12 and its reported detrimental effect on joint position sense in older people.14 In contrast, the malleability of the soft soles could have contributed to reducing the ML COM excursion during the uneven surface trials when the platforms were oriented mediolaterally because compression of the sole would counteract the destabilizing effect of the slope. Walking stability measures were similar in the tread sole and standard shoe conditions except that subjects generally walked faster and had greater step length on the even surface when wearing the tread sole shoes. The increased coefficient of friction that the tread pattern provides on a dry surface34 may

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assist in reducing shear forces at the shoe sole – floor interface, in turn, improving propulsion efficiency. The significantly lower ratings of perceived comfort and stability reported for the elevated heel shoes compared with the control shoes indicate that both young and older subjects were aware of the balance threat imposed by the elevated heel and adjusted their walking pattern accordingly. However, only the older participants, whose plantar tactile sensitivity was less than the younger group, rated the soft sole shoes to be as stable as the control shoes. Because of their capacity to disperse localized loads, soft sole shoes are commonly recommended to individuals such as neuropathic patients who suffer from foot pain.35 All subjects perceived the soft sole shoe to be no more comfortable than the standard and hard sole shoes, which is in conflict with a previous finding that subjects report soft sole shoes to be the most comfortable.13 However, the fact that older people, especially those with somatosensory deficits, perceived the soft sole shoes to be both comfortable and stable presents a potential hazard in that they may be more likely to purchase these shoes,36 unaware of the detrimental effect they have on balance. Study Limitations It is acknowledged that the study has certain limitations. First, given the small study sample and the wide range of testing conditions, the statistical power of the study might not have been sufficient to detect potential significant differences between shoe conditions. However, several hypothesized results pertaining to the elevated heel and the soft sole were confirmed and presented moderate-to-large effect sizes. Second, some of the associations uncovered may have occurred by chance, but it is likely that the comparisons with moderate and large effect sizes reflect true associations. Third, it has been reported that experience with wearing elevated heels shoes may affect walking stability in low heel shoes1 and gait patterns in high heel shoes.37 Unfortunately, the number of female participants who were or had been habitual high heel shoe wearers in the present study was not recorded. The absence of significant sex by shoe interactions for the dependent variables and the finding that elevated heel shoes significantly affected walking pattern and balance control suggest that any habituation to high heel shoes was not evident. Fourth, given that the reliability of the 5-point scales used to assess perceived comfort and stability was not established in this study, the subjective data findings should be interpreted with caution. Finally, step length variability when subjects walked on the uneven surface was surprisingly low. This contradicted our expectation that the challenging walking task would lead to more irregular foot placements and, in turn, more variability between trials. Low step length variability as well as an increased step width on the uneven surface may have resulted from the experimental task, which forced subjects to adopt specific foot placements. CONCLUSIONS The findings of this study suggest that increased shoe heel height and sole softness caused a more conservative walking pattern and impaired ML balance control, respectively, in both young and older subjects. In contrast, increased sole hardness (above that found in a standard shoe), a tread sole, and a raised collar height did not improve walking stability in either group. Thus, although shoes with elevated heels and soft soles should not be recommended for older people, a laced shoe with a low collar and a sole of standard hardness with or without a tread might provide them optimal dynamic balance when walking on even and uneven surfaces. Arch Phys Med Rehabil Vol 89, October 2008

