Differences in foot shape when wearing wedge-heeled shoes with elevated forefoot height and heel height

Three-dimensional (3D) foot scanning was performed on 35 females and the 14 foot dimensions were measured. Increased forefoot height generated larger lengths (foot, ball and out ball), smaller girths (ball and instep) and heights (instep and navicular) (p < 0.05). Thus, when the forefoot height increased, the foot became longer, slimmer and flatter. Moreover, elevated heel height resulted in larger dimensions for girths (ball and instep), heights (instep and navicular), and smaller dimensions for lengths (foot, ball and out ball), widths (diagonal and horizontal) and toe 5 angles of the foot (p < 0.01). That means shorter, narrower and more convex foot shapes were observed when heel height increased. Subjective measurements implied that increased forefoot height significantly enhanced perceived comfort, whereas increased heel height diminished comfort. It was found that forefoot elevation could result in less deformation and discomfort which accompanied heel elevation, especially in the low heel-toe drop combinations (10 × 30 and 20 × 30 mm). The findings provide valuable references for enhancing shoe fitting and comfort for wedge-heeled shoes by providing dimensional data on the toe, ball, arch and instep regions.

To improve the visual leg-to-body ratio, a great number of women choose to wear shoes with heel elevation, despite studies indicating their negative effects on foot health (Au & Goonetilleke, 2007; McRitchie et al., 2018). As one of the main female footwear characteristics, shoe elevation has become a ubiquitous element in the design of women’s footwear. Surveys have reported that 37% to 69% of women wear shoes with stack height on a daily basis (Kannan et al., 2019). It has been proven that passive heel elevation leads to changes in foot shape and greatly increases plantar pressure in the forefoot region, causing musculoskeletal symptoms, such as osteoarthritis, hallux valgus, and pain (Buldt & Menz, 2018).

Foot shape is a pivotal factor in human performance related to footwear, as it reveals deformations through foot dimensions. Over the past few decades, researchers have conducted several studies on the analysis of foot shape using two-dimensional (2D) anthropometry, three-dimensional (3D) scanning, and modeling approaches. Three-dimensional foot scanners were recommended to collect foot measurements because of their comparatively higher precision, accuracy, and robustness (Lee et al., 2014). It is widely believed that heel elevation causes changes in foot shape that has a relationship with foot function (Jo et al., 2022). Knowledge of foot shape can benefit footwear design (Kim & Do, 2019), thereby mitigating the discomfort and risk of injury due to ill-fitting shoes (Branthwaite & Chockalingam, 2019). Kouchi and Tsutsumi (2000) quantitatively clarified the changes in foot shape caused by heel heights of 0, 40, and 80 mm. Lee and Hong (2005) determined that heel elevation increases medial forefoot pressure and perceived discomfort based on experimental heel heights of 10 (flat), 51 (low), and 76 mm (high). Wan et al. (2017) detected forefoot shape changes in raised fourth and fifth metatarsophalangeal joints caused by heel height elevation. Quantifying the foot shape can be used to inform footwear design by integrating shoe fitting (McRitchie et al., 2018), foot shape (Stanković et al., 2020) or comfort (Matthias et al., 2021) factors.

Additionally, subjective wearing comfort was another important factor in the design and selection of women’s footwear. Results from earlier qualitative studies have reported a decrease in comfort with increased heel height (Lee & Hong, 2005; Melvin et al., 2019). Au and Goonetilleke (2007) noted that shoe-fitting preferences were related to the toe, metatarsophalangeal, and arch regions. To minimise foot deformation and discomfort, Witana et al. (2009) emphasised that the footbed shape should be optimised, especially the heel wedge angle. Moreover, Branthwaite et al. (2013) indicated that ill-fitting footwear can be detrimental to foot health, especially in the forefoot area. Wearing comfort influences wearability and impacts physical mobility, performance, and foot-related complaints (Matthias et al., 2021).

Generally, forefoot shape is an important criterion in footwear design (Krauss et al., 2010). Contrastingly, the heel region is hardly affected by heel elevation or other footwear characteristics (Buldt & Menz, 2018). Hence, in addition to the heel height, the forefoot height of a footbed (also known as forefoot stack height) also should be noted. Furthermore, heel height-related studies were only applicable to high-heeled shoes; therefore, the results cannot be applied to other types of shoes. Although women’s shoes with stack height are commonly seen as stilettos with elevated heels, wedge-heeled shoes are also routinely worn in work and other settings. Wedge-heeled shoes are formed by elevating both the forefoot and heel. The wedge-heeled footbed functioned as both the heel and sole with a raised platform. Forefoot height and heel height are defined as the stack height difference between the forefoot and heel regions (Mo et al., 2020; Xiong et al., 2008). Compared with high-heeled shoes, wedge-heeled shoes can provide extra height in the forefoot region and reduce heel-toe drop (the height difference between the forefoot and heel region of the shoe), which affects the comfort and cushioning of the shoe (Mo et al., 2020). For consumers, wedge-heeled shoes are a good compromise for balancing comfort and aesthetics while also enhancing the visual leg-to-body ratio. To elevate the forefoot region, wedges are frequently constructed by extending and elongating the heel. Most of the previous studies have investigated high-heeled shoes (Branthwaite & Chockalingam, 2019). Information regarding the dimensional differences when wearing wedge-heeled shoes is still lacking. It is uncertain whether the increased forefoot height provided by the wedge-shaped footbeds would have an impact on foot shape. Thus, the research questions (RQs) of the present study are listed as follows:

RQ 1::

How does the foot dimensions change when forefoot height and heel height are elevated in wedge-heeled shoes?

