![Change (%) in pain/function and various plantarflexor muscle function measures with resistance exercise over time in individuals with Achilles tendinopathy: a pain and function measured by the Victorian Institute of Sports Assessment (VISA) self-administered Achilles questionnaire [23, 39, 46, 48, 49], Pain Disability Index [50], visual analog scale (VAS) [24–26, 41] or proportion that had pain with activity [51]; b strength, i.e. peak concentric torque [24, 26, 41, 48, 50], isometric torque [45] or repetition maximum [23]; c endurance, i.e. work done during isotonic contractions [26, 39, 49], heel raise repetitions [23, 46, 51] or mean concentric torque over repeated repetitions; d power, i.e. isotonic heel raise power](https://www.researchgate.net/publication/349960282/figure/fig5/AS:1000048372486147@1615441520910/Change-in-pain-function-and-various-plantarflexor-muscle-function-measures-with_Q320.jpg)
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S Y S T E M A T I C R E V I E W Open Access
Are Plantarflexor Muscle Impairments
Present Among Individuals with Achilles
Tendinopathy and Do They Change with
Exercise? A Systematic Review with Meta-
analysis
Fatmah Hasani
1,2*
, Patrick Vallance
1
, Terry Haines
3
, Shannon E. Munteanu
4,5
and Peter Malliaras
1
Abstract
Background: Understanding plantarflexor muscle impairments among individuals with Achilles tendinopathy (AT)
may help to guide future research and inform clinical management of AT. Therefore, the aim of this review is to
evaluate plantarflexor muscle impairments among individuals with AT and whether plantarflexor muscle function
changes following resistance training interventions.
Methods: We searched relevant databases including Cochrane Central Register of Controlled Trials, Ovid (MEDLINE,
EMBASE, AMED) and EBSCO (CINAHL Plus and SPORTDiscus) up to September 2020. Studies investigating plantarflexor
muscle function were included if they met the following criteria: (1) any study design enabled comparison of plantarflexor
muscle function between individuals with and without AT, or the affected and unaffected side of individuals with unilateral
AT, and (2) any studies enabled investigation of change in plantarflexion muscle function over time with use of resistance
training intervention. We included studies that recruited adults with either insertional or mid-portion AT of any duration.
Study selection, quality assessment and data extraction were undertaken independentlybytworeviewers.Discrepancies
were resolved via discussion, or by consulting a third reviewer where necessary. The Joanna Briggs Institute (JBI) critical
appraisal tools specific to each study design were used to assess the methodological quality of included studies. Grading the
strength of evidence for each outcome was determined according to the quality and number of studies.
(Continued on next page)
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* Correspondence: Fatmah.hasani@monash.edu
1
Physiotherapy Department, School of Primary and Allied Health Care,
Monash University, Frankston, Victoria 3199, Australia
2
Physiotherapy Department, Security Forces Hospital, Riyadh 11481, Saudi
Arabia
Full list of author information is available at the end of the article
Hasani et al. Sports Medicine - Open (2021) 7:18
https://doi.org/10.1186/s40798-021-00308-8
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(Continued from previous page)
Results: A total of 25 studies (545 participants) met inclusion. Participants’mean age was 40 ± 7 years old. Six studies were
high quality for all domains, while the remaining were susceptible to the risk of bias (e.g. selection criteria, reporting findings).
This review identified moderate evidence that individuals with AT have impairment in maximal plantarflexor torque (seven
studies including one with a mixed population) on their affected side, compared with the unaffected side. Impairments were
modest (9% and 13% [pooled effect divided by mean of the unaffected side scores]) and of uncertain clinical importance.
The remaining evidence, primarily among individuals with mid-portion AT, showed conflicting impairments for plantarflexor
function (i.e. explosive strength and endurance) between sides. There was limited to very limited evidence for improvement
in plantarflexor endurance (7% and 23%) but not power or strength (five studies including one with a mixed population for
strength) over time, despite individuals undertaking several weeks of resistance training.
Conclusions: Plantarflexor impairments appearmorecommonbetweensidesthancomparedwithcontrolgroupsbut
given limitations in the literature further exploration of these relationships is needed.
Registration: PROSPERO Database; number CRD42019100747.
Keywords: Achilles tendinopathy, Function, Capacity, Neuromuscular, Torque, Power, Work
Key Points
It is not clear whether plantarflexor muscle
impairments exist among people with Achilles
tendinopathy (AT), and whether plantarflexor
muscle function changes with resistance training
interventions.
Apart from impairments in maximal plantarflexor
torque, there were conflicting findings for
impairments in plantarflexor function in affected
and unaffected side comparisons.
There was conflicting evidence for impairment in all
plantarflexor muscle function between people with
AT and healthy controls.
There was also limited to very limited evidence for
improvement in plantarflexor endurance but not the
other measures (torque, power) over time after a
minimum of 12 weeks of resistance training.
Background
The Achilles tendon is the largest in the body and has
an important function in storing and releasing energy
during human locomotion [1]. Achilles tendinopathy
(AT) is highly prevalent in both active [2] and less active
individuals [3] and is characterised by local tendon path-
ology and pain that can persist for years. Achilles tendi-
nopathy can present either unilaterally or bilaterally, and
at either the mid-portion (2 to 6 cm proximal to inser-
tion) or the insertion into the calcaneus. The aetiology
of tendinopathy is multifactorial [4], involving both ex-
trinsic factors such as training errors and poor technique
and intrinsic factors like strength. Key pathology features
include increased cellularity and altered cell phenotype,
accumulation of a ground substance, disruption of the
collagen matrix, a larger cross-sectional area (CSA) and
a decrease in tendon stiffness (force resistance) [5].
Pre-existing plantarflexor impairment has been found to
be a risk factor for developing AT [6]. Cross-sectional
studies have identified plantarflexor function impairments,
including reduced maximal plantarflexor torque output
[7–10], rate of force development [11], endurance [7], and
altered muscle activation [12–18] among individuals with
AT in comparison to the unaffected side or healthy con-
trols. McAullife et al. recently reviewed this literature and
concluded that people with AT have strength impairments
(i.e. maximal, reactive and explosive strength) compared
with the uninjured or asymptomatic side [19]. This con-
clusion was based on pooled data where comparisons be-
tween the affected and unaffected side and comparisons
between healthy controls and people with AT were com-
bined. Given this pooling of data, the review by McAuliffe
et al. is not able to determine whether impairments exist
on the unaffected side or compared with controls. This
has important clinical implications; if impairments exist
on both sides, clinicians managing AT should not use the
unaffected side as a benchmark when setting plantarflexor
functional targets [20].
It is also important to determine whether plantarflexor
muscle function (and any plantarflexor muscle impair-
ments that exist) can improve over time. Progressive re-
sistance training targeting plantarflexor muscle function
is a key component of recommended management for
AT [21,22], yet the extent to which these training inter-
ventions influence plantarflexor function is not clear. If
current resistance interventions do not improve and re-
cover strength that may signal a need to review current
approaches to regaining function among people with
AT. Several studies have assessed plantarflexor muscle
function before and after resistance training interven-
tions [23–26] but, to the best of the authors’knowledge,
this literature has not been reviewed.
The primary aim of this review is to perform a system-
atic review of existing literature to identify, critique and
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 2 of 18
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
summarise the evidence for plantarflexor muscle impair-
ment among individuals with AT. Specifically, we will
focus on studies that explore the affected compared with
unaffected side, or AT compared with controls. The sec-
ondary aim is to review the studies that assess the
change in plantarflexor muscle function over time
among individuals undertaking resistance training inter-
ventions for AT. This knowledge will help clinicians
make informed decisions about potential impairments to
consider among this clinical group.
Methods
Searches
The PRISMA statement for systematic reviews was used
to guide the reporting of this review [27]. The review
was registered at the International Prospective Register
of Systematic Reviews (PROSPERO Database; number
CRD42019100747). We searched relevant databases in-
cluding the Cochrane Central Register of Controlled Tri-
als, Ovid (MEDLINE, EMBASE, AMED) and EBSCO
(CINAHL Plus and SPORTDiscus) from inception up to
September 2020. The reference lists of all retrieved jour-
nal articles were searched for additional articles and for-
ward searches of studies citing eligible studies from our
yield were conducted in Google Scholar. The search
strategy with keyword terms and specific subject head-
ings within each database was used. Searches spanned
three categories: neuromuscular, tendinopathy, and
Achilles. For each Keyword search, the Boolean com-
mand “OR”was used and categories were linked with
the Boolean command “AND”. The search terms for
Ovid MEDLINE are shown in Table 1.
Study Inclusion and Exclusion Criteria
Any study design that addressed our aims was included.
This could include studies comparing muscle function
between the affected and unaffected side, studies com-
paring individuals with AT and healthy controls (i.e.
cross-sectional, cohort or randomised trial [any type e.g.
parallel, factorial]), and studies that enabled investigation
of change in these functions over time (i.e. case series,
prospective cohort or randomised trial [any type e.g.
parallel, factorial]). For the second aim, we were inter-
ested in within group change in the measures of interest
rather than comparative between group analyses so we
could determine firstly whether these measures actually
change over time. There was no restriction on the date
of publication. Animal studies, case reports, abstracts,
non-peer reviewed studies, unpublished studies, letters,
reviews, and opinion studies were excluded. Studies pub-
lished in languages other than English were also
excluded.
For prospective studies that enabled the investiga-
tion of change in plantarflexor function over time, any
resistance training protocol was accepted. This in-
cluded isometric, eccentric, concentric or isotonic ex-
ercise used to treat AT. The resistance training
protocol had to be applied for four weeks or longer so
that the effects could be observed [28]. For studies
that included co-interventions alongside the exercise,
such as manual therapy or electrotherapy, we included
the exercise only arm if available. Otherwise, we in-
cluded the study and we planned subgroup analyses to
compare findings in studies that did and did not in-
clude co-interventions.
Types of Participants
We included studies that recruited participants aged
eighteen years and older with either insertional or mid-
portion AT of any duration. Studies were included re-
gardless of how they diagnosed AT, whether through
clinical or imaging, or whether they described diagnosis
at all. We planned subgroup analyses to compare effects
from studies that diagnosed AT based on established
clinical recommendations [21]. Established diagnosis of
AT was based on localised pain at the Achilles tendon
insertion (insertional) or two to six centimetres above
the calcaneus (mid-portion), pain during or after phys-
ical activities that loaded the tendon, or pain that was
worse in the morning or upon weight bearing after a
period of rest [21]. Studies investigating symptomatic
AT (imaging pathology on ultrasound or MRI) were eli-
gible for inclusion. Imaging pathology could include ten-
don thickening, hypoechoic areas or Doppler signal on
ultrasound, and thickening or increased signal on MRI
[29].
Studies were excluded if they included participants who:
i) Had a complete Achilles tear or rupture, based on
presentation or imaging findings.
ii) Had undergone previous Achilles surgery or
injection for their currently affected Achilles tendon
problem in the last three months.
iii) Had been diagnosed with a neurological disorder
(e.g. multiple sclerosis) or systemic inflammatory
condition (e.g. rheumatoid arthritis).
Exceptions were made for studies that presented data
separately for our population of interest.
Table 1 Search terms in MEDLINE database
Neuromuscular OR force OR strength OR RFD OR rate of force
development OR proprioceptive deficits OR Proprioception OR constant
force OR force match OR MVC OR MVIC OR maximum voluntary
contraction OR torque OR power OR muscle bulk OR atrophy AND
Tendinopathy AND Achilles Tendon
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 3 of 18
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Types of Outcome Measures
To be included, studies must have one or more mea-
sures of plantarflexor function. We included any meas-
ure of plantarflexor muscle function including:
i) Strength (e.g. maximal voluntary isometric contrac-
tion [MVIC] or maximal voluntary isotonic contrac-
tion—peak torque in newtons metre [Nm], isotonic
plantarflexor contraction [Nm]or peak force [N]).
ii) Power (e.g. isotonic toe raises or isotonic plantar-
flexor contraction [joules per second or watts (W)]).
iii) Explosive strength (i.e. rate of producing force
[RFD in Ns]).
iv) Endurance or work done (e.g. heel raise work per
repetitions [joules (J)]).
v) Motor aspects of proprioception (e.g. force sensing
tasks).
Work is often measured over several repetitions, so we
categorised this outcome as an endurance measure.
Compound measures of plantarflexor muscle function
that involved multiple joints and muscles (e.g. squatting,
jumping, hopping) were excluded.
Study Selection
The search yield was downloaded into Endnote version
X8 (Thomson Reuters, Philadelphia, USA) and dupli-
cates were removed. Two authors (FH, PV) independ-
ently screened titles and abstracts for potentially eligible
literature based on a predetermined checklist of inclu-
sion criteria. The full text of studies that were not ex-
cluded at this stage was retrieved and independently
assessed by the same two authors to determine eligibil-
ity. When there was a disparity between the assessors, a
third reviewer (PM) was consulted to determine
eligibility.
Study Quality Assessment
The quality assessment was performed independently by
two authors (FH, PV). A third reviewer was available to
assess conflicts if they occurred (PM), but was not
needed. The Joanna Briggs Institute (JBI) critical ap-
praisal tools specific to each study design were used to
assess the methodological quality of included studies
[30,31]. The JBI has a range of critical appraisal tools
developed specifically for use in systemic reviews that
address both quality and bias. All items in the JBI critical
appraisal are rated as “Yes”,“No”,“Unclear”,or“Not ap-
plicable”. There is no total scoring for these tools. We
considered a study to be high quality if all the criteria
within a scale were satisfied.
For our first aim—evaluating potential differences in
plantarflexor muscle function among individuals with
AT (the affected compared with unaffected side, or
AT compared with controls)—the JBI case-control ap-
praisal tool was used. This tool assesses
representativeness of the control group, definition of
the source population, recruitment, validity and reli-
ability of the methods of assessing the condition,
identification and adjustment for confounders, expos-
ure period, and statistical analyses.
For our second aim—evaluating change in plantar-
flexor muscle function following resistance training
interventions—the JBI case series appraisal tool was
themostappropriatetoolsincewewereinterestedin
within-group change. The tool assesses the inclusion
criteria (clarity), standardised, reliable and valid as-
sessment of the condition, whether cases were con-
secutive (or randomly allocated if it was a trial),
inclusion (case series) or retention (trial) of all partic-
ipants, reporting of participant demographics and
clinical information, clear reporting of outcomes, clear
reporting of recruitment source, appropriate statistical
analyses.
Data Extraction Strategy
Two authors (FH, PV) independently extracted data
onto a separate standard data extraction form. Any dis-
crepancies in study selection or extraction were resolved
via discussion, or by consulting a third author where ne-
cessary (PM).
The following data were extracted from each study:
i. Study characteristics (first author name, year of
publication, study design).
ii. Participant characteristics (mean age [years], mean
height [cm], mean weight [kg], sex [number of
men], mean duration of symptoms [number of
months], activity level, site of injury [insertion or
mid-portion], side of injury (unilateral, bilateral),
number of participants, inclusion/exclusion
criteria).
iii. Outcome of interest, and how the measurement
was done.
iv. Mean and standard deviation (SD) of plantarflexor
muscle function measure in each group.
v. Clinical pain and function measured by patients'
self-report at baseline (e.g. the Victorian Institute of
Sports Assessment self-administered Achilles ques-
tionnaire (VISA-A), visual analog scale (VAS), or
numeric pain rating scale (NPRS).
The standard error or p- value and t-value were used
to calculate the standard deviation, if missing [32].
Otherwise, we contacted authors for missing data; this
occurred twice, over two weeks [23,33]. Mean and SD
were extracted from graphs if data were not reported in
the manuscript or the authors did not provide the rele-
vant information.
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Data Synthesis and Presentation
Effect sizes—mean difference (MD) and 95% confidence
intervals (CIs)—were calculated for continuous data
measuring plantarflexor muscle function. Meta-analysis
was performed with Cochrane Collaboration statistical
software, Review Manager 5.3 (RevMan 2014). Data were
pooled where possible to address the first aim—compari-
son of AT and healthy controls or of the affected and
non-affected side. Effect sizes were pooled (MD or
standard mean difference [SMD] if data were on differ-
ent scales) where two or more studies had similar popu-
lation characteristics (age, activity level, site of injury
[mid-portion or insertional]) and utilised the same out-
come. We planned to report the back-translated MD
and 95% CI when possible by multiplying the SMD value
and each confidence band by the SD of the highest-
weighted study. We also planned to report—but not
pool—data for the second aim investigating the pro-
spective change in plantarflexor function, given that data
for this question were pre to post within a group rather
than between-group data. The Consensus on Exercise
Reporting Template (CERT) was used to evaluate the
reporting of exercise doses in interventional studies.
A random effects model was chosen a priori for all
analyses, given clinical and methodological heterogeneity
are likely to exist between studies. The level of statistical
heterogeneity for pooled data was established using I
2
statistic (i.e. interpreted as not important (< 50%), mod-
erate (50–75%) or high (> 75%)) [34]. Where data could
not be pooled, we reported effect estimates and 95% CIs
in narrative form.
We plotted the percentage change in pain and/or
function outcome against various plantarflexor muscle
measures over time. For the pain and/or function out-
come we used VISA-A (the only disease-specific out-
come), otherwise another composite pain and function
outcome (if reported) or overall pain measured with
VAS or NRS. If there were multiple measures reported
for a strength construct (e.g. torque), we selected the
one with the highest percentage change. If there were
multiple trial arms, we only extracted data for the arms
that involved exercise (or exercise only if co-
interventions were added to exercise in other arms).
Levels of reported evidence were determined based on
a modified version of the van Tulder guidelines [35].
Levels of evidence were determined as the following:
i) Strong evidence: Consistent findings among multiple
studies, including at least three high-quality studies.
ii) Moderate evidence: Consistent findings among mul-
tiple trials, including at least three moderate/high-quality
studies or two high-quality studies.
iii) Limited evidence: Consistent findings among mul-
tiple studies, including multiple low/moderate quality
studies, or one high-quality study.
iv) Very limited: Findings from one low/moderate
quality study.
v) Conflicting evidence: Conflicting findings among
multiple studies.
Results
From the 1319 citations in the search yield, a total of 25
studies met our selection criteria. This included 15 stud-
ies evaluating the plantarflexor muscle function among
individuals with AT (question 1) [8–11,26,36–45] and
14 studies evaluating the change in plantarflexor muscle
function following resistance training interventions
(question 2) [23–26,33,39,41,45–51]. There were four
studies addressing both aims [26,39,41,45]. Figure 1
represents the results of the study selection process.
Study Characteristics
A total of 545 participants (353 men and 126 women
with mean age of 40 years ± 7, mean VISA-A of 60 ± 15
out of 100 points, and BMI of 25 kg/m2 [range 21–30])
were included. One study did not specify the sex distri-
bution of participants [39]. Characteristics of the studies,
participants, interventions (prospective studies only) and
outcomes are shown in Additional file 4:Table S1.
For the first question, all studies adopted a cross-
sectional design (i.e. affected vs unaffected (n=11)[11,
26,36,37,39–45], healthy vs AT (n= 6)) [8–10,36,38,
45]. The majority of the studies included mid-portion
AT (n= 11). One study included insertional AT [10],
two studies included a mixed sample [44,45] and one
did not specify the location of pain [9]. AT diagnosis
was based solely on physical examination (i.e. palpation,
site of pain, other clinical tests) [8,9,39,45] or physical
examination and imaging [10,11,26,36–38,40–44].
For the second question, studies were mostly rando-
mised trials [24–26,33,36,39,46–51]. Most studies in-
cluded only mid-portion AT, though one study did not
state the location of AT [24] and one had a mixed popu-
lation [45]. Resistance training interventions were be-
tween 12 weeks and six months in duration and
included isotonic loading (i.e. eccentric or concentric-
eccentric loading protocols), where load and volume
progressed over time (see Table 2for details).
Plantarflexor Muscle Function Measures
Twelve studies [8,9,24,26,36,40–44,48,50]
assessed plantarflexion peak torque using isokinetic
dynamometry and different modes of contraction (i.e.
concentric and eccentric), with the knee bent or ex-
tended. Four studies [10,37,38,45] assessed maximal
voluntary isometric contraction, and one [23] mea-
sured six repetition maximum (RM). Two studies [39,
49] assessed plantarflexion power using the isotonic
concentric and eccentric heel-raise tests. Two studies
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[11,37] measured the plantarflexion explosive
strength. Endurance was measured as isotonic total
work or average work in ten studies [8,25,26,36,
39,40,43,44,47,49] or heel raise test (repetitions to
fatigue) in six studies [23,25,39,46,49,51]. Three
studies did not provide sufficient detail on the out-
comes of management [33,47,50]. For instance,
Mayer et al strength and pain and function data were
extracted from graphs because the values were not re-
ported [50]. Data from Horstmann et al. [47]and
Boesen et al. [33] were not included in the analysis
(Fig. 2) as there were insufficient data reported (in
text or in the graphs). No study assessed
proprioception.
Study Quality Assessment
Quality assessment of the includedstudiesissum-
marised in Table 3. For the first question, evaluating if
there is a difference in plantarflexor function impair-
ment among individuals with AT, four studies scored
yes for all items and were considered high quality [11,
36,37,40]. Most studies (n= 13) appropriately
matched the AT and control participants. All studies
described the method of measurement of the AT (n=
15). One study was at risk of selection bias, as it did not
adequately define the criteria for study inclusion and
exclusion [9]. One study did not provide details regard-
ing participants with unilateral symptoms; therefore,
the validity and reliability of methods used in the
Fig. 1 PRISMA flow diagram of the search results
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 6 of 18
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Table 2 Description of exercise interventions using CERT checklist
Study Alfredson
et al. [26,
41]
Boesen
et al. [47]
Masood
et al. [45]
Mayer et al.
[50]
Neeter
et al. [51]
Rabusin
et al. [46]
Tumilty
et al. [48]
Silbernagel et al.
[25,39,49]
Sancho
et al. [23]
Yu et al.
[24]
Yu et al. [24]
Intervention Eccentric
exercises
Eccentric
exercises
Eccentric
exercises
Eccentric
exercises
Toe-raises Eccentric
exercises
Eccentric
exercises
Toe-raises Hopping
intervention
Eccentric
exercises
Concentric exercises
Exercise
details
Straight
and bend
knee heel
drop off
step
Straight
and bend
knee heel
drop off
step
Straight
and bend
knee heel
drop off
step
Standardised
physiotherapy
with eccentric
training 10
single 30-min
sessions
Two-legged
concentric–
eccentric
toe-raises
Straight
and bend
knee heel
drop off
step
Straight
and bend
knee heel
drop off
step
Two-legged and
one-legged concen-
tric–eccentric toe-
raises, and eccentric
and fast rebounding
toe-raises
4 levels of
exercise
include
isometric,
isotonic,
jump and
hopping
Straight
and bend
knee heel
drop off
step
Heel raises using theraband with
straightened knee, sit on a chair and
lift the heels. Hold onto the wall and
lift the heels of both feet and
progress to injured one. In addition
to Hamstring and calf muscle
stretching
Equipment Backpack
and a
weight
machine
U Backpack U U Backpack
and a
weight
machine
U Backpack and a
weight machine
U Bag with
dumbbell
Elastic band
Provider Physical
therapist
Physical
therapist
U Physical
therapist
Physical
therapist
Podiatrist Physical
therapist
Physical therapist Physical
therapist
Research
assistant
Research assistant
Delivery Individually Individually Individually Individually Individually Individually Individually Individually Individually Individually Individually
Supervision U U U U U U U Y Y U U
Reporting of
adherence
NNNN N Y NU Y NN
Motivation
strategies
NNNN N NNN N NN
Decision rules
for
progressing
Based on
pain level
Based on
pain level
Based on
pain level
U Based on
pain level
Based on
pain level
Based on
pain level
Based on pain level Based on
load
tolerance
Based on
pain level
Based on pain level
Illustrations of
ex
YYYU Y YYY Y UU
Home
programme
content
YYUY Y YYY Y UU
Nonexercised
components
Running
activity
was
allowed if
not agg
pain
sport
activities
were
allowed if
not agg
pain
N U N N U U Running
activity was
allowed if
not agg
pain
NN
Incidence of
adverse
events
documented
NUNN Y Y NY Y NN
Location of
exercises
UUUU U UUU U UU
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Table 2 Description of exercise interventions using CERT checklist (Continued)
Study Alfredson
et al. [26,
41]
Boesen
et al. [47]
Masood
et al. [45]
Mayer et al.
[50]
Neeter
et al. [51]
Rabusin
et al. [46]
Tumilty
et al. [48]
Silbernagel et al.
[25,39,49]
Sancho
et al. [23]
Yu et al.
[24]
Yu et al. [24]
performed
Dosage* 3, 15 reps,
twice a
day 7
days/week,
for 12
weeks, NS
3, 15 reps,
twice a
day 7
days/week,
for 12
weeks, NS
3, 15 reps,
twice a
day 7
days/week,
for 12
weeks, NS
3, 15, Freq ns,
2-3 time/week,
for 4 weeks,
NS
2, 30 reps, 3
times/day
for 10
weeks, NS
3, 15 reps,
twice a
day 7
days/week,
for 12
weeks, NS
3, 15 reps,
twice a
day 7
days/week,
for 12
weeks, NS
3, 10–15 reps, once
a day for 12 weeks
to 6 months, NS
Starts with 5
sets of 45 s,
three times
a day for 12
weeks, BW
3, 15 reps,
Freq ns,
for 8
weeks, NS
3, 15 reps, Freq ns, for 8 weeks, NS
Tailoring Generic,
intensity
was based
on the
patients’
status
Generic,
intensity
was based
on the
patients’
status
Generic,
intensity
was based
on the
patients’
status
U Generic,
progression
was based
on the
patients’
status
Generic,
intensity
was based
on the
patients’
status
Generic,
intensity
was based
on the
patients’
status
Generic, intensity
and number of
repetitions were
based on the
patients’status
Generic,
progression
was based
on the
patients’
status
Individual Individual
Starting level BW U BW U U BW U Bilateral Isometric
seated heel
raises with
BW
Bilateral Bilateral
Exercise
intervention
is delivered as
planned
UUUU U UUU Y UU
Y,yes; N, no; U, unclear; NS, not specified; BW, body weight; ex., exercise; *dosage [sets, repetitions, duration, intensity]
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 8 of 18
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
assessment of conditions were rated as unclear [39]. Six
studies did not clearly report the precise statistical find-
ings (pvalues or 95% CI) or variability data (SD) for the
main outcomes [8,26,41–44].
For the second question, evaluating the change in plantar-
flexor muscle function following resistance training interven-
tions,therewereonlytwohigh-qualitystudiesoutofthe
included studies [23,49]. Two studies did not define their in-
clusion criteria clearly [25,48]. Seven studies were at risk of
selection bias, as they did not indicate a consecutive inclusion
in participant recruitment [24,33,45–47,50,51]. Two
studies lacked precision of their statistical analysis, as they
did not report the 95% CI of the outcome data [26,41].
Quantitative Synthesis for Review Question 1: Is
There a Difference in Plantarflexor Function
Impairment Among Individuals with AT (the
Affected Compared with Unaffected Side, or AT
Compared with Controls)?
Plantarflexor Function in Affected vs Unaffected Side
Strength
All findings relate to mid-portion AT, unless stated.
There was moderate evidence (seven studies, 121
Fig. 2 Difference between affected and unaffected sides in aisotonic plantarflexion peak torque and bendurance. CI, confidence interval;
J, Joules
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 9 of 18
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participants) for lower plantarflexor concentric peak
torque at 90°/s (MD = −8.74 Nm [95%CI = −13.91 to −
3.56], I
2
= 0%, Fig. 2) and 225°/s (MD = −4.83 Nm
[95%CI = −7.59 to −2.08], I
2
= 0%, Fig. 2) and lower
plantarflexor eccentric peak torque at 90°/s (MD = −
12.98 Nm [95%CI = −25.75 to −0.22], I
2
= 0%, Fig. 2)
on the affected side when tested with the knee bent [26,
36,40–44]. The included studies recruited individuals
with mid-portion AT apart from one study that included
a mixed population [44].
There was limited evidence (one study, 34 partici-
pants) for no difference in plantarflexor concentric peak
torque at 90°/s (MD = −3.50 Nm [95%CI = −11.23 to
4.23], at 225°/s (MD = −1.70 Nm [95%CI = −5.93 to
2.53], and eccentric peak torque at 90°/s (MD = −0.50
Nm [95%CI = −17.26 to 16.26], Figure S1) when tested
with the knee extended [36].
There was very limited evidence (one study, 11 par-
ticipants, mixed population of insertional and midpor-
tion AT) for no difference in plantarflexor isometric
peak force (MD = −149 N, [95%CI = −302.92 to
4.92], Figure S1)[45]. Similarly, there was limited evi-
dence (one study, 14 participants) that plantarflexor
isometric peak torque was not different between the
sides (MD = −8.20 Nm, [95%CI = −20.54 to 4.14],
Figure S1)[37].
Table 3 Methodological quality assessment for the included studies
Study Q1 Q2 Q3 Q4 Q5 Q6 Q7 Q8 Q9 Q10 Total of “yes”scores
Review Question 1: Is there a difference in plantarflexor function impairment among people with AT (the affected compared with unaffected side, or
AT compared with controls)?
Wang et al. [11] YYYYYYYYYY 10
Wang et al. [37] YYYYYYYYYY 10
Silbernagel et al. [39] YYYYYYUUYY 8
Ohberg et al. [40] YYYYYYYYYY 10
Alfredson et al. [41] YYYYYYYYYU 9
Alfredson et al. [26] YYYYYYYYYU 9
Alfredson et al. [42] YYYYYYYYYU 9
Alfredson et al. [43] YYYYYYYYYU 9
Alfredson et al. [44] YYYYYYYYYU 9
O'Neill et al. [36] YYYYYYYYYY 10
Chimenti et al. [10] YYUYYYYYYY 9
Masood et al. [45] YYUUUYYYYY 7
Child et al. [38] YUYUYYYYYY 8
McCrory et al. [8] YYUYYYYYYU 8
Haglund-Akerlind and Eriksson [9]NNUUUNNY Y Y 3
Review Question 2: Is plantarflexor function changed over time following exercise interventions?
Rabusin et al. [46] YYYUUYYYYY 8
Sancho et al. [23] YYYYYYYYYY 10
Boesen et al. [47] YYYUYYYYYY 9
Masood et al. [45] YYYUUYYYYY 8
Yu et al. [24] YYYUYYYYYY 9
Horstmann et al. [33] YYYUNYYNNY 6
Silbernagel et al. [49] YYYYYYYYYY 10
Silbernagel et al. [39] YYYYYUYYYY 9
Tumilty et al. [48] NUUYYYYYYY 7
Mayer et al. [50] YYYUYYYYYY 9
Neeter et al. [51] YYYUUYYYYY 7
Silbernagel et al. [25] NYNYNYYYYY 7
Alfredson et al. [41] YYYYYYYYYU 9
Alfredson et al. [26] YYYYYYYYYU 9
Y, yes; N, no; U, unclear; NA, not applicable
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 10 of 18
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Power
There was very limited evidence (one study, 25 partici-
pants) for no difference in isotonic concentric toes raise
power (MD = −42 W, [95%CI = −114.15 to 30.15]),
and no difference in and eccentric toes raise power (MD
=−54 W, [95%CI = −133.06 to 25.06], Figure S1)[39].
Explosive Strength
There was limited evidence (one study, 14 participants) for
reduction in normalised RFD on the affected side when mea-
suredbetween0to30,0to50and0to100ms(MD=−60
Ns [95%CI = −99.41 to −20.59], MD = −69.6 Ns [95%CI =
−112.92 to −26.28], MD = −64.7 Ns [95%CI = −99.14 to −
30.26], respectively, Figure S1)[37]. There was limited evi-
dence (one study, 17 participants) for no difference in RFD
when measured over different time periods (first quarter,
half, three quarters and entire time) to reach peak force (MD
=−226.7 Ns [95%CI = −573.96 to 120.56], MD = −332.9
Ns [95%CI = −670.83 to 5.03], MD = −203.5 Ns [95%CI =
−502.53 to 95.53], MD = −103.4 Ns [95%CI = −250.75 to
43.95], respectively, Figure S1)[11].
Endurance
There was limited evidence (two studies, 24 participants, one
containing a mixed insertional and mid-portion AT popula-
tion [44]) for a reduction in concentric plantarflexor total
work at 90°/s (MD = −35 .18 Nm [95%CI = −69.16 to −
1.20]), and at 225 degrees (MD = −32.85 Nm [95%CI = −
63.68 to −2.02]) in the affected side, but no reduction in ec-
centric plantarflexor total work at 90°/s (MD = −14.73 Nm
[95%CI = −58.74 to 29.28], I
2
=0%,Fig.2)[43,44].
There was also limited evidence (one study, 39 partici-
pants) for no difference between sides in plantarflexor total
work during maximal concentric-eccentric effort at 90°/s
(MD = −177 Nm [95%CI = −394.88 to 40.88], Figure S1)
[36], and very limited evidence (one study, 24 participants)
for no difference in toe raise test for endurance (MD = −180
J[95%CI=−747.18 to 387.18], Figure S1)[39]. Similarly,
there was very limited evidence (one study, 15 participants)
for no difference between sides in concentric plantarflexor
average work at 90 degrees (MD = −5.70 J [95%CI = −
14.31 to 2.91]), at 225°/s (MD = −3.30 J [95%CI = −6.96 to
0.36]), and eccentric average work at 90°/s (MD = −10.80 J
[95%CI = −43.08 to 21.48], Figure S1)[26].
Plantarflexor Function in AT vs Healthy Controls
Strength
There was limited evidence (one study, 39 AT participants
and 38 healthy controls) that isotonic plantarflexor concentric
peak torque at 90°/s (MD = −17.30 Nm [95%CI = −25.73 to
−8.87]), concentric peak torque at 225°/s (MD = −8.10 Nm
[95%CI = −13.54 to −2.66]), and eccentric peak torque at
90°/s (MD = -109.70 Nm [95%CI = -129.09 to −90/31], Fig-
ure S2) was lower in those with AT when tested with knee
bent [36]. In the same study, with the knee extended, com-
parable reductions in isotonic plantarflexor concentric peak
torque at 90°/s (MD = −26.10 [95%CI = −35.45 to −16.75]),
concentric peak torque at 225°/s (MD = −14.80 Nm [95%CI
=−21.01 to −8.59]), and eccentric peak torque at 90°/s (MD
= -55.50 Nm [95%CI = −73.46 to −37.54], Figure S2)were
found [36]. Similarly, there was very limited evidence (one
study, 10 AT participants and 10 healthy controls, not re-
ported whether insertional or mid-portion AT) that isotonic
plantarflexor eccentric peak torque at 30 degree/sec, 60 de-
gree/sec, 120°/s and 180°/s peak torque was lower in AT
(MD = −17.60 Nm [95%CI = −33.22 to −1.98], MD = −21
Nm [95%CI = −40.04 to −1.96], MD = −19.60 Nm [95%CI
=−35.95 to −3.25], MD = −19.70 Nm [95%CI = −32.84 to
−6.56], respectively, Figure S2)[9].
There was very limited evidence (two studies, 41 AT
participants and 68 healthy controls) for no difference in
plantarflexor peak torque at 60°/s (MD = −4.51 Nm
[95%CI = −12.10 to 3.08], I
2
= 0%, Fig. 3), and conflict-
ing evidence between the studies at 180°/s (MD = −0.81
Nm [95%CI = −11.64 to 10.02], I
2
= 82%, Fig. 3). Sub-
stantial heterogeneity may be explained by the uncharac-
teristically low SD reported in the study by McCrory
et al. [8] (confirmed as SD and not standard error in
email communication with author).
There was very limited evidence (two studies, 24 AT
participants and 25 healthy control) for no reduction in
maximal isometric plantarflexion peak force (MD = −44
N [95%CI = −268 to 181], I
2
= 72%, Fig. 3)[38,45]. In
contrast, there was very limited evidence that maximal
isometric plantarflexion peak torque was lower among
individuals with insertional AT (one study, 20 AT partic-
ipants and 20 healthy controls (MD = −18.30 Nm,
[95%CI = −34.33 to −2.27], Figure S2)[10].
Power
There was very limited evidence (one study, 31 AT par-
ticipants and 58 healthy controls) for no difference in
isotonic plantarflexion average power (MD = −8.76
[95%CI = −19.61 to 2.09], Figure S2)[8].
Explosive Strength
No study investigated differences in explosive strength.
Endurance
There was limited evidence (one study, 39 AT partici-
pants and 38 healthy controls) for a reduction in
plantarflexor total work of 20 maximal effort concentric-
eccentric plantarflexor at 90°/s (MD = −613.50 Nm
[95%CI = −833.17 to −393.83], Figure S2)[36]. Further-
more, there was very limited evidence (one study, 31 AT
participants and 58 healthy controls) for no difference in
plantarflexor total and average work of 30 repetitions at
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 11 of 18
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
180°/s (MD = −89.10 Nm [95%CI = −181.70 to 3.50],
Figure S2)[8].
Quantitative Synthesis for Review Question 2:
Does Plantarflexion Function Change over Time in
Individuals Undertaking Resistance Training
Interventions for AT?
Strength
Overall, there was conflicting evidence for improvement
in strength. There was limited evidence (two studies, 29
AT participants) for no change in plantarflexor concen-
tric and eccentric peak torque at 90°/s (MD = 8.76 Nm
[95%CI = −3.45 to 20.97] and MD = 21.29 Nm [95%CI
=−8.08 to 50.67, respectively] and concentric peak
torque at 225°/s (MD = 3.76 Nm [95%CI = −1.60 to
9.12], Fig. 4) after 12 weeks of the Alfredson protocol
[26,41]. In contrast, there was limited quality evidence
(one study, 15 AT participants) for increase in six RM
after 12 weeks of pain guided progressive exercise
among runners (MD = 18.70 kg [95%CI = 7 to 30], Fig-
ure S3)[23]. Similarly, there was very limited evidence
(one study, 20 AT participants) for improved concentric
plantarflexion peak torque after 12 weeks of eccentric
exercise [48] (MD = 59.20 to 84.20, Figure S3) and very
limited evidence (one study, 32 AT participants, not re-
ported whether insertional or mid-portion AT) for
improved concentric plantarflexion peak torque at 30°/s
after eight weeks of eccentric (MD = 8.7 Nm [95%CI =
4.6 to 12.8]) and concentric exercise [24] (MD = 7.97
Nm [95%CI = 2.0 to 13.9], Figure S3). There was very
limited evidence (one study, 10 AT participants and 10
healthy controls, mixed population of insertional and
mid-portion AT) for no change in maximal isometric
plantarflexion peak force among individuals with mid-
portion and insertional AT (MD = 194 N [95%CI = −
0.26 to 388], Figure S3)[45].
Power
There was limited evidence (two studies, 51 AT partici-
pants) for no change in concentric-eccentric plantar-
flexor isotonic power six weeks and one year after the
Silbernagel resistance training programme (MD=26.00
W [95%CI = −0.29 to 0.80] and MD = 59.32 W [95%CI
=−3.60 to 122.23], I
2
= 0%, respectively, Fig. 4)[39,49].
Explosive Strength
No study investigated the change in explosive strength
over time.
Endurance
There was limited evidence (two studies, 49 AT partici-
pants) for improvement in plantarflexor heel raise test
Fig. 3 Difference between tendinopathic group and healthy controls in aplantarflexor isotonic concentric-eccentric peak torque and b
plantarflexor isometric peak force. CI, confidence interval
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 12 of 18
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work after 12 weeks of the Silbernagel programme for
loading intervention (MD = 616.46 J [95%CI = 173.39 to
1059.54], Fig. 4)[39,49]. Similarly, there was limited evi-
dence (four studies, 105 mid-portional AT participants)
that plantarflexion heel raise test repetitions increased
significantly after 12 weeks of resistance training (Silber-
nagel programme [25,51] a resistance training and hop-
ping intervention [23] and Alfredson programme [46]
(Fig. 4). There was very limited evidence (one study, 32
AT participants, not reported whether insertional or
mid-portion AT) that concentric-eccentric plantarflexion
endurance (mean torque over 20 repetitions) signifi-
cantly improved after eight weeks of both concentric
and eccentric exercise intervention (MD ranged from
7.68 and13.56 Nm respectively, Figure S3)[24]. In con-
trast, very limited evidence (one study, 15 AT partici-
pants) demonstrated no change in isotonic
plantarflexion endurance at 90 and 225°/s with the
Alfredson programme at 12 weeks (MD ranged from
2.20 and 4.70 J respectively, Figure S3)[26].
Figure 5shows that the percentage improvement
in pain and/or function outcomes ranges from 20.5%
to 95.0% at 12 weeks, and further gains appear to
be more gradual beyond 12 weeks. In contrast, per-
centage improvement in plantarflexor muscle func-
tion is 14.9% to 19.2% for strength, 0.8% to 33.3%
for endurance, and 9.4% to 25.6% for power at 12
weeks.
Fig. 4 Change over time among individuals with Achilles tendinopathy in aisotonic plantarflexor concentric-eccentric peak torque, b
plantarflexion power, cisotonic total work, and dheel raise to fatigue. CI, confidence interval; J, Joules
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 13 of 18
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Discussion
This review identified moderate evidence that individ-
uals with AT have impairment in maximal plantarflexor
torque (seven studies including one with a mixed popu-
lation) on their affected side, compared with the un-
affected side. Impairments were modest (9% and 13%
[pooled effect divided by mean of the unaffected side
scores]) and of uncertain clinical importance. The
remaining evidence, primarily among individuals with
mid-portion AT, showed conflicting impairments for
plantarflexor function (i.e. explosive strength and endur-
ance) between sides. One study among individuals with
insertional AT reported reduced isometric plantarflexor
torque (27%) compared with healthy controls. There
were no differences for all remaining plantarflexor
muscle measures (i.e. isotonic torque, power and endur-
ance) between primarily mid-portion AT and healthy
control groups. These findings are different from the
conclusion in the previous review by McAullife et al
[19], where all plantarflexor functional data—side to side
versus healthy to AT comparisons, as well as isometric
and isotonic—were combined. The authors of this re-
view reported that there were deficits in plantarflexor
function in individuals with AT. Our review provides a
more nuanced investigation of how strength may be im-
paired in AT and suggests that, besides torque and iso-
metric strength impairment on the affected side, other
impairments are conflicting and uncertain. There was
limited to very limited evidence for improvement in
plantarflexor endurance (7% and 23% [pooled effect di-
vided by mean of the unaffected side scores]) but not
power or strength (five studies including one with a
mixed population for strength) over time, despite indi-
viduals undertaking several weeks of resistance training.
Reduction in strength (isotonic and isometric) on
the affected side among individuals with AT may be
related to apprehension or fear [39,52]. In a prior
trial among individuals with AT, recovery in calf
strength was impaired among individuals with greater
fear avoidance beliefs [39]. This suggests that fear of
loading the affected Achilles tendon may drive some
individuals to adopt avoidance-type behaviours, which
may manifest during—and influence—maximal
strength testing. Impaired strength between sides
could also be related to atrophy or reduced muscle
activation, which may occur secondary to pain [53].
Lastly, it may be possible that strength impairments
were present prior to developing pain [6]. Mahieu
et al. investigated prospective risk factors for the de-
velopment of AT among a cohort of 69 military re-
cruits over six weeks and found a reduction in
plantarflexor peak torque for both the right and left
leg (MD = −16.82 Nm [95%CI = −26.65 to −6.99]
and (MD = −18.56 Nm [95%CI = −32.15 to −4.97],
respectively [6]. Currently, there is an incomplete un-
derstanding of the mechanisms underlying impaired
plantarflexor muscle function among individuals with
AT.
The findings of our review are contrary to the theory
that individuals with tendinopathy have bilateral
strength impairments. Heales et al [54] synthesised the
evidence for bilateral sensory and motor deficits involve-
ment in all unilateral tendinopathy, not specifically AT.
The meta-analysis of the data demonstrated contralateral
sensory (i.e. pressure and thermal pain thresholds, reac-
tion time) and motor (i.e. speed in movement) system
deficits in individuals with unilateral pathology. They
proposed that there may be motor cortex inhibition af-
fecting strength on both sides and suggested that the un-
affected side is not a healthy comparator that can be
used in clinical assessment. The difference in findings in
our review may be explained by the populations investi-
gated. In the Heales et al. [54] review, 18 out of the 20
studies included were among individuals with upper
limb tendinopathy (lateral elbow or rotator cuff tendino-
pathy). The role of muscle inhibition in the upper limb
and lower limb requires consideration in longitudinal
studies. It may be that bilateral inhibition and strength
impairments manifest for some individuals at varying
stages of tendinopathy disease, or that these impair-
ments are specific to the site of tendinopathy.
There were conflicting findings for impairments in
plantarflexor muscle function in affected versus un-
affected side comparisons, and for the healthy versus
controls comparisons. This is surprising, as these mea-
sures relate to functional tasks that are impaired among
individuals with AT during such as walking [55,56],
running [16,18,57] and hopping [12,49]. For example,
calf function relates to walking speed, which has been
shown to be impaired in AT [58]. The confidence of our
findings may be limited by the limited number of stud-
ies, quality of the literature, population size or character-
istics. There is also the possibility that differences in the
study populations explain the findings. Although studies
in this review involved active participants, mostly run-
ners, only two of the six cross-sectional studies specified
the duration of AT symptoms. Conflicting findings could
also be explained by variability in strength among indi-
viduals with AT that may relate to the severity or dur-
ation of disease, or other factors [59]. Lastly, conflicting
findings could also be because of differences in testing
protocols for plantarflexor outcomes. For instance, one
study used a heel raise endurance test until fatigue [39],
while another study used twenty maximal effort
concentric-eccentric plantarflexion contractions [36]. A
third used the middle 30 repetitions of 32 repetitions
performed at an angular velocity of 180° as an indicator
of muscular endurance [26].
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Gains in strength after rehabilitation were conflicting.
Only endurance measures increased which may be ex-
plained by the relatively high-volume resistance pro-
grammes prescribed among the studies in this review
(15 to 30 repetitions). At face value, it is surprising that
we did not observe consistent gains in strength (e.g.
torque) because the resistance training prescribed should
lead to strength and hypertrophy gains [28]. This finding
is less surprising when considered in the context of con-
flicting plantarflexor function impairments across
Fig. 5 Change (%) in pain/function and various plantarflexor muscle function measures with resistance exercise over time in individuals with Achilles
tendinopathy: apain and function measured by the Victorian Institute of Sports Assessment (VISA) self-administered Achilles questionnaire [23,39,46,
48,49], Pain Disability Index [50], visual analog scale (VAS) [24–26,41] or proportion that had pain with activity [51]; bstrength, i.e. peak concentric
torque [24,26,41,48,50], isometric torque [45] or repetition maximum [23]; cendurance, i.e. work done during isotonic contractions [26,39,49], heel
raise repetitions [23,46,51] or mean concentric torque over repeated repetitions; dpower, i.e. isotonic heel raise power
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 15 of 18
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studies in our review and may reflect the participants
having adequate strength when they commenced re-
habilitation. Alternatively, the lack of plantarflexor gains
in some prospective studies may be related to inad-
equate prescribed exercise dose (e.g. volume or intensity)
or inadequate exercise adherence; both factors were in-
completely reported in the included studies (Table 2).
