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Objectives: To determine the incidence of Achilles tendinopathy in a large group of recreational runners and to determine risk factors for developing AT. Design: Observational cohort study. Methods: Runners registering for running events (5-42 km) in the Netherlands were eligible for inclusion. Main inclusion criteria were: age ≥18 years, and registration ≥2 months before the running event. The digital baseline questionnaire obtained at registration consisted of demographics, training characteristics, previous participation in events, lifestyle and previous running-related injuries. All participants received 3 follow-up questionnaires up to 1 month after the running event with self-reported AT as primary outcome measure. To study the relationship between baseline variables and AT onset, multivariable logistic regression analyses were performed. Results: In total, 2378 runners were included, of which 1929 completed ≥1 follow-up questionnaire, and 100 (5.2%, 95%CI [4.2;6.2]) developed AT. Runners registered for a marathon (7.4%) had the highest incidence of AT. Risk factors for developing AT were use of a training schedule (odds ratio (OR) = 1.8 (95%Confidence Interval(CI)[1.1;3.0])), use of sport compression socks ((OR = 1.7, 95%CI[1.0;2.8]) and AT in the previous 12 months (OR = 6.3, 95%CI[3.9;10.0]). None of the demographic, lifestyle or training-related factors were associated with the onset of AT. Conclusion: One in twenty recreational runners develop AT. AT in the preceding 12 months is the strongest risk factor for having AT symptoms. Using a training schedule or sport compression socks increases the risk of developing AT and this should be discouraged in a comparable running population. Trial registration number: The Netherlands Trial Register (ID number: NL5843).
Content may be subject to copyright.
Please
cite
this
article
in
press
as:
Lagas
IF,
et
al.Incidence
of
Achilles
tendinopathy
and
associated
risk
factors
in
recreational
runners:
A
large
prospective
cohort
study.
J
Sci
Med
Sport
(2019),
https://doi.org/10.1016/j.jsams.2019.12.013
ARTICLE IN PRESS
G Model
JSAMS-2222;
No.
of
Pages
5
Journal
of
Science
and
Medicine
in
Sport
xxx
(2019)
xxx–xxx
Contents lists available at ScienceDirect
Journal
of
Science
and
Medicine
in
Sport
journal homepage: www.elsevier.com/locate/jsams
Original
research
Incidence
of
Achilles
tendinopathy
and
associated
risk
factors
in
recreational
runners:
A
large
prospective
cohort
study
Iris
F.
Lagasa,,
Tryntsje
Fokkemab,
Jan
A.N.
Verhaara,
Sita
M.A.
Bierma-Zeinstraa,b,
Marienke
van
Middelkoopb,
Robert-Jan
de
Vosa
aDepartment
of
Orthopaedic
Surgery
and
Sports
Medicine,
Erasmus
MC,
University
Medical
Center
Rotterdam,
The
Netherlands
bDepartment
of
General
Practice,
Erasmus
MC,
University
Medical
Center
Rotterdam,
The
Netherlands
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
28
May
2019
Received
in
revised
form
10
December
2019
Accepted
15
December
2019
Available
online
xxx
Keywords:
Athletes
Athletic
injuries/prevention
&
control
Ankle
injuries
Epidemiology
a
b
s
t
r
a
c
t
Objectives:
To
determine
the
incidence
of
Achilles
tendinopathy
in
a
large
group
of
recreational
runners
and
to
determine
risk
factors
for
developing
AT.
Design:
Observational
cohort
study.
Methods:
Runners
registering
for
running
events
(5–42
km)
in
the
Netherlands
were
eligible
for
inclusion.
Main
inