ChapterPDF Available

Performance of EVA foam in running shoes

Authors:
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
1
Performance of EVA foam in running shoes
Raquel Verdejo and Nigel Mills
Metallurgy and Materials, University of Birmingham, Birmingham,
U.K
Abstract: Both in-shoe measurements of plantar pressure, and Finite Element
Analysis of the stress distribution in the heelpad and shoe midsole, were used to study
the mechanics of heelstrike in running. The heelpad properties were deduced from
published force-deflection data. The ASTM 1614 method of midsole testing produces
significantly different peak pressures and stress distribution than does the human
heelpad compressing an EVA foam midsole. Midsole deterioration was measured in
controlled running tests, and air loss from the foam shown to cause reduced heelstrike
cushioning.
INTRODUCTION
Running involves a series of heel-strike impacts on the ground. The midsole foams of
running shoes, by absorbing energy, limit the peak impact force in the heel-strike. In
the course of a long run, there is a reduction of the air content of the foam cells,
reducing the cushioning. The foam does not appear to fully recover after a run.
The foamed copolymer of ethylene and vinyl acetate (EVA) is widely used in
running shoe midsoles. The density of the closed-cell, crosslinked foams ranges from
150 to 250 kg m
-3. Misevich and Cavenagh (1984) performed repeated rapid-
compression tests on EVA foam - in these the stress was constant through the foam.
They showed that the midsole force-deflection response changes with cycle number.
They claimed there was an initially positive internal gas pressure in the foam cells;
this declined during repeated loading, as air diffused through the cell faces. Barlett
(1995) discussed the cell geometry seen in sectioned EVA midsoles, claiming that
cells next to the outsole became flattened after 3200 km of running, and that some cell
faces fractured while others buckled. Mills and Perez (2001) modelled creep loading
of EVA foams, showing that gas diffusion contributed significantly to the creep
process. However they did not consider the loading history experienced in a shoe.
Sensor insoles can determine the pressure distribution on the upper foam surface,
under the athlete’s foot. Finite Element Analysis (FEA) is needed to analyse the large
geometric changes in the heelpad and shoe foam, and to determine the stress field in
the foam. Aerts et al (1995) described the force-deflection response of the human
heelpad, while Gefen et al (2001) gave the mean pressure at the interface between the
heelpad and a flat rigid surface as a function of an average heelpad thickness strain.
However, they gave no heelpad stress-strain data. Thompson et al’s (1999) FEA
analysis considered the axially symmetric indentation of an EVA foam midsole by a
rigid ASTM heelform, revealing some stress variation in the foam. However, there
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
2
has been no FEA of the stresses in a human heelpad, or its interaction with a shoe
midsole. This paper performs such an analysis, and compares the results with
experimentally measured foot pressure distributions during a run.
MATERIAL CHARACTERISATION
The midsole of the Reebok Aztrek DMX shoe was composed of EVA foams; a
section of area 20 mm by 40 mm on the lateral side of the heel was coloured grey, and
the rest white. The densities, measured using a hydrostatic balance were 170 and 173
kg/m3 for the white and grey foams respectively, i.e. approximately the same. The
foams were analysed with a Mettler Differential Scanning Calorimeter (DSC) 30. The
degree of crystallinity was calculated by dividing the measured enthalpy of fusion by
that for pure polyethylene (286.8 J/g). The results (table 1) are similar. Hence there is
approximately 18 % VA in the copolymer.
Table 1 Characterisation of EVA foams in Reebok midsole.
Sample Melting Point (ºC) Melting Enthalpy (J/g) Crystallinity (%)
White 82.2 56.7 19.8
Grey 82.0 53.6 18.7
PLANTAR PRESSURE DISTRIBUTION IN RUNNING
Three healthy male long distance runners, aged 34, 37, and 49, weighed 91.4, 82.1,
63.4 kg, with UK shoe size 11, 9, 8 respectively. They were all rearfoot strikers, did
not use orthotics, and reported no lower extremity injury for the past year. The study
was approved by the University ethical committee.
