RESEARCH ARTICLE
The effect of changing foot progression angle
using real-time visual feedback on rearfoot
eversion during running
Seyed Hamed MousaviID1,2,3*, Laurens van Kouwenhove1, Reza Rajabi2,
Johannes Zwerver3,4, Juha M. Hijmans1
1 Department of Rehabilitation Medicine, University Medical Center Groningen, University of Groningen,
Groningen, The Netherlands, 2 Department of Health and Sport Medicine, Faculty of Physical Education and
Sport Sciences, University of Tehran, Tehran, Iran, 3 Center for Human Movement Science, University
Medical Center Groningen, University of Groningen, Groningen, The Netherlands, 4 Department of Sports
Medicine, Gelderse Vallei Hospital, Ede, The Netherlands
* s.h.mousavi@umcg.nl, mousavihamed84@yahoo.com
Abstract
Atypical rearfoot in/eversion may be an important risk factor for running-related injuries.
Prominent interventions for atypical rearfoot eversion include foot orthoses, footwear, and
taping but a modification derived from gait retraining to correct atypical rearfoot in/eversion
is lacking. We aimed to investigate changes in rearfoot in/eversion, subtalar pronation,
medial longitudinal arch angle, and selected lower limb joint biomechanics while performing
toe-in/toe-out running using real-time visual feedback. Fifteen female runners participated in
this study. Subjects performed toe-in/toe-out running using real-time visual feedback on foot
progression angle, which was set ±5˚ from habitual foot progression angle. 3D kinematics of
rearfoot in/eversion, subtalar supination/pronation, medial longitudinal arch angle, foot pro-
gression angle, hip flexion, ab/adduction and internal/external rotation, knee flexion, ankle
dorsiflexion, and ankle power were analyzed. A repeated-measures ANOVA followed by
pairwise comparisons was used to analyze changes between three conditions. Toe-in run-
ning compared to normal and toe-out running reduced peak rearfoot eversion (mean differ-
ence (MD) with normal = 2.1˚; p<0.001, MD with toe-out = 3.5˚; p<0.001), peak pronation
(MD with normal = -2.0˚; p<0.001, MD with toe-out = -3.4; p = <0.001), and peak medial lon-
gitudinal arch angle (MD with normal = -0.7˚; p = 0.022, MD with toe-out = -0.9; p = 0.005).
Toe-out running significantly increased these kinematic factors compared to normal and
toe-in running. Toe-in running compared to normal running increased peak hip internal rota-
tion (MD = 2.3; p<0.001), and reduced peak knee flexion (MD = 1.3; p = 0.014). Toe-out run-
ning compared to normal running reduced peak hip internal rotation (MD = 2.5; p<0.001),
peak hip ab/adduction (MD = 2.5; p<0.001), peak knee flexion (MD = 1.5; p = 0.003), peak
ankle dorsiflexion (MD = 1.6; p<0.001), and peak ankle power (MD = 1.3; p = 0.001). Run-
ners were able to change their foot progression angle when receiving real-time visual feed-
back for foot progression angle. Toe-in/toe-out running altered rearfoot kinematics and
medial longitudinal arch angle, therefore supporting the potential value of gait retraining
focused on foot progression angle using real-time visual feedback when atypical rearfoot in/
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February 10, 2021
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OPEN ACCESS
Citation: Mousavi SH, van Kouwenhove L, Rajabi
R, Zwerver J, Hijmans JM (2021) The effect of
changing foot progression angle using real-time
visual feedback on rearfoot eversion during
running. PLoS ONE 16(2): e0246425. https://doi.
org/10.1371/journal.pone.0246425
Editor: Nizam Uddin Ahamed, University of
Pittsburgh, UNITED STATES
Received: March 4, 2020
Accepted: January 19, 2021
Published: February 10, 2021
Copyright: © 2021 Mousavi et al. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Data Availability Statement: All relevant data are
within the manuscript and its Supporting
Information files.
Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
eversion needs to be modified. It should be considered that changes in foot progression
angle when running is accompanied by changes in lower limb joint biomechanics.
Introduction
Running-related injuries (RRIs) are very common in athletes; sports clinicians are frequently
consulted about these injuries [1]. Abnormal kinematics as intrinsic risk factors are considered
to have an important role in the high incidence of RRIs [2]. Rearfoot eversion is among the
most commonly reported kinematic factors in the studies investigating foot function and/or
risk factors for lower-limb injuries [2–4]. As the subtalar coordination axis is not aligned with
the foot coordination axes, and besides, no anatomical landmark exists on the talus, rearfoot
eversion is predominantly measured as a surrogate to describe subtalar pronation [5, 6]. Much
debate exists on whether atypical rearfoot eversion contributes to injury [4, 7]. This is mainly
because most studies investigating rearfoot eversion for RRIs have either a case-control or a
cross-sectional design that cannot prove causality; the results of prospective studies are mainly
based on a small sample size [8]. There are several studies reporting that rearfoot eversion may
be a potential risk factor for RRIs [2, 9, 10]. In a recent systematic review [2] we showed that
peak rearfoot eversion may be associated with iliotibial band syndrome, patellar tendinopathy,
and posterior tibial tendon dysfunction in runners. Female runners with atypical rearfoot ever-
sion may be more prone to RRIs, specifically female runners with tibial stress fracture showed
greater peak rearfoot eversion [11] and female runners with iliotibial band syndrome showed
lower peak rearfoot eversion compared to non-injured runners [2, 12].