RQ 2::

Does the perceived comfort level decrease while the forefoot and heel heights elevated?

Therefore, this study aimed to determine the effects of forefoot height (10, 20, and 30 mm) and heel height (30, 50, 70, and 90 mm) on foot shape and perceived comfort when wearing wedge-heeled shoes. Objective (14 foot dimensions) and subjective (perceived comfort) measurements were collected for evaluation. The findings can provide valuable anthropometric information on foot shape for footwear designers and manufacturers when designing wedge-heeled shoes within the corresponding heights of footbeds.

Participants

Thirty-five female adults with a mean age of 22.1 ± 1.5 years were recruited for the current study. The mean body weight and height of the participants were 50.9 ± 6.3 kg and 161.9 ± 3.3 cm, respectively. The inclusion criteria of the participant was the female adult who gets used to wear wedge-heeled shoes in daily. An experienced wearer was referred to the study of Henderson and Piazza (2004) and was defined as an individual who had worn shoes with a minimum heel height of 40 mm more than twice a week for at least 8 h a day. The mentioned criteria was applied to avoid the bias of food morphology caused by the wearing experience lead to the unbalanced standing while scanning. The shoe size of participants ranged from EU 37 to 38. Additionally, the participants reported that they were free of any musculoskeletal disorders or lower extremity pain for at least 1 year. Three-dimensional foot scanning data were collected from May 2021 to November 2021. Written informed consent was obtained from all participants prior to the experiment. The experimental protocols were approved by the Institutional Ethics Committee of the university.

Footbeds

Figure 1a shows 12 footbeds of commercially available wedge-heeled shoes that were used for evaluation. The footbeds were customised in the same wedge style but with different forefoot and heel heights (Fig. 1b). Three forefoot heights (10, 20, and 30 mm) and four heel heights (30, 50, 70, and 90 mm) were used. The forefoot heights used in the study were selected from commercial available wedge-heeled shoes. The levels of heel height were determined in accordance with the guidelines of the AKA64-WMS system. Accordingly, the 30-, 50-, 70-, and 90-mm heel heights were classified as low, medium, medium–high, and high, respectively. Each footbed was designed and manufactured exclusively for this investigation to eliminate possible errors caused by the footwear design. The wedge-heeled footbeds used the same 2D bottom pattern shape based on the AKA64 design system. Moreover, the arch curve designs under different heel heights and bottom toe curves in this study were referenced by Luximon (2021). All footbeds have a pointed toe box and a 15° toe spring to position the foot naturally.

Twelve wedge-heeled footbeds used in the study (A) and the illustration of wedge-heeled shoes (B)

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Experimental apparatus

A 3D foot scanner (INFOOT USB scanning system, IFU-S01, I-ware Laboratory Co., Ltd., Japan) was used to obtain 3D foot models. The 3D scanner employs eight charged-coupled device cameras and four laser projectors to construct digital foot models with an accuracy of 1.0 mm. The heel height adjustment range was maintained within the allowance of the scanning volume (L400 × B200 × H150 mm). The reliability and validity of the scanner with respect to 3D foot shape collection for ergonomic and medical applications have been confirmed (Lee et al., 2014).

Experimental procedures

Prior to the experiment, the demographic data of the participants, such as age, height, weight, foot size, and wearing experience, were recorded. Foot-fitting practice was performed before scanning. The participants were provided with sufficient time to place their feet on different footbeds and were required to stand naturally (without support) while wearing each footbed. An example of a participant while scanning was demonstrated in Fig. 2.

An example for scanning participant’s foot while wearing in a shoe condition

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Before data collection, participants’ feet were disinfected and dried. Subsequently, a well-trained research assistant placed markers on specific anatomic points on the right foot. Four anatomical positions were used to increase the accuracy of data measurements. The four landmarks were metatarsal tibial (MT), metatarsal fibular (MF), arch point (AP), and junction point (JP). MT and MF were located on the most medial prominence of the first metatarsal-phalangeal joints (MPJ) and the lateral prominence of the fifth MPJ, respectively. AP was at the tubercle of the navicular. JP was on the junction of the leg and foot on the dorsal aspect (crossing point of the tendon to the fifth toe and the crease between the leg and the foot) (Fig. 3). The anatomical points were also adopted in Hill et al. (2017).

