J Syst Sci Syst Eng ISSN: 1004-3756 (Paper) 1861-9576 (Online) DOI: 10.1007/s11518-011-5171-0 CN11-2983/N

Systems Engineering Society of China & Springer-Verlag Berlin Heidelberg 2011

GETTING TO THE BOTTOM OF FOOTWEAR CUSTOMIZATION

Thilina W. WEERASINGHE1 Ravindra S. GOONETILLEKE2 1,2 Human Performance Laboratory, Department of Industrial Engineering and Logistics Management, Hong

Kong University of Science and Technology, Clear Water Bay, Hong Kong, China 1thilina@ust.hk

2ravindra@ust.hk ()

Abstract

Footwear have been evaluated mostly using commercially available products, while some

researchers have used custom shoes. Hence, the understanding of the effects of various parameters of a

shoe is quite limited. The footbed simulator invented in recent years allows a range of parameters to be

studied in quiet standing. It can be used to evaluate perceived feel and center of pressure changes to

changes in heel height, seat length, material, wedge angle and toe spring. This paper is meant to show

the value of the footbed simulator in terms of research and the actual production of shoes. A study

performed with two heel heights, three combinations of seat length and material and three wedge

angles showed that the perceived feel is closely related to the center of pressure. The results also show

the optimum footbed has a significantly different perceived feel. Thus, the footbed simulator is an ideal

way to generate custom footwear designs.

Keywords: Footwear, insole, shoe shape, footbed, wedge angle, center of pressure, comfort, mid-foot,

stability

1. Introduction Footwear to protect one’s feet is not new;

they have been existence for a very long time

(Pinhasi et al. 2010, Linder & Saltzman 1998,

Thompson & Coughlin 1994). In the earliest

days, footwear was used to protect feet from

external hazards (DeMello 2009, Trinkaus 2005).

More recently, they are claimed to have style,

performance, protection, high slip resistance,

minimum energy expenditure in addition to

being comfortable (Chiou et al. 1996, Luximon

& Goonetilleke 2003, Kuklane 2009, Lake 2000,

Dahmen et al. 2001, Stefanyshyn & Nigg 2000).

In order to truly experience these claims, it is

necessary to have a good fit between the foot

and footwear. Past research has focused on using

anthropometry, due to human diversity (Edgren

2006, Tan et al. 2010), to generate shoe lasts that

fit feet (Baba 1975, Bunch 1988, Luximon et al.

2001) with primary concerns being on the joints

and the top covering of footwear. When a person

moves, the soft tissue deforms, and to

accommodate these deformations, various

tolerances are built-into the last used to make

footwear. These have been positive and

sometimes negative tolerances depending on the

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

location. For example, a negative tolerance

around the ball of the foot is necessary to

provide a functional fit between the foot and the

shoe (Witana et al. 2004).

A high variety of shoes designs exist today.

Last manufacturers generally manipulate the

forefoot shape to derive different designs while

keeping the rear part of a last pretty much the

same, as the variations among feet in the

rearfoot part are supposedly small (Cheng &

Perng 1999). With numerous bones on the feet,

most designers assume that the foot can bend

into any shape. However, the lengths of the

various bones and the way they are joined

together constrain the movement and the

deformation.

The surface of the shoe that contacts the foot

is known as the footbed. A good fit between the

sole of the foot and the footbed is essential for

functionality as all body weight acts on this

surface. Hence, the contact and the distribution

of force can alter the pressure distribution, gait

characteristics and perceived feel. For example,

high contact pressures are related to discomfort

(Dai et al. 2011, Godfrey et al. 1967, Hodge et al.

1999) and have been mitigated primarily

through the use of materials, contoured insoles,

and outsole design (Fuller et al. 2001, Brown et

al. 2004, Long et al. 2007). The center of

pressure (COP) determines a person’s sense of

balance and it also plays a crucial role when

determining the function and form of a shoe.

COP shifts have been primarily linked to heel

design (Xu et al. 1999). Studies have also shown

that the center of pressure on the foot moves

forward when wearing high-heeled shoes

(Shimizu & Andrew 1999, Snow & Williams

1994, Gefen et al. 2002, McBride 1991, Han et

al. 1999). High-heeled shoes tend to have higher

plantar foot pressures as well (Holtom 1995).

Corrigan et al. (1993) found that a 1¾ inch heel

had 50% more pressure under the ball of the foot

when compared to wearing a ¾ inch heel. Very

few studies have evaluated the design of the

footbed of shoes except with orthotics, which

conform to the sole side of the foot thereby

limiting and restricting the shock absorption

qualities of the foot arch. Hence, the objective of

this paper is to present a relatively new

invention that simulates the footbed, and which

can be used to evaluate the changes on various

subjective and objective parameters so that more

comfortable and functional shoes can be

manufactured and the variations of COP shifts

can be quantified.