The Tekscan F-Scan system consists of a flexible, 0.18 mm thick sole-shape,
having 960 pressure sensors, each 4 by 3 mm. The resistance of the pressure-sensitive
ink, contained between 2 polymer-film substrates, decreases as the pressure, applied
pressure to the substrate, increases (Ahroni, 1998). The sensor insole was trimmed to
fit the subjects’ right shoe. It was calibrated by the known weight of the test subject
standing on one foot. Data was recorded at 150 Hz for 4 sec.
The subjects ran, at 2.61 m/s for 10 min, on a Quinton Instrument Co. 640
treadmill; this short experiment avoided fatigue. They wore trainers that were brand-
new at the start of the experiment. The plantar distribution was recorded at the
beginning, middle and end of the run. The subjects were asked to run on hard surfaces
(track or roads) using the shoes for their training, and to keep a running distance
diary. Every 15 days the plantar pressure distribution was measured, waiting 24 h
since the last use to standardise the recovery time. They will run 500 to 1000 km
using the shoes, then the cellular structure of the midsole will be compared with that
in an unused pair of shoes, to investigate the foam damage.
The Tekscan measurements show the in-shoe pressure distribution as a function of
the running time. Figure 1 shows composite maps of the peak pressure at each pixel
throughout the footstrike, at 1 and 10 minutes in a session. The peak pressures in the
heel region (table 2) increased with run time in both sessions, with a greater increase
in the second session, as result of prior use. The 1 minute peak pressures in the 2nd
session are close to those in the 1st, due to the 1 day of recovery prior to testing.
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
3
Table 2 Peak pressures values (kPa) for the runners
session first second
runner 1 min
5 min
10 min
Distance
Run (km) 1 min
5 min
10 min
1 280 301 311 106.2 286 321 342
2 245 262 273 30.9 250 283 310
3 232 259 275 52.3 240 279 306
Fig 1 Maximum pressures, at each sensor pixel, for footstrikes after 1 and 10 minutes.
Repeat impact ASTM method.
In this test a weight falls a fixed vertical distance onto the shoe midsole in the heel
region (ASTM, 1999) with an impact kinetic energy of 5.0 J. The striker was the flat
end of a steel cylinder of diameter 45 mm, with edge of 1.0 mm radius. The total drop
mass of 5.9 kg fell 85 mm, guided by low friction bearings on 2 vertical cables. The
signal from an accelerometer on the striker was captured digitally and analysed by
computer to give the striker force versus the deflection of the top surface of the
midsole (Mills, 1994).
The outsole was removed from the shoe heel giving a flat surface, which was
fixed to a horizontal rigid fixed anvil. The midsole thickness was 20.1 mm. Data were
collected from the first and the 26th impact. The sample was allowed to recover for 15
min, and 100 cycles were performed on it, recording data every 25 cycles. The foam
force-deflection was studied as a function of time. Table 3 shows how the peak
deflection and force increase slightly with impact number, while the energy absorbed
in the loading-unloading cycle decreases.
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
4
The peak pressure is calculated as the average value across the heel surface. The
values are significantly higher than in the human trials, while foam performance
changes in far fewer impacts than in the human trials.
Table 3 ASTM test results for the EVA foam midsole.
session Impact
number Maximum
displacement
(mm)
Maximum
pressure
(kPa)
Maximum
force
(kN)
Energy
absorbed
(J)
1 1 7.31 764 1.21 3.28
1 26 7.74 817 1.30 2.58
2 1 7.57 799 1.27 2.91
2 25 7.73 817 1.29 2.60
2 50 7.76 863 1.37 2.17
2 75 8.03 787 1.25 2.80
2 100 7.79 872 1.39 1.93
Figure 2 showed the force vs. deflection graphs for both sessions. The greatest
shape change occurs in the first session, probably due to softening of the polymer part
of the foam structure. The midsole response appears to approach an ‘equilibrium
response when impact number is high. The hysteresis in this response could in part be
due to heat transfer from the compressed air in the cells to the cell walls.