Atypical medial longitudinal arch angle (MLAA) is another contributing factor predisposing
athletes to musculoskeletal overuse injuries [13]. MLAA collapse causes the calcaneus to evert in
relation to the tibia, resulting in rearfoot eversion [14]. Normal rearfoot eversion and MLAA are
essential for optimal shock absorption of the foot during the stance phase of gait. Atypical rearfoot
eversion and MLAA may influence: 1. distribution of weight through the lower extremity, increas-
ing force to the medial aspects of the foot and reducing shock absorption and postural balance
abilities; and 2. alignment of the lower-limb kinematic chain, such as knee and hip mechanics
[15]. For instance, excessive rearfoot eversion may result in excessive tibia internal rotation and
hip internal rotation. This can result in faulty knee flexion/extension biomechanics which in turn
produce a compensatory reaction in the tibiofemoral joint and may subsequently lead to patellofe-
moral symptoms [16]. Reduced shock absorption and malalignment are considered to play an
important role in the development of running-related overuse injuries of the lower extremity [17].
In recent decades there has been increasing clinical and scientific interest in modifying
atypical rearfoot eversion in order to prevent or manage RRI [18, 19]. External supports such
as foot orthoses, motion control shoes, and therapeutic adhesive taping are the most common
interventions studied to reduce excessive rearfoot eversion [20–22]. These are widely pre-
scribed to realign or correct atypical rearfoot eversion. It is reported that foot orthoses may
cause dependency and long-term negative psychological effects [23]. Most importantly, their
effectiveness remains controversial [18]. So far, only a few studies have investigated the effect
of a training program on excessive rearfoot eversion [24–28], showing sensory-motor training
to be superior to either foot orthoses [26] or taping for correcting excessive rearfoot eversion
[28]. More research into sports-related functional training interventions that modify rearfoot
eversion is thus warranted.
Gait retraining is an increasingly used biomechanical modification intervention to practice
a movement task [25, 29]. Gait retraining applying real-time feedback from an instrumented
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treadmill and/or motion capture through a projection and sound system is a novel way to
induce motor strategies to alter movement patterns. Data obtained by tracking the biomechan-
ics of the body are used to produce real-time feedback. Likewise, feedback modalities such as
auditory and/or visual cues are provided for a training task to practice it at a given point/posi-
tion. As these feedback modalities are given for any steps and individuals can perceive their
own performance in the real time, this may help perform the training task more effectively
than verbal feedback given by a coach or clinician. Step rate, step width, step length, foot strike,
and hip adduction are the most common gait parameters on which virtual reality feedback is
currently given to modify other running-related biomechanical risk factors [30–33]. These
studies report successful findings for modifying biomechanical risk factors using the afore-
mentioned parameters. Moreover, real-time visual feedback of kinematics or kinetics was
reported as the most successful strategy to reduce high-risk factors for RRIs [31]. Therefore,
gait retraining may be a useful approach to alter rearfoot biomechanics during running.
Previous studies show that rearfoot motion and MLAA might be influenced by changing
foot progression angle (FPA) during running and walking [34–36]. In addition, FPA (foot
abduction) is postulated as having a positive association with subtalar eversion because it is
one of the subtalar pronation movement components [6, 37]. The assumption is that while
running toe-out or toe-in, rearfoot kinematics are changed to more or less eversion, respec-
tively. As inconsistent results have been reported about the effect/association of FPA on/with
rearfoot eversion [34, 38–40], well-designed studies to investigate the effects of changing FPA
on rearfoot kinematics are needed. Several studies have intervened FPA during walking using
gait retraining to improve pain or reduce knee adduction moment, a contributing factor to
knee osteoarthritis [41–43]. Nevertheless, it is unknown whether toe-in or toe-out positioning
during running affects frontal plane rearfoot motion. Accordingly, it is postulated that running
retraining with changing FPA may be useful to correct atypical rearfoot kinematics.
The main aim of this study is to investigate changes in rearfoot in/eversion while perform-
ing toe-in/toe-out running using real-time visual feedback. The secondary aim is to investigate
changes in subtalar pronation/supination, MLAA, hip flexion, ab/adduction and internal/
external rotation, knee flexion, ankle dorsiflexion, and ankle power while performing toe-out/
in running. We hypothesized that toe-in running reduces peak rearfoot eversion, subtalar pro-
nation, and MLAA, and toe-out running increases these factors relative to a natural FPA.
From a clinical perspective, the findings might be helpful toward controlling atypical rearfoot
kinematics when managing RRI.
Methods
Study design
This is a cross-sectional pilot study conducted to determine the feasibility and effects of chang-
ing FPA using real-time visual feedback on rearfoot in/eversion, subtalar supination/prona-
tion, and MLAA.
Setting
Data were collected at the Motion Lab of the Center for Rehabilitation, University Medical
Center Groningen between January 2019 and April 2019.
Participants
Seventeen female runners recruited by our advertisements and social media from the Univer-
sity of Groningen and local running clubs volunteered to participate. Inclusion criteria were:
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female, aged 18–40, minimum 1 year running experience, training distance >10 km/week,
habitual rearfoot striker, free of self-reported lower-limb injuries or pain during the last 6
months, no musculoskeletal disorders and/or pain, no foot medial arch disorder determined
using the navicular drop test, and no atypical static rearfoot in/eversion [44] prior to data col-
lection. Two volunteers were excluded who had flat foot and/or excessive static rearfoot ever-
sion. Fifteen volunteers who met the inclusion criteria participated in this study. Ethical
approval was obtained through the local Medical Ethics Committee (METc 2018/086) of Uni-
versity Medical Center Groningen. Subjects signed an informed consent form and completed
a self-developed questionnaire for demographic information prior to data collection.
