Vol.:(0123456789)
1 3
European Journal of Applied Physiology (2022) 122:1179–1187 
https://doi.org/10.1007/s00421-022-04903-9
ORIGINAL ARTICLE
Acute intense fatigue does not modify the effect of EVA and TPU 
custom foot orthoses on running mechanics, running economy 
and perceived comfort
Ken Van Alsenoy1,2 · Joong Hyun Ryu3 · Olivier Girard1,4 
Received: 23 August 2021 / Accepted: 29 January 2022 / Published online: 24 February 2022 
© The Author(s) 2022
Abstract
We determined whether fatigue modifies the effect of custom foot orthoses manufactured from ethyl-vinyl acetate (EVA) 
and expanded thermoplastic polyurethane (TPU) materials, both compared to standardized footwear (CON), on running 
mechanics, running economy, and perceived comfort. Eighteen well-trained, males ran on an instrumented treadmill for 
6 min at the speed corresponding to their first ventilatory threshold (13.8 ± 1.1 km/h) in three footwear conditions (CON, 
EVA, and TPU). Immediately after completion of a repeated-sprints exercise (8 × 5 s treadmill sprints, rest = 25 s), these run 
tests were replicated. Running mechanics, running economy and perceived comfort were determined. Two-way repeated 
measures ANOVA [condition (CON, EVA, and TPU) × fatigue (fresh and fatigued)] were conducted. Flight time shortened 
(P = 0.026), peak braking (P = 0.016) and push-off (P = 0.032) forces decreased and vertical stiffness increased (P = 0.014) 
from before to after the repeated-sprint exercise, independent of footwear condition. There was a global fatigue-induced 
deterioration in running economy (− 1.6 ± 0.4%; P < 0.001). There was no significant condition × fatigue [except mean 
loading rate (P = 0.046)] for the large majority of biomechanical, cardio-respiratory [except minute ventilation (P = 0.020) 
and breathing frequency (P = 0.019)] and perceived comfort variables. Acute intense fatigue does not modify the effect of 
custom foot orthoses with different resilience characteristics on running mechanics, running economy and perceived comfort.
Keywords Fatigue · Orthotics · Material resilience · Stride pattern · Economy of locomotion · Footwear comfort
Abbreviations
ANOVA  Repeated-measures analysis of variance
CFO 
 Custom foot orthotics
CON 
 Control condition
EVA 
 Ethyl-vinyl acetate
GRF 
 Ground reaction force
RE 
 Running economy
RPE 
 Ratings of perceived exertion
TPU 
 Thermoplastic polyurethane
Introduction
Custom foot orthoses (CFOs), which refer to shoe inserts 
built from a three-dimensional representation of the ath-
lete’s feet, have become a contemporary topic in footwear 
biomechanics literature. Wearing CFOs is used to provide 
foot support and shock absorption during ground contact 
through re-distribution of plantar loading and a better 
maintenance of foot stability (Crago et al. 2019). Prior 
studies investigating CFOs effects on key biomechani-
cal indicators have reported mixed results with beneficial 
(Worobets et al. 2014; Wilkinson et al. 2018) or unchanged 
(Lewinson et al. 2013) adjustments in stride pattern. Dis-
crepant findings may relate to intrinsic properties (i.e., 
energy return and longitudinal bending stiffness) of tested 
insoles provoking specific biomechanical modifications 
Communicated by Jean -Rene Lacour.
 * Ken Van Alsenoy 
 
Ken.VanAlsenoy@aspetar.com
 * Olivier Girard 
 
olivier.girard@uwa.edu.au
1 
Aspetar, Orthopaedic and Sports Medicine Hospital, FIFA 
Medical Centre of Excellence, Doha, Qatar
2 
Centre for Health, Activity and Rehabilitation Research 
(CHEARR), Queen Margaret University, Edinburgh, UK
3 
Sports Science Department, Aspire Academy, Doha, Qatar
4 
School of Human Sciences (Exercise and Sport Science), The 
University of Western Australia, Perth, WA, Australia
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1 3
in running gait (Sinclair et al. 2016). Quantifying stride 
mechanical adjustments in response to CFOs is crucial to 
better understand how footwear features may eventually 
influence the energetic cost of running (Moore 2016).
Running economy (RE), the steady-state oxygen uptake 
at a constant submaximal speed, is considered a key physi-
ological measure for distance runners (Barnes and Kilding 
2015). The effect of inserts (foot orthoses and sock absorb-
ing insoles) on RE in distance runners has produced incon-
sistent results (Crago et al. 2019). In a study by Burke 
and Papuga (2012), six recreational athletes consumed at 
least 3% less oxygen while running with CFOs compared 
to their shoe-fitted insoles. In contrast, the effect of wear-
ing CFOs manufactured from ethyl-vinyl acetate (EVA) 
and expanded thermoplastic polyurethane (TPU) materi-
als, both compared to standardized (shoe only), on RE 
was considered negligible and marginally improved (albeit 
not significantly), respectively (Van Alsenoy et al. 2019). 
