A Review of Research on Shock Absorbing Sports Soles

Jiao Wang

1,2,*

and Xupeng Wang

1,2

Research Center for Civil-Military Integration and Protection Equipment Design Innovation, Xi’an 710054, China

School of Art and Design, Xi'an University of Technology, Xi’an 710054, China

Keywords:

Material Shock Absorption, Structural Shock Absorption, Shock Absorption Performance Evaluation, Sole

Molding Technology.

Abstract: The shock-absorbing sports sole can reduce the risk of sports injury and perform physical exercise, and can

be widely used in sports fitness, auxiliary rehabilitation and other scenarios. The research progress of shock-

absorbing sports soles is systematically expounded. the research on shock-absorbing sports soles at home and

abroad is reviewed. Classing accord to shock-absorbing materials, shock-absorbing structures, shock-

absorbing performance evaluation. Summarizing emphatically the characteristics of shock-absorbing

materials performance and innovative applications, shock absorption principle of shock absorbing structure,

analysis of advantages and disadvantages of shock absorbing structure, methods of shock absorption

performance evaluation. Analyzing and prospecting the key technologies and future development trends of

sole shock absorption material, shock absorption structure, shock absorption performance evaluation.

1 INTRODUCTION

With the development of society, our country has

entered the stage of population aging. With the

growth of age, the physiological functions of the

elderly gradually decline. Various chronic diseases of

the elderly increase. Their needs for health are more

urgent. According to the relevant experimental

research of the American Exercise Association,

appropriate and reasonable physical exercise can

improve the physical health of the elderly to a certain

extent (Zixin X, 2018). As an essential equipment for

fitness exercise, sports shoes must have the following

characteristics: lightweight, air permeability,

elasticity, shock absorption and stability, anti-skid,

wear resistance and bending flexibility, among which

the shock absorption performance of sports shoes can

enhance the impact resistance of sports shoes and

avoid damage to the body caused by impact (Benmin

Z, 2020).

In 2012, Yuancai Zhang applied novel plantar

pressure test system and shock absorption impact test

instrument to test the plantar pressure and shock

absorption performance of 5 pairs of sports shoes

with arch structure design. The results showed that

the shock absorption of the sole was related to the sole

material and sole structure design. When the sole

material was softer, the stress of the sole structure was

balanced, the smaller the pressure on the human foot,

the better the shock absorption effect. In 2016, Yi

Zheng et al. in order to analyze the impact of sole

hardness on shoe cushioning performance and sports

injuries, tested sports shoes with different sole

hardness through running experiments, and collected

the kinematic indicators of human lower limbs. The

results showed that 30° and 40° sole hardness had a

good cushioning effect, which could reduce the risk

of sports injuries on the feet. In 2019, Sicheng Ke et

al. conducted 20 / 40 / 60cm landing tests on 12

subjects that wore three different shock-absorbing

soles, in order to study the shock-absorbing

performance of the soles. The experimental data

showed that the softness of the soles affected the peak

pressure of the soles, smaller the peak pressure of the

soles, better the shock-absorbing effect. In 2021,

Hongying Jiang arranged 10 subjects to conduct

landing experiments in order to study the impact of

different sports sole hardness on the biomechanics of

lower limbs. The results showed that a softer sole

would reduce the impact of ground reaction force on

lower limbs.

The above research shows that the shock

absorption performance of sports shoes is related to

sole materials and sole structure. Therefore, this paper

mainly expounds sole shock absorption materials,

Wang, J. and Wang, X.

A Review of Research on Shock Absorbing Sports Soles.

DOI: 10.5220/0012019100003633

In Proceedings of the 4th International Conference on Biotechnology and Biomedicine (ICBB 2022), pages 245-252

ISBN: 978-989-758-637-8

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

245

sole shock absorption structure, shock absorption

performance evaluation and sole forming technology.

2 SHOCK ABSORPTION OF

MATERIAL

The so-called shock absorption performance refers to

the ability of the sole to absorb and reduce the

vibration wave caused by movement (Pei Y, 2017).