Instrumentation
Running assessments were performed on an instrumented split-belt treadmill with two inte-
grated force plates of the Gait Real-time Analysis Interactive Lab (GRAIL) system (Motekforce
Link, The Netherlands) [45]. Ground reaction force (GRF) signals were recorded at 1000Hz,
combined with a 10-camera integrated motion capture system (Vicon Bonita 10; Vicon
Motion Systems, Oxford, UK) and further processed to kinematic and kinetic variables in
D-Flow (Version 3.28; Motekforce Link, The Netherlands) at a 100 Hz sampling frequency.
Real-time filtering of the marker data was performed with a low-pass second-order zero phase
Butterworth filter with a 6 Hz cut-off frequency.
Marker placement
Thirty-four markers were attached to the subject’s body by the same investigator (SHM). Of
these, 26 markers were attached according to the human body model 2 (HBM2) (Fig 1) [29]
and 8 markers were attached to both feet at the first metatarsal head, navicular bone tuberosity,
medial side of calcaneus and posterior part of calcaneus for measuring rearfoot in/eversion
and MLAA. Two markers attached along the vertical bisection of the heel counter and the
Fig 1. Marker placement. Twenty-six markers were attached to the body according to the HBM model; 8 markers
were attached to the feet, to be used for calculating rearfoot eversion and MLAA.
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marker on the medial aspect of the heel counter were considered for the rearfoot segment.
Additionally, four holes were cut in the shoes: at the first metatarsal head, navicular bone
tuberosity, medial side and posterior part of calcaneus (Fig 1). The holes were cut to uncover
these aspects of the foot in order to attach markers directly to the skin, allowing measurement
of foot movement and not shoe movement. All subjects wore the same brand of neutral shoes
(Dr Comfort, refresh, USA) with the same neutral insole.
Baseline measurement
To set a certain running speed for giving real-time feedback and generalize it to all runners,
treadmill speed was set at 8 km/h for all conditions. After a 5-minute warm-up period, a
20-second baseline dataset was collected containing at least 20 strides. Baseline FPA was calcu-
lated using the average FPA in midstance (the value in 50% of stance phase) for the first 20
strides. FPA was considered the angle between the line connecting the markers on the calca-
neus and second metatarsal head with the longitudinal axis of the treadmill.
Feedback
A custom-made application was developed on the D-flow software to produce real-time
feedback for toeing-out and toeing-in FPA. A clock with a red pointer set to FPA (degrees)
was designed to reflect FPA during midstance in real time (Fig 2). A 5˚ target range was
shown on the clock (green, Fig 2). To perform toeing-out FPA, the target range was set at 5˚
more than the baseline FPA average with an area of ±2.5˚ deviation from this point. Like-
wise, to perform toeing-in FPA, the target range was set at 5˚ less than the baseline FPA
average with an area of ±2.5˚ deviation from this point. The 5˚ deviation from the baseline
FPA was selected based on pilot testing. In the pilot testing, five runners were asked to run
with various FPA (±5˚, ±10˚, and ±15˚) relative to their baseline FPA. The 5˚ deviation
from the baseline FPA (±5˚ FPA) was the only FPA that all runners ran with no difficulty.
Prior to the feedback session, subjects were asked to practice the conditions with their dom-
inant foot in the standing position to become familiar with the feedback. When the red
pointer was located within the given target range, the area became green (positive feed-
back)–otherwise it became red (negative feedback). The red pointer was fixed on FPA in
midstance and updated on each step. Before doing each task, subjects had a 2-min running
with FPA feedback. An extra minute was allowed if needed. Subjects were then asked to
run, and after 1-min running a 20-second dataset was collected. The order of the experi-
mental tasks was randomized.
Outcome measurement
The rearfoot segment coordinate system was established according to International Society
of Biomechanics (ISB) recommendations [46] and calculated as rotation of local calcaneus
coordination system relative to the fixed laboratory coordinate system using the rotation
sequence defined by ISB. Subtalar pronation/supination angle was calculated using the
Isman and Inman method [5], with the detailed explanation described in the study of van
den Bogert et al. [29]. Because the pronation/supination axis is not aligned with the foot
coordinate axes, it is defined in the foot reference frame using the average subtalar joint.
MLAA was calculated based on the angle formed between three markers: first metatarsal
head, navicular bone tuberosity and medial aspect of calcaneus. Hip, knee and ankle kine-
matics and ankle power are standard measures of HBM2 computed as explained by van
den Bogert et al. [29]. GRF data were used to identify the stance phase, with a threshold of
10N vertical GRF for touchdown and toe-off. Kinematic and GRF data were filtered using
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low-pass zero phase second-order Butterworth filters with the same 6Hz cutoff frequency.
Outcomes of ten consecutive steps following the given FPA were calculated and averaged
within subjects before being averaged within conditions. Kinematic data during the stance
phase were time-normalized to 100% of stance phase. Peak angles were explained as the
maximum angle during the stance phase. Timing of peak angles was expressed as percent-
age of the stance phase. Angle excursions were expressed as range of motion from touch-
down to peak angle. A custom MATLAB script (Version R2018a, Natick, MA, USA) was
used to analyze data.