Additionally, it is known that the amount of cushioning 
material in the shoe can influence RE (Tung et al. 2014), 
and that these effects might be mediated by comfort per-
ception (Mundermann et al. 2003).
Footwear comfort is paramount since it largely influ-
ences the adherence to ongoing use of CFOs. The percep-
tion of load attenuation (i.e., perceived comfort), result-
ing from the level of somatosensory feedback experienced 
(Robbins and Hanna 1987), has been related to RE (Luo 
et  al. 2009; Lindorfer et  al. 2020). Comfort-induced 
changes, as a result of load attenuation on certain anatomi-
cal foot structures, potentially contribute to reduction in 
metabolic demands via more economical stride character-
istics (Moore et al. 2014). Analyzing the runner’s percep-
tion of the CFOs’ cushioning properties and the associated 
biomechanical adjustments induced by different inserts 
features (yet with identical geometry) may also help to 
better elucidate their effectiveness at protecting against 
fatigue effects (Hintzy et al. 2015).
Tolerance to ground impact is often compromised as 
fatigue appears, which in turn may limit performance and/or 
increase injury risk, notably by increasing the magnitude and 
rate of loading (Li et al. 2020). To date, little attention has 
been paid to the effectiveness of CFOs at reducing impact 
loading in situations of intense fatigue (i.e., repeated ‘all out’ 
efforts; Girard et al. 2020), with participants typically tested 
in ‘fresh’ conditions only. Reportedly, CFOs reduced plantar 
loading under the hallux, medial midfoot, and lateral midfoot 
compared to prefabricated insoles by ~ 30–35% post-fatigue 
(12 min at treadmill speed of ~ 14.4 km/h) (Lucas-Ceuvas 
et al. 2014). It is therefore plausible that the use of CFOs 
may become a more important protective mechanism for 
excessive mechanical constraints in the lower extremities 
once runners become fatigued. Because increased fatigue 
differently affects the biomechanical pattern of running 
(Brocherie et al. 2016), assessing the effects of CFOs for 
spatio-temporal, spring-mass model and antero-posterior 
variables is relevant.
This study determined whether acute intense fatigue 
modifies the effect of CFOs manufactured from ethyl-vinyl 
acetate (EVA) and expanded thermoplastic polyurethane 
(TPU) materials, both compared to standardized footwear 
(CON), on running mechanics, RE, and perceived comfort.
Methods
Participants
Eighteen male well-trained athletes (mean ± SD age, 
38.9 ± 5.1 years; body height, 175.3 ± 5.8 cm; body mass 
74.9 ± 7.7 kg; maximal oxygen uptake, 49.1 ± 6.6 mL/min/
kg; maximal aerobic speed, 18.4 ± 1.6 km/h) were recruited 
for this study. They trained on average 8.8 ± 3.7 h per week 
in the 3 months leading up to the data collection with an 
average weekly running distance of 37.6 ± 26.7 km. Thirteen 
were rear-foot strikers, one was a midfoot striker and four 
were forefoot strikers at 10 km/h. Written informed consent 
was obtained from participants, and the study was approved 
by Anti-Doping Laboratory Ethics Committee in Qatar (IRB 
Application Number 2017000201) and conducted according 
to the Declaration of Helsinki.
Study design
About 1 week before testing, participants undertook a pre-
liminary session. They completed a continuous, maximal 
incremental running test where the individual ventilatory 
threshold, and corresponding running speed that was used 
for the three following intervention sessions, were deter-
mined. Briefly, participants started running at 9 km/h with 
speed increases of 0.5 km/h every 30 s. The test ended with 
voluntary exhaustion of the participants. Verbal encourage-
ment was only given by the researcher guiding the runners 
throughout the session. Ventilatory threshold was deter-
mined using the criteria of an increase in minute ventila-
tion/oxygen uptake with no increase in minute ventilation/
carbon dioxide and the departure from linearity of minute 
ventilation (Davis 1985).
On three occasions, participants performed (in a counter-
balanced randomized crossover design), at the same time 
of day (± 1 h) and 4–5 days apart, an exercise protocol (see 
below) in different footwear conditions: a control session 
where participants ran with standardized (i.e., only shoe 
liner inserted) footwear, CFOs made of EVA and TPU. After 
arrival to the laboratory, CFOs were inserted bilaterally in 
participants’ shoes. The participants and the researcher who 
was directly involved in guiding the session were visually 
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1 3
blinded from the CFO materials. Participants were asked 
to avoid strenuous exercise in the 12 h, as well as refrain 
from food and caffeine for 4 h preceding their visits to the 
laboratory and were encouraged to replicate their diet and 
training pattern for all visits. Laboratory conditions were 
similar throughout all running sessions (mean ± SD tempera-
ture 20.7 ± 0.2 °C, relative humidity 60.4 ± 0.6%). Time of 
day was standardized for each participant over all sessions.