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Acknowledgment: We thank Sarah Rabley for her assistance with data collection and processing. References 1. Gabell A, Simons MA, Nayak US. Falls in the healthy elderly: predisposing causes. Ergonomics 1985;28:965-75. 2. Kerse N, Butler M, Robinson E, Todd M. Wearing slippers, falls and injury in residential care. Aust N Z J Public Health 2004;28:180-7. 3. Tencer AF, Koepsell TD, Wolf ME, et al. Biomechanical properties of shoes and risk of falls in older adults. J Am Geriatr Soc 2004;52:1840-6. 4. Keegan TH, Kelsey JL, King AC, Quesenberry CP Jr, Sidney S. Characteristics of fallers who fracture at the foot, distal forearm, proximal humerus, pelvis, and shaft of the tibia/fibula compared with fallers who do not fracture. Am J Epidemiol 2004;159:192-203. 5. Sherrington C, Menz HB. An evaluation of footwear worn at the time of fall-related hip fracture. Age Ageing 2003;32:310-4. 6. Koepsell TD, Wolf ME, Buchner DM, et al. Footwear style and risk of falls in older adults. J Am Geriatr Soc 2004;52:1495-501. 7. Menz HB, Morris ME, Lord SR. Footwear characteristics and risk of indoor and outdoor falls in older people. Gerontology 2006;52: 174-80. 8. Larsen RE, Mosekilde L, Foldspang A. Correlates of falling during 24 h among elderly Danish community residents. Prev Med 2004;39:389-98. 9. Menz HB, Lord SR. Footwear and postural stability in older people. J Am Podiatr Med Assoc 1999;89:346-57. 10. Hijmans JM, Geertzen JH, Dijkstra PU, Postema K. A systematic review of the effects of shoes and other ankle or foot appliances on balance in older people and people with peripheral nervous system disorders. Gait Posture 2007;25:316-23. 11. 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;47:681-4. 12. Perry SD, Radtke A, Goodwin CR. Influence of footwear midsole material hardness on dynamic balance control during unexpected gait termination. Gait Posture 2007;25:94-8. 13. Robbins S, Gouw JG, McClaran J. Shoe sole thickness and hardness influence balance in older men. J Am Geriatr Soc 1992;40: 1089-94. 14. Robbins S, Waked E, McClaran J. Proprioception and stability: foot position awareness as a function of age and footwear. Age Ageing 1995;24:67-72. 15. Esenyel M, Walsh K, Walden JG, Gitter A. Kinetics of highheeled gait. J Am Podiatr Med Assoc 2003;93:27-32. 16. Lord SR, Bashford GM. Shoe characteristics and balance in older women. J Am Geriatr Soc 1996;44:429-33. 17. Lord SR, Sherrington C, Menz HB. Falls in older people. 2nd ed. Cambridge: Cambridge Univ Pr; 2001. 18. Patla AE. Strategies for dynamic stability during adaptive human locomotion. IEEE Eng Med Biol Mag 2003;22:48-52. 19. Janisse DJ. The art and science of fitting shoes. Foot Ankle 1992;13:257-62. 20. Lord SR, Menz HB, Tiedemann NA. A physiological profile approach to falls risk assessment and prevention. Phys Ther 2003; 83:237-52. 21. Perry SD. Evaluation of age-related plantar-surface insensitivity and onset age of advanced insensitivity in older adults using vibratory and touch sensation tests. Neurosci Lett 2006;392:62-7.

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22. Perry SD, Radtke, Goodwin C. Uneven terrain as a perturbation to the centre of mass motion during locomotion. In: Proceedings of the 13th Biennial Conference of the Canadian Society for Biomechanics: 2004 Aug; Halifax (NS). 23. Perry SD, Bombardier E, Radtke A, Tiidus PM. Hormone replacement and strength training positively influence balance during gait in post-menopausal females: a pilot study. J Sport Sci Med 2005; 4:372-81. 24. Winter DA. Biomechanics and motor control of human movement. 3rd ed. Toronto: John Wiley & Sons; 1990. 25. Perry SD, Santos LC, Patla AE. Contribution of vision and cutaneous sensation to the control of centre of mass (COM) during gait termination. Brain Res 2001;913:27-34. 26. Tabachnick BG, Fidell LS. Using multivariate statistics. 5th ed. Amsterdam: Elsevier; 2007. 27. Little RJ, Rubin DB. The analysis of social-science data with missing values. Sociol Methods Res 1989;18:292-326. 28. Dempster AP, Laird NM, Rubin DB. Maximum likelihood from incomplete data via EM algorithm. J R Stat Soc Ser B Stat Meth 1977;39:1-38. 29. Perneger TV. What’s wrong with Bonferroni adjustments. BMJ 1998;316:1236-8. 30. Marigold DS, Patla AE. Strategies for dynamic stability during locomotion on a slippery surface: effects of prior experience and knowledge. J Neurophysiol 2002;88:339-53. 31. Snow RE, Williams KR. High heeled shoes: their effects on center of mass position, posture, three-dimensional kinematics, rearfoot motion, and ground reaction forces. Arch Phys Med Rehabil 1994;74:568-76. 32. You SH, Granata KP, Bunker LK. Effects of circumferential ankle pressure on ankle proprioception, stiffness, and postural stability: a preliminary investigation. J Orthop Sports Phys Ther 2004;34: 449-60. 33. Menz HB, Lord SR, Fitzpatrick RC. A tactile stimulus applied to the leg improves postural stability in young, old and neuropathic subjects. Neurosci Lett 2006;406:23-6. 34. Menz HB, Lord SR, McIntosh AS. Slip resistance of casual footwear: implications for falls in older adults. Gerontology 2001; 47:145-9. 35. Charanya G, Patil KM, Narayanamurthy VB, Parivalavan R, Visvanathan K. Effect of foot sole hardness, thickness and footwear on foot pressure distribution parameters in diabetic neuropathy. Proc Inst Mech Eng [H] 2004;218:431-43. 36. Munro BJ, Steele JR. Household-shoe wearing and purchasing habits. A survey of people aged 65 years and older. J Am Podiatr Med Assoc 1999;89:506-14. 37. Lee KH, Shieh JC, Matteliano A, Smiehorowski T. Electromyographic changes of leg muscles with heel lifts in women: therapeutic implications. Arch Phys Med Rehabil 1990;71:31-3. Suppliers a. Gadean Footwear, 139 Buxton St, Mount Hawthorn, WA 6016, Australia. b. Northern Digital Inc, 103 Randall Dr, Waterloo, ON N2V 1C5, Canada. c. Advanced Mechanical Technology Inc, 176 Waltham St, Watertown, MA 02472. d. Version 14.0; SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.