Fig 2. Picture representing real-time visual feedback for changing FPA. The training process: Real-time visual feedback is provided to the subject via the big
screen. The red pointer represents the FPA of the right foot that is fixed in the midstance (50% stance phase) and updated on each step. The target area is a
wedge with a 5˚ range, with its middle point specifying the subject’s normal FPA +5˚ for toe-out and -5˚ for toe-in. The aim is to turn the target area green
(positive feedback) by keeping the red pointer (FPA) inside the target area. If the red pointer leaves the target area, the target area turns red (negative feedback).
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Statistical analysis
Data were analyzed using IBM SPSS version 23 (IBM Corp., Armonk, NY, USA). The normal-
ity of data was checked by Shapiro-Wilk tests and QQ plots. A repeated-measures one-way
ANOVA was performed for each outcome to identify statistically significant differences
between conditions: baseline, toeing-out, and toeing-in trials. Significant main effects were fol-
lowed up using pairwise comparisons with Bonferroni adjustment. Significance level (α) was
set at 0.05.
Results
Table 1 shows participants’ characteristics. All assumptions for repeated-measures ANOVA
were met (no significant outliers, normal distribution, and sphericity). Table 2 shows results of
one-way repeated-measures ANOVA analysis for all measured variables; Fig 3 shows the
ensemble average curves of measured variables. The average FPA in midstance for normal,
toe-out and toe-in running were -6.2˚, -10.4˚, and -1.8˚, respectively; the average individual
standard deviations were 1.1˚, 1˚ and 0.9˚, respectively.
We found significant main effects of FPA conditions on peak rearfoot eversion (p<0.001),
rearfoot eversion at touchdown (p = 0.001), and rearfoot eversion excursion (p<0.001). There
was no significant main effect of FPA conditions on time to peak rearfoot eversion (p = 0.462).
Post-hoc tests showed a significant difference in peak rearfoot eversion between normal and
toe-out (mean difference (MD) = 1.4; p<0.001), between normal and toe-in (MD = -2.1;
p<0.001), and between toe-out and toe-in (MD = -3.5; p<0.001). Post-hoc tests for rearfoot
eversion excursion showed a significant difference between normal and toe-in (MD = -1.4;
p = 0.001), and between toe-out and toe-in (MD = 1.9; p = 0.001). There was no significant dif-
ference in rearfoot eversion excursion between normal and toe-out (MD = -0.5; p = 0.1). Post-
hoc tests for rearfoot eversion at touchdown showed significant differences between normal
and toe-out (MD = 1.1; p = 0.037), and between toe-out and toe-in (MD = -2.0; p = 0.011).
We found significant main effects of FPA conditions on peak pronation (p<0.001), supina-
tion/pronation at touchdown (p<0.001), time to peak pronation (p = 0.019), and pronation
excursion (p = 0.042). Pairwise comparisons showed a significant difference in peak pronation
between normal and toe-out (MD = -1.4; p = 0.002), between normal and toe-in (MD = 2;
p<0.001), and between toe-out and toe-in (MD = 3.4; p<0.001). Post-hoc tests for pronation
at touchdown showed a significant difference between normal and toe-out (MD = -1.4;
p = 0.015), between normal and toe-in (MD = 1.2; p = 0.004), and between toe-out and toe-in
(MD = 2.6; p<0.001). Post-hoc tests showed a significant difference in time to peak pronation
between toe-out and toe-in (MD = 2.5; p = 0.035). There were no significant differences in
time to peak pronation between normal and toe-in (MD = 0.5; p = 0.999) and between normal
and toe-out (MD = -2.1; p = 0.219).
We found significant main effects of FPA conditions on peak MLAA (p = 0.001), time to
peak MLAA (p = 0.04), and MLAA excursion (p<0.001). There was no significant main effect
Table 1. Participants characteristics.
Variable
Mean (SD)
Range
Age, y
27.5 (6.3)
21–40
Height, cm
170 (5)
164–182
Weight, kg
61.4 (6.1)
50–72
Running experience, y
6.3 (4.4)
2–17
Weekly distance, km
32.7 (17.4)
10–65
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of FPA conditions on MLAA at touchdown (p = 0.816). Bonferroni post-hoc tests showed a
significant difference in peak MLAA between normal and toe-in (MD = 0.7; p = 0.022), and
between toe-out and toe-in (MD = 0.9; p = 0.005). There was no significant difference in peak
MLAA between normal and toe-out (MD = -0.2; p = 0.876). Post-hoc tests also showed a sig-
nificant difference in time to peak MLAA between normal and toe-out (MD = -2.6; p = 0.033)
Table 2. Statistical results of the all variablesa for each FPA condition.
