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Control of the motion of the body's center of mass in relation to the center of pressure during high-heeled gait
Gait & Posture 38 (2013) 391–396
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Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Control of the motion of the body’s center of mass in relation to the center of pressure during high-heeled gait Hui-Lien Chien a, Tung-Wu Lu a,b,*, Ming-Wei Liu c a
Institute of Biomedical Engineering, National Taiwan University, Taiwan, ROC Department of Orthopaedic Surgery, School of Medicine, National Taiwan University, Taiwan, ROC c Department of Surgery, Taiwan Adventist Hospital, Taiwan, ROC b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 May 2012 Received in revised form 17 December 2012 Accepted 26 December 2012
High-heeled shoes are associated with instability and falling, leading to injuries such as fracture and ankle sprain. Knowledge of the motion of the body’s center of mass (COM) with respect to the center of pressure (COP) during high-heeled gait may offer insights into the balance control strategies and provide a basis for approaches that minimize the risk of falling and associated adverse effects. The study aimed to investigate the influence of the base and height of the heels on the COM motion in terms of COM–COP inclination angles (IA) and the rate of change of IA (RCIA). Fifteen females who regularly wear high heels walked barefoot and with narrow-heeled shoes with three heel heights (3.9 cm, 6.3 cm and 7.3 cm) while kinematic and ground reaction force data were measured and used to calculate the COM and COP, as well as the temporal-distance parameters. The reduced base of the heels was found to be the primary factor for the reduced normalized walking speed and the reduced frontal IA throughout the gait cycle. This was achieved mainly through the control of the RCIA during double-leg stance (DLS). The heel heights affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. These results suggest young adults adopt a conservative strategy for balance control during narrow-heeled gait. The results will serve as baseline data for future evaluation of patients and/or older adults during narrow-heeled gait with the aim of reducing the risk of falling. ß 2012 Elsevier B.V. All rights reserved.
Keywords: Gait High heels Center of mass Center of pressure Balance
1. Introduction Surveys on shoe use show that between 39% and 69% of women wear high-heeled shoes on a daily basis [1–3]. High-heeled shoes increase the difficulty of maintaining balance and thus the risk of falling [4–6], leading to injuries such as fractures [7]. Wearing shoes with heels higher than 2.5 cm nearly doubles the risk of falling in older people [5]. Apart from heel heights, narrow heels or smaller sole contact also significantly increase the risk of a fall [5,8]. This may be related to the increased difficulty in controlling the body’s center of mass (COM) subject to a narrow and constantly changing base of support (BOS). Knowledge of the motion of the COM with respect to the BOS may offer insights into the balance control strategies and provide a basis for protocols that minimize the risk of falling and associated adverse effects. Of prime importance to an individual with narrow and highheeled shoes is the stability or balance on the heel of the shoe
* Corresponding author at: Institute of Biomedical Engineering, National Taiwan University, No. 1, Sec. 1, Jen-Ai Road, Taipei 100, Taiwan, ROC. Tel.: +886 2 33653335; fax: +886 2 33653335. E-mail address: [email protected] (T.-W. Lu). 0966-6362/$ – see front matter ß 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gaitpost.2012.12.015
during the stance phase. While the kinematic and kinetic changes of the lower limb joints during high-heeled gait and their relationship with overuse injuries have been studied extensively [9–13], the balance control of the body and its relationship with risks of falling have not been studied as widely [4]. The current knowledge of the body’s control with high-heeled shoes was formed primarily from the data obtained using clinical tests or swaymeter measurements [4,14]. However, they provide limited insights into stability under more challenging, dynamic conditions and cannot directly represent the motion of the whole body and its control. The motion of the COM in relation to the BOS has been used to represent the balance control of the whole body [15,16]. A person is thought to be in balance as long as the downward vertical projection of the COM is kept within the BOS during standing [15,16], or while the person maintains control of the COM subject to a constantly changing and moving BOS during walking [17]. Previous studies showed that with increased heel height the changes of the positions of the COM relative to the BOS were different between quiet standing and level walking [6,18]. For studying the motion of the COM during high-heeled gait, one faces the difficulty of defining the dynamic BOS not only during singleleg stance (SLS), but also during double-leg stance (DLS). This
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difficulty can be resolved by describing the motion of the COM relative to the center of pressure (COP) that can be directly measured using forceplates [19,20]. Among the relevant variables, COM–COP inclination angles (IA), i.e., angles formed by the vertical line and the line connecting the COP and COM, and the rate of change of IA (RCIA) have been used to describe the body’s dynamic control during locomotion without the influence of stature differences among subjects [19–21]. The greater the inclination angle, the further the COM will diverge from the COP, and the more difficult it will be to bring the COM back to be right above the COP unless accompanied with an appropriate rate of change of IA. A similar argument for the motion of the COM relative to the COP has been proposed by Pai and Patton [22]. The IA and RCIA are also useful for quantifying the effects of the heel base and height on the body’s COM control during walking. The purposes of this study were to investigate the influence of the base and height of the shoes’ heels on the COM motion in terms of IA and RCIA during walking. It was hypothesized that compared to barefoot walking, wearing narrow-heeled shoes would decrease the IA and RCIA which would also be significantly affected by heelheight. 2. Materials and methods 2.1. Subjects Fifteen female adults (age: 24.4 3.4 years; height: 158.9 5.7 cm; mass: 49.2 5.1 kg) participated in the current study with informed written consent, as approved by the Institutional Research Board. They were free from any neuromusculoskeletal pathology that might have affected their normal gait. They had worn shoes with narrow heels of more than 3 cm height at least three times per week, 6 h per day for at least two years. 2.2. Data collection Each subject walked at a self-selected pace on an 8-m walkway either barefoot, or with low-heeled (3.9 cm), medium-heeled (6.3 cm), or high-heeled shoes (7.3 cm). All the shoes were narrow-heeled (2.0 cm 1.6 cm) and had similar construction, including foot contact points, vamp and shape of the toe box (Fig. 1). Subjects were fitted with the most suitable test shoes from several different sizes, and were allowed to familiarize themselves with the walkway for each pair of test shoes. Each subject wore 39 retroreflective markers for tracking body segmental motions [23]. Foot markers, namely big toe, fifth metatarsal base and heel, were put on the corresponding positions on the shoes for shoe conditions. Markers on the navicular tuberosity and malleoli were not affected by the shoes. Threedimensional trajectories of the markers were measured using a motion capture system (Vicon 512, OMG, UK) at a sampling rate of 120 Hz, and were low-pass filtered using a fourth-order Butterworth filter with a cut-off frequency of 5 Hz [19]. The ground reaction forces (GRF) were collected from two forceplates (AMTI, USA) at a frequency of 1080 Hz [23]. Six successful trials, three for each limb, for each condition were obtained. The order of the test conditions was randomized. 2.3. Data analysis The body’s COM position data were calculated as the weighted sum of those of all the body segments, including head and neck, trunk, pelvis, arms, forearms, thighs, shanks and feet [23,24]. The COP position was calculated using forces and moments measured by the forceplates [25]. The medial/lateral positions of the COM and COP were described relative to the line of progression that bisected the medial/lateral range of motion of the COM during a gait cycle, a positive value being to the side of the contralateral limb [21] (Fig. 2). The anterior/posterior positions of the COM and COP were described parallel to the direction of progression, a zero value being the position of heel-strike and a positive value being anterior to that position. The IA in the sagittal plane (a) and frontal plane (b) were then calculated as follows [19] (Fig. 2). *
t¼
*
PCOM *
COP
jPCOM
*! Z
(1)
COP j
a ¼ sin1 ðtx Þ
(2)
b ¼ sin1 ðty Þ *
(3) *
where PCOM COP was the vector pointing from the COP to the COM, and Z was the unit vector of the global vertical axis. With the current forceplate setup, a and b were
Fig. 1. The three narrow-heeled shoes used in the current study: (a) low-heeled (3.9 cm), (b) medium-heeled (6.3 cm), and (c) high-heeled (7.3 cm). The maximum area of the base of support of the foot is much greater than that of the high-heeled shoe (d). calculated from the beginning of the single-leg stance to the subsequent heel-strike. The rates of changes of a and b were also calculated by smoothing and differentiating their trajectories using the generalized cross-validatory spline method [26]. During the gait cycle, the transition between SLS and DLS, i.e., heel-strike of the contralateral leg and toe-off of the ipsilateral leg, are critical instances at which
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2.4. Statistical analysis All calculated variables were analyzed using one-way repeated measures ANCOVA with normalized gait speed as a covariate (a = 0.05). If a significant main effect was found, pair-wise comparisons between test conditions using an independent t-test with Bonferroni correction (a = 0.05/6 = 0.0082) was performed. All statistical analysis was performed using SPSS (version 11.0, SPSS Inc., USA).
3. Results No significant differences in gait speed, stride length, stride time and cadence were found between any conditions (Table 1). After normalization, these variables except cadence and stride time were significantly decreased for all shoe conditions when compared to the barefoot condition (p < 0.002). Stance time and DLS time for all shoe conditions were increased (p < 0.0001) but only high-heeled shoes showed a smaller SLS time (p = 0.005) when compared to the barefoot condition. Frontal IA at contralateral heel-strike, toe-off and those averaged over the DLS were reduced for all shoe conditions when compared to barefoot, but no differences were found between heel heights (Fig. 3 and Table 2). For variables related to sagittal IA, the values of the high-heeled condition at contralateral heel-strike (p = 0.046) were smaller than for barefoot (Table 2). While the IA during the SLS phase were either constant in the frontal plane or increased anteriorly in the sagittal plane, rapid posterior and lateral increases in the inclination angles during DLS were found (Fig. 3). ANCOVA analysis suggests that normalized gait speed did not impose a limitation on the interpretation of the IA results. While the RCIA were small and remained almost constant during SLS, those during DLS varied significantly (Fig. 3). At the first peak of the ensemble-averaged curves of the RCIA during DLS, narrow-heeled shoes showed smaller lateral RCIA than barefoot (p < 0.0001), while high-heeled shoes had smaller lateral RCIA than low-heeled shoes (p < 0.0001), Table 2. In the sagittal plane, high-heeled and medium-heeled shoes showed smaller posterior RCIA than barefoot, and those for high-heeled shoes were smaller than those for low-heeled shoes, Table 2. At the second peak, no significant differences were found in the frontal plane, while statistically smaller values in the sagittal plane for barefoot and low-heeled shoes were found compared to those for middle-heeled and high-heeled shoes (p < 0.0001), Table 2. Compared to barefoot, all shoes had significantly smaller RCIA at contralateral heel-strike in the frontal plane (p < 0.0001), while only those for high-heeled and medium-heeled shoes were smaller in the sagittal plane (p = 0.014). Values of the RCIA averaged over the DLS were significantly smaller than those of the barefoot condition both in the sagittal and frontal planes (p < 0.008). No significant differences were found between heel heights. When gait speed was covaried (ANCOVA), the same stastical findings were obtained. 4. Discussion Fig. 2. (a) Trajectories of the COM and COP of a typical subject during a gait cycle in the transverse plane. (b) Sagittal COM–COP inclination angle (a). (c) Frontal COM– COP inclination angle (b). COP1 is the COP position at the end of single leg stance of the reference leg, while COP2 is the COP position at toe-off of the reference leg.
maintaining body stability is expected to be more difficult [19]. Therefore, the values of the IA and RCIA at these instances were obtained. The values of IA and RCIA were also averaged over the DLS and SLS, respectively. The peak RCIA during DLS was also extracted. Temporal-distance parameters (gait speed, stride length, step width, stride time, cadence, stance time, DLS time and SLS time) for all conditions were also calculated. For between-condition comparisons, gait speed, stride length, step width, stride time, cadence, and COP and COM positions were normalized following Hof [27]. Leg length was defined as the distance between the ipsilateral anterior superior iliac spine and the medial malleolus for barefoot gait, and the shoe height was added to this distance for shod gait.
The current study aimed to compare the balance control during walking between barefoot and narrow-heeled shoes of different heel heights, in terms of the COM motion and its coordination with the COP. The reduced BOS of narrow-heeled shoes was found to be the primary factor for the reduced normalized walking speed, and the reduced frontal IA throughout the gait cycle. Together with heel height, the reduced BOS also contributed to the reduced frontal and sagittal RCIA during DLS. These results suggest that the subjects used a conservative strategy for balance control during narrow-heeled gait. The reduced BOS of narrow-heeled shoes appeared to be the main factor for the reduction of the normalized walking speed. Decreased normalized gait speeds were found for all shoe conditions when compared to the barefoot condition, while no
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Table 1 Means (SD) of temporal-distance parameters during barefoot and narrow-heeled shoes gait. Barefoot
Parameters Gait speed m/s dimensionless Stride length m %LL Step width cm %LL Stride time s dimensionless Cadence steps/min dimensionless Stance time % Double leg suport time % Single leg support time % * ** ***
Low heel
Medium heel
High heel
p
1.15(0.11) 0.41(0.04)
1.09(0.09) 0.38(0.03)
1.09(0.09) 0.37(0.03)
1.07(0.09) 0.37(0.03)
0.129 0.002*
1.18(0.07) 146.8(7.7)
1.16(0.05) 137.8(5.8)
1.15(0.05) 132.4(6.1)
1.16(0.05) 132.0(6.5)
0.414 <0.0001*
8.32(2.26) 10.3(2.7)
6.53(1.43) 7.8(1.9)
6.33(1.68) 7.3(2.0)
5.85(2.10) 6.6(2.3)
0.005* 0.0002*
1.03(0.06) 3.60(0.24)
1.07(0.08) 3.66(0.26)
1.06(0.07) 3.56(0.24)
1.08(0.08) 3.62(0.27)
0.223 0.740
117.36(7.38) 0.56(0.04)
112.63(7.91) 0.55(0.04)
114.09(7.39) 0.57(0.04)
111.40(7.81) 0.56(0.04)
0.176 0.756
61.2(1.2)
63.1(1.1)
63.7(1.6)
64.2(1.4)
<0.0001**
10.3(1.3)
12.2(1.0)
12.9(1.7)
13.3(1.4)
<0.0001**
38.5(1.3)
37.7(1.1)
37.2(1.5)
36.6(1.6)
0.005***
Statistical significance, p < 0.05: barefoot > low heel, medium heel, high heel. Statistical significance, p < 0.05: barefoot < low heel, medium heel, high heel. Statistical significance, p < 0.05: barefoot > high heel.
Table 2 Means (standard deviations) of the sagittal and frontal COM–COP inclination angles (IA) and their rates of change (RCIA) at heel-strike of contralateral leg (CHS), toe-off of ipsilateral leg (TO) and their average values during single-leg stance (SLS) and double-leg stance (DLS), as well as the two peaks of sagittal and frontal RCIA during barefoot (B), low-heeled (L), medium-heeled (M) and high-heeled (H) gait. Statistical results using repeated measures ANCOVA with normalized gait speed as a covariate are also shown. COM–COP inclination angle (IA,8)
Sagittal plane CHS TO Average DLS Average SLS Peak1 Peak2 Frontal CHS TO Average DLS Average SLS Peak1 Peak2
Rate of change of COM–COP inclination angle (RCIA,8/s)
Barefoot
Low
Medium
High
10.2(1.6) 10.8(1.3) 1.7(0.6) 0.2(0.4) – –
9.3(1.1) 10.4(0.9) 1.7(0.5) 0.6(0.4) – –
9.1(1.0) 11.0(1.2) 1.4(0.8) 0.5(0.5) – –
8.5(1.2) 11.0(1.2) 1.2(0.7) 0.5(0.4) – –
1.8(0.4) 2.2(0.4) 0.2(0.2) 1.9(0.4) – –
1.6(0.5) 2.0(0.4) 0.1(0.3) 1.7(0.3) – –
2.7(0.7) 2.7(0.5) 0.3(0.3) 2.7(0.5) – –
2.1(0.6) 2.20(0.4) 0.2(0.2) 2.0(0.4) – –
p
Barefoot
Low
Medium
High
p
0.046+ 0.139 0.379 0.304 – –
158.0(114.9) 3.7(13.5) 196.9(42.5) 51.8(5.5) 479.6(88.5) 344.5(72.9)
77.8(88.6) 4.0(22.9) 147.6(23.9) 49.3(4.5) 396.5(102.6) 342.8(66.6)
39.1(62.7) 1.2(17.5) 147.8(33.1) 52.3(4.6) 302.4(126.1) 477.1(134.4)
48.1(64.5) 10.6(21.9) 135.5(28.8) 50.4(5.1) 271.9(88.4) 465.0(110.3)
0.014+++ 0.057 <0.008++ 0.069 0.0002* <0.0001**
<0.0001++ 0.002++ 0.995 <0.0001++ – –
48.6(32.8) 9.8(4.2) 50.3(10.8) 0.8(0.8) 110.2(22.9) 79.3(19.8)
18.4(12.0) 9.8(5.1) 32.2(7.5) 0.5(0.5) 77.3(22.3) 69.6(21.5)
12.6(9.8) 11.6(4.9) 29.8(8.4) 0.3(0.7) 55.6(27.0) 81.7(26.4)
10.5(9.2) 12.5(3.8) 24.9(6.1) 0.5(0.3) 50.9(16.9) 76.1(25.8)
<0.0001++ 0.051 <0.0001++ 0.226 <0.0001*** 0.546
+, ++, +++, *, **, ***: statistical significance, p < 0.05. Post hoc comparisons (based on the absolute values of the variables): +, B > H;
++
, B > L, M, H;
significant differences were found between height conditions. This suggests that a reduction of the normalized gait speed was adopted as a strategy to meet the balance requirements in association with the reduced BOS, instead of for heel height. The control of the RCIA during DLS seemed to be critical for the balance of the body while wearing narrow-heeled shoes. Through the control of the RCIA, the range of the IA can be controlled. The observed frontal RCIA changes appeared to be related to the narrow heels of the shoes, especially to those of the contralateral limb around its heel-strike. The significantly reduced RCIA at contralateral heel-strike indicated the need for a better control of the transfer of the body weight to the relatively reduced BOS of the contralateral limb. The reduced step width also contributed to this requirement. The rate of body weight acceptance of the contralateral limb was also reduced as indicated by the reduced peak RCIA shortly after heel-strike. This was even more pronounced when the heel height reached 7.3 cm. After this altered loading response around heel-strike, the RCIA
+++
, B > M, H; *: B > M, H; L > H; **: B, L < M, H; ***: B > L, M, H; L > H.
remained unchanged for the rest of the DLS during which the contralateral foot was in full contact with the floor with a greater BOS. This conservative strategy of RCIA control led to reduced IA in the frontal plane during DLS, which may be helpful for reducing the risk of falling related to unsuccessful weight transfer to the narrow heels around heel-strike. A strategy of RCIA control similar to that in the frontal plane was also found in the sagittal plane except during the second half of the DLS. The significantly reduced RCIA around contralateral heelstrike and during the first half of the DLS in the sagittal plane also suggested the need for a better control of the transfer of the body weight to the relatively reduced BOS of the contralateral limb. After this period, the sagittal RCIA was increased during the second half to maintain the continuing progression of the COM. This phenomenon was more pronounced while wearing shoes with higher heels, most likely because narrow heels had less of an effect on the longitudinal component of the BOS of the individual shoes. The specific RCIA control in the sagittal plane enabled the subjects
H.-L. Chien et al. / Gait & Posture 38 (2013) 391–396
A/P COM and COP Position
(a)
SLS
DLS
CSLS
COM
100
%LL
395
50
COP
0 -50
Sagittal IA
Degree
20
0
-20
Sagittal RCIA Degree/s
200 0 -200 -400
HS
CTO
20
40
60
80
100
TO
CHS
HS
% Gait cycle M/L COM and COP Position
(b) %LL
CSLS
DLS
SLS
10
COP
0
COM -10
Frontal IA
Degree
5 0 -5
Frontal RCIA Degree/s
50 0 -50
HS
CTO
20
40
60
CHS
TO
80
100
HS
% Gait cycle Fig. 3. Ensemble-averaged (a) A/P COP and COM positions, sagittal COM–COP inclination angle (IA), and sagittal rate of change of IA (RCIA), (b) M/L COP and COM positions, frontal COM–COP inclination angle (IA), and frontal rate of change of IA (RCIA) when walking barefoot (thin curves, solid) and with narrow-heeled shoes (thick curves) with low heel height (dashed), medium heel height (dotted) and high heel height (solid). The anterior/posterior (A/P) positions of the COM and COP were described parallel to the direction of progression, a zero value being the position of heel-strike and a positive value being anterior to that position. The medial/lateral (M/L) positions of the COM and COP were described relative to the line of progression that bisected the M/L range of motion of the COM during a gait cycle, a positive value being to the side of the contralateral limb. Vertical lines indicate the critical times (HS, heel-strike; CTO, toe-off of the contralateral leg; CHS, heel-strike of the contralateral leg; TO, toe-off). SLS, single-leg stance; DLS, double-leg stance; CSLS, single-leg stance of contralateral leg.
to maintain unaltered IA throughout the DLS, which was in contrast to the reduced IA in the frontal plane. During the single-leg stance of narrow-heeled gait the unaltered RCIA indicated that the reduced frontal IA was a
continuation of the reduced IA during the preceding double-leg stance. The unchanged RCIA appeared to be maintained by keeping a constant relative position between the COM and COP (Fig. 3b), which was different from double-leg stance where the control of
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the RCIA was mainly through control of the velocity of the COP (Fig. 3b). The reduced IA suggests that females in narrow-heeled shoes tried to minimize the separation of the COM and COP in the frontal plane, which helped maximize the body’s stability. This conservative strategy may be useful for fall prevention while walking with a reduced sole contact area which is associated with an increased risk of a fall [5]. The current study revealed that the narrow heels of the shoes, instead of the heel height, was the main factor leading to the reduced frontal IA. Similarly, no significant difference was found in the lateral COM-BOS margin between standard shoes and shoes with elevated wide-base heels [6]. In the current young subjects, the height of the heels affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. Since high heels increase the ankle plantarflexion [18,28], affecting the motion and stability of foot/ankle, the activities of the associated muscles, and thus the posture of the body [29], the altered RCIA may lead to altered IA in the elderly whose muscles around the ankle may not be strong enough to maintain foot/ankle stability and body balance. Another factor that may affect the balance control during high-heeled gait is the accelerated fatigue of the muscles [28,29], which may limit the ability to control the foot stability and the COM in response to postural perturbations, leading to an increased risk of ankle sprains and/or falls [29]. Further study on the effects of high-heeled shoes and muscle fatigue in an older population is needed to better clarify the relationship between reduced BOS, increased heel heights and the increased risk of a fall in the elderly [5]. 5. Conclusions The reduced base of the heels was found to be the primary factor for the reduced normalized walking speed, and the reduced frontal IA throughout the gait cycle. This was achieved mainly through the control of the RCIA during DLS. The height of the heels affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. This suggest that young adults adopt a conservative strategy for balance control during narrow-heeled gait. The current results will serve as baseline data for future evaluation of patients and/or older adults during narrow-heeled gait with the aim of reducing the risk of falling. Acknowledgements The authors wish to thank Hao-Ling Chen for her help with data analysis and gratefully acknowledge the financial support from Taiwan Adventist Hospital of Taiwan (100-E-11). Conflict of interest statement None. References [1] Frey C, Thompson F, Smith J, Sanders M, Horstman H. American Orthopaedic Foot and Ankle Society women’s shoe survey. Foot and Ankle 1993;14:78–81.
[2] American Podiatric Medical Association (APMA). 2003 High heel survey. http://www.apma.org/s_apma/doc.asp?CID=1233&DID=17112. 2003/. [3] Esenyel M, Walsh K, Walden JG, Gitter A. Kinetics of high-heeled gait. Journal of the American Podiatric Medical Association 2003;93:27–32. [4] Lord SR, Bashford GM. Shoe characteristics and balance in older women. Journal of the American Geriatrics Society 1996;44:429–33. [5] Tencer AF, Koepsell TD, Wolf ME, Frankenfeld CL, Buchner DM, Kukull WA, et al. Biomechanical properties of shoes and risk of falls in older adults. Journal of the American Geriatrics Society 2004;52:1840–6. [6] 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. Archives of Physical Medicine and Rehabilitation 2008;89:1970–6. [7] Sherrington C, Menz HB. An evaluation of footwear worn at the time of fallrelated hip fracture. Age and Ageing 2003;32:310–4. [8] Keegan TH, Kelsey JL, King AC, Quesenberry Jr CP, 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. American Journal of Epidemiology 2004;159:192–203. [9] Opila-Correia KA. Kinematics of high-heeled gait. Archives of Physical Medicine and Rehabilitation 1990;71:304–9. [10] Opila-Correia KA. Kinematics of high-heeled gait with consideration for age and experience of wearers. Archives of Physical Medicine and Rehabilitation 1990;71:905–9. [11] Snow RE, Williams KR, Holmes Jr GB. The effects of wearing high heeled shoes on pedal pressure in women. Foot and Ankle 1992;13:85–92. [12] Mandato MG, Nester E. The effects of increasing heel height on forefoot peak pressure. Journal of the American Podiatric Medical Association 1999;89: 75–80. [13] Lee CM, Jeong EH, Freivalds A. Biomechanical effects of wearing high-heeled shoes. International Journal of Industrial Ergonomics 2001;28:321–6. [14] Arnadottir SA, Mercer VS. Effects of footwear on measurements of balance and gait in women between the ages of 65 and 93 years. Physical Therapy 2000;80: 17–27. [15] Kuo AD. An optimal control model for analyzing human postural balance. IEEE Transactions on Biomedical Engineering 1995;42:87–101. [16] Patla AE, Frank JS, Winter DA. Assessment of balance control in the elderly: major issues. Physiotherapy Canada 1990;42:89–97. [17] Patla AE. Strategies for dynamic stability during adaptive human locomotion. IEEE Engineering in Medicine and Biology Magazine 2003;22:48–52. [18] Snow RE, Williams KR. High heeled shoes: their effect on center of mass position, posture, three-dimensional kinematics, rearfoot motion, and ground reaction forces. Archives of Physical Medicine and Rehabilitation 1994;75: 568–76. [19] Hsu WC, Wang TM, Liu MW, Chang CF, Chen HL, Lu TW. Control of body’s center of mass motion during level walking and obstacle-crossing in older patients with knee osteoarthritis. Journal of Mechanics 2010;26:229–37. [20] Lee HJ, Chou LS. Detection of gait instability using the center of mass and center of pressure inclination angles. Archives of Physical Medicine and Rehabilitation 2006;87:569–75. [21] Huang SC, Lu TW, Chen HL, Wang TM, Chou LS. Age and height effects on the center of mass and center of pressure inclination angles during obstaclecrossing. Medical Engineering and Physics 2008;30:968–75. [22] Pai YC, Patton J. Center of mass velocity-position predictions for balance control. Journal of Biomechanics 1997;30:347–54. [23] Lu TW, Chien HL, Chen HL. Joint loading in the lower extremities during elliptical exercise. Medicine and Science in Sports and Exercise 2007;39: 1651–8. [24] Dempster WT, Gabel WC, Felts WJ. The anthropometry of the manual work space for the seated subject. American Journal of Physical Anthropology 1959;17:289–317. [25] Hsieh HJ, Lu TW, Chen SC, Chang CM, Hung C. A new device for in situ static and dynamic calibration of force platforms. Gait and Posture 2011;33:701–5. [26] Woltring HJ. A FORTRAN package for generalized, cross-validatory spline smoothing and differentiation. Advances in Engineering Software 1986;8. [27] Hof AL. Scaling gait data to body size. Gait and Posture 1996;4:222–3. [28] Ebbeling CJ, Hamill J, Crussemeyer JA. Lower extremity mechanics and energy cost of walking in high-heeled shoes. Journal of Orthopaedic and Sports Physical Therapy 1994;19:190–6. [29] Gefen A, Megido-Ravid M, Itzchak Y, Arcan M. Analysis of muscular fatigue and foot stability during high-heeled gait. Gait and Posture 2002;15:56–63.
Contents lists available at SciVerse ScienceDirect
Gait & Posture journal homepage: www.elsevier.com/locate/gaitpost
Control of the motion of the body’s center of mass in relation to the center of pressure during high-heeled gait Hui-Lien Chien a, Tung-Wu Lu a,b,*, Ming-Wei Liu c a
Institute of Biomedical Engineering, National Taiwan University, Taiwan, ROC Department of Orthopaedic Surgery, School of Medicine, National Taiwan University, Taiwan, ROC c Department of Surgery, Taiwan Adventist Hospital, Taiwan, ROC b
A R T I C L E I N F O
A B S T R A C T
Article history: Received 5 May 2012 Received in revised form 17 December 2012 Accepted 26 December 2012
High-heeled shoes are associated with instability and falling, leading to injuries such as fracture and ankle sprain. Knowledge of the motion of the body’s center of mass (COM) with respect to the center of pressure (COP) during high-heeled gait may offer insights into the balance control strategies and provide a basis for approaches that minimize the risk of falling and associated adverse effects. The study aimed to investigate the influence of the base and height of the heels on the COM motion in terms of COM–COP inclination angles (IA) and the rate of change of IA (RCIA). Fifteen females who regularly wear high heels walked barefoot and with narrow-heeled shoes with three heel heights (3.9 cm, 6.3 cm and 7.3 cm) while kinematic and ground reaction force data were measured and used to calculate the COM and COP, as well as the temporal-distance parameters. The reduced base of the heels was found to be the primary factor for the reduced normalized walking speed and the reduced frontal IA throughout the gait cycle. This was achieved mainly through the control of the RCIA during double-leg stance (DLS). The heel heights affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. These results suggest young adults adopt a conservative strategy for balance control during narrow-heeled gait. The results will serve as baseline data for future evaluation of patients and/or older adults during narrow-heeled gait with the aim of reducing the risk of falling. ß 2012 Elsevier B.V. All rights reserved.
Keywords: Gait High heels Center of mass Center of pressure Balance
1. Introduction Surveys on shoe use show that between 39% and 69% of women wear high-heeled shoes on a daily basis [1–3]. High-heeled shoes increase the difficulty of maintaining balance and thus the risk of falling [4–6], leading to injuries such as fractures [7]. Wearing shoes with heels higher than 2.5 cm nearly doubles the risk of falling in older people [5]. Apart from heel heights, narrow heels or smaller sole contact also significantly increase the risk of a fall [5,8]. This may be related to the increased difficulty in controlling the body’s center of mass (COM) subject to a narrow and constantly changing base of support (BOS). Knowledge of the motion of the COM with respect to the BOS may offer insights into the balance control strategies and provide a basis for protocols that minimize the risk of falling and associated adverse effects. Of prime importance to an individual with narrow and highheeled shoes is the stability or balance on the heel of the shoe
* Corresponding author at: Institute of Biomedical Engineering, National Taiwan University, No. 1, Sec. 1, Jen-Ai Road, Taipei 100, Taiwan, ROC. Tel.: +886 2 33653335; fax: +886 2 33653335. E-mail address: [email protected] (T.-W. Lu). 0966-6362/$ – see front matter ß 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.gaitpost.2012.12.015
during the stance phase. While the kinematic and kinetic changes of the lower limb joints during high-heeled gait and their relationship with overuse injuries have been studied extensively [9–13], the balance control of the body and its relationship with risks of falling have not been studied as widely [4]. The current knowledge of the body’s control with high-heeled shoes was formed primarily from the data obtained using clinical tests or swaymeter measurements [4,14]. However, they provide limited insights into stability under more challenging, dynamic conditions and cannot directly represent the motion of the whole body and its control. The motion of the COM in relation to the BOS has been used to represent the balance control of the whole body [15,16]. A person is thought to be in balance as long as the downward vertical projection of the COM is kept within the BOS during standing [15,16], or while the person maintains control of the COM subject to a constantly changing and moving BOS during walking [17]. Previous studies showed that with increased heel height the changes of the positions of the COM relative to the BOS were different between quiet standing and level walking [6,18]. For studying the motion of the COM during high-heeled gait, one faces the difficulty of defining the dynamic BOS not only during singleleg stance (SLS), but also during double-leg stance (DLS). This
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difficulty can be resolved by describing the motion of the COM relative to the center of pressure (COP) that can be directly measured using forceplates [19,20]. Among the relevant variables, COM–COP inclination angles (IA), i.e., angles formed by the vertical line and the line connecting the COP and COM, and the rate of change of IA (RCIA) have been used to describe the body’s dynamic control during locomotion without the influence of stature differences among subjects [19–21]. The greater the inclination angle, the further the COM will diverge from the COP, and the more difficult it will be to bring the COM back to be right above the COP unless accompanied with an appropriate rate of change of IA. A similar argument for the motion of the COM relative to the COP has been proposed by Pai and Patton [22]. The IA and RCIA are also useful for quantifying the effects of the heel base and height on the body’s COM control during walking. The purposes of this study were to investigate the influence of the base and height of the shoes’ heels on the COM motion in terms of IA and RCIA during walking. It was hypothesized that compared to barefoot walking, wearing narrow-heeled shoes would decrease the IA and RCIA which would also be significantly affected by heelheight. 2. Materials and methods 2.1. Subjects Fifteen female adults (age: 24.4 3.4 years; height: 158.9 5.7 cm; mass: 49.2 5.1 kg) participated in the current study with informed written consent, as approved by the Institutional Research Board. They were free from any neuromusculoskeletal pathology that might have affected their normal gait. They had worn shoes with narrow heels of more than 3 cm height at least three times per week, 6 h per day for at least two years. 2.2. Data collection Each subject walked at a self-selected pace on an 8-m walkway either barefoot, or with low-heeled (3.9 cm), medium-heeled (6.3 cm), or high-heeled shoes (7.3 cm). All the shoes were narrow-heeled (2.0 cm 1.6 cm) and had similar construction, including foot contact points, vamp and shape of the toe box (Fig. 1). Subjects were fitted with the most suitable test shoes from several different sizes, and were allowed to familiarize themselves with the walkway for each pair of test shoes. Each subject wore 39 retroreflective markers for tracking body segmental motions [23]. Foot markers, namely big toe, fifth metatarsal base and heel, were put on the corresponding positions on the shoes for shoe conditions. Markers on the navicular tuberosity and malleoli were not affected by the shoes. Threedimensional trajectories of the markers were measured using a motion capture system (Vicon 512, OMG, UK) at a sampling rate of 120 Hz, and were low-pass filtered using a fourth-order Butterworth filter with a cut-off frequency of 5 Hz [19]. The ground reaction forces (GRF) were collected from two forceplates (AMTI, USA) at a frequency of 1080 Hz [23]. Six successful trials, three for each limb, for each condition were obtained. The order of the test conditions was randomized. 2.3. Data analysis The body’s COM position data were calculated as the weighted sum of those of all the body segments, including head and neck, trunk, pelvis, arms, forearms, thighs, shanks and feet [23,24]. The COP position was calculated using forces and moments measured by the forceplates [25]. The medial/lateral positions of the COM and COP were described relative to the line of progression that bisected the medial/lateral range of motion of the COM during a gait cycle, a positive value being to the side of the contralateral limb [21] (Fig. 2). The anterior/posterior positions of the COM and COP were described parallel to the direction of progression, a zero value being the position of heel-strike and a positive value being anterior to that position. The IA in the sagittal plane (a) and frontal plane (b) were then calculated as follows [19] (Fig. 2). *
t¼
*
PCOM *
COP
jPCOM
*! Z
(1)
COP j
a ¼ sin1 ðtx Þ
(2)
b ¼ sin1 ðty Þ *
(3) *
where PCOM COP was the vector pointing from the COP to the COM, and Z was the unit vector of the global vertical axis. With the current forceplate setup, a and b were
Fig. 1. The three narrow-heeled shoes used in the current study: (a) low-heeled (3.9 cm), (b) medium-heeled (6.3 cm), and (c) high-heeled (7.3 cm). The maximum area of the base of support of the foot is much greater than that of the high-heeled shoe (d). calculated from the beginning of the single-leg stance to the subsequent heel-strike. The rates of changes of a and b were also calculated by smoothing and differentiating their trajectories using the generalized cross-validatory spline method [26]. During the gait cycle, the transition between SLS and DLS, i.e., heel-strike of the contralateral leg and toe-off of the ipsilateral leg, are critical instances at which
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2.4. Statistical analysis All calculated variables were analyzed using one-way repeated measures ANCOVA with normalized gait speed as a covariate (a = 0.05). If a significant main effect was found, pair-wise comparisons between test conditions using an independent t-test with Bonferroni correction (a = 0.05/6 = 0.0082) was performed. All statistical analysis was performed using SPSS (version 11.0, SPSS Inc., USA).
3. Results No significant differences in gait speed, stride length, stride time and cadence were found between any conditions (Table 1). After normalization, these variables except cadence and stride time were significantly decreased for all shoe conditions when compared to the barefoot condition (p < 0.002). Stance time and DLS time for all shoe conditions were increased (p < 0.0001) but only high-heeled shoes showed a smaller SLS time (p = 0.005) when compared to the barefoot condition. Frontal IA at contralateral heel-strike, toe-off and those averaged over the DLS were reduced for all shoe conditions when compared to barefoot, but no differences were found between heel heights (Fig. 3 and Table 2). For variables related to sagittal IA, the values of the high-heeled condition at contralateral heel-strike (p = 0.046) were smaller than for barefoot (Table 2). While the IA during the SLS phase were either constant in the frontal plane or increased anteriorly in the sagittal plane, rapid posterior and lateral increases in the inclination angles during DLS were found (Fig. 3). ANCOVA analysis suggests that normalized gait speed did not impose a limitation on the interpretation of the IA results. While the RCIA were small and remained almost constant during SLS, those during DLS varied significantly (Fig. 3). At the first peak of the ensemble-averaged curves of the RCIA during DLS, narrow-heeled shoes showed smaller lateral RCIA than barefoot (p < 0.0001), while high-heeled shoes had smaller lateral RCIA than low-heeled shoes (p < 0.0001), Table 2. In the sagittal plane, high-heeled and medium-heeled shoes showed smaller posterior RCIA than barefoot, and those for high-heeled shoes were smaller than those for low-heeled shoes, Table 2. At the second peak, no significant differences were found in the frontal plane, while statistically smaller values in the sagittal plane for barefoot and low-heeled shoes were found compared to those for middle-heeled and high-heeled shoes (p < 0.0001), Table 2. Compared to barefoot, all shoes had significantly smaller RCIA at contralateral heel-strike in the frontal plane (p < 0.0001), while only those for high-heeled and medium-heeled shoes were smaller in the sagittal plane (p = 0.014). Values of the RCIA averaged over the DLS were significantly smaller than those of the barefoot condition both in the sagittal and frontal planes (p < 0.008). No significant differences were found between heel heights. When gait speed was covaried (ANCOVA), the same stastical findings were obtained. 4. Discussion Fig. 2. (a) Trajectories of the COM and COP of a typical subject during a gait cycle in the transverse plane. (b) Sagittal COM–COP inclination angle (a). (c) Frontal COM– COP inclination angle (b). COP1 is the COP position at the end of single leg stance of the reference leg, while COP2 is the COP position at toe-off of the reference leg.