Fig. 2 Force versus deflection for the ASTM heel impacts on EVA foam midsole.
STRESS ANALYSIS OF THE FOOT AND SHOE HEEL
The ABAQUS FEA program (HKS) was used. Unpublished work showed that the
response of EVA foam midsoles could be modelled in compression and tension using
a single modulus version of the Ogden hyperfoam material. The problems tackled
were axisymmetric, with a vertical axis of rotational symmetry. The large deformation
option is used. Meshing was chosen to maximise the computation stability;
nevertheless most simulations became unstable at high deformations.
The geometry of the calcaneus bone of the heel is grossly simplified to be a
hemisphere of radius 15 mm, attached to the end of a 20 mm long vertical cylinder of
radius 15 mm. The heel pad is assumed to be bonded to the heelbone surface. This
allows some of the load to be transferred by shear to the cylindrical surface.
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
5
The properties of the heel pad were adjusted until the force deflection response in
problem (1) matched the data of Aerts et al (1995). Its outer geometry was a vertical
cylinder of radius 30 mm and a flat end face, with a 10 mm radius to the edge. No
account is taken of the confining effect of the shoe sides. The EVA midsole foam was
taken as vertical cylinder of radius 35 mm and height 22 mm, with flat end faces.
Three problems were considered:
1) The ASTM F1614 ‘heel’ compressing an EVA foam midsole, resting on a
flat rigid surface. The ‘heel’ is a vertical steel cylinder, of 45 mm diameter
and flat end, with a 1 mm radius to the edge. The Ogden hyperfoam data for
the EVA foam were µ = 100 kPa, α = 0.5, Poisson’s ratio = 0.
2) The deformation of the human heel pad in contact with a flat rigid surface.
The heelpad was simulated using the Ogden hyperelastic material. The shear
moduli were µ1 = µ2 = 200 kPa, the exponents α1= 2 and α2=-2, and the
inverse bulk modulus D= 1.0 x 10-9 Pa-1. This is equivalent to the Mooney
Rivlin equation for rubber, but with moduli typical of gels. Although the
material model cannot simulate hysteresis on unloading, it can predict the
shape of the mean of the loading and unloading curves.
3) The deformation of the heelpad, in contact with an EVA midsole, resting on
a flat rigid surface, using the EVA foam parameters of the 1st analysis and
the heelpad parameters of the 2nd analysis.
Figure 3a and b show the vertical compressive stress σ22 contours, respectively for the
ASTM test and the heel/heelpad model at deformations of 11 and 17 mm respectively.
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
6
Fig 3 σ22 stress distribution for a) ASTM heel on EVA midsole at 11 mm deflection
b) Heelbone and pad on EVA midsole at 17 mm deflection. Contours in kPa.
In the ASTM test, there is a sudden transition between nearly uniformly
compressed foam under the ‘heel’ and the surrounding foam. While the stress in the
majority of the foam directly under the heel is in the range 250 to 300 kPa, it reaches
a peak of 450 kPa at the heel edge. This is for a load of 0.55 kN, only 42% of the
typical peak load in the ASTM experiments. Hence, the stresses would be higher if
the simulation had reached the peak load of the experiment.
In the heel plus midsole simulation, the upper midsole surface is concave, while
the heelpad has spread laterally at a load of 0.70 kN. The maximum foam stress is
300 kPa, at the centre of the contact area on the foam upper surface (fig 3b). This is
confirmed by the 240 kPa area of diameter approximately 20 mm in the Tekscan
pressure map of the midsole upper surface. Peak footstrike forces (Nigg, 1990) are
higher at 1.5 to 2 kN, but the FEA has ignored load transfer from other parts of the
foot to the ground. The Tekscan pressure map, at the moment of maximum heelstrike,
indicates that about 40% of the total force is transmitted through the heel region.