Exercise protocol
After a 10 min warm-up at 10 km/h, followed by a 3 min 
break used to put on the mask to collect expired gases, par-
ticipants ran for 6 min at the speed associated with their 
first ventilatory threshold (13.8 ± 1.1 km/h) whereby run-
ning mechanics, RE and perceived comfort were evaluated. 
Participants were then allowed 5 min to rest in a standing 
position prior to undertaking a fatiguing task that consisted 
of performing eight, 5 s sprints separated by 25 s of rest 
(Girard et al. 2020). Lastly, 2 min after the termination of 
the fatiguing task, participants repeated the 6 min run trial. 
The complete timing sequence from warm-up to finish was 
strictly controlled and guided by visual and verbal cues.
Footwear
During all running, the participants used neutral-like run-
ning shoes (Pearl Izumi N2v2, Colorado, US) with an 
average European shoe size of 43.6 ± 1.6, a stack height 
of 23–24 mm and a heel drop of 4 mm. The two pairs of 
CFOs used by participants were based on an individual non-
weight-bearing 3D scan of the foot using a Delcam iCube 
scanner (Elinvision, Karmėlava, Lithuania). CFOs were 
designed by a sport podiatrist with nearly 20 years of expe-
rience, using the Orthomodel Pro CAD software (Autodesk, 
California, USA). Briefly, scans were imported into the soft-
ware, markers were placed over the heel, first- and fifth met-
atarsal and medial arch. A base model surface was adjusted 
to match the contour of the foot using cross-sectional views 
from the heel to the forefoot. The thickness of the orthotic 
was arbitrary set to 8 mm in an attempt to maximize the 
potential of the TPU beats inside the  Infinergy® material 
(BASF, Ludwigshafen, Germany). All CFOs were direct-
milled out of EVA and TPU materials and manually fin-
ished to fit inside the shoes. Wear-in time between the first 
and second intervention session was 4.5 ± 2.5 days and 
4.6 ± 2.8 days between the second and last intervention ses-
sion. The mass of the three footwear conditions was on aver-
age 600.3 ± 32.0 g, 647.3 ± 36.0 g and 681.1 ± 35.7 g for the 
shoes with its original liners (CON), with the custom EVA 
orthoses (EVA) and with the custom TPU orthoses (TPU), 
respectively.
Running mechanics
An instrumented treadmill [ADAL3D-WR, Medical Devel-
opment—HEF Tecmachine, France; for details, see Belli 
et al. (2001)] was used for all running conditions. Briefly, 
it is mounted on a highly rigid metal frame, set at 0°grade 
incline, fixed to the ground through four piezoelectric force 
transducers (KI 9077b; Kistler, Winterthur, Switzerland) and 
installed on a specially engineered concrete slab to ensure 
maximal rigidity of the supporting ground (Girard et al 
2017). In this study, the treadmill function was switched 
to either constant speed mode (i.e., to measure the constant 
speed running pattern with direct ground reaction force 
measurement) or constant motor torque mode (i.e., to allow 
participants to perform sprints; Morin et al. 2010).
Over the last 2 minutes of each 6 min run, three-dimen-
sional ground reaction force was continuously sampled 
at 1000 Hz. Ten consecutive steps recorded after running 
for ~ 4 min 15 s, ~ 4 min 45 s, ~ 5 min 15 s and ~ 5 min 45 s 
were subsequently averaged for final analysis. After appro-
priate filtering (Butterworth-type 30 Hz low-pass filter), 
instantaneous data of vertical and antero-posterior ground 
reaction forces were averaged for each support phase when 
the vertical force was above 30 N. These data were deter-
mined by measurement of the main spatio-temporal varia-
bles: contact time (s), flight time (s) and step frequency (Hz) 
were reported. Peak braking and peak push-off forces (BW) 
along with duration of braking and push-off phases (s) were 
determined. Finally, average vertical loading rate (BW/s) 
was calculated as the mean value of the time-derivate of 
vertical force signal within the first 50 ms of the support 
phase (Li et al. 2020).
A linear spring-mass model paradigm was used to inves-
tigate the main mechanical integrative variables character-
izing the lower limb behavior during running (McMahon 
and Cheng 1990). Vertical stiffness (kN/m) was calculated 
as the ratio of peak vertical forces (N) to the maximal verti-
cal downward displacement of center of mass (m), which 
was determined by double integration of vertical accel-
eration of center of mass over time during ground con-
tact (Cavagna 1975). Leg stiffness (kN/m) was calculated 
as the ratio of peak vertical forces to the maximum leg 
spring compression [maximal vertical downward displace-
ment +  L0-√L0
2–(0.5 × running speed × contact time)2, in 
m], both occurring at mid-stance (Morin et al. 2005). Initial 
leg length  (L0, great trochanter to ground distance in a stand-
ing position) was determined from participant’s stature as 
 L0 = 0.53 × stature (Morin et al. 2005).