The most important factor that directly affects the

shock absorption performance of sports sole is sole

material and structure. Material shock absorption

refers to the use of special materials with shock

absorption performance in the midsole of sports shoes

(Li L, 2015). The material deformation occurs after

impact to prolong the impact time. The midsole refers

to the part between the outsole with anti-skid effect

and the shoe body, which is an important component

to ensure that sports shoes have good cushioning,

shock absorption and energy return. The midsole

directly affects the function and comfort of sports

shoes.

EVA (thermoplastic resin material) and TPU

(thermoplastic elastomer) materials are widely used

in the midsole of sports shoes, which are light weight

and strong comfort, good shock absorption effect.

TPU shoe materials account for about 40% of the

domestic TPU market share (Tingting N, 2021;

Wenhui Y, 2015). As a high-performance material,

GEL and P4U material can be applied to high strength

protection market, which has excellent performance

such as shock absorption, energy absorption and

cushioning (Heaven Has Feathers, 2021). So this

paper describes four kinds of shock absorption

materials, EVA, TPU, GEL, P4U.

2.1 EVA Shock Absorption Material

EVA material is a thermoplastic resin material, which

is widely used in the field of shoe materials and the

most commonly used material for shoe midsole. In

2001, Mills et al. studied the shock absorption of

EVA material and found that the smaller the volume

of the internal void of EVA material, the worse the

shock absorption effect. In 2004, Verdejo et al.

discovered that long-term exercise could cause

structural damage to the EVA foam on the sole, thus

losing the cushioning effect. The disadvantage of this

material is that the internal structure of the material is

easy to be damaged by extrusion and lose the effect

of shock absorption.

In 2018, Jiyu Liang et al. analyzed the properties

and structures of EVA and BIIR / EVA foamed

materials, the study found that the increase of EVA

composition of BIIR / EVA foamed materials would

improve the hardness and mechanical strength of

foamed materials.

2.2 TPU Shock Absorption Material

TPU thermoplastic polyurethane, a kind of linear

block copolymer composed of soft and hard

segments, is one of the most widely used engineering

thermoplastic elastomers at present (Tongyu Z,

2020). In 2016, Yanna Zhang et al. investigated the

properties of new TPU foamed material, found that

the TPU foamed material was light and elastic. In

2020, Zhiyuan Xin et al. carried out modification

experiments on TPU materials, the results showed

that the modified TPU materials had better softness

and elasticity, the compression set of TPU / OBC

infuse modified materials was the smallest.

Compared with EVA material, TPU foamed material

has lower modulus, higher energy efficiency and

lower compressive brittle deformation, which is

applied in the field of sports’ shoes (Wei W, 2019).

The material can be applied not only in footwear

products, but also in cushioning packaging structure.

In 2019, Xiaoyi Chen printed TPU ( semi soft 3D

material ) into buffer pad with one or two layers of

hollow structure, the experimental data showed that

the buffer pad of TPU material after 3D printing had

good cushioning performance.

2.3 Shock Absorbing Gel Material

In 1986, Asics introduced a shock-absorbing gel,

which is a soft rubber material between solid and

liquid. Comparing with the same volume of EVA

foam, GEL material reduces the weight by half, and

increases 10% elasticity and 20% shock absorption.

GEL material provides elastic and energy feedback

for all kinds of sneakers. Asics's gel-kayano shock-

absorbing shoes series will distribute the GEL in front

and back of the sole in order to provide better

buffering effect (Zhi Q, 2013). In addition to GEL,

there is also an intelligent P4U material, which has

been used in sports. In 2019, the Peak team released

its own adaptive midsole technology-P4U, a gel

material based on mechanical state transition

principle. P4U material dynamic mechanical

properties can achieve transient state transition and

make adaptive adjustment. P4U material is in a fluid

state at low speed of human body sports, and in a solid

state at high speed of human body sports. P4U

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material has impact resistance, protection, shock

absorption and other properties. This cushioning gel

material is not only used in sports field, but also used

in electronic equipment, automotive protection,

modification materials, military and other aspects

(Clifton P, 2011).

