RESEARCH ARTICLE
The effect of footwear on mechanical
behaviour of the human ankle plantar-flexors
in forefoot runners
Jason BonacciID1*, Wayne Spratford2,3,4, Claire Kenneally-Dabrowski1,
Danielle TrowellID1, Adrian Lai5
1 Centre for Sports Research, School of Exercise and Nutrition Sciences, Deakin University, Geelong,
Australia, 2 Movement Science, Australian Institute of Sport, Canberra, Australia, 3 Discipline of Sport and
Exercise Science, Faculty of Health, University of Canberra, Canberra, Australia, 4 University of Canberra
Research Institute for Sport and Exercise (UCRISE), University of Canberra, Australia, 5 Lululemon
Athletica, Vancouver, Canada
* jason.bonacci@deakin.edu.au
Abstract
Purpose
To compare the ankle plantar-flexor muscle-tendon mechanical behaviour during barefoot
and shod forefoot running.
Methods
Thirteen highly trained forefoot runners performed five overground steady-state running tri-
als (4.5 ± 0.5 m.s-1) while barefoot and shod. Three-dimensional kinematic and ground reac-
tion force data were collected and used as inputs for musculoskeletal modelling. Muscle-
tendon behaviour of the ankle plantar-flexors (soleus; medial gastrocnemius; and lateral
gastrocnemius) were estimated across the stance phase and compared between barefoot
and shod running using a two-way multivariate analysis of variance.
Results
During barefoot running peak muscle-tendon unit (MTU) power generation was 16.5% (p =
0.01) higher compared to shod running. Total positive MTU work was 18.5% (p = 0.002)
higher during barefoot running compared to shod running. The total sum of tendon elastic
strain energy was 8% (p = 0.036) greater during barefoot compared to shod running, how-
ever the relative contribution of tendon and muscle fibres to muscle-tendon unit positive
work was not different between conditions.
Conclusion
Barefoot forefoot running demands greater muscle and tendon work than shod forefoot run-
ning, but the relative contribution of tendon strain energy to overall muscle-tendon unit work
was not greater.
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OPEN ACCESS
Citation: Bonacci J, Spratford W, Kenneally-
Dabrowski C, Trowell D, Lai A (2022) The effect of
footwear on mechanical behaviour of the human
ankle plantar-flexors in forefoot runners. PLoS ONE
17(9): e0274806. https://doi.org/10.1371/journal.
pone.0274806
Editor: Nili Steinberg, The Wingate College of
Physical Education and Sports Sciences at the
Wingate Institute, IL, ISRAEL
Received: November 30, 2021
Accepted: September 3, 2022
Published: September 19, 2022
Copyright: © 2022 Bonacci 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 paper 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.
Introduction
The human ankle plantar-flexors, the soleus (SOL), medial gastrocnemius (MG) and lateral
gastrocnemius (LG), perform an important biomechanical function during running. Experi-
mental and musculoskeletal modelling studies have demonstrated that the ankle plantar-flex-
ors generate force up to 12 times body weight (BW) during running [1]; the greatest force of
all the lower-limb muscle groups. They are also the dominant contributors to support and hor-
izontal propulsion of the body’s centre of mass during running [2, 3]. The ankle plantar-flexors
have relatively short muscle fibres that insert onto the calcaneus via a compliant Achilles ten-
don. This configuration favours the capacity to store elastic strain energy, which contributes a
considerable amount of the mechanical work performed by the musculotendon units [4, 5].
Previous modelling studies have demonstrated that elastic strain energy stored in the Achilles
tendon during steady-state running provides a greater contribution to muscle-tendon unit
(MTU) propulsive work compared to the muscle fibres [6–8]. The contribution of tendon
strain energy to overall MTU propulsive work increases as running speed advances from slow
running towards maximum sprinting [8]. However, the effects of footwear and foot-strike type
on the energetics of the muscle fibres and tendon in the ankle plantar-flexors during running
remain largely unknown.
Sinclair et al. [9] utilised musculoskeletal modelling to examine the effect of footwear on
ankle plantar-flexor muscle forces during running. They reported a 32% increase in MG mus-
cle force during barefoot running compared to shod running, but no difference in LG or SOL
muscle forces between conditions. Foot-strike pattern was not controlled in the study. For
example, the ankle was more plantarflexed at contact in the barefoot running condition sug-
gesting that participants switched from a rearfoot strike during the shod condition to a mid/
forefoot strike during the barefoot condition. It is not possible to discern if the increase in
force developed by the MG was due to differences in footwear or foot-strike type. Running
barefoot and in minimalist shoes has been associated with greater peak internal ankle plantar-
flexion moments and plantar-flexor impulse during stance [10, 11]. These greater demands on
the ankle plantar-flexors are due to a lack of shoe cushioning, which increases the muscular
effort required to attenuate ground impact forces while potentially increasing the metabolic
cost of running [12, 13]. However, running barefoot and in a minimalist shoe are more eco-
nomical than running in a cushioned shoe, even when shoe mass is accounted for [11, 14]. If
barefoot running is more economical but has greater ankle plantar-flexor demands, this dis-
crepancy may suggest that the ankle plantar-flexors utilise greater Achilles tendon elastic strain
energy during barefoot compared to shod running.
Perl et al. [11] postulated that the elevated heel during shod running and greater lower
extremity elastic energy storage may explain the moderate metabolic benefits of barefoot or
minimally shod running. The authors indirectly measured Achilles tendon strain using the
overall length change of the entire triceps surae MTU complex. This method does not distin-
guish between the length changes of the muscle fibre and tendon components, which previous
in-vivo studies of the ankle plantar-flexors during running have shown are decoupled from
that of the MTU [15, 16]. As a result, it is not possible to differentiate the work done by the
MTU in the ankle plantar-flexors into the elastic strain energy stored in the tendon and the
work done by the muscle fibres. Therefore, the aim of the study was to investigate the effect of
footwear on the mechanical behaviour of the ankle plantar-flexor muscle fibre and tendon
components during running. Specifically, we used experimental kinematic and kinetic data in
conjunction with musculoskeletal modelling to compute the mechanical power and work per-
formed by the MTU, muscle fibres and tendon in the SOL, MG and LG during barefoot and
shod running. As the purpose of this study was to examine the effect of footwear rather than
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foot strike on plantar-flexor MTU behaviour, only habitual forefoot runners were recruited.
This is because rearfoot strikers often switch to a forefoot strike when running barefoot [17],
which could confound the results. We hypothesised that in comparison to shod running, bare-
foot running would result in: (i) a greater amount of tendon elastic strain energy in the ankle
plantar-flexors and; (ii) a greater relative contribution of tendon elastic strain energy contribu-
tion to MTU positive work compared to muscle fibre work.
Methods
Participants
Thirteen highly trained distance runners (8 males and 5 females; mean ± SD; age, 29.9 ± 5.9
years; height, 176.7 ± 7.5 cm; body mass, 64.9 ± 8.8 kg) were recruited for the study. Based on
a priori sample size calculation, 13 participants would be sufficient to generate an effect size of
0.93 at a power of 80% and α of < 0.05 [9]. All participants were training for competition
(training history: 14.3 ± 1.9 km per session, 7.4 ± 1.9 sessions per week, 109.6 ± 27.9 km per
week) with the average personal best 10 km time in the previous year of 33.7 ± 3.7 minutes. All
participants self-reported as a forefoot striker and this was confirmed during data collection.
No participants were suffering from any pre-existing musculoskeletal injury that might affect
their ability to participant in the study. Written informed consent was obtained from all partic-
ipants and ethical approval was attained from Deakin University and Australian Institute of
Sport human research ethics committees.
Experimental data collection
Running trials were conducted on a 110 m indoor synthetic track in the Biomechanics Labora-
tory at the Australian Institute of Sport, Canberra. Three-dimensional kinematic data were col-
lected using a 22-camera motion analysis system (VICON, Oxford Metrics Ltd, Oxford, UK)
sampling at 250 Hz. The calibrated capture volume was approximately 20 m in length and situ-
ated ~60 m along the track, which allowed sufficient distance for participants to accelerate,
hold a steady-state speed through the capture volume and then safely decelerate to rest. Retro-
reflective markers (14 mm diameter) were placed at predefined locations on the pelvis and
lower limbs [10]. Individual markers were placed on the left and right anterior superior iliac
spines and posterior superior iliac spines. The thigh segment was defined by a three-marker
cluster affixed laterally and aligned with the head of the femur and lateral femoral condyle.
The lower leg was defined by a three-marker cluster aligned with the lateral femoral condyle
and lateral malleolus. Markers placed on the superoposterior aspect of the calcaneus, and first
and fifth metatarsals defined the foot. In the shod condition, these markers were placed on the
shoe. Individual retroflective markers were also placed on the medial and lateral femoral con-
dyles and medial and lateral malleoli to define the knee and ankle joint centres, respectively.
Ground reaction force (GRF) data were collected using eight in-ground force plates (Kistler
Instrument Corp., Dimensions: 900 x 600 mm, Amherst, New York, USA) sampling at 1500
Hz. The force plates were embedded into the synthetic running track directly adjacent to each
other spanning a total length of 7.2 m. Marker trajectories and GRF data were filtered using
fourth-order, low-pass Butterworth filters with the same cut-off frequency of 20 Hz [18].
The data collection protocol involved two experimental conditions: shod (lightweight rac-
ing flat, NIKE LunaRacer 2) and barefoot. The racing flat had a low heel-forefoot offset (6
mm) and mean mass of 184.2 ± 19.4 g. All participants were required to complete a 10-day
familiarisation period prior to testing to get accustomed to the barefoot and shod conditions.
The average distances completed by all participants during the familiarisation period in the
barefoot and shod conditions were 4.3 ± 3.2 km and 20.7 ± 11.5 km in 2.6 ± 0.6 and 3 ± 0.6
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sessions, respectively. The volume of barefoot familiarisation was lower than shod running to
minimise likelihood of acute overload during this unfamiliar condition [19, 20]. All partici-
pants habitually wore a standard cushioned running shoe for most of their training volume.
Participants performed a standardised warm-up prior to data collection that involved five
overground running trials within the capture volume. Participants then performed a static cali-
bration trial and five overground steady-state running trials in each of the two randomly
ordered conditions. The desired steady-state running speed for each participant was set at 90%
of participant’s best 10 km time in the previous year. The mean desired steady-state running
speed for all participants was 4.5 ± 0.5 m.s-1. Average steady-state speed for each trial were
obtained using timing gates (Speedlight Telemetry Timing, Swift Performance Equipment,
Walcol, QLD, Australia) placed at the start and end of the calibrated capture volume. Trials
were accepted if the average speed was within ±5% of the desired speed. Steady state running
was confirmed post testing via examination of the net horizontal force impulse (S1 Dataset).
Forefoot strike was defined as a foot strike in which the point of first contact of the foot with
the ground was the forefoot or the front half of the shoe sole. Two classification techniques
were used to confirm that all participants had forefoot strike patterns. The first classification
technique used the presence of an initial ankle plantar-flexion angle in kinematic data and
absence of impact peak in the vertical GRF profile [17] while the second classification tech-
nique used the markers placed on the heel and the toe to define foot strike patterns [21]. The
difference in vertical position of the heel and first metatarsal markers was calculated during
both the static trial and at initial contact during running. The vertical difference between
markers during the static trial was then subtracted from the difference at initial contact. Partic-
ipants were classified as forefoot strikers if the final value was 40 mm or less [21]. Both classifi-
cation techniques verified that all participants had forefoot strike patterns in both the barefoot
and shod conditions. All participants completed both experimental conditions.
Musculoskeletal model
The skeletal system was modelled as a 12 segment, 31 degree of freedom (DOF) mechanical
linkage system, similar to that described by Hamner at al. [22]. The lower limb joints were
modelled as follows: the pelvis was free to translate and rotate in space (6 DOF), the hip was a
ball-and-socket joint (3 DOF), the knee was a hinge joint (1 DOF), and the ankle-subtalar
complex was a universal joint (2 DOF) comprised of two non-intersection hinge joints. While
the model contained the metatarsophalangeal joint, this was locked for all simulations and the
mid and fore- foot acted as a rigid segment. The model was actuated by 96 MTU actuators.
Each MTU was modelled as a Hill-type muscle consisting of in-parallel active and passive mus-
cle fibre elements attached in-series with a series elastic element. Hereafter, the series elastic
element will be termed tendon as a result of the significant influence of the free tendon on
series elasticity compared with other elastic connective tissue (e.g. aponeurosis). Maximum
shortening velocity was set to 15 optimal fibre lengths per second to be consistent with previ-
ous modelling studies investigating running [23, 24]. The maximum isometric force of all mus-
cles was increased three-fold, as required for successful simulations of highly dynamic
movement such as gait [25, 26].
The SOL, MG and LG were assumed to be three separate MTUs with three independent
tendon elements representing the Achilles tendon. The tendon strains at maximum isometric
force generation were informed by previously reported data. Experimental data from three
rearfoot runners [16] were used to evaluate the effect of modifying plantar-flexor tendon
strains on model-based estimates of SOL and MG muscle fibre lengths. Simulated fibre lengths
were compared to that measured in-vivo using ultrasound during rearfoot running [6, 16, 27]
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across a range of tendon strains from 3.3%-10%. A tendon strain of 5% for the SOL, MG and
LG gave the most comparable model-based muscle fibre length changes in the SOL and MG
during stance phase. Tendon strains at maximum isometric contraction for the ankle plantar-
flexors were consistent with reported in-vivo measurements of the Achilles tendon of 4.9 ± 1%
[28] and 5.1 ± 1.1% [29]. Furthermore, simulated tendon strains during running were within
the range of tendon strains reported using dynamic ultrasound measurements of the SOL and
MG at equivalent running speeds [15, 16]. These two consistencies support our decision to
increase the tendon strain for the ankle plantar-flexors at maximum isometric force genera-
tion. Tendon compliance in other MTUs remained unchanged at 3.3% of maximum isometric
force generation [30].
Computational simulations
Muscle modelling simulations were performed using OpenSimTM [31]. Subject-specific mus-
culoskeletal models were attained by scaling a generic model to the participant’s height and
body mass. Individual musculoskeletal models were created for each participant and experi-
mental condition (i.e. barefoot and shod). The same generic model (with identical marker
placement) was used during scaling for each condition. A set of joint angles for each time
instant was calculated using an inverse kinematic analysis where the sum of the squares of the
differences between experimental markers trajectories and virtual markers in the model was
minimised [32].
An inverse dynamics analysis in conjunction with a computed muscle control algorithm
(CMC) were used to predict muscle forces and activations [33, 34]. A standard inverse
dynamic approach was used to compute net joint torques generated about the torso, hip,
knee and ankle joints. Residual reduction analysis (RRA) was used to reduce dynamic incon-
sistencies between joint kinematics and the measured GRF. The errors in the residual forces
and dynamically-consistent kinematics were within the recommended bounds of the analysis
[35]. Muscle forces and activations were computed using CMC in accordance with the physi-
ological force-length and force-velocity properties of muscle fibre and tendon, as well as the
geometric and dynamic constraints of the system. CMC solved the muscle redundancy prob-
lem by predicting a set of muscle excitations that drove a model forward in time (simulation
time window of 0.01 s) such that the sum of squared muscle activations was minimised and
the kinematics of the model tracked the dynamically consistent joint kinematics obtained
from RRA. Muscle excitations were bounded between 0 (no muscle activation) and 1 (full
muscle activation) with activation and deactivation time constants of 10 and 30 ms, respec-
tively [30].
The mechanical power developed by the MTU, muscle fibre and tendon elements were cal-
culated by multiplying MTU, muscle fibre and tendon force by their corresponding contrac-
tion velocity at each time instant. Negative and positive power represented power absorption
and generation, respectively. The positive work done by the MTU, muscle fibre and tendon
was found by integrating the MTU, muscle fibre and tendon power curves over the duration
of the stance phase where power was generated. All participants were forefoot strikers, thus the
tendon and MTU lengthened and performed negative work during early stance followed by a
period of positive work during mid- to late stance. The recovery of tendon elastic strain energy
was represented by the positive work done by the tendon after the tendon performed negative
work and after the MTU started generating positive power.
In this study, we were specifically interested in the percentage contributions of positive
muscle fibre work and tendon elastic strain energy to the positive work done by the MTU (i.e.,
propulsion energy). The calculation of these contributions are detailed in a previous paper [8].
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Briefly, the contributions were calculated using the following equation,
%MTU contribution ¼
W
WMTU
 100
where WMTU is the positive work done by the MTU and W is the area under the WMTU curve
attributable to positive muscle fibre work or tendon elastic strain energy.
Data analysis
Data for each participant were averaged over five stance phases for each experimental condi-
tion; time normalised to 0–100% of the stride cycle and used to calculate group mean ± SD val-
ues. Muscle force was normalised by the participant’s body weight while mechanical power
and work were normalised by body mass (kg). Stride parameters of stance duration and stride
length were calculated, along with the relative heel-toe marker position to classify footstrike
position. The outcome variables included SOL, MG and LG peak muscle force (BW), peak
MTU positive power (W.kg-1), total MTU positive work (J.kg-1) and the percentage contribu-
tions of positive muscle fibre work and tendon elastic strain energy to the positive work done
by the MTU. The total sum of tendon elastic strain energy stored in all three ankle plantar-flex-
ors was also calculated. Data are presented as mean ± SD. The Shapiro-Wilk test was con-
ducted to determine if any data violated the assumption of normality. Stance duration, stride
length, footstrike position and total sum of tendon elastic strain energy were compared
between shod and barefoot conditions using a two-tailed paired sample t-test with α set
at < 0.05. A two-way multivariate analysis of variance (MANOVA) was used to identify the
effect of footwear and muscle on ankle plantar-flexor peak muscle forces, peak MTU power
generation, total positive MTU work, and contribution of tendon and muscle fibre to positive
MTU work. Where a main effect was found, post-hoc tests with Bonferroni correction were
used to test for differences between means. Standardised mean differences (SMD) were calcu-
lated to express the magnitude of difference between conditions and interpreted according to
the following criteria: calculated SMD of 0.2–0.49, small change; SMD of 0.5–0.79, moderate
change; and SMD  0.8, large change [36]. All statistical analysis was conducted using the Sta-
tistical Package for the Social Sciences v27 (IBM Statistics, Chicago, USA).
Results
Stance duration and footstrike position were not different between barefoot and shod running,
though there was a small decrease in stride length during barefoot compared to shod running
(Table 1). Peak muscle force, peak MTU power generation, total positive MTU work and rela-
tive contribution of tendon and muscle fibre to MTU positive work are plotted in Fig 1. Plan-
tar-flexor muscle fibre, tendon and MTU length curves across the stance phase are presented
in S1 Appendix.
Table 1. Group mean ± SD values and the difference between footwear conditions for stride parameters and footstrike position.
Barefoot
Shod
Mean difference [95% CI]
p-value
SMD
Stance duration (s)
0.2 ± 0.01
0.2 ± 0.01
0.00 [-0.00, 0.00]
0.391
0.098
Stride length (m)
3.0 ± 0.3
3.1 ± 0.3
-0.1 [-0.11, -0.07]
0.001
0.29†
Footstrike position (mm)#
34.4 ± 3.6
33.2 ± 3.6
1.2 [-0.3, 2.8]
0.099
0.34
#A lower relative value indicates more ankle plantarflexion at initial contact (forefoot strike < 40 mm).
Significant difference between barefoot and shod conditions. † Small change
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The total sum of tendon elastic strain energy was 8.3% (1.2 to 15.5%, SMD = 1.17,
p = 0.036) higher during barefoot (0.9 [0.1] J.kg-1) compared to shod (0.8 [0.1] J.kg-1) running.
The two-way MANOVA revealed a main effect for footwear (p = 0.032) but no footwear by
muscle interaction effects (p = 0.669). Univariate analysis demonstrated a significant differ-
ence in peak MTU power generation (p = 0.01) and total positive MTU work (p = 0.002) for
footwear conditions. Compared to shod running, peak MTU power generation was 16.5% (4
to 28.9%, SMD = 0.7) higher when running barefoot. Total positive MTU work was 18.5% (7
to 30%, SMD = 0.92) higher during barefoot compared to shod running. There was no effect
of footwear on peak muscle force (p = 0.21) or tendon and muscle fibre contribution to posi-
tive MTU work (p = 0.46 & 0.37, respectively).
Fig 1. (A) Peak MTU muscle force; (B) peak MTU power generation; (C) total positive MTU work; (D) relative tendon and muscle fibre contributions to
positive work (%) during the stance phase of barefoot and shod running.
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Discussion
The aim of this study was to investigate the mechanical behaviour of the plantar-flexor muscle
fibre and tendon components during barefoot and shod forefoot running. Tendon elastic
strain energy was significantly greater (8.3%, SMD = 1.2) during barefoot running compared
to shod running, confirming our first hypothesis. Achilles tendon elastic strain energy contrib-
uted the majority of MTU positive work during both shod and barefoot running. This is con-
sistent with previous studies of shod running [6–8], although estimates of SOL relative tendon
contribution were approximately 10% greater than previously reported at comparative speeds
[7, 8]. This discrepancy may be due to methodological differences, as previous studies did not
control footstrike pattern and modelled the LG and MG as a single MTU [7, 8]. The relative
contribution of the tendon to plantar-flexor MTU positive work was not different between
barefoot and shod conditions. This indicates that both tendon and muscle fibre work are
increased when running barefoot. This finding refutes our second hypothesis.
The sum of muscle fibre and tendon contributions to MTU positive work exceeded 100%
for all plantar-flexors during both barefoot and shod running. However, this is expected and
can be explained by the transfer of energy from the muscle fibre to the tendon. During early
stance, muscle fibres of the plantar flexors shorten and do positive work, while the tendon and
MTU lengthen and do negative work [16]. This behaviour is common in MTUs where the ten-
don is compliant [30], such as the plantar-flexors. Positive work done by the muscle fibres dur-
ing early stance is transmitted to the tendon as it stretches. This results in energy stored in the
tendon, which is later returned as the tendon shortens during propulsion, in late stance.
Because of this additional energy returned via the tendon, the sum of tendon and muscle fibre
positive work exceeds 100% of MTU positive work.
Peak muscle force was not significantly greater during barefoot compared to shod forefoot
running. In contrast, Sinclair et al. [9] found large increases in MG (32%) peak forces during
barefoot compared to shod running. This disparity may be due to differences in footstrike pat-
terns between studies. In the current study, all participants used a forefoot strike during shod
and barefoot running. In comparison, a switch from rear to mid/forefoot was noted between
shod and barefoot running in the previous study. The observed change in footstrike pattern
has previously been reported in habitual rearfoot shod runners when transitioning to barefoot
running [17]. The large increases observed by Sinclair et al. [9] are likely due to a lack of cush-
ioning combined with a change in footstrike pattern during barefoot running. An absence of
shoe cushioning during barefoot running increases the ankle plantarflexion moment during
stance due to an increased muscular effort to absorb impact forces [10, 11]. The increase in
MG muscle forces and Achilles tendon strain energy during barefoot forefoot running must
be carefully considered as a rapid transition out of footwear may overload this complex.
Peak MTU power generation and MTU total positive work increased when running bare-
foot compared to shod running. Previous studies have reported that greater positive work is
required at the ankle joint as shoe cushioning decreases or is removed when running barefoot
[10, 37]. This is likely caused by greater propulsive forces when running barefoot [38]. As the
plantar-flexors are the primary contributors to ankle joint torque and positive work generation
during propulsion, it is expected that greater work is required of the plantar-flexor MTUs
when running barefoot. As previously noted, this increased MTU positive work was a result of
increases in both tendon elastic strain energy and muscle fibre work. There is some evidence
that barefoot running has economical benefits [11, 14]. However, the mechanism behind this
is unclear. Perl et al. [11] postulated that greater elastic energy storage and release could
explain greater economy when running barefoot. While the current study showed an increase
in tendon elastic strain energy during barefoot running, the relative contribution of the tendon
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to MTU positive work was not greater. Thus, barefoot running was not associated with greater
tendon contribution to overall MTU positive work. The muscle fibre contributions to overall
MTU positive work were similar in both conditions. The overall increase in MTU work and
similar tendon and muscle fibre contributions to MTU positive work are unlikely to contribute
to an economical advantage during barefoot running.
We found increased Achilles tendon elastic strain energy while running barefoot compared
to shod. While increased tendon elastic strain energy may be beneficial, we must also consider
how tendon compliance affects the operating range of muscle fibres on the force-length curve.
Increased tendon elastic-energy storage and recovery may result in muscle fibres operating
almost isometrically, which is most beneficial if near the optimal operating length on the
force-length curve [39]. However, a trade-off may occur if tendon compliance results in mus-
cle fibres operating at lengths which are further from optimal. This was observed by Uchida
et al. [40], whereby increased tendon compliance of the MG resulted in muscle fibres remain-
ing shorter during running, and operating far from their optimal lengths. As a result, greater
activation was required to produce the necessary plantarflexion moment and the metabolic
power requirement of running was increased. A similar trend has been observed when exam-
ining plantar-flexor muscle-tendon behaviour at varying running speeds [8]. As speed
increases, tendon contribution to MTU positive work increases; however, muscle fibres oper-
ate at progressively unfavourable regions of the force-length curve. In the current study, mus-
cle fibre lengths and shortening were similar between barefoot and shod conditions (S1
Appendix). Therefore, in both conditions the plantar-flexor muscle fibres were at similar oper-
ating lengths. Both tendon contributions to work, and the resulting effects on muscle fibre
length operating range must be considered when understanding the effects of fibre and tendon
behaviour on running economy.
Previous studies show that increased ankle work when running barefoot or in minimalist
shoes is associated with a decrease in mechanical work at the knee [10, 37]. Fuller et al. [37]
suggested the shift in mechanical work towards the ankle may allow greater elastic-energy stor-
age and recovery in the Achilles tendon, and therefore improve mechanical efficiency. The
plantar-flexors have short fibres and a long, compliant tendon while the MTUs supporting the
knee are generally larger with relatively shorter tendons [41]. Therefore, it could be more eco-
nomical to rely on the plantar-flexors to produce power during stance [37]. A more holistic
view of lower-limb positive work production may suggest that increased reliance on the ankle
will result in increased elastic-energy storage and recovery, as this is better facilitated by the
plantar-flexors. Further studies which examine the muscle and tendon mechanical behaviour
of the MTUs supporting both the ankle and knee during forefoot shod and barefoot running
are required to further explore this theory.
We observed an increase in total Achilles tendon elastic strain energy when running bare-
foot. Footwear also limits the compression and recoil of the elastic elements supporting the
longitudinal arch of the foot [42]. The current study used a rigid mid and fore- foot model that
does not consider the compression and recoil of the foot longitudinal arch. This is a limitation
of the current study, as a simplified foot model may result in overestimation of predicted ankle
joint power and therefore plantar-flexor MTUs power [43]. It is possible that this could influ-
ence differences in MTU power of the plantar-flexors between barefoot and shod running.
Further, the small decrease (0.1 m) in stride length during barefoot running may explain the
changes in plantar-flexor MTU behaviour between barefoot and shod running.
Predictions of plantar-flexor energetics are sensitive to the musculoskeletal model input
parameters. The model and its underlying parameters were scaled to each subject’s height and
mass; however subject-specific bone geometries and musculotendon measures were not uti-
lised. In particular, Achilles tendon compliance can vary greatly in humans [44] and influences
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the magnitude of muscle fibre work and tendon elastic strain energy [39]. This is important to
acknowledge within the context of this study. However, a robust method was used to deter-
mine the most appropriate tendon compliance, and this resulted in fibre [6, 16, 27] and tendon
[15, 16] strains that were consistent with ultrasound measurements during steady-state run-
ning. The Achilles tendon was also represented within the model as three separate tendons,
rather than one common tendon. While the exact effects of this design on predictions of plan-
tar-flexor energetics is unclear, non-uniform deformation of tendinous regions arising from
each plantar-flexor have been reported [45] and may support this modelling approach. We
only examined the acute effects of barefoot forefoot running on plantar-flexor energetics.
Long-term use of minimalist shoes and running barefoot can result in adaptations to the
mechanical properties of the tendon. Runners with four years of running in minimalist shoes
displayed greater Achilles tendon stiffness compared to traditionally shod runners [46] and
the Achilles tendon adapts to increased loading when exposed to minimalist shoe running by
increasing stiffness [47]. This suggests that greater Achilles tendon stiffness might also be seen
in habitual barefoot runners. The mechanical properties of the tendon were the same during
the barefoot and shod simulations. This study does not account for adaptations to the mechan-
ical properties of the tendon that may occur with prolonged barefoot running, which may
influence predicted plantar-flexor energetics. Finally, we used the same performance criterion
to estimate muscle-tendon parameters during barefoot and shod running. It is possible that
participants utilised a different cost function to minimise muscle excitations than the one used
in this study; however, there is currently no evidence to support this.
In conclusion, running barefoot with a forefoot strike increased plantar-flexor positive
work and power generation and Achilles tendon elastic strain energy when compared to shod
forefoot running. The relative contribution of tendon to plantar-flexor MTU positive work
remained similar between shod and barefoot forefoot running. These results indicate that
barefoot forefoot running does not preferentially favour elastic energy over muscle fibre work
more than shod running. Those who adopt barefoot forefoot running should be aware of the
greater demand on the plantar-flexors and Achilles tendon compared to shod forefoot run-
ning. Future studies should consider how mechanical adaptations of the Achilles tendon in
habitual barefoot runners may influence plantar-flexor energetics.
Supporting information
S1 Appendix. Plantar-flexor muscle fibre, tendon and musculotendon normalised lengths.
(TIF)
S1 Dataset. Individual data for the plantar-flexor muscle, tendon and musculotendon
units during barefoot and shod forefoot running.
(XLSX)
Acknowledgments
The authors would like to Dr Amy Hicks for her assistance with data collection and analysis
and Cody Lindsay for his assistance in reviewing the manuscript.
Author Contributions
Conceptualization: Jason Bonacci.
Formal analysis: Jason Bonacci, Adrian Lai.
Investigation: Jason Bonacci, Wayne Spratford.
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Methodology: Jason Bonacci, Wayne Spratford, Adrian Lai.
Project administration: Jason Bonacci.
Resources: Wayne Spratford, Adrian Lai.
Visualization: Jason Bonacci, Claire Kenneally-Dabrowski, Danielle Trowell.
Writing – original draft: Jason Bonacci, Claire Kenneally-Dabrowski, Danielle Trowell,
Adrian Lai.
Writing – review & editing: Jason Bonacci, Wayne Spratford, Claire Kenneally-Dabrowski,
Danielle Trowell, Adrian Lai.
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The effect of footwear on mechanical behaviour of the human ankle plantar-flexors in forefoot runners.
Bonacci, Jason,Spratford, Wayne,Kenneally-Dabrowski, Claire,Trowell, Danielle,Lai, Adrian