The predicted force versus deflection relationships are shown in figure 4. With the
deformable heel model, rather than the ‘rigid’ ASTM heel, the deflection is 20%
higher for a given compressive force. Hence a footstrike of a given kinetic energy will
produce a lower peak force than in the ASTM test with the same kinetic energy. The
predicted ASTM test force-deflection relationship has the same shape as, but is
slightly lower than, the experimental unloading data (figure 2). Hence a better
material model is required for the FEA of the loading response of EVA foam.
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0 2 4 6 8 10 12 14 16
force kN
deflection mm
ASTM
Heel model
Fig 4 Force vs deflection for EVA midsole compressed with ASTM rigid heel and
heel/heelpad.
Gas loss modelling
The loss of air from the EVA foam under creep loading was modelled using one-
dimensional finite difference methods (Mills and Perez, 2001). The air flow was
along the direction of the compressive stress. The model predicts that a gas-depleted
layer develops at the upper and lower surfaces of the foam. In rapid heelstrikes, a
similar process should occur, but the timescale is difficult to predict. Applying these
results in a qualitative manner to heelstrikes on EVA midsoles suggests that a gas
Pp 580-587, The Engineering of Sport 4, Eds S. Ujihashi and S.J. Haake, Blackwell (2002).
7
depleted layer should develop near the upper surface of the midsole, in the high
pressure region indicated by the Tekscan results.
DISCUSSION
The ASTM test, used for shoe midsole testing, produces a different pressure
distribution on the upper surface of an EVA midsole than either those recorded in-
shoe, or that predicted by FEA of the heelpad plus EVA foam. The peak forces in the
ASTM test are too high, because it ignores forces transmitted through the rest of the
shoe during heelstriker, producing excessive pressures on the foam. FEA indicates a
high pressure region on the foam at the edge of the ASTM heel, which is quite
different from the in-shoe measurements with runners, or the FEA prediction for a
heel/heelpad model on a midsole. The ASTM test probably causes a more rapid
deterioration in the midsole response than does running. Further FEA will be done of
the effect of air-depleted foam in upper central region of the midsole.
The modelling of gas loss from the foam under uniform-stress creep conditions
indicates the process rate, and the development of a depleted gas region near the foam
upper and lower surfaces. However when shoes are used for running
a) The foam pressure distribution is non-uniform, with peak values near the
centre of the heel contact area.
b) The impacts, repeated at 0.3 s intervals, may cause fatigue damage to the
foam.
The above suggests that foam cell flattening should be seen near the upper surface of
the midsole, near the heel centre. We will check the shoes after 500 km of use. The
observations quoted by Bartlett (1985) may be wrong, as the process of moulding the
midsole into its final form can cause the flattening of surface cells.
The thin cell faces of EVA foam are not perfect for containing air. As the air
provides a major shock cushioning mechanism in the foam, its loss reduces the
midsole performance. There is a complex interaction between the heel-strike stress
field in the foam and the gas diffusion from the foam. Although the gas diffusion can
be modelled during uniform creep loading, it is not yet possible to consider all aspects
of the stress-diffusion interaction.
REFERENCES
Aerts P. et al, (1995) J. Biomech. 28, 1299-1304.
Ahroni J. H. et al, (1998) Foot & Ankle Int., 19, 668- 673.
Barlett R., (1995) Sports Biomechanics, Spon.
Gefen A. et al, (2001) J. Biomech. 34, 1661-1665.
Misevich K.W. & Cavenagh K.W., (1984) Ch. 3 in Sports Shoes and Playing
surfaces, Eds. EC. Frederick, Human Kinetics Inc.
Mills N.J., (1994) Impact response, in Low density cellular plastics, Eds. Hilyard
N.C. & Cunningham A., Chapman & Hall, London.
Mills N.J. & Rodriguez-Perez M.A., (2001) Cell. Polym., 20, 79-100.
Nigg B., (1990) Ed. Biomechanics of running shoes, Shoe Trades Publ. Co., p 29.
Thompson R.D. et al, (1999) Sports Engng. 2, 109-120.
ASTM F1614 Shock Attenuating Properties of Materials Systems for Athletic
Footwear, American Society for Testing and Materials, 1999.
... The most common materials for shoe midsoles are flexible EVA and polyurethane foams (Mills, 2007;Verdejo and Mills, 2002;Brückner et al., 2010). However, there has been a recent interest in the use of polyurea foams based on the superior impact mitigation properties of the bulk . ...
... t 3 ) gradient with a laminate density of q À Ã ¼ 0:147 is shown to outperform EML230 and EML350 in terms of efficiency and ideality at lower stress levels while having a lighter weight. This laminate can concurrently outperform EML95 in terms of strength, therefore offering longer durability (Verdejo and Mills, 2002;Verdejo and Mills, 2004). ...
Article
Several sports-related injuries and orthopedic treatments need the necessity of corrective shoes that can assuage the excessive pressure on sensitive locations of the foot. In the present work, we study the mechanical and energy absorption characteristics of density-graded foams designed for shoe midsoles. The stress-strain responses of polyurea foams with relative densities (nominal density of foam divided by the density of water) of 0.095, 0.23, and 0.35 are obtained experimentally and used as input to a semi-analytical model. Using this model, three-layered foam laminates with various gradients are designed and characterized in terms of their weight, strength, and energy absorption properties. We show that, in comparison with monolithic foams, significant improvement in strength and energy absorption performance can be achieved through density gradation. Our findings also suggest that there is not a single gradient that offers a superior combination of strength, energy absorption, and weight. Rather, an optimal gradient depends on the plantar location and pressure. Depending on the magnitude of the local plantar pressure, density gradients that lead to the highest specific energy absorption are identified for normal walking and running conditions.
... La problématique de la protection des biens et des personnes est toujours d'actualité aujourd'hui et gagne même en intérêt au vue de la généralisation de la mobilité et donc de la multiplicité des modes de déplacement. On retrouve cette problématique dans le sport Verdejo et Mills, 2002], dans le transport de matières dangereuses [Kraiem, 2016], dans l'automobile, dans l'aéronautique, le militaire, ... C'est un des domaines dans lequel les composites à base de liège semblent être prometteurs . ...
Thesis
Full-text available
In order to lightening structures, the improvement and reinforcement of materials are becoming a growing concern in the aeronautics and aerospace industries. The suitability of certain materials to sweep across a wide spectrum of physical properties would make it possible to bring substantial mass savings to many existing applications. Agglomerated cork is one of these versatile materials thanks to its chemical composition and natural cellular structure. It is thus today considered for shock absorption applications and the addition of fibre reinforcements could be a way of improving its performance. The study of the absorption capacity of this material according to temperature and strain rate allows us to refine our understanding of the mechanical behaviour of multi-scale polymeric cellular materials under severe loading. The characterisation of the structure at different scales has highlighted the highly heterogeneous character of cork agglomerates as well as a strongly oriented mesoscopic structure. Mechanical tests in quasi-static regime carried out at different scales of observation showed the predominant role of this mesostructure on the mechanisms of deformation, damage and rupture, in compression as well as in tension at breakage. In order to study the coupled influence of strain rate and temperature, the visco-elastic behaviour of the materials making up the agglomerates was investigated using double sweep vibration tests in time and temperature (DMA tests). A set-up specifically designed for this work made it possible to carry out compression tests in dynamic regime (70 to 250 s -1 ) at different temperatures (from 20 to 73°C) in order to try to decorrelate material and structure effects in the dependence of polymeric cellular materials on the strain rate.
... Some of these approaches are implemented in finite element (FE) codes. One widely used material model is Hyperfaom in the computer aided engineering (CAE) software ABAQUS , which was e.g.employed by Erdemir et al. (2005) ; Lemmon, Shiang, Hashmi, Ulbrecht, and Cavanagh (1997) ; Lyn and Mills (2001) ; Mills and Gilchrist (20 0 0) ; Petre, Erdemir, and Cavanagh (2006) , Thomson , N. (2016a) ; Verdejo and Mills (2002) . Another suitable material model in ABAQUS is Low Density Foam ( N., 2016b ). ...
Article
The complex mechanical deformation behaviour of closed-cell foams is governed by morphological and physical properties such as cell structure and crystallinity. In this study, the micro-, meso- and macroscopic scale of commercially available bead foam was analysed. Statistical distribution of the cell structure including cell size, wall thickness and shape was determined using optical microscopy and micro computed-tomography. Local crystallinity was investigated by DSC scans and X-ray scattering. The results confirm the important influence of the multi-step manufacturing process on the physical properties of bead foams. Mesoscopic and macroscopic numerical analyses of the mechanical behaviour of polymeric closed-cell bead foams are performed. With regard to the manufacturing influence on local physical properties of bead foams, the suggested approach takes into account the density and crystallinity-specific material properties and the generation of associated material cards, using virtual test methods. To represent the mesoscopic foam morphology considered here, statistical volume elements (SVE) are generated using the Laguerre tessellation method. Crystallinity-dependent base material properties are used in SVE material cards to investigate tensile and compressive behaviour. For validation of the suggested approach, four-point bending tests are conducted on macroscopic scale and compared with the numerically predicted results. The paper shows the advanced forecast capability of locally resolved modelling of closed-cell bead foam structures and underlines the huge potential of the multi-scale modelling approach.
... To simulate shoe sole components, block-shape ethylene vinyl acetate copolymer (EVA) foam/styrene butadiene rubber (SBR) laminated materials were prepared as test specimens for this study. The EVA foam [13] and SBR [14] are typically used in the mid-sole and outer sole in sports shoes. The effects of EVA foam thickness and normal force on the contact area and the static and dynamic friction coefficients were investigated using a dead-weight-type tribometer under non-lubricated conditions. ...
Article
In this study, we investigate the frictional behavior of an ethylene vinyl acetate copolymer (EVA) foam/styrene butadiene rubber (SBR) laminated block using a dead-weight-type tribometer to identify the design guidelines for shoe tread patterns to provide variable friction control. The friction coefficient was highest when the ratio of the EVA foam thickness with respect to the total block thickness was high (i.e., 0.2–0.8) because of the increased contact area. Further, the friction coefficient of the laminated block specimen increased with the normalized parameter, which is the ratio of shear deformation and bending deformation in the bulk material. These results indicate that increasing the shear deformation component prevents the detachment of the specimen's trailing edge and maintains the contact area at the specimen–floor interface, resulting in increased friction coefficients. Our observations are expected to contribute to a detailed understanding of how to control the friction of the resin-foam/rubber-laminated block materials.
... Dynamic Mechanical Thermal Analysis (DMTA) [20][21][22] is acknowledged in the field to correlate material properties and impact performances, also accounting for aging effects. 23 This method consists in applying an oscillatory force to a beam sample and analyzing its viscoelastic frequency-dependent mechanical response. DMTA is of relevant importance since this kind of equipment is subjected to large temperature changes during use and storage. ...
Article
Objectives: Winter sports are high-energy outdoor activities involving high velocities and acrobatic maneuvers, thus raising safety concerns. Specific studies on the impact mechanics of back protectors are very limited. In this study analytical and numerical models are developed to rationalize results of impact experiments and propose new design procedures for this kind of equipment. Design: Different soft-shell solutions currently available on the market are compared. In particular, the role of dynamic material constitutive properties and of environmental temperature (which affects mainly material stiffness) on energy absorption capability are evaluated. Methods: Starting from dynamic mechanical-thermal characterization of the closed-cell polymeric foams constituting the protectors, we exploited analytical modeling and Finite Element Method simulations to interpret experimental data from drop weight impact test and to characterize protectors at different temperatures and after multiple impacts. Results: The temperature and frequency dependent properties of these materials characterize their impact behavior. Modeling results are in good agreement with impact tests. Results demonstrate how ergonomic soft-shell solution provides an advantage with respect to traditional hard-shell in terms of impact protection. Moreover, it can maintain nearly unaltered its protective properties after multiple impacts on the same point. Conclusions: The coupled analytical-simulation approach here presented could be extensively used to predict the impact behavior of such equipment, starting from material characterization, allowing to save costs and time for physical prototyping and tests for design and optimization.
... Closed-cell-structured ethylene-vinyl acetate (EVA) foam is light and provides high cushioning and is thus widely used in running shoe midsoles [13][14][15]. To investigate whether the EVA foam can be applied as the outer sole material that contacts roads or floors and requires high grip, Yamaguchi et al. [16] investigated the friction coefficient of EVA blocks with different porosities slid against a smooth steel surface under different sliding velocities. ...
Article
Full-text available
In this study, the effect of porosity on the dry sliding fiction of ethylene-vinyl acetate (EVA) foams was investigated under different normal load conditions. EVA foam blocks with varying porosities were slid against a smooth stainless steel plate under dry conditions. The friction coefficient increased with increasing porosity under all of the normal load conditions. In addition, the contact area was estimated using a contact model considering elastic buckling of the cell walls (elastic collapse). The elastic collapse area in the anterior portion of the EVA foam block increased with increasing normal load and porosity, which resulted in an increased contact area. Furthermore, the friction coefficient was positively correlated with the estimated contact area divided by the normal load, indicating that adhesion friction increases with increasing porosity of polymer foams. These results may contribute to the design of high-friction, lightweight shoe sole tread blocks prepared using polymer foam blocks.
... The protrusion height of the layered texture on the bottom of the artificial heel was evaluated with the Footscanner insole 2.39 system under impact. These protrusions were altered continuously until reproducing a pressure distribution equivalent to the measured one, thus avoiding excess pressures [26], ensuring an optimum simulation of the impact conditions. The edges of the impact module were rounded by a radius of at least 1.2 mm to prevent adverse specimen tearing, with a 4265 mm 2 surface area delivering the impact. ...
Article
Full-text available
Long-distance running is known to induce joint overloading and elevate cytokine levels, which are the hallmarks for a variety of running-related injuries. To address this, footwear systems incorporate cushioning midsoles to mitigate injurious mechanical loading. The aim of this study was to evaluate the effect of athlete body mass on the cushioning capacity of technical footwear. An artificial heel was prototyped to fit the impact pattern of a heel-strike runner and used to measure shock attenuation by an automated drop test. Impact mass and velocity were modulated to simulate runners of various body mass and speeds. The investigation provided refined insight on running-induced impact transmission to the human body. The examined midsole system was optimized around anthropometric data corresponding to an average (normal) body mass. The results suggest that although modern footwear is capable of attenuating the shock waves occurring during foot strike, improper shoe selection could expose an athlete to high levels of peak stress that could provoke an abnormal cartilage response. The selection of a weight-specific cushioning system could provide optimum protection and could thus prolong the duration of physical exercise beneficial to maintaining a simulated immune system.
Article
Elastomeric foams display a nonlinear viscoelastic mechanical response, which is relevant to impact protection applications. In this paper, we present a methodology for the experimental characterization and constitutive modeling of the rate-dependent, large-deformation mechanical behavior of isotropic, viscoelastic foam materials under quasi-static and moderate loading rates. We conduct large-deformation, homogeneous simple compression/tension experiments under quasi-static and moderate strain rates for a closed-cell elastomeric foam to inform a phenomenological, isotropic constitutive model. The model is based on a multi-mechanism strategy, consisting of a rate-independent, hyperelastic contribution and several rate-dependent, viscoelastic contributions. This structure is motivated by the rheological concept of a hyperelastic spring in parallel with several nonlinear Maxwell elements. The hyperelastic modeling utilizes the invariants of the logarithmic strain and accounts for high compressibility and strong volumetric-distortional coupling. For the rate-dependent mechanisms, we employ the framework of the multiplicative decomposition of the deformation gradient into non-equilibrium elastic and viscous parts. Validation experiments that involve inhomogeneous deformation and deformation-rate fields are conducted, which include spherical indentation, simple-shear-like deformation both without and with a fixed pre-compression, and tension of a specimen with circular holes at different deformation rates. The responses from the validation experiments are compared against model predictions obtained from finite-element-based numerical simulations, demonstrating that the experimental results are accurately captured by the proposed model.
Article
The shock-absorbing performance of single ethylene-vinyl acetate (EVA) foams and functionally graded laminates of these foams were evaluated with simulated gait experiments. Mechanical properties such as the hardness and density were measured, along with the viscoelastic behavior. The shock-absorbing performance was found to be dependent on the cellular structure, namely the density, elastic modulus, and loss tangent of the single EVA foams. However, for the laminated samples, there were slight variations in the shock-absorption efficiency for samples with the same density. The graded materials showed higher shock-absorption efficiency compared with non-graded single materials with the same density. In addition, laminates with elastic modulus values slightly higher than those of the single EVA samples showed higher shock-absorption efficiency. Finally, since laminates with varied properties were produced, a design concept for a suitable functionally graded material is presented for footwear applications.
Article
A technique is introduced for simultaneous measurements of the heel pad tissue deformation and the heel-ground contact stresses developing during the stance phase of gait. Subjects walked upon a gait platform integrating the contact pressure display optical method for plantar pressure measurements and a digital radiographic fluoroscopy system for skeletal and soft tissue motion recording. Clear images of the posterior-plantar aspect of the calcaneus and enveloping soft tissues were obtained simultaneously with the pressure distribution under the heel region throughout the stance phase of gait. The heel pad was shown to undergo a rapid compression during initial contact and heel strike, reaching a strain of 0.39 +/- 0.05 in about 150 ms. The stress-strain relation of the heel pad was shown to be highly non-linear, with a compression modulus of 105 +/- 11 kPa initially and 306 +/- 16 kPa at 30% strain. The energy dissipation during heel strike was evaluated to be 17.8+/-0.8%. The present technique is useful for biomechanical as well as clinical evaluation of the stress-strain and energy absorption characteristics of the heel pad in vivo, during natural gait.
Ch. 3 in Sports Shoes and Playing surfaces
  • K W Misevich
  • K W Cavenagh
Misevich K.W. & Cavenagh K.W., (1984) Ch. 3 in Sports Shoes and Playing surfaces, Eds. EC. Frederick, Human Kinetics Inc
Sports Engng. 2, 109-120. ASTM F1614 Shock Attenuating Properties of Materials Systems for Athletic Footwear
  • R D Thompson
Thompson R.D. et al, (1999) Sports Engng. 2, 109-120. ASTM F1614 Shock Attenuating Properties of Materials Systems for Athletic Footwear, American Society for Testing and Materials, 1999. 7
  • R Barlett
Barlett R., (1995) Sports Biomechanics, Spon.
  • P Aerts
Aerts P. et al, (1995) J. Biomech. 28, 1299-1304.
  • N J Mills
  • M A Rodriguez-Perez
Mills N.J. & Rodriguez-Perez M.A., (2001) Cell. Polym., 20, 79-100.
  • J H Ahroni
Ahroni J. H. et al, (1998) Foot & Ankle Int., 19, 668-673.
  • A Gefen
Gefen A. et al, (2001) J. Biomech. 34, 1661-1665.
Impact response, in Low density cellular plastics
  • N J Mills
Mills N.J., (1994) Impact response, in Low density cellular plastics, Eds. Hilyard N.C. & Cunningham A., Chapman & Hall, London.