Although the shock-absorbing material is high

protection performance and shock-absorbing

performance, which can be placed in the midsole to

absorb impact force. But the material itself is in a soft

gel state and can not be combined with the shock

absorbing structure of the sole to play a dual role in

reducing vibration.

3 SHOCK ABSORPTION OF

STRUCTURE

Previous studies on running shoes focused on the

occurrence and prevention of plantar injury. In 2014,

Yaodong Gu et al. found that the unstable sole

structure could adjust and control the movement of

lower limbs through the sole experiment, and then

verified that the unstable sole structure could adjust

the walking posture and guide the rehabilitation

training. In 2015, Agnes Huebner et al. conducted the

experiment on safety shoes with cushioning heel

insert, found that adjustable heel insert could reduce

muscle activity and strain fatigue. In 2017, Weijie Fu

et al. learned through human landing experiment that

the shock absorption of shoes played a limited role in

mitigating impact force, but it could reduce the peak

impact force during accidental landing and avoid

impact damage. In 2020, Alfred Gatt et al. studied the

effect of the best rigid cushioning shoes on the

clinical treatment of diabetic foot, the results showed

that the best rigid cushioning shoes could reduce the

plantar peak pressure of diabetics.

Sports shoes with shock absorption function can

absorb the impact and reduce sports injury, protect the

human body, so the development of sole shock

absorption structure has great significance to people

in sports. The existing shock absorbing structure

include filling structure, bionic structure, and

electromechanical structure.

3.1 Filled Structure

The working principle of airbag shock absorption is

that the midsole is subjected to huge impact force, and

the gas volume in air cushion will be compressed to

complete the process of force relief (Dengxin W,

2013). The air cushion structure includes half palm air

cushion (Fig. 1 (a)) and full palm air cushion (Fig. 1

(b)), the half palm air cushion is mainly placed in the

heel, while the full palm air cushion is placed in the

whole sole. Air-cushioned shoes has obvious shock

absorption effect, but it can not provide thrust to assist

athletes in sports (Hui S, 2005).

(a) Half palm air cushion

(b) Full palm air cushion

Figure 1: Filled sole structure.

Liquid pad is a cavity structure composed of one

or more chambers, which is filled the whole chamber

with liquid, similar to air cushion structure. The liquid

pad can transmit the huge impact force through the

liquid to the outer wall of shock absorber. The result

is shock absorber deformation and converting the

impact force into upward feedback force. The fluid

shock absorption device (Fig. 1 (c)) used Hydroflow

technology is designed based on the working

principle of liquid pad (Piaolin P, 2014). The stability

of liquid shock absorber pad is good, but the outer

wall of liquid shock absorber pad is squeezed for a

long time, which will cause compressed permanent

deformation and even leak liquid.

3.2 Bionic Structure

The arch structure is deformed under compression to

realize effective cushioning and rebound, reduce the

impact force transmit to foot (Yong T, 2008). This

half-horseshoe arch structure (Fig. 2 (a)) is composed

of elastic bow piece, tension spring, balance base

A Review of Research on Shock Absorbing Sports Soles

247

(Zhao Z, 2006). The elastic bow piece reduces the

impact force from the ground through appropriate

deformation to avoid injury to the body. The tension

spring provides the extension force for the elastic

bow. The balance base is responsible for the stability

of the elastic bow and tension spring in operation.

(a) Arched structure

(b) Wave structure

(d) Grid structure

(e) Blade structure

Figure 2: Bionic sole structure.

The wave structure (Fig. 2 (b)) utilizes the

mechanics of bow to disperse the impact force (Lan

Y, 2016). The wave structure is not only applied in

the field of footwear, but also in the field of

transportation / packaging, such as corrugated paper

structure (Fig. 2 (c)). A bionic structure similar to the

ocean wave, which disperse the impact force through

pressure deformation and protect the good from

damage.