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1 3
Cardio‑respiratory variables
Expired gases were collected by a metabolic cart (Jeager™ 
Oxycon Mobile, Carefusion, Hoechberg, Germany). Prior 
to each session, calibration of gas sensor was completed 
for ambient air and a known gas mixture (16% oxygen, 
5% carbon dioxide). Turbine was calibrated using a 3 Liter 
(± 0.4%) syringe and automated high and low flow ventila-
tion. Breath-by-breath gas samples were first averaged every 
15 s and subsequently expressed as the average of the last 
2 minutes of each 6 min run. Oxygen uptake expressed in 
both absolute (mL/min) and relative (mL/kg/min) terms, 
minute ventilation (L/min), breathing frequency (breaths/
min), tidal volume (L) were determined. Heart rate (beats/
min) was continuously measured by short-range telemetry 
(Polar, Kempele, Finland). RE was calculated as the oxygen 
uptake per body mass over speed, expressed in milliliters of 
oxygen consumed per kilogram per kilometer (mL/kg/km). 
The metabolic cart was suspended from the ceiling next to 
participants, so they did not have to support the additional 
weight of the system when running.
Perceptual and comfort measures
Within the first minute after finishing low-speed and high-
speed runs, a global (6 min run) rating of perceived exertion 
value was collected using the 6–20 Borg scale. A modified 
version of the footwear comfort assessment tool, developed 
and tested on reliability by Mundermann (2002), was used 
to assess comfort associated with wearing each footwear 
condition using an iPad mini (Apple, California, US). This 
scale was used in previous studies to assess perceived com-
fort (McPoil et al. 2011; Burke and Papuga 2012). For this 
study, only six of the nine items (‘overall comfort’, ‘heel 
cushioning’, ‘forefoot cushioning’, ‘medio-lateral control’, 
‘arch height’ and ‘heel cup fit’) were scored on a digital, 
150 mm visual analogic scale where 0 was defined as ‘not 
comfortable at all’ and 150 ‘most comfortable condition 
imaginable’.
Statistical analysis
Values are presented as mean ± SD. Two-way repeated 
measures analysis of variance (ANOVAs) [Condition (CON, 
EVA, TPU) × Fatigue (fresh and fatigued)] were used to 
compare investigated variables. To assess assumptions of 
variance, Mauchly’s test of sphericity was performed using 
all ANOVA results. A Greenhouse–Geisser correction was 
performed to adjust the degree of freedom if an assumption 
was violated, while post hoc pairwise-comparisons with 
Bonferroni-adjusted P values were performed if a signifi-
cant main effect was observed. Partial eta-squared (ηp
2, with 
ηp
2 ≥ 0.06 representing a moderate effect and ηp
2 ≥ 0.14 a 
large effect) values were calculated. All statistical calcula-
tions were performed using SPSS statistical software V.26.0 
(IBM Corp., Armonk, USA). The significance level was set 
at P < 0.05.
Results
Running mechanics (Table 1)
There was a significant main condition effect for mean 
loading rate, push-off duration and push-off peak force (all 
P ≤ 0.017; 0.27 ≤ ηp
2 ≤ 0.47). A significant main fatigue 
effect was noted for four out of nine variables studied: 
flight time, vertical stiffness, as well as braking and push-
off durations (all P ≤ 0.032; 0.26 ≤ ηp
2 ≤ 0.32). There was 
no significant condition × fatigue [except mean loading 
rate (P = 0.046; ηp
2 = 0.19)] interactions for any stride 
mechanical variable.
Cardio‑respiratory variables (Table 2)
There was a significant main condition effect for heart rate 
(P = 0.027; ηp
2 = 0.19) only. All examined cardiorespira-
tory variables changed significantly from fresh to fatigued 
state (all P ≤ 0.021; 0.28 ≤ ηp
2 ≤ 0.83), except tidal volume 
(P = 0.507; ηp
2 = 0.03). Only minute ventilation (P = 0.020; 
ηp
2 = 0.21) and breathing frequency (P = 0.019; ηp
2 = 0.21) 
displayed significant condition × fatigue interactions.
Perceptual and Comfort measures (Table 3)
There was a significant main condition effect for 
medio-lateral control and arch height (all P ≤ 0.027; 
0.20 ≤ ηp
2 ≤ 0.30). Increased ratings of perceived exertion 
(P < 0.001; ηp
2 = 0.71) occurred under fatigue, while heel 
cushioning (P = 0.021; ηp
2 = 0.29) was also rated as more 
comfortable.