Only one out of 14 interventional studies transparently
revealed the adherence and fidelity of the resistance
training programme [23]. This finding highlights the
need to develop strategies for more effective loading
programmes to address impairments.
The findings of this review also raise questions about
the mechanisms of pain improvement in AT. Despite im-
provements in pain and function with loading interven-
tions in the included trials, parallel plantarflexor function
often improved more modestly (see Fig. 5) and often plan-
tarflexor function improvements were not significant.
These findings demonstrate that even with substantial
pain and function improvement, improvement in plantar-
flexor muscle function may be minimal. This questions
how important plantarflexor muscle improvements are in
improving AT symptoms [60]. It is important to note that
we could only observe parallel changes in pain and plan-
tarflexor function at the group rather than individual level
[61]. Even if it is not a mechanism for improved pain, re-
covering plantarflexor muscle function is presumably im-
portant to allow individuals to return to activities of daily
living such as walking and sport.
An important finding in this review is that plantar-
flexor impairment is likely to be a heterogeneous finding
among individuals with AT, but simply, not everyone
with AT will have strength impairments. The challenge
that this presents is identifying individuals with impaired
strength. Currently, there is an incomplete understand-
ing of what constitutes a plantarflexor muscle impair-
ment for specific populations. Adequate strength is
likely to be very individual and depends on activity levels
(e.g. athletes of different levels versus non-athletic
people), age, and other factors [62]. Efforts to define nor-
mal plantarflexor function among the community that
includes both healthy individuals and those with lower
limb pathology are needed.
Limitations
There are several limitations related to the literature—par-
ticularly, the low individual study quality—which limited
robust conclusions. It is important to note that there were
differences in diagnostic criteria between studies that may
have contributed to heterogeneity in the condition. For
example, there may have been a range of pathologies in-
cluding paratenon and plantaris tendon involvement, or
even partial tears, and these pathologies may impact plan-
tarflexor muscle function differently. Further, we
identified few studies investigating strength among people
with insertional AT which means we have very little cer-
tainty about plantarflexor impairments in this population.
There were also limitations related to the review. The
main review limitation was a limited number of studies
that both met our criteria and had similar populations, in-
terventions, and outcomes. This precluded meta-analysis
of the data. Another limitation is that we could not iden-
tify studies for some of our outcomes of interest. Specific-
ally, although muscle force steadiness has been
investigated in the healthy, young and elderly populations,
we could not find a study in the AT population. It is also
important to note that a majority of studies for the af-
fected vs unaffected side comparison for plantarflexor
torque were by the same author’s group (Alfredson et al.
[26,41–44]). Finally, although addressing our aim to in-
vestigate within-group change, including non-randomised
studies means that biases such as natural history and re-
gression to the mean may threaten the internal validity of
any within-group change that was observed.
Conclusion
There was moderate evidence that individuals with AT
have impairments in maximal plantarflexor torque and
limited evidence for impairment in concentric endurance
on their affected side. However, there was conflicting evi-
dence for other plantarflexor function—explosive strength,
power, and other endurance measures—between sides,
and for all measures when compared with healthy con-
trols. There was limited to very limited evidence for im-
provement in plantarflexor endurance but not in strength
or power after undertaking 12 weeks of resistance training.
There is a need for high-quality studies to investigate
plantarflexor impairment in individuals with AT, and to
identify optimal interventions to address impairments.
Supplementary Information
The online version contains supplementary material available at https://doi.
org/10.1186/s40798-021-00308-8.
Additional file 1: Figure S1. Difference between affected and
unaffected sides in a) isotonic plantarflexion peak torque; b) maximal
isometric plantarflexion strength; c) isotonic power; d) explosive strength;
e) endurance. Abbreviation: CI, confidence interval; RFD, normalised rated
of force development; ms, millisecond.
Additional file 2: Figure S2. Difference between tendinopathic group
and healthy controls in a) isotonic concentric-eccentric plantarflexor peak
torque between mid-portion pathological group and healthy controls; b)
isometric plantarflexor peak torque between insertional pathological
group and healthy controls; c) isotonic power; d) endurance. Abbrevi-
ation: CI, confidence interval; J, Joules.
Additional file 3: Figure S3. Change over time among individuals with
Achilles tendinopathy in a) isotonic plantarflexor concentric-eccentric
peak torque; b) isotonic power; c) endurance. Abbreviation: CI, confidence
interval; W, Watt unit; J, Joules.
Additional file 4: Table S1. Summary of the characteristics of the
included studies.
Hasani et al. Sports Medicine - Open (2021) 7:18 Page 16 of 18
Content courtesy of Springer Nature, terms of use apply. Rights reserved.
Acknowledgements
Fatmah Hasani acknowledges the support from the Saudi Arabian Ministry of
Higher Education Scholarship. Patrick Vallance acknowledges the support
from the Australian Governments Research Training Program Scholarship.
Authors’Contributions
FH, TH, SEM and PM contributed to the conception and design. FH, PV and
PM conducted the comprehensive search, critical appraisal and analyses. FH
drafted the manuscript and all co-authors provided critical revisions to the
clinical and intellectual content. All authors have read and approved the
manuscript.
Authors’Information
Fatmah Hasani, PhD candidate, Physiotherapy Department, Monash
University.
Funding
No financial support was received for the conduct of this study or
preparation of this article.
Availability of Data and Materials
The datasets generated and/or analysed during the current study are
available from the corresponding author on reasonable request.
Ethics Approval and Consent to Participate
Not applicable.
Consent for Publication
Not applicable.
Competing Interests
Fatmah Hasani, Patrick Vallance, Terry Haines, Shannon Munteanu and Peter
Malliaras declare that they have no conflicts of interest.
Author details
1
Physiotherapy Department, School of Primary and Allied Health Care,
Monash University, Frankston, Victoria 3199, Australia.
2
Physiotherapy
Department, Security Forces Hospital, Riyadh 11481, Saudi Arabia.
3
School of
Primary and Allied Health Care, Faculty of Medicine, Nursing, and Health
Sciences, Monash University, Frankston, Victoria 3199, Australia.
4
Discipline of
Podiatry, School of Allied Health, Human Services and Sport, College of
Science, Health and Engineering, La Trobe University, Melbourne, Victoria
3086, Australia.
5
La Trobe Sport and Exercise Medicine Research Centre,
School of Allied Health, Human Services and Sport, College of Science,
Health and Engineering, La Trobe University, Melbourne, Victoria 3086,
Australia.
Received: 19 June 2020 Accepted: 14 February 2021
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