maintaining body stability is expected to be more difficult [19]. Therefore, the values of the IA and RCIA at these instances were obtained. The values of IA and RCIA were also averaged over the DLS and SLS, respectively. The peak RCIA during DLS was also extracted. Temporal-distance parameters (gait speed, stride length, step width, stride time, cadence, stance time, DLS time and SLS time) for all conditions were also calculated. For between-condition comparisons, gait speed, stride length, step width, stride time, cadence, and COP and COM positions were normalized following Hof [27]. Leg length was defined as the distance between the ipsilateral anterior superior iliac spine and the medial malleolus for barefoot gait, and the shoe height was added to this distance for shod gait.
The current study aimed to compare the balance control during walking between barefoot and narrow-heeled shoes of different heel heights, in terms of the COM motion and its coordination with the COP. The reduced BOS of narrow-heeled shoes was found to be the primary factor for the reduced normalized walking speed, and the reduced frontal IA throughout the gait cycle. Together with heel height, the reduced BOS also contributed to the reduced frontal and sagittal RCIA during DLS. These results suggest that the subjects used a conservative strategy for balance control during narrow-heeled gait. The reduced BOS of narrow-heeled shoes appeared to be the main factor for the reduction of the normalized walking speed. Decreased normalized gait speeds were found for all shoe conditions when compared to the barefoot condition, while no
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Table 1 Means (SD) of temporal-distance parameters during barefoot and narrow-heeled shoes gait. Barefoot
Parameters Gait speed m/s dimensionless Stride length m %LL Step width cm %LL Stride time s dimensionless Cadence steps/min dimensionless Stance time % Double leg suport time % Single leg support time % * ** ***
Low heel
Medium heel
High heel
p
1.15(0.11) 0.41(0.04)
1.09(0.09) 0.38(0.03)
1.09(0.09) 0.37(0.03)
1.07(0.09) 0.37(0.03)
0.129 0.002*
1.18(0.07) 146.8(7.7)
1.16(0.05) 137.8(5.8)
1.15(0.05) 132.4(6.1)
1.16(0.05) 132.0(6.5)
0.414 <0.0001*
8.32(2.26) 10.3(2.7)
6.53(1.43) 7.8(1.9)
6.33(1.68) 7.3(2.0)
5.85(2.10) 6.6(2.3)
0.005* 0.0002*
1.03(0.06) 3.60(0.24)
1.07(0.08) 3.66(0.26)
1.06(0.07) 3.56(0.24)
1.08(0.08) 3.62(0.27)
0.223 0.740
117.36(7.38) 0.56(0.04)
112.63(7.91) 0.55(0.04)
114.09(7.39) 0.57(0.04)
111.40(7.81) 0.56(0.04)
0.176 0.756
61.2(1.2)
63.1(1.1)
63.7(1.6)
64.2(1.4)
<0.0001**
10.3(1.3)
12.2(1.0)
12.9(1.7)
13.3(1.4)
<0.0001**
38.5(1.3)
37.7(1.1)
37.2(1.5)
36.6(1.6)
0.005***
Statistical significance, p < 0.05: barefoot > low heel, medium heel, high heel. Statistical significance, p < 0.05: barefoot < low heel, medium heel, high heel. Statistical significance, p < 0.05: barefoot > high heel.
Table 2 Means (standard deviations) of the sagittal and frontal COM–COP inclination angles (IA) and their rates of change (RCIA) at heel-strike of contralateral leg (CHS), toe-off of ipsilateral leg (TO) and their average values during single-leg stance (SLS) and double-leg stance (DLS), as well as the two peaks of sagittal and frontal RCIA during barefoot (B), low-heeled (L), medium-heeled (M) and high-heeled (H) gait. Statistical results using repeated measures ANCOVA with normalized gait speed as a covariate are also shown. COM–COP inclination angle (IA,8)
Sagittal plane CHS TO Average DLS Average SLS Peak1 Peak2 Frontal CHS TO Average DLS Average SLS Peak1 Peak2
Rate of change of COM–COP inclination angle (RCIA,8/s)
Barefoot
Low
Medium
High
10.2(1.6) 10.8(1.3) 1.7(0.6) 0.2(0.4) – –
9.3(1.1) 10.4(0.9) 1.7(0.5) 0.6(0.4) – –
9.1(1.0) 11.0(1.2) 1.4(0.8) 0.5(0.5) – –
8.5(1.2) 11.0(1.2) 1.2(0.7) 0.5(0.4) – –
1.8(0.4) 2.2(0.4) 0.2(0.2) 1.9(0.4) – –
1.6(0.5) 2.0(0.4) 0.1(0.3) 1.7(0.3) – –
2.7(0.7) 2.7(0.5) 0.3(0.3) 2.7(0.5) – –
2.1(0.6) 2.20(0.4) 0.2(0.2) 2.0(0.4) – –
p
Barefoot
Low
Medium
High
p
0.046+ 0.139 0.379 0.304 – –
158.0(114.9) 3.7(13.5) 196.9(42.5) 51.8(5.5) 479.6(88.5) 344.5(72.9)
77.8(88.6) 4.0(22.9) 147.6(23.9) 49.3(4.5) 396.5(102.6) 342.8(66.6)
39.1(62.7) 1.2(17.5) 147.8(33.1) 52.3(4.6) 302.4(126.1) 477.1(134.4)
48.1(64.5) 10.6(21.9) 135.5(28.8) 50.4(5.1) 271.9(88.4) 465.0(110.3)
0.014+++ 0.057 <0.008++ 0.069 0.0002* <0.0001**
<0.0001++ 0.002++ 0.995 <0.0001++ – –
48.6(32.8) 9.8(4.2) 50.3(10.8) 0.8(0.8) 110.2(22.9) 79.3(19.8)
18.4(12.0) 9.8(5.1) 32.2(7.5) 0.5(0.5) 77.3(22.3) 69.6(21.5)
12.6(9.8) 11.6(4.9) 29.8(8.4) 0.3(0.7) 55.6(27.0) 81.7(26.4)
10.5(9.2) 12.5(3.8) 24.9(6.1) 0.5(0.3) 50.9(16.9) 76.1(25.8)
<0.0001++ 0.051 <0.0001++ 0.226 <0.0001*** 0.546
+, ++, +++, *, **, ***: statistical significance, p < 0.05. Post hoc comparisons (based on the absolute values of the variables): +, B > H;
++
, B > L, M, H;
significant differences were found between height conditions. This suggests that a reduction of the normalized gait speed was adopted as a strategy to meet the balance requirements in association with the reduced BOS, instead of for heel height. The control of the RCIA during DLS seemed to be critical for the balance of the body while wearing narrow-heeled shoes. Through the control of the RCIA, the range of the IA can be controlled. The observed frontal RCIA changes appeared to be related to the narrow heels of the shoes, especially to those of the contralateral limb around its heel-strike. The significantly reduced RCIA at contralateral heel-strike indicated the need for a better control of the transfer of the body weight to the relatively reduced BOS of the contralateral limb. The reduced step width also contributed to this requirement. The rate of body weight acceptance of the contralateral limb was also reduced as indicated by the reduced peak RCIA shortly after heel-strike. This was even more pronounced when the heel height reached 7.3 cm. After this altered loading response around heel-strike, the RCIA
+++
, B > M, H; *: B > M, H; L > H; **: B, L < M, H; ***: B > L, M, H; L > H.