Grid structure (Fig. 2 (d)) uses the elastic of the

racket line to absorb the impact force and slow down

the vibration caused by the impact force (Katherine

K,2008). This mesh structure with high elasticity

rubber cords is applied directly under the heel.

The blade structure imitates the movement

posture of athlete that is the center of the human body

tilts forward and forms an angle of less than 90

degrees with the ground, provides movement

potential energy and thrust. This midsole structure

(Figure 2 (e)) is designed with 16 independent blades

to absorb impact force and convert it into thrust to

provide energy for users (Haitao W, 2017). Compares

with other shock-absorbing running shoes, this

running shoe has low stability and high elasticity.

3.3 Electromechanical Structure

The sensing sole structure (Fig. 3 (a)) senses the

vibration of sole movement through sensors and

transmits the vibration to the chip processor (

Dengxin

W, 2013)

. The built-in chip processor is connected

with the electric motor and shock absorbing unit with

cables. The hardness of the shock absorbing unit can

be changed by adjusting the cables through the chip

processor to improve the shock absorbing efficiency

(Fig. 3 (b)). Although this structure can freely change

the performance of the shock absorbing unit, its

deficiency is that the sensing components and built-in

chip can not withstand repeated motion impact, which

will cause component damage and loss of sensitivity

of the sensor.

(a) Sensing sole

(b) Schematic of shock absorption

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Figure 3: Electromechanical sensing structure.

Mechanical structure shock absorption is to

absorb the force through the physical structure of the

shock absorption system. When the structural unit is

compressed to produce deformation, the force will be

transformed into elastic potential energy and stored.

When you take the next step, the structure will

rebound and release the elastic potential energy to

produce the force in the same motion direction to

realize the force feedback. In 1984, Nike introduced

the spring shock absorption structure (Fig. 3 (c)),

which was placed in the middle of the sports sole, the

shock absorption and elasticity of sports shoes are

enhanced (Yong T, 2008).

SHOCK ABSORPTION

PERFORMANCE

EVALUATION

There are two ways to evaluate the shock absorption

performance of sole: finite element simulation

evaluation and sole impact test. Finite element

simulation evaluation is to predict the shock

absorption effect of the unformed sports shoe sole

structure and material, optimizing the sole structure

and material in advance according to the prediction

result. Sole impact test is to evaluate the shock

absorption effect of formed sports sole.

4.1 Finite Element Simulation

Evaluation

Finite element method refers to the finite element

method, which is a research method of Engineering

Mechanics similar to solving continuous domain

problems. The solution process is shown in Figure 4,

the stress and strain of the output element are solved

by introducing load and boundary conditions into the

discrete element matrix, and the internal mechanical

characteristics are analyzed (Tao L, 2002).

Figure 4: Finite element solution diagram.

In 2007, TM Cheung et al. established a finite

element model to evaluate the biomechanical effect

of heel support on foot ankle joint in order to alleviate

the symptoms of plantar fasciitis, the results showed

that heel support could reduce the symptoms and pain

of plantar fasciitis. In 2012, Wenquan Xu et al. used

the impact simulation software ANSYS / LS-DYNA

to evaluate the shock absorption of sports shoes in

order to provide relevant data of shock absorption

performance for the structural design of sports shoes

sole. It was simulated that drill bit impacted the heel

of sports shoes in the software, the maximum impact

force and maximum deformation of forefoot and heel

of sports shoes were 790 N, 6.36 mm and 850 N, 6.18

mm. In 2020, Sicheng Ke et al. established a foot-

shoe-ground finite element simulation model to

simulate the stress process of sports shoes when they

landed from 20cm, 40cm and 60cm heights in order

to analyze the pressure distribution of soles and foots

when they landed on the ground. By using three-

dimensional force measuring platform and plantar

pressure test system, evaluated the shock absorption

performance of the sole and the verified effectiveness

of the simulation model. The experimental results

showed that the foot-shoe-ground finite element

simulation model was effective and could evaluate

the shock absorption performance of the sole.