Most high-heeled shoe wearers experience

high pressures because the footbed does not

provide adequate support for the foot. Figure 1

shows an example of a “see-thru” shoe where

the foot rests on the heel and the forefoot areas

with hardly any support in the midfoot region.

The result is a di-pod type of support which

makes the foot and body quite unstable. If the

lateral side of the foot can be supported well, the

issues of high pressure and poor stability can be

addressed. The shape of a footbed is determined

by the bottom surface of a shoe last and very

little research exists in relation to this

component of a shoe. The parameters that can be

varied on a footbed include the heel height, seat

length, shank shape, wedge angle and so on

(Figure 2). Unfortunately, these parameters have

not been systematically explored. Instead,

footbed designs have been evaluated using

commercially available shoes (Snow & Williams

1994, Nyska et al. 1996, Mandato & Nester

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

1999, Hansen & Childress 2004, Hong et al.

2005, Chiu & Wang 2007, Lin et al. 2007) that

generally have very standard shapes. More

recently, a footbed simulator has been developed

by Goonetilleke & Witana (2010) with which

Witana et al. (2009a, b) evaluated the effects of

various footbed parameters. The simulator

(Figure 3) allows the footbed shape to be

changed at any heel height. The ideal footbed

shape will vary depending on heel height. One

shape cannot be used to manufacture shoes with

different heel heights because the foot is not a

rigid object. The numerous bones and soft tissue

will deform depending on relative height

between the forefoot and the rear foot. Thus,

optimizing the shank shape, wedge angle, seat

length to obtain a comfortable profile is not an

easy task and cannot be done manually. The

footbed simulator has a range of mechanical and

electrical controllers that allow a wide variety of

parameters to be changed. The subject

essentially “rides” on the platform and the

preferred shape can be obtained using the

electro-mechanical controls within a few

minutes. Once the subject is satisfied with the

feeling on the foot, the subject gets-off the

platform and the shape is digitized (Figure 4).

The digitized shape is then designed-in to a last

using custom software (Figure 5), cut on a CNC

machine, and the shoe is produced thereafter.

The shoe produced using this system have no

gaping holes in the midfoot (Figure 6). The

preferred shapes have desirable pressure patterns,

thus validating the subjective preferences with

the objective pressure distributions (Figure 7).

Witana et al. (2009a) showed that the ideal

wedge angle for heel heights of 25, 50, and 75

mm were 4-50, 10-110 and 16-180 for optimum

perceived feel, respectively. It has also been

shown that the midfoot shape can change

independent of the rearfoot and forefoot regions

(Witana et al. 2009b), and people are capable of

identifying the differences in shape with distinct

preferences towards some shapes.

Those two studies were somewhat

independent of each other and only the

perceived feel was evaluated in addition to the

forces and stiffness of materials. Thus, this paper

is focused on examining the combinations and

interactions among wedge angle, materials, and

anthropometric characteristics on perceived feel

and center of pressure.

Essentially, we use an axiomatic design (Suh

1990) approach to understand the effect of

various design parameters on the requirements

related to perceived feel and COP as follows:

Figure 1 A designer shoe with no midfoot support.

The lack of support is clearly seen as it is “see-thru”

Figure 2 Design parameters of the footbed of a shoe.

A= toe spring; B= seat length; C=heel height;

D=wedge angle

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

Perceived Feel

COP

=

Wedge Anglea b c

Seat Lengthd e f

Material

(1)

The above design may not be ideal as the

number of functional requirements is less than

the number of design parameters. In addition,

there could be a relationship between perceived

feel and COP as well.

Figure 3 Footbed simulator that allows a subject to stand

with one shoe on one foot and the other foot on the

simulator platform so that comparisons can be made

Figure 4 A sample footbed profile captured from the simulator

Stevens’ power law relates the sensation

magnitude (S) with the physical magnitude of a

stimulus (P) in the form of S = k (P−P0)α,where

k is a dimensional constant and α is a power

exponent that varies for different types of stimuli.

For example, the power exponent, α, is 0.85 for

heaviness of weights and 0.65 for pressure on

the palm (Gescheider 1985). The power

exponent has been found to be 1 for line length

estimation. In other words, S = k (P−P0) and

hence people are able to estimate line lengths

relative to a standard line quite accurately. Thus,

line lengths are generally used as a screening

test of a person’s ability to make subjective

judgments.

This paper evaluates the existence of any

relationships between the footbed parameters,

comfort and COP so that the validity of Stevens’

power law for perceived sensation can be

ascertained.

