Study participants and design
The survey participants comprised 15 female adult volunteers (age: 20.89±3.04 years; height: 1.61±0.05 m; weight: 53.80±5.75 kg; body mass index: 20.73±2.05 kg/m2) with a foot size of 37 (European standard). Most of the participants wore HHSs once to four times per week. None of the participants reported foot-related disorders, skin lesions, or health-related problems. Signed and written consent was obtained from all participants before the experiments. The study design was approved by the Ethics Committee of Huashan Hospital. The participants were examined under seven experimental conditions: (A1) thick HHS with 3-cm heels, (A2) thick HHS with 6-cm heels, (A3) thick HHS with 8.2-cm heels, (B1) thin HHS with 3-cm heels, (B2) thin HHS with 6-cm heels, (B3) thin HHS with 8.2-cm heels, and (C) flat shoes (Figure 1). All shoes were made at the Shanghai Second Leather Shoes Factory using the same kind of material.
SA motion
A custom‐made platform, with force plate, embedded staircase, was used in the study. The staircase consisted of three overall steps (0.18 m × 0.29 m × 0.6 m, height × depth × width).
The infrared 3-dimensional gait analysis system (Motion Analysis Corp., Santa Rosa, CA, USA) captured the three-dimensional trajectory data of the 25 markers affixed to the subject’s bilateral anatomical landmarks. It consists of 10-mm reflective markers, eight infrared digital cameras (sampling frequency of 60 Hz), a workstation and video processor, high-performance computer, an information conversion controller, and the image collection software EvaRT4.2.
Cameras were located on 10 m x 6-m walls at a mean height of 2.2 m from the floor; the direction of the cameras could be adjusted; they had a visual angle >568o and a resolution >1/60,000. Force plates measuring 60 cm x 60 cm (Advanced Mechanical Technology Inc., Watertown, MA, USA) were embedded in the floor. The sampling frequency was 1000 Hz; the output voltage was 10 mV. A custom wooden staircase, without banister, was used. All data were collected by Cortex-64 5.5.0.1579 (Motion Analysis Corp.) and processed using the OthroTrak 6.6.4 research software.
Before the experiments, the participants had several minutes to get accustomed to the HHSs and were required to gradually walk along a track thrice in each pair of shoes. The positive values of joint positions in the current study referred to anterior, lateral, and proximal translation, external rotation, eversion, and dorsiflexion, whereas the negative values responded to the opposite; for peak values of kinematic data, smaller negative values represented a larger change in the direction. The range of motion (ROM) was calculated as the maximal value minus the minimum value.
FE model
Computer tomography (CT) scans (Light Speed, GE, Milwaukee, MI, USA) were acquired from the right foot of a healthy female subject wearing the flat shoes (age: 24 years, height: 160 cm, weight: 53 kg). She had no musculoskeletal pathology, previous foot surgery, or foot deformity. A custom ankle-foot orthosis was used to fix the ankle in a neutral unloaded position during CT. Scans were taken from 10 cm above the tibiotalar joint through the toe tip, with the thickness, in-plane resolution, voltage, current, and matrix of 0.67 mm, 0.35 mm, 120 kV, 200 mA, and 512 × 512 pixels, respectively. Based on the CT scan, the FE model was developed. The images were scanned at a 1-mm slice interval and had a pixel size of 0.625 mm.
A surface model of the foot bones and skin surfaces was created using MIMICS v17.0 (Materialise, Leuven, Belgium). The surface model of the foot structures was imported into Geomagic Studio 12.0 (Geomagic, Morrisville, NC, USA) to form the solid model. Thereafter, the FE package ABAQUS v6.18 (Hibbitt, Karlsson & Sorensen, Inc., Pawtucket, RI, USA) was used to create the FE mesh and subsequent analysis. The developed FE model consisted of 28 distinct bony segments, including the distal parts of the tibia and fibula, talus, calcaneus, cuboid, navicula, three cuneiforms, five metatarsals, 14 phalanges, 78 ligaments, and the plantar. Interactions among the bone segments were simulated as frictionless contacting bodies, whereas the surface interaction between the plantar foot and external support surface was assigned with a coefficient of friction of 0.1[18].
Shoe design software SolidWorks (Dassault Systemes, Waltham, MA, USA) was used to develop a 3-dimensional geometrical model of the HHSs according to the volunteer’s shoe size (EU size: 37). A 6-cm heel height for the HHSs was chosen because of its high popularity and sustainability for wearers. The HHS model consisted of a 4-mm thick outsole, a 6-cm heel, 2-mm thick shankpiece, and homogenous, regularly shaped upper HHS. The material properties of each component of the HHS and foot tissues were selected from the literature and are listed in Table 1.
Simulation of SA motion with HHSs
The ground support was vertically displaced to establish contact with the shoe heel. Loading and boundary conditions for the three simulated stance instances were as follows: heel strike (29%), midstance (57%), and push off (73%). They were applied accordingly for the subsequent steps[19]. The vertical GRFs were collected during SA motion with HHSs in the same subject using the Motion Analysis and Advanced Mechanical Technology force platform. For simplification, other GRFs and tibial angle components in the transverse and coronal planes were not defined. The vertical GRFs (136%, 113%, and 142% of body weight, 530 N) were measured via the rigid bottom layer of the ground support for heel strike, midstance, and push off phases, respectively.
Validation of the FE shoe model
To validate the FE shoe model, the predicted plantar pressure and distribution were compared to those measured from the same subject who wore the same 6-cm HHSs by using the FScan insole pressure measurement system (Tekscan Inc., South Boston, MA, USA).
Table 1. Material properties assigned to the foot tissues and HHS models
Material
|
Element Types
|
Young's
modulus,
E (MPa)
|
Poisson's
ratio, ν
|
Bony
|
C3D6
|
7300
|
0.3
|
Soft tissue
|
C3D6
|
0.48
|
0.48
|
Ligaments
|
T3D2
|
10
|
0.3
|
Cartilage
|
C3D6
|
20
|
0.3
|
Shoe soles
|
C3D6
|
1000
|
0.42
|
Shoe heels
|
C3D6
|
3000
|
0.3
|
Statistical analyses
All analyses were performed for the right side (dominant side)[20]. In order to analyze the influences of heel height and heel type on gait parameters, we used a Heel Height × Heel Type mixed analysis of variance test, with Heel Height (4 levels: control group, low [3 cm], medium [6 cm], and high [8.2 cm]) as the within-group factor, and Heel Type (3 levels: control group, thin, and thick) as the between-group factor. For each statistical test, the significance was set at 0.05. When the tests were significant, the least significant difference criterion was adopted for post hoc comparisons. Statistical analyses were performed with the built-in functions of SPSS 21 (IBM Corp., Armonk, NY, USA).