remained unchanged for the rest of the DLS during which the contralateral foot was in full contact with the floor with a greater BOS. This conservative strategy of RCIA control led to reduced IA in the frontal plane during DLS, which may be helpful for reducing the risk of falling related to unsuccessful weight transfer to the narrow heels around heel-strike. A strategy of RCIA control similar to that in the frontal plane was also found in the sagittal plane except during the second half of the DLS. The significantly reduced RCIA around contralateral heelstrike and during the first half of the DLS in the sagittal plane also suggested the need for a better control of the transfer of the body weight to the relatively reduced BOS of the contralateral limb. After this period, the sagittal RCIA was increased during the second half to maintain the continuing progression of the COM. This phenomenon was more pronounced while wearing shoes with higher heels, most likely because narrow heels had less of an effect on the longitudinal component of the BOS of the individual shoes. The specific RCIA control in the sagittal plane enabled the subjects
H.-L. Chien et al. / Gait & Posture 38 (2013) 391–396
A/P COM and COP Position
(a)
SLS
DLS
CSLS
COM
100
%LL
395
50
COP
0 -50
Sagittal IA
Degree
20
0
-20
Sagittal RCIA Degree/s
200 0 -200 -400
HS
CTO
20
40
60
80
100
TO
CHS
HS
% Gait cycle M/L COM and COP Position
(b) %LL
CSLS
DLS
SLS
10
COP
0
COM -10
Frontal IA
Degree
5 0 -5
Frontal RCIA Degree/s
50 0 -50
HS
CTO
20
40
60
CHS
TO
80
100
HS
% Gait cycle Fig. 3. Ensemble-averaged (a) A/P COP and COM positions, sagittal COM–COP inclination angle (IA), and sagittal rate of change of IA (RCIA), (b) M/L COP and COM positions, frontal COM–COP inclination angle (IA), and frontal rate of change of IA (RCIA) when walking barefoot (thin curves, solid) and with narrow-heeled shoes (thick curves) with low heel height (dashed), medium heel height (dotted) and high heel height (solid). The anterior/posterior (A/P) positions of the COM and COP were described parallel to the direction of progression, a zero value being the position of heel-strike and a positive value being anterior to that position. The medial/lateral (M/L) positions of the COM and COP were described relative to the line of progression that bisected the M/L range of motion of the COM during a gait cycle, a positive value being to the side of the contralateral limb. Vertical lines indicate the critical times (HS, heel-strike; CTO, toe-off of the contralateral leg; CHS, heel-strike of the contralateral leg; TO, toe-off). SLS, single-leg stance; DLS, double-leg stance; CSLS, single-leg stance of contralateral leg.
to maintain unaltered IA throughout the DLS, which was in contrast to the reduced IA in the frontal plane. During the single-leg stance of narrow-heeled gait the unaltered RCIA indicated that the reduced frontal IA was a
continuation of the reduced IA during the preceding double-leg stance. The unchanged RCIA appeared to be maintained by keeping a constant relative position between the COM and COP (Fig. 3b), which was different from double-leg stance where the control of
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the RCIA was mainly through control of the velocity of the COP (Fig. 3b). The reduced IA suggests that females in narrow-heeled shoes tried to minimize the separation of the COM and COP in the frontal plane, which helped maximize the body’s stability. This conservative strategy may be useful for fall prevention while walking with a reduced sole contact area which is associated with an increased risk of a fall [5]. The current study revealed that the narrow heels of the shoes, instead of the heel height, was the main factor leading to the reduced frontal IA. Similarly, no significant difference was found in the lateral COM-BOS margin between standard shoes and shoes with elevated wide-base heels [6]. In the current young subjects, the height of the heels affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. Since high heels increase the ankle plantarflexion [18,28], affecting the motion and stability of foot/ankle, the activities of the associated muscles, and thus the posture of the body [29], the altered RCIA may lead to altered IA in the elderly whose muscles around the ankle may not be strong enough to maintain foot/ankle stability and body balance. Another factor that may affect the balance control during high-heeled gait is the accelerated fatigue of the muscles [28,29], which may limit the ability to control the foot stability and the COM in response to postural perturbations, leading to an increased risk of ankle sprains and/or falls [29]. Further study on the effects of high-heeled shoes and muscle fatigue in an older population is needed to better clarify the relationship between reduced BOS, increased heel heights and the increased risk of a fall in the elderly [5]. 5. Conclusions The reduced base of the heels was found to be the primary factor for the reduced normalized walking speed, and the reduced frontal IA throughout the gait cycle. This was achieved mainly through the control of the RCIA during DLS. The height of the heels affected mainly the peak RCIA during DLS, which were not big enough to affect the IA. This suggest that young adults adopt a conservative strategy for balance control during narrow-heeled gait. The current results will serve as baseline data for future evaluation of patients and/or older adults during narrow-heeled gait with the aim of reducing the risk of falling. Acknowledgements The authors wish to thank Hao-Ling Chen for her help with data analysis and gratefully acknowledge the financial support from Taiwan Adventist Hospital of Taiwan (100-E-11). Conflict of interest statement None. References [1] Frey C, Thompson F, Smith J, Sanders M, Horstman H. American Orthopaedic Foot and Ankle Society women’s shoe survey. Foot and Ankle 1993;14:78–81.
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