4.2 Sole Impact Test

The sole impact test is also called material test that

mainly adopts the impact testing machine, seting the

metal impact head to impact tested shoes at a fixed

height, and measuring the impact force on the impact

head to analyze the ability of shoes or insoles to

reduce the impact load.

In 2012, Jun Gao mainly elaborated the test

conditions and test process of the drop hammer

impact method. The test conditions included test

equipment (45mm impact hammer) and samples

(5mm ~ 35mm soles), impact energy (5J and 7J ). The

test process was that the 8.5kg impact hammer freely

A Review of Research on Shock Absorbing Sports Soles

249

fallen to the surface of the insole at a specified height,

simulating the stress of the sole that the human body

fallen from a high place. It recorded and analyzed the

maximum impact force, maximum compression

displacement, and the ratio of maximum acceleration

to gravity acceleration. In 2014, Jianfeng Chen

researched a method to test the dynamic compressive

deformation of rubber shoes soles that was tested by

repeatedly impacting the soles with an impact

hammer. when the more the number of impacts, the

dynamic compression set rate of rubber shoes not

changed much and tended to be stable. In 2016, Yunqi

Tang et al. tested four sports shoes by using two

methods of sole pressure test and sole impact test in

order to find the evaluation index of shock absorption

performance of sports shoes, the results showed that

the pressure index of foot pressure test and the energy

absorption index of impact test were available

indexes.

5 KEY TECHNOLOGY

ANALYSIS

(1) Materials and comfort. Because the shock

absorption function of sports shoes is reflected in the

sole material, and the heavier the shoe material, the

greater the energy consumption of human movement.

In addition, long-term exercise will cause sole wear.

Therefore, certain requirements are put forward for

the strength, quality and elasticity of shock-absorbing

shoe materials. Shock absorbing shoe materials need

to have the characteristics of high strength,

lightweight and high elasticity. Shoe materials (EVA

material and foamed TPU) are selected according to

the design requirements and material characteristics

to meet the daily needs of people for exercise.

(2) Structure and comfort. In terms of shock-

absorbing structure, the existing shock-absorbing

sports shoes focus on reducing the pressure of the

sole, but this kind of design rarely considers the

service cycle and stability of the structure. Therefore,

study the pressure distribution area of the sole

combine with anatomy and human sports mechanics

to optimize the existing shock-absorbing structure. It

ensures that the sports shoes absorb and disperse the

sports impact beyond the bearing range of the human

body, reducing the local structural damage caused by

pressure concentration, improving the service cycle

and stability of the sole.

(3) Evaluation of shock absorption performance.

The evaluation of sole shock absorption performance

is divided into virtual evaluation before sole forming

(finite element simulation) and actual evaluation after

shoe forming (sole impact test). Both evaluate the

sole shock absorption performance through the

evaluation indexes such as sole pressure, maximum

impact force and deformation of sole. The accuracy

of its evaluation affects the production and use of

sole, so the combination of finite element simulation

evaluation and sole impact test will greatly improve

the accuracy of shock absorption performance

evaluation. When the data comparison difference

between finite element simulation evaluation and sole

impact test is smaller, it proves that the shock

absorption performance evaluation result of sole is

more accurate, and vice versa.

6 CONCLUSION

As the base of the elderly population increases, and

the demand for health industry will also increase. The

elderly sports health service industry will be the focus

of future development. With the increasing demand of

the elderly for auxiliary rehabilitation and physical

exercise, the design of shock-absorbing sneakers will

have a very broad prospect.

By analyzing and summarizing the development

of sole materials of shock-absorbing sports shoes, the

working principle of shock-absorbing structure, the

evaluation method of shock-absorbing performance,

readers have a deeper understanding of shock-

absorbing sports shoes. In addition, future researchers

should strengthen the research on the gait and foot

pressure of human lower limbs, summarize the gait

law and foot pressure distribution of human body,

innovate the shoe material and structure of shock-

absorbing sports shoes on this basis.

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