Variable
FPA condition
One-way repeated measures analysis
Normal FPA
Toe-out 5˚
Toe-in 5˚
F-value
P-value
Eta squared
Foot progression angle (°)
-6.2 (3.1)
-10.4 (3.2) †
-1.9 (3.2) ‡
444.66
< 0.001
0.97
Average individual SD for FPA(°)
1.1 (0.3)
1.0 (0.2)
0.9 (0.2)
1.77
0.189
0.11
Peak rearfoot eversion (˚)
-8.5 (2.2)
-9.9 (2.6) †
-6.4 (2.2) ‡
103.16
< 0.001
0.88
Time to peak rearfoot eversion (% stance)
46.3 (2.7)
47.3 (3.0)
47.3 (3.7)
0.76
0.478
0.05
Rearfoot eversion at touchdown (˚)
3.2 (2.1)
2.1 (2.1) †
4.2 (2.3)
9.51
0.001
0.41
Rearfoot eversion excursion (˚)
-11.5 (3.3)
-12.0 (3.6) †
-10.1 (3.2) ‡
20.46
< 0.001
0.59
Peak pronation (˚)
4.4 (4.5)
5.8 (4.5) †
2.4 (4.6) ‡
66.36
< 0.001
0.83
Time to peak pronation (% stance)
70.1 (15.2)
72.1 (17.3) †
69.6 (16)
4.56
0.019
0.25
Supination/pronation at touchdown (˚)
-2.3 (4.9)
-1 (4.8) †
-3.5 (4.9) ‡
24.69
< 0.001
0.64
Pronation excursion (˚)
6.7 (4.2)
6.8 (3.4)
5.9 (4) ‡
3.57
0.042
0.20
Peak MLAA (˚)
6.2 (2.2)
6.4 (2.2) †
5.5 (2.3) ‡
9.7
0.001
0.41
Time to peak MLAA (% stance)
54 (8.1)
56.6 (7.1) 
54.1 (9)
3.61
0.040
0.21
MLAA at touchdown (˚)
-1.0 (2.3)
-1 (2.5)
-0.9 (2.7)
0.21
0.816
0.01
MLAA excursion (˚)
7.2 (1.7)
7.4 (1.7) †
6.5 (1.7) ‡
13.02
< 0.001
0.48
Peak hip internal rotation
7.6 (4.8)
5.1 (5.2) †
9.8 (5.1) ‡
71.28
< 0.001
0.84
Time to peak hip internal rotation (% stance)
58.3 (38.8)
59.9 (35.7)
51.9 (39.6)
1.70
0.214
0.11
Hip internal rotation at touchdown (˚)
4.5 (5.7)
1.6 (5.9) †
7.0 (5.8) ‡
82.25
< 0.001
0.86
Hip internal rotation excursion (˚)
3.0 (2.7)
3.5 (2.5)
2.8 (3.0)
1.21
0.305
0.08
Peak hip ab/adduction
14.0 (3.6)
11.6 (3.4) †
13.0 (4.6)
12.38
< 0.001
0.47
Time to peak hip ab/adduction (% stance)
43.2 (3.6)
43.4 (4.0)
45.7 (5.5)
4.02
0.055
0.22
Hip ab/adduction at touchdown (˚)
7.1 (2.6)
6.3 (2.4)
6.0 (3.3)
3.06
0.063
0.18
Hip ab/adduction excursion (˚)
6.9 (2.5)
5.3 (2.3) †
7.0 (2.9)
28.45
< 0.001
0.67
Peak hip flexion
36.8 (5.5)
37.1 (5.6)
36.5 (5.2)
0.80
0.455
0.05
Time to peak hip flexion (% stance)
15.1 (16.0)
15.4 (16.3)
14.0 (16.9)
0.51
0.494
0.04
Hip flexion at touchdown (˚)
35.3 (4.9)
35.8 (5.1)
35.4 (4.8)
0.62
0.546
0.04
Hip flexion excursion (˚)
1.5 (2.2)
1.3 (1.9)
1.1 (1.6)
1.49
0.245
0.10
Peak knee flexion
41.5 (4.5)
40.0 (4.6) 
40.2 (4.9) ‡
8.81
0.001
0.39
Time to peak knee flexion (% stance)
48.1 (1.9)
48.1 (2.5)
48.6 (1.4)
0.83
0.445
0.06
Knee flexion at touchdown (˚)
9.3 (4.2)
9.9 (4.2)
10.4 (3.8)
1.83
0.179
0.11
Knee flexion excursion (˚)
32.1 (5.5)
30.2 (4.3) 
30.0 (4.7) ‡
11.46
< 0.001
0.45
Peak ankle dorsiflexion
24.0 (3.7)
22.4 (3.6) †
23.4 (3.7)
10.30
< 0.001
0.42
Time to peak ankle dorsiflexion (% stance)
56.9 (2.1)
56.8 (2.5)
57.1 (2.3)
0.22
0.807
0.02
Ankle dorsiflexion at touchdown (˚)
5.3 (3.9)
5.4 (2.9)
5.3 (3.7)
0.08
0.927
0.01
Ankle dorsiflexion excursion (˚)
18.7 (2.5)
17.0 (2.7) 
18.0 (3.1)
5.50
0.010
0.28
Peak ankle power
13.7 (2.5)
12.3 (2.0) †
13.3 (2.5)
11.7
<0.001
0.46
a Values expressed as mean (SD), FPA foot progression angle, TD touchdown, MLAA medial longitudinal arch angle,
 significant difference between normal and toe-out FPA p<0.05,
† significant difference between toe-out and toe-in p<0.050,
‡ significant difference between normal and toe-in FPA p<0.05.
https://doi.org/10.1371/journal.pone.0246425.t002
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and a significant difference in MLAA excursion between normal and toe-in (MD = 0.8;
p = 0.005), and between toe-out and toe-in (MD = 1; p<0.001).
Fig 3. Ensemble average curves of measured variables for three FPA conditions, with 0% representing heel strike
and 100% toe-off. Solid curves = normal FPA, dotted curves = toe-out condition, dashed curves = toe-in condition.