2. Methodology An experiment was conducted with 12

participants. The footbed simulator was used to

generate the various footbed shapes. Two heel

heights (50 and 75 mm), three wedge angles (4°,

10°, 14° at 50 mm; 14°, 18°, 22° at 75 mm) and

three different mid-foot support conditions

(45-PU, HL-PU, HL-SS) were simulated while

the toe spring was controlled at 0°. The 45 and

HL corresponded to a seat length of 45 mm and

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

a length corresponding to an anatomical

landmark on the foot (Witana et al. 2009a). The

PU (Polyurethane) and SS represented two

materials that were different in the rebound

resilience and the flexural rigidity (Witana et al.

2009b). Due to the many different combinations

that were possible, only some combinations

were chosen for testing. Hence the test

conditions did not allow a truly factorial design.

Therefore, the results would be analyzed as nine

separate conditions at two different heel heights.

The toe spring was controlled to be 0°.

Each subject was tested in every condition

with two replicates. Only the second replicate

was analyzed as the first was meant for the

subject to get accustomed to the test procedure.

Figure 5 The scanned last and digitized footbed shape is imported into the software and deformed to obtain the final last shape

Figure 6 A custom made high-heeled shoe using the footbed simulator

Figure 7 Foot pressure profiles can be changed by

adjusting the simulator. The left foot shows the

pressure profile when wearing a normal shoe. The

right foot is on the adjusted simulator showing a more

distributed pressure pattern

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

3. Procedure The experimental task of the subject was to

quietly stand, on left and right footbed simulator

units that were set to one of the experimental

conditions, for 30 seconds. The different

conditions were randomized. Each subject stood

on the ground barefooted (called foot-flat) and

was asked to rate the various footbed shapes

relative to the perceived feeling of the ground.

They were thereafter tested at each of the

experimental settings. The subjects were asked

to rate the experimental conditions relative to

that when standing on the ground bare-footed.

The question posed to each participant was as

follows:

“Assume that when you are standing

barefoot on a flat floor with equal load on both

feet, the whole body comfort feeling you have in

your feet is 100. When compared with that

feeling, what value would you give for whole

body comfort now? A higher number represents

a higher comfort level.”

Prior to the test, the subjects were screened

to ensure that they had the ability to give valid

ratings for a physical variable using line length

estimation (Kee & Karwowski 2001) based on

the well-known free modulus method

(Gescheider 1985) of psychophysics. All

participants passed this screening test. Pressure

between the foot and the simulator was

measured using a F-Scan (Tekscan 2011) unit.

The F-scan unit was first calibrated when

standing on the simulator and the software was

set to record the pressure under the foot at

100Hz for 30 s. The heel to heel distance was

controlled at 17 cm due to equipment limitations.

The subject movement was monitored using a

Motion Analysis system and are reported

elsewhere.

4. Results and Analysis The perceived feeling scores were

normalized with respect to each subject’s

maximum and minimum ratings. The mean

scores of each condition at each heel height are

shown in Figure 8.

The F-Scan unit software was used to

generate the contact area, center of pressure

(COP) measured from the back of the heel, and

peak pressure. The contact area between the

footbed and the foot was calculated from the

number of active sensors in the F-Scan sensor

(area of one sensor is 0.256 cm2). The pressure

under the toes was not considered for the

analysis. The pressure measures and the

perceived feeling of comfort were all subjected

to an Analysis of variance (ANOVA). Since the

wedge angles at the two heel heights were

different, two separate ANOVAs were performed

corresponding to the 50 and 75 mm heel heights.

Wedge angle (p<0.05), type of mid-foot

(p<0.05) and their interactions (p<0.001) had a

significant effect on perceived feeling of

comfort. The interactions were further analyzed

and the simple effect analysis results showed

that at 50 mm, the 10° wedge angle and HL-SS

mid-foot condition had the highest level of

perceived comfort. At 75 mm, the 18° wedge

angle and HL-SS mid-foot condition had a

significantly higher perceived comfort rating

over the other conditions. Generally, when

comparing the types of mid-foot conditions,

HL-SS had the highest rating and the 45-PU

condition had the lowest rating (Figure 8).

An ANOVA on the plantar foot contact area

showed that at both 50 mm and 75 mm heel

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

heights, the mid-foot condition had a significant

effect (p < 0.01). The post-hoc test showed that

the HL conditions have a significantly higher

contact area than the 45 mm mid-foot support

conditions (Figure 9).

The ANOVA on COP showed that at heel

heights of 50 mm and 75 mm, the only

significant factor was the type of midfoot

(p<0.001) with the HL-SS having a significantly

lower COP value compared to the other

conditions (Figure 10). Another ANOVA was

conducted to test the differences between each

of the nine conditions and the foot flat condition.