Discussion
Running mechanics
One strength of our study is that running mechanics dur-
ing constant submaximal runs were derived from direct 
ground reaction forces’ recording in both vertical and 
antero-posterior directions, as opposed to previous studies 
using tri-axial accelerometers to assess the effects of CFOs 
before and after an intense run (Lucas-Cuevas et al. 2014; 
Lucas-Cuevas et al. 2017). Lower mean loading rates were 
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1 3
recorded while wearing CFO made of TPU compared to 
EVA or no insert (CON), likely due to the higher resilience 
material properties (not measured). Contrary to the present 
study, previous studies observed lower loading rates in 
stiffer shoes (Henning et al. 1996; Milani et al. 1997). The 
often-used statement that, based upon their perception, 
runners adapt their technique with stiffer footwear to avoid 
high impact rates over the heel by cushioning the ground 
is not supported.
Biomechanical manifestation of fatigue resulting from 
completion of a repeated-sprint treadmill exercise was gen-
erally not modified by inserts. Out of nine biomechanical 
variables, only contact time, mean loading rate and brak-
ing phase duration displayed statistically significant con-
dition × fatigue interactions, with no systematic advantage 
of one or the other CFO condition. With fatigue, however, 
wearing CFOs (EVA or TPU) only generated subtle kinetic 
(mean loading rate: 1–2 BW/s) and kinematic (contact time 
and braking phase duration: 1–3 ms) adjustments compared 
to CON. It must therefore be questioned whether such small 
fatigue-related differences between footwear conditions are 
clinically relevant. Despite different resilience character-
istics but similar configuration of tested CFOs, our novel 
findings indicate that participants who wore CFOs, either 
Table 1  Changes in 
biomechanical parameters 
for shoe only (CON), shoe 
with Ethyl-Vinyl Acetate 
orthotic (EVA), and shoe with 
Thermoplastic Poly-Urethane 
orthotic (TPU) conditions 
before (Fresh) and after 
(Fatigued) the completion of 
a repeated-sprint treadmill 
exercise
Bold values indicate statistically significant ANOVA P values (P < 0.05)
Values are mean ± SD. C and F, respectively, refer to ANOVA main effects of condition and fatigue and 
interaction between these two factors with P value and partial eta-squared (η2) in parentheses. Bold values 
indicate statistically significant findings
† Significantly different from TPU, P < 0.05
*Significantly different from Fresh, P < 0.05
Variables
CON
EVA
TPU
ANOVA P value (η2)
C
F
C × F
Contact time (ms)
 Fresh
223 ± 17
225 ± 17
226 ± 17
0.121
0.332
0.404
 Fatigued
225 ± 18
225 ± 18
226 ± 17
(0.12)
(0.06)
(0.06)
Flight time (ms)
 Fresh
117 ± 18
116 ± 19
117 ± 17
0.354
0.026
0.571
 Fatigued
112 ± 18*
113 ± 17*
115 ± 17*
(0.06)
(0.27)
(0.03)
Step frequency (Hz)
 Fresh
2.97 ± 0.15
2.96 ± 0.14
2.94 ± 0.14
0.076
0.069
0.953
 Fatigued
2.95 ± 0.15
2.94 ± 0.12
2.94 ± 0.12
(0.15)
(0.19)
(0.01)
Mean loading rate (BW/s)
 Fresh
62.3 ± 13.8†
62.4 ± 14.0†
55.0 ± 10.6
 < 0.001
0.684
0.046
 Fatigued
60.4 ± 14.1†*
62.2 ± 14.6†
56.1 ± 12.3
(0.46)
(0.01)
(0.19)
Vertical stiffness (kN/m)
 Fresh
34.8 ± 4.5
35.5 ± 4.3
35.2 ± 4.6
0.617
0.014
0.878
 Fatigued
36.0 ± 5.3*
36.4 ± 4.5*
36.4 ± 5.0*
(0.02)
(0.32)
(0.01)
Leg stiffness (kN/m)
 Fresh
15.7 ± 2.5
15.6 ± 2.2
15.5 ± 2.2
0.88
0.788
0.152
 Fatigued
15.4 ± 2.1
15.5 ± 2.1
15.7 ± 1.9
(0.01)
(0.01)
(0.11)
Braking phase duration (ms)
 Fresh
109 ± 9
109 ± 8
110 ± 8
0.828
0.423
0.32
 Fatigued
110 ± 9
110 ± 8
109 ± 7
(0.01)
(0.040)
(0.07)
Push-off phase duration (ms)
 Fresh
114 ± 11†
115 ± 12
116 ± 12
0.003
0.309
0.368
 Fatigued
115 ± 12†
115 ± 13
117 ± 12
(0.31)
(0.06)
(0.06)
Peak braking force (kN)
 Fresh
0.59 ± 0.11
0.58 ± 0.13
0.60 ± 0.11
0.253
0.016
0.895
 Fatigued
0.58 ± 0.11*
0.57 ± 0.13*
0.59 ± 0.11*
(0.08)
(0.31)
(0.01)
Peak push-off force (kN)
 Fresh
0.41 ± 0.06†
0.39 ± 0.07
0.39 ± 0.06
0.017
0.032
0.867
 Fatigued
0.40 ± 0.06†*
0.39 ± 0.06*
0.38 ± 0.06*
(0.27)
(0.26)
(0.1)
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European Journal of Applied Physiology (2022) 122:1179–1187
1 3
made of EVA or TPU materials, produced essentially similar 
fatigue-induced adjustments in their stride pattern at con-
stant treadmill speed.