Shaded area represents ±1 SD of the normal FPA condition.
https://doi.org/10.1371/journal.pone.0246425.g003
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We found significant main effects of FPA conditions on peak hip internal/external rota-
tion (p<0.001), and hip internal/external rotation at touchdown (p<0.001). There were no
significant main effects of FPA conditions on time to peak hip internal/external rotation
(p = 0.214), and hip internal/external rotation excursion (p = 0.305). Post-hoc tests showed
a significant difference in peak hip internal/external rotation between normal and toe-out
(MD = 2.5; p<0.001), between toe-out and toe-in (MD = -4.8; p<0.001), and between nor-
mal and toe-in (MD = -2.3; p<0.001). Post-hoc tests also showed a significant difference in
hip internal/external rotation at touchdown between normal and toe-out (MD = 3.0;
p<0.001), between toe-out and toe-in (MD = -5.4; p<0.001), and between normal and toe-
in (MD = -2.5; p<0.001).
We found significant main effects of FPA conditions on peak hip ab/adduction (p<0.001),
and hip ab/adduction excursion (p<0.001). There were no significant main effects of FPA con-
ditions on hip ab/adduction at touchdown (p = 0.063), and time to peak hip ab/adduction
(p = 0.055). Post-hoc tests showed a significant difference in peak hip ab/adduction between
normal and toe-out (MD = 2.5; p<0.001), and between toe-out and toe-in (MD = -1.4;
p = 0.044). There was no significant difference in peak hip ab/adduction between normal and
toe-in (MD = 1.0; p = 0.292). Post-hoc tests also showed a significant difference in hip ab/
adduction excursion between normal and toe-out (MD = 1.6; p<0.001), and between toe-out
and toe-in (MD = -1.7; p = 0.044). There was no significant difference in hip ab/adduction
excursion between normal and toe-in (MD = -0.1; p = 0.999).
We found no significant main effects of FPA conditions on peak hip flexion (p = 0.455),
time to peak hip flexion (p = 0.494), hip flexion at touchdown (p = 0.546), and hip flexion
excursion (p = 0.245).
We found significant main effects of FPA conditions on peak knee flexion (p = 0.001), and
knee flexion excursion (p<0.001). There were no significant main effects of FPA conditions
on time to peak knee flexion (p = 0.445), and knee flexion at touchdown (p = 0.179). Post-hoc
tests showed a significant difference in peak knee flexion between normal and toe-out
(MD = 1.5; p = 0.003), and between normal and toe-in (MD = 1.3; p = 0.014). There was no
significant difference in peak knee flexion between toe-out and toe-in (MD = -0.2; p = 0.999).
Post-hoc tests showed a significant difference in knee flexion excursion between normal and
toe-out (MD = 2.0; p = 0.003), and between normal and toe-in (MD = 2.4; p = 0.005). There
was no significant difference in knee flexion excursion between toe-out and toe-in (MD = 0.4;
p = 0.999).
We found significant main effects of FPA conditions on peak ankle dorsiflexion (p<0.001),
and ankle dorsiflexion excursion (p = 0.010). There were no significant main effects of FPA
conditions on time to peak ankle dorsiflexion (p = 0.807), and ankle dorsiflexion at touchdown
(p = 0.927). Post-hoc tests showed a significant difference in peak ankle dorsiflexion between
normal and toe-out (MD = 1.6; p<0.001), and between toe-out and toe-in (MD = -1.0;
p = 0.011). There was no significant difference in peak ankle dorsiflexion between normal and
toe-in (MD = 0.6; p = 0.653). Post-hoc tests also showed a significant difference in ankle dorsi-
flexion excursion between normal and toe-out (MD = 1.8; p = 0.001). There was no significant
difference in ankle dorsiflexion excursion between normal and toe-in (MD = 0.6; p = 0.916),
and between toe-out and toe-in (MD = -1.1; p = 0.258).
We found significant main effects of FPA conditions on peak ankle power (p<0.001).
Post-hoc tests showed a significant difference in peak ankle power between normal and toe-
out (MD = 1.3; p = 0.001), and between toe-out and toe-in (MD = -0.9; p<0.005). There
was no significant difference in peak ankle power between normal and toe-in (MD = 0.4;
p = 590).
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Discussion
The main aim of this study was to investigate the immediate effects of toe-in/toe-out running
using real-time visual feedback on rearfoot in/eversion, subtalar pronation/supination, and
MLAA during running. In support of our hypothesis, peak rearfoot eversion, peak subtalar
pronation and peak MLAA were reduced by toe-in running and increased by toe-out running
compared to normal running. Additionally, toe-in running reduced rearfoot eversion excur-
sion, MLAA excursion, and subtalar pronation at touchdown compared to normal and toe-
out running. Nowadays gait retraining is increasingly used in clinical practice to prevent and
manage a variety of sports injuries. Since modifying atypical rearfoot in/eversion is of great
interest to biomechanical research and clinical practices, our study constitutes a feasible and
applicable basis for using real-time visual feedback to perform toe-in/toe-out running in order
to change rearfoot in/eversion, subtalar supination/pronation, and MLAA.
No study has so far investigated the kinematic effects of changing FPA using real-time visual
feedback during running. We considered ±5˚ differences from the subject’s normal FPA as target
points. Participants generally responded in accordance with the target points. The target area on
the clock was set ±2.5˚ from the target point for both toe-in and toe-out conditions. The results of
the averaged individual SDs show that participants successfully changed their FPA based on the
target area (2SD = 2 for toe-out and 1.8 for toe-in). None reported any problems with changing
the FPA when asked about any difficulties during performing tasks. The average change of FPA
relative to normal FPA was 4.2˚ for toe-out running and 4.4˚ for toe-in running. In fact, the FPA
display in the real-time feedback during familiarization helped subjects adapt to the experiment.