For the 50 mm heel height, all HL mid-foot

conditions were not different (p > 0.05) with the

foot flat condition COP. At the higher heel

height of 75 mm, only HL-SS was not

significantly different with the foot-flat COP

(Figure 10).

A regression analysis showed the following

relationship between perceived feeling of

comfort and COP position: Comfort = 87.2 – 0.798*COP; (2)

R2 = 0.704

(a)

(b)

Figure 8 The Comfort rating for the different experimental conditions at (a) 50 mm (b) 75 mm

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

Figure 9 Mean contact area for the three mid-foot conditions at a heel height of 75 mm

(a)

(b)

Figure 10 The variation of COP for the different experiment conditions at (a) 50 mm (b) 75 mm

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

5. Discussion

With a trend towards mass customization and

real-time customization (Tien 2008, Tien et al.

2004) understanding the customer needs and

creating products and services that are valued

are essential for a competitive advantage

(Hamilton 2004, Qian & Tang 2009). Footwear

is not different. In the past, researchers have

studied shoes available in stores or those made

by shoe manufacturers. The footbed simulator

(Goonetilleke & Witana 2010) allowed a range

of footbed parameters to be tested so that the

ideal footbed design corresponding to a

“neutral” posture may be obtained. The

perceived feeling is significantly different with

the right combinations of wedge angle, seat

length and material, as evidenced from the

significant interaction among the variables.

COP is an important aspect for body balance

and stability. When standing on flat ground, the

foot contact area is relatively high and the COP

tends to be closer to the heel when standing

erect. With most high-heeled shoes the load on

the forefoot tends to increase, shifting the COP

forward requiring more muscle force to support

the upright posture (Joseph & Nightingale 1956,

Lee et al. 2001). The varied conditions tested in

this study allowed the shift to be quantified

through a regression analysis between comfort

rating and COP. The lower COP results in a

higher value of comfort (Figures 8 and 10). In

other words, a shift of COP towards the heel is

preferred. Surprisingly, the wedge angle had no

significant effect on COP. Only the midfoot

condition comprising seat length and material

influenced COP. The load shift and the COP

shift of high-heeled shoes has generally been

accepted by high-heel shoe wearers. This study

shows that the design of the footbed can

significantly impact the loading on the foot, the

COP and thereby comfort. These aspects have

not been evaluated before due to the lack of an

instrument that can simulate differing

conditions.

The comparison with the foot-flat condition

also shows a pattern. At lower heel heights, the

seat length plays an important role in the COP

shift. At higher heel heights, the seat length, and

the midfoot condition have to be right in order to

be comfortable. In other words, the complete

design of the footbed is critical for a high-heel

shoe wearer to be comfortable. These results can

be explained considering the foot structure. The

foot can articulate better than any other part in

the body due to its anatomy, comprising the

various soft tissue and bones. However, the foot

cannot bend at areas other than at joints. Hence

the footbed has to conform to not just the

superficial plantar surface but to the bony

structure as well. Thus, the seat length plays an

important role to position the heel correctly. The

HL distance of the participants ranged from 62 –

67 mm. This distance essentially represents the

“length” of the calcaneus. With a seat length of

45 mm, a clear mismatch exists that result in

poor contact of the foot with the footbed.

The study does have some weaknesses. Only

12 participants were evaluated. However, based

on the experimental data, it was found that the

minimum required sample size (N) to be 6.28.

Furthermore, only quiet standing was tested and

more research is needed to extend this study

with custom fitting shoes having the optimal

parameters as predicted from this study.

Weerasinghe and Goonetilleke: Getting to the Bottom of Footwear Customization J Syst Sci Syst Eng

6. Conclusion This study found that the perceived feel is

linearly related with COP through adjustments to

footbed parameters. Previous studies performed

with commercial footwear have limited findings

as they tend to be unique to the footwear tested.

The footbed simulator is a versatile test bed to

evaluate the effects of footbed shape and other

variables on comfort and stability.

Acknowledgments The authors would like to thank the Research

Grants Council of Hong Kong for funding this

study under grant GRF 613607, the Sino

Software Research Institute for funding the

software development and the reviewers to help

improve the paper.

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Ravindra Goonetilleke is a Professor in the

Department of Industrial Engineering and

Logistics Management at the Hong Kong

University of Science and Technology. He

received his Ph.D. from the State University of

New York at Buffalo in human factors

engineering in 1990. His main interests are

cognitive engineering, culture-friendly product

design and footwear customization.

Thilina Weerasinghe is a Ph.D. candidate in the

Department of Industrial Engineering and

Logistics Management at the Hong Kong

University of Science and Technology. He

obtained his B.Sc. (Eng) degree from the

University of Moratuwa, Sri Lanka and a M.Phil.

degree from the Hong Kong University of

Science and Technology. His main interests are

product design.