The anatomical points used in the study: metatarsal tibial (MT), metatarsal fibular (MF), arch point (AP), and junction point (JP)

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During the scanning process, a footbed was positioned inside the scanner to facilitate a good simulation of foot shape when wearing wedge-heeled shoes. Sequences of the 12 footbeds for each participant were randomly assigned. Only the dominant foot (the right foot) was scanned. The dominant foot is defined as the foot most frequently used for manipulating or mobilising actions. The left foot was positioned on the same shoe to ensure an even distribution of body weight. Each right foot was scanned twice to ensure image quality and scanned in the late morning to avoid foot volume deformation throughout the day (Lee & Wang, 2015). To prevent fatigue, each participant rested for at least 5 min between each footbed. After successful scanning, the digital foot model with a footbed and four landmarks was generated and stored in the STL format for further foot dimension extraction.

Additionally, to evaluate the perceived comfort of wearing, an 11-point Likert scale was used to evaluate the wearing comfort. The subjective measurements were obtained from Matthias et al. (2021). Participants were asked to rate their wearing comfort under the 12 shoe conditions on a scale of 0 (not comfortable at all) to 10 (very comfortable).

Data extraction and foot dimensions

The foot dimensions used in the study were measured from scanned digital foot models using virtual tools (PolyWorks Software, InnovMetric Software, Quebec, QC, Canada). PolyWorks allows precise measurements along the sagittal (x) and transverse (y) axes and realigned foot axis (Schwarz-Müller et al., 2021; Whitson et al., 2018). The foot axis is defined as the line joining the pternion (Pte, the most posteriorly projecting point of the heel) and the centre point (CP) of the vertical cross-section passing through the MT and MF (Lee & Wang, 2015).

The scanned data were first aligned to avoid the influence of foot orientation differences on the scanning foot data. In particular, the 3D coordinate systems of the generated digital mesh foot models were realigned. The heel centreline of the foot was consistent with the longitudinal axis of the scanned models to draw and measure linear distances, perimeters, and angles as follows:

• Align the X–Y plane with the footbed plane. The X–Y plane is perpendicular to the vertical plane that includes the foot axis and passes through the MT.
• Locate the Pte and CP then re-establish the foot axis.
• Align the foot axis with the longitudinal axis (X-axis) and the 3D coordinate system of Polyworks is overlapped with the actual foot axis.
• Set the Pte as the origin of the coordinate system. Using the foot axis as the X-axis, the foot width in the horizontal direction is used as the Y-axis and the upward direction of the lower leg is used as the Z-axis.
• Construct three cross-sections of the ball girth, instep girth, and short heel girth when the alignment process is completed.
• The foot dimensions were measured using the points, baselines, and cross-sections determined based on each definition.

This procedure was adopted from previous studies by Schwarz-Müller et al. (2021), and Tsung et al. (2003). Each experimental condition was performed twice, and the mean of each foot dimension was calculated for further statistical analysis. Eventually, 14 dimensions were measured from each digital foot model to evaluate the deformation of foot shape when wearing different wedge-heeled shoes. In total, three lengths, three widths, three girths, three heights, and two toe angles were measured. The definitions of the 14 foot dimensions are illustrated in Table 1, including foot length (FL), ball of FL (BFL), outside ball of FL (OBFL), foot width diagonal (FWD), foot width horizontal (FWH), heel width (HW), ball girth (BG), instep girth (IG), short heel girth (SHG), instep height (IH), navicular height (NH), toe height (TH), toe 1 angle (T1A), and toe 5 angle (T5A). The dimensions were derived from the shoe-last template based on the corresponding foot regions (Wang, 2010).

Statistical analysis

A two-way analysis of variance (ANOVA) was performed for data analysis. The independent variables were the forefoot height (10, 20, and 30 mm) and heel height (30, 50, 70, and 90 mm). The dependent variables were the 14 foot dimensions and perceived comfort measures. Duncan’s multiple range test (MRT) was performed for post hoc comparisons of the significant variables. Effect size statistics (partial η²) and observed power were calculated for the main outcome variables. For each statistical test, a 95% confidence level (p > 0.05) was used to identify the significance. All data were analysed using IBM SPSS Statistics software (version 26, IBM, Armonk, NY, USA).

Effects of forefoot heights on foot dimensions

The various forefoot heights of the wedge-heeled shoes had significant effects on seven of the 14 foot dimensions, as presented in Table 2 (all p < 0.05). When the forefoot height was elevated, foot length (η² = 0.175, power = 1.000), ball of foot length (η² = 0.185, power = 1.000), and outside ball of foot length (η² = 0.070, power = 0.999) measurements increased based on Duncan’s MRT results. In contrast, ball girth (η² = 0.020, power = 0.740), instep girth (η² = 0.026, power = 0.856), instep height (η² = 0.378, power = 1.000), and navicular height (η² = 0.044, power = 0.980) significantly decreased when forefoot height increased. Moreover, Table 3 reported that wearing a wedge-heeled shoe with a 30-mm forefoot height yielded the largest foot length (225.35 mm), ball of foot length (165.04 mm), and outside ball of foot length (142.40 mm), and the smallest ball girth (224.72 mm), instep girth (225.33 mm), instep height (65.51 mm), and navicular height (25.94 mm). Additionally, no significant differences were found in width- and angle-related dimensions.