Because the pointer was aligned with the subject’s FPA, it could be easily perceived.
We hypothesized that toe-in running reduces peak rearfoot eversion and toe-out running
increases peak rearfoot eversion. Excessive rearfoot eversion is a modifiable risk factor for
overuse injuries in athletes [47]. We showed that moving from toe-out to toe-in resulted in a
reduction in peak eversion and subtalar pronation, thus supporting the potential value of
changing FPA as a method for gait retraining in order to modify atypical rearfoot in/eversion.
These alterations in peak rearfoot in/eversion by toe-in/toe-out running might be partially due
to lateral and medial shifting of foot pressure during foot roll-over by toe-in and toe-out,
respectively [48]. Our results showed that toe-in running reduces peak rearfoot eversion by
2.1˚ –a promising result, as a recent systematic review and meta-analysis investigating rearfoot
eversion in injured runners and controls showed that a 2˚ increase in peak rearfoot eversion
distinguishes injured from healthy runners [2]. Hence toe-in/toe-out running with a 4–5˚ dif-
ference from the preferred FPA may have clinical significance in the control of atypical rear-
foot in/eversion when preventing and managing RRI.
Subtalar pronation at touchdown and time to peak pronation were significantly reduced by
toe-in running relative to normal running; however, rearfoot eversion at touchdown and time
to peak rearfoot eversion were not significant between different FPA conditions. Rearfoot
eversion excursion was also reduced during toe-in running compared to normal and toe-out
running. Supination and/or inversion are directly attributed to tarsal joint locking in either the
early or the late stance phase [49]. Accordingly, supination helps the foot turn to a rigid lever
where needed. It is documented that greater pronation excursion leads to a delayed supination
in the late stance [50]. According to our results, in toe-in running the foot is in a more supi-
nated position relative to normal and toe-out running. Therefore, in individuals who have
greater foot pronation, toe-running may help foot stabilization at touchdown and even in late
stance phase during running.
We found a smaller MLAA during toe-in running compared with normal and toe-out run-
ning. The effect of changing FPA on MLAA during running is not yet well documented. Only
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few studies with conflicting results investigated the association between MLAA and FPA dur-
ing running or walking [36, 51, 52], examining only the correlation between MLAA and FPA
but not how changeable MLAA is when changing FPA. We found a decreased MLAA of
approximately 0.7˚ and 1˚ during toe-in running compared to normal and toe-out running,
respectively. This amount of change might appear small compared to changes possibly
required to correct an atypical MLAA. An explanation for a small change in MLAA might be
that the current study was conducted for effects on healthy subjects in order to assess the
potential of FPA modification on MLAA. Further research on individuals with atypical MLAA
is therefore warranted to determine how effective FPA modifications are on MLAA. This is
even more important because the correction of atypical MLAA has often been suggested as
one of the potential corrective strategies for atypical rearfoot eversion [50, 53].
Our results showed that changes in FPA are accompanied by changes in lower limb joint
biomechanics. Specifically, these changes occurred in the peak hip internal/external rotation,
hip ab/adduction, knee flexion, ankle dorsiflexion, and ankle power. Compared to running
with normal FPA, both toe-out and toe-in running reduced peak hip adduction. As increased
peak hip adduction is associated with iliotibial band syndrome and patellofemoral pain syn-
drome in runners [12, 54], FPA modification might be used as a potential gait retraining to
reduce peak hip adduction. Toe-out running was accompanied by increased hip external rota-
tion which is reported as a risk factor for medial tibial stress syndrome [55]. In contrast, toe-in
running was accompanied by increased hip internal rotation. Toe-in running, therefore, may
be used as potential gait retraining to reduce increased hip external rotation. Toe-out running
compared to running with normal FPA reduced ankle power, possibly resulting in reducing
the effectiveness of the ankle in providing positive push-off power. Gait retraining studies have
shown that the peak knee adduction moment, a contributing factor to knee osteoarthritis, is
increased/decreased with changes in FPA [41, 56]. This can subsequently load different aspects
of the knee. As a result, clinician and researchers should consider changes in lower limb joint
biomechanics when using FPA to modify rearfoot eversion.
Rearfoot in/eversion has been commonly used in the literature as an alternative way to
express subtalar supination/pronation during walking or running [4, 6]. Our results show that
although peak rearfoot eversion can be a proper representative of peak subtalar pronation dur-
ing running, there are considerable differences between the other variables such as angle at
touchdown, time to peak and excursion. Rearfoot eversion only describes one aspect of subta-
lar pronation and the other aspects of subtalar pronation may distinguish it from rearfoot ever-
sion during running. Isman and Inman [5] presented a 3D kinematic approach defining true
supination/pronation angle occurring in the subtalar joint as used in the current study. It is
therefore suggested that future studies apply proper terminology (supination/pronation and/
or rearfoot in/eversion) based on the study objective.
One major concern about rearfoot eversion is that no standardized norm or clinical defini-
tion exists for classification of rearfoot eversion during running to specify the extent to which
rearfoot eversion falls into typical or atypical movements [4]. In the current study, the use of
the terms typical or atypical (greater or lower) rearfoot eversion during running is based on
the results of studies investigating rearfoot eversion between injured (history of injury) and
non-injured runners. Therefore, further studies warrant to determine to what extent rearfoot
eversion can be considered as typical or atypical.