Inspection of changes in biomechanical variables induced 
by the repeated-sprint treadmill exercise indicates that glob-
ally running technique is not profoundly modified under 
acute intense fatigue. Regardless of footwear condition, 
the runners mainly reduced their flight time and increased 
their vertical stiffness to maintain treadmill speed constant, 
while the magnitude of these adjustments also did not dif-
fer between the two running speeds. Previously, both low 
(10 km/h) and high (20 km/h) constant speed running pat-
terns were found unchanged from before to ~ 3 min after 
repeated-sprint exercises [four sets of five, 6 s sprints with 
24 s recovery and 3 min between sets (Morin et al. 2012); 
three sets of five, 5 s sprints with 25 s recovery and 3 min 
between sets (Girard et al. 2017)] using the ADAL treadmill. 
Despite exacerbated cardio-respiratory and perceptual (rat-
ing of perceived exertion) responses occurring after sprint-
ing repeatedly, tested individuals may have not reached 
exertion levels where biomechanical manifestation of fatigue 
would cause more stressful ground impacts.
Cardio‑respiratory variables
As expected, physiological responses were elevated between 
before and after the fatiguing exercise. No significant inter-
action was found between fatigue and footwear condi-
tions for key cardio-respiratory variables (i.e., heart rate, 
oxygen uptake). This finding provides evidence that both 
types of CFOs (TPU and EVA materials) were not able 
to protect against fatigue-related deterioration in RE and/
or elevations in physiological strain compared to the shoe 
only (CON) condition. This may not be surprising since no 
fatigue-induced differences in mechanical variables could 
be detected across footwear conditions. Similarly, wearing 
CFOs that alters neuromuscular control during a submaxi-
mal 1 h treadmill run was found ineffective to reduce the 
aerobic cost of running (Kelly et al. 2011). In our study, 
the biomechanical effect of wearing CFOs with or without 
fatigue was probably too small to induce meaningful differ-
ences in physiological variables (expect minute ventilation 
that was lower in TPU compared to CON and to a lower 
extent EVA) across conditions.
Another interesting observation was that RE at a speed 
corresponding to the first ventilatory threshold deterio-
rated by ~ 1.7% with fatigue. Results from Day and Hahn 
Table 2  Changes in cardio-
respiratory parameters for shoe 
only (CON), shoe with Ethyl-
Vinyl Acetate orthotic (EVA), 
and shoe with Thermoplastic 
Poly-Urethane orthotic (TPU) 
conditions before (Fresh) and 
after (Fatigued) the completion 
of a repeated-sprint treadmill 
exercise
Bold values indicate statistically significant ANOVA P values (P < 0.05)
Values are mean ± SD. C and F, respectively, refer to ANOVA main effects of condition and fatigue and 
interaction between these two factors with P value and partial eta-squared (η2) in parentheses. Bold values 
indicate statistically significant findings
† Significantly different from TPU, P < 0.05
*Significantly different from Fresh, P < 0.05
Variables
CON
EVA
TPU
ANOVA P value (η2)
C
F
C × F
Heart rate (bpm)
 Fresh
164 ± 13†
163 ± 13
162 ± 13
0.027
 < 0.001
0.904
 Fatigued
172 ± 13†*
171 ± 12*
170 ± 13*
(0.19)
(0.60)
(0.01)
Running economy (mL/kg/km)
 Fresh
190 ± 11
185 ± 14
188 ± 11
0.077
0.020
0.746
 Fatigued
193 ± 12*
189 ± 14*
191 ± 13*
(0.14)
(0.28)
(0.02)
Oxygen uptake (mL/kg/min)
 Fresh
43.7 ± 4.0
42.5 ± 4.5
43.4 ± 5.0
0.087
0.021
0.796
 Fatigued
44.4 ± 4.8*
43.4 ± 4.5*
44.0 ± 5.5*
(0.13)
(0.28)
(0.02)
Minute ventilation (L/min)
 Fresh
101 ± 16
101 ± 16
100 ± 15
0.058
 < 0.001
0.020
 Fatigued
119 ± 20†*
116 ± 21†*
112 ± 19*
(0.15)
(0.71)
(0.21)
Breathing frequency (breath/min)
 Fresh
42.6 ± 6.8
44.9 ± 10.7
44.5 ± 8.6
0.496
 < 0.001
0.019
 Fatigued
50.9 ± 8.4†*
50.2 ± 8.9*
49.0 ± 8.3*
(0.03)
(0.83)
(0.21)
Tidal volume (L)
 Fresh
2.39 ± 0.43
2.31 ± 0.57
2.30 ± 0.48
(0.204)
0.507
0.420
 Fatigued
2.37 ± 0.50
2.35 ± 0.56
2.33 ± 0.51
(0.09)
(0.03)
(0.05)
1185
European Journal of Applied Physiology (2022) 122:1179–1187 
1 3
(2019) suggest that optimal footwear longitudinal bend-
ing stiffness to improve RE in fresh conditions is speed 
dependent. Whereas most participants running at 14 km/h 
elicited a minimum metabolic rate in the normal shoe, an 
increased number of participants were more economical in 
the stiff shoe (despite it weighing an extra 50 g compared 
to the normal shoe) at 17 km/h. In our study, EVA and TPU 
were ~ 50 g (+ 8%) and ~ 80 g (+ 14%) heavier compared to 
CON, respectively. The relationship between shoe mass and 
energy cost suggests that energy demands during running 
are greater as shoe mass is increased (i.e., + 1% for every 
added 100 g per shoe; Frederick 1984). Consequently, we 
cannot exclude that the additional weight of inserts may 
have confounded any protective effect of CFOs on RE when 
fatigue sets in. While our approach did not account for dif-
ferent footwear mass, imposing running speeds relative to 
our participants’ physiological capacity rather than absolute 
speeds, as commonly done in the CFO-related literature on 
RE (Crago et al. 2019), was a strength.
Comfort measures
Significantly improved perceived comfort for medio-lateral 
control (∼20%) and arch height (∼25%) was reported for 
both EVA and TPU, yet with no difference between the two 
inserts, compared to CON. Despite statistical significance 
was not reached, other perceived comfort-related metrics 
(heel and forefoot cushioning, heel cup fit) displayed simi-
lar trends. However, the presumably greater levels of rigid-
ity of the sole of the EVA insert did not generate greater 
discomfort for the runners, also with similar or improved 
(e.g., lower loading rates) running biomechanics. Overall, 
in line with previous research in fresh conditions (Lindorfer 
et al. 2020), increase in comfort at the foot/shoe interface 
did not lead to improved RE and meaningful changes in 
biomechanical variables. Perhaps functional biomechanical 
variables that were not measured in this study (i.e., plantar 
pressure distribution, muscle activity, ankle and knee joint 
moments), known to be influenced by perceived comfort, 
Table 3  Changes in rating of 
perceived exertion (RPE) and 
comfort parameters for shoe 
only (CON), shoe with Ethyl-
Vinyl Acetate orthotic (EVA), 
and shoe with Thermoplastic 
Poly-Urethane orthotic (TPU) 
conditions before (Fresh) and 
after (Fatigued) the completion 
of a repeated-sprint treadmill 
exercise
Bold values indicate statistically significant ANOVA P values (P < 0.05)
RPE was assessed using a 6–20 Borg scale and other comfort parameters were measures using a Visual 
Analog Scale (0–150 mm); Values are mean ± SD. Values are mean ± SD. C and F, respectively, refer to 
ANOVA main effects of condition and fatigue and interaction between these two factors with P value and 
partial eta-squared (η2) in parentheses. Bold values indicate statistically significant findings
# Significantly different from EVA, P < 0.05
† Significantly different from TPU, P < 0.05
*Significantly different from Fresh, P < 0.05
Variables
CON
EVA
TPU
ANOVA P value (η2)
C
F
C × F
RPE
 Fresh
12.7 ± 3.1
13.2 ± 3.1
13.0 ± 3.0
0.256
 < 0.001
0.738
 Fatigued
14.7 ± 4.2*
15.1 ± 3.1*
13.7 ± 3.1*
(0.08)
(0.71)
(0.01)
Overall comfort
 Fresh
86 ± 32
93 ± 31
97 ± 25
0.089
0.163
0.121
 Fatigued
81 ± 31
105 ± 21
101 ± 23
(0.15)
(0.12)
(0.14)
Heel cushioning
 Fresh
83 ± 33
96 ± 23
89 ± 25
0.090
0.021
0.813
 Fatigued
85 ± 27*
102 ± 22*
93 ± 26*
(0.15)
(0.29)
(0.01)
Forefoot cushioning
 Fresh
88 ± 34
96 ± 28
102 ± 23
0.079
0.326
0.086
 Fatigued
84 ± 29
105 ± 24
104 ± 22
(0.16)
(0.06)
(0.16)
Medio-lateral control
 Fresh
83 ±  32#†
98 ± 26
101 ± 25
0.027
0.080
0.361
 Fatigued
84 ±  30#†
107 ± 22
100 ± 24
(0.20)
(0.18)
(0.06)
Arch height
 Fresh
74 ±  36#†
92 ± 33
96 ± 24
0.009
0.346
0.060
 Fatigued
72 ±  32#†
102 ± 28
95 ± 26
(0.30)
(0.06)
(0.18)
Heel cup fit
 Fresh
86 ± 29
95 ± 23
88 ± 29
0.135
0.206
0.416
 Fatigued
85 ± 29
102 ± 22
93 ± 26
(0.12)
(0.10)
(0.05)
1186
 
European Journal of Applied Physiology (2022) 122:1179–1187
1 3
could explain observed differences between footwear con-
ditions (Dinato et al. 2015). While ground reaction forces 
are commonly (similar to current approach) used as proxy 
measurements to reflect biomechanical loads imposed on the 
lower extremities as a whole, directly quantifying tissue and/
or structure-specific strain (i.e., longitudinal arch, Achilles’ 
tendon) remains a challenge (Verheul et al. 2020).
Unexpectedly, perceived comfort ratings in different 
regions of the foot were in fact improved after completion 
of the repeated-sprint treadmill exercise. While the condi-
tion × fatigue interaction was not significant, there was a 
trend for the two CFOs conditions to become more com-
fortable in the fatigued state compared to CON. Contrast-
ingly, during a 13 km run, decrement in perceived overall 
footwear comfort became significant only after 44 min of 
exercise (~ 7.8 km) (Hintzy et al. 2015). Discrepant findings 
between our results (i.e., assessment before and after a short 
and intense fatigue protocol) and previous studies (i.e., regu-
lar assessments during prolonged running at lower inten-
sity; Hintzy et al. 2015; Jimenez-Perez et al. 2021) could be 
explained by the methodology used for measuring biome-
chanical manifestation of fatigue and the nature/degree of 
fatigue attained by participants. The clinical implication of 
our findings is that well-trained runners wearing CFOs made 
of either EVA or TPU materials should not fear deteriorated 
comfort ratings with acute intense fatigue.
Limitations
Several limitations must be considered. First, the inclusion 
of only male runners who were mainly habitual rear-foot 
strikers (~ 70%). Our findings may not be generalizable to 
runners with habitual midfoot/forefoot strike patterns and 
female population since biomechanical variables may dif-
fer across various foot strikes and between genders (Moore 
2016). Whereas the foot strike pattern of tested athletes was 
determined, our sample size of 18 participants (with only 
one and four forefoot and midfoot strikers) was too small 
to allow meaningful comparisons between groups. Addi-
tionally, to reflect ecological situations, participants should 
undertake over-ground runs with their foot strikes recorded 
by a number of force plates laid in series. Because inher-
ent characteristics of running shoes per se can alter running 
mechanics and/or RE (Hoogkamer et al. 2018), participants 
were not allowed to use their own running shoes. Standardi-
zation of footwear conditions across participants (also with 
the use of personalized inserts) is a strength of our study 
from a methodological standpoint. Nonetheless, one could 
speculate that any protective effect of fatigue may have more 
apparent if individuals were wearing their habitual footwear 
or other types of foot orthoses.
Conclusion
Acute intense fatigue does not modify the effect of custom 
foot orthoses with different resilience characteristics (EVA 
or TPU materials both compared to standardized footwear) 
on running mechanics, running economy and perceived 
comfort. When facing acute intense fatigue, well-trained 
runners should not expect any protective effects from wear-
ing CFOs.
Acknowledgements The authors also thank Pr. Jean-Benoit Morin 
from the Université of Lyon, Saint Étienne, France, for his comment 
on our draft and help in providing the running mechanics data process-
ing custom software.
Author contributions OG and KVA conceived and designed research. 
OG and KVA conducted experiments. All authors analyzed data and 
interpreted results of experiments. OG and KVA drafted manuscript 
and prepared tables. All authors edited and revised manuscript. All 
authors approved final version of manuscript.
Funding Open Access funding provided by the Qatar National Library. 
Data were collected using an instrumented treadmill funded by a QNRF 
grant (NPRP 4–760-3–217).
Declarations 
Conflict of interest No potential conflict of interest was reported by 
the authors.
Open Access This article is licensed under a Creative Commons Attri-
bution 4.0 International License, which permits use, sharing, adapta-
tion, distribution and reproduction in any medium or format, as long 
as you give appropriate credit to the original author(s) and the source, 
provide a link to the Creative Commons licence, and indicate if changes 
were made. The images or other third party material in this article are 
included in the article's Creative Commons licence, unless indicated 
otherwise in a credit line to the material. If material is not included in 
the article's Creative Commons licence and your intended use is not 
permitted by statutory regulation or exceeds the permitted use, you will 
need to obtain permission directly from the copyright holder. To view a 
copy of this licence, visit http:// creat iveco mmons. org/ licen ses/ by/4. 0/.
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Acute intense fatigue does not modify the effect of EVA and TPU custom foot orthoses on running mechanics, running economy and perceived comfort.
Van Alsenoy, Ken,Ryu, Joong Hyun,Girard, Olivier