The current study showed the feasibility and effects of toe-in/toe-out running using real-
time visual feedback on rearfoot in/eversion, subtalar pronation/supination, and MLAA. To
successfully achieve a target FPA during running an advanced system is needed, as running is
a fast-cyclic motion that makes real-time feedback difficult. Current approaches for giving
feedback on FPA mainly require camera-based motion capture, limiting FPA measurement
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or/and training to laboratory settings (thus hindering FPA training outside the laboratory).
Recent studies present valid means using insole- or shoe-embedded sensors to estimate FPA
and give real-time feedback during over-ground gait [57, 58]. However, these means were vali-
dated during walking only, therefore usability investigations during running are required.
Also, compared to the motion capture values an absolute error of 1.7˚ should be taken into
account when using these means. To see the effectiveness of gait retraining on rearfoot ever-
sion over time or to be sure that change in rearfoot eversion is at the desired level, tracking
rearfoot eversion might be useful. As application of 3D motion capture systems is not easy in
clinical practice, 2D measurement of rearfoot eversion using smartphone application can be
used as a surrogate to 3D measurement [59].
Limitations and recommendations
There are a few limitations in the current study. All participants were healthy female runners
so results cannot be extrapolated to male runners and/or injured runners. Further research is
needed to investigate whether our results have the same effect on runners with atypical in/
eversion, supinated or pronated feet, and/or MLAA. All participants ran with rearfoot strike.
Since foot strike pattern affects the ankle and foot biomechanics, our results cannot be general-
ized to those with midfoot or forefoot strike. Based on our results regarding the differences
between rearfoot eversion and subtalar pronation it was suggested that future research con-
ducted to investigate the biomechanics of subtalar pronation in individuals should consider
the difference between rearfoot in/eversion and subtalar supination/pronation, and choose
one or both based on the study objective. Running speed was set at 8km/h, so it is not clear
whether the same results would be found at higher or slower speeds. Changes in lower limb
biomechanics such as hip ab/adduction, hip internal/external rotation, knee flexion, ankle dor-
siflexion, and ankle power should be taken into account when changing FPA is used to modify
rearfoot eversion. As our study aimed to investigate the acute effect of changing FPA, it is
unknown whether runners would retain it in the long term. Therefore, further studies should
be undertaken to investigate the viability of toe-in/toe-out running in the long term.
Conclusion
This study showed that female healthy runners were able to change their FPA when receiving
real-time visual feedback for FPA. Toe-in running using real-time visual feedback reduced
peak rearfoot eversion, peak pronation, and peak MLAA compared to normal and toe-out
running. Toe-out running, instead, increased these kinematic factors compared to normal and
toe-in running. Rearfoot in/eversion is not an appropriate surrogate to predict all supination/
pronation parameters. Our study provides new knowledge and lays the foundation for future
research into modifying atypical rearfoot in/eversion, subtalar supination/pronation, and
MLAA using gait retraining (toe-in and toe-out running) by real-time visual feedback during
running. Clinicians and researchers should take it into account that changes in FPA when run-
ning is accompanied by changes in lower limb joint biomechanics.
Supporting information
S1 Fig. One-way repeated measure ANOVA results for foot progression angle (FPA).
(DOCX)
S2 Fig. One-way repeated measure ANOVA results for rearfoot eversion variables.
(DOCX)
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S3 Fig. One-way repeated measure ANOVA results for pronation variables.
(DOCX)
S4 Fig. One-way repeated measure ANOVA results for medial longitudinal arch angle.
(DOCX)
S5 Fig. One-way repeated measure ANOVA results for hip internal rotation.
(DOCX)
S6 Fig. One-way repeated measure ANOVA results for hip ab/adduction.
(DOCX)
S7 Fig. One-way repeated measure ANOVA results for hip flexion.
(DOCX)
S8 Fig. One-way repeated measure ANOVA results for knee flexion.
(DOCX)
S9 Fig. One-way repeated measure ANOVA results for ankle dorsiflexion.
(DOCX)
S10 Fig. One-way repeated measure ANOVA results for peak ankle power.
(DOCX)
Acknowledgments
We gratefully thank all participants for having volunteered to be part of this study.
Author Contributions
Conceptualization: Seyed Hamed Mousavi, Laurens van Kouwenhove, Reza Rajabi, Johannes
Zwerver, Juha M. Hijmans.
Data curation: Seyed Hamed Mousavi, Laurens van Kouwenhove, Reza Rajabi.
Formal analysis: Seyed Hamed Mousavi.
Funding acquisition: Seyed Hamed Mousavi, Reza Rajabi.
Methodology: Laurens van Kouwenhove.
Project administration: Seyed Hamed Mousavi, Johannes Zwerver, Juha M. Hijmans.
Software: Laurens van Kouwenhove.
Supervision: Laurens van Kouwenhove, Reza Rajabi, Johannes Zwerver, Juha M. Hijmans.
Visualization: Seyed Hamed Mousavi, Johannes Zwerver, Juha M. Hijmans.
Writing – original draft: Seyed Hamed Mousavi, Johannes Zwerver, Juha M. Hijmans.
Writing – review & editing: Seyed Hamed Mousavi, Laurens van Kouwenhove, Reza Rajabi,
Johannes Zwerver, Juha M. Hijmans.
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The effect of changing foot progression angle using real-time visual feedback on rearfoot eversion during running.
Mousavi, Seyed Hamed,van Kouwenhove, Laurens,Rajabi, Reza,Zwerver, Johannes,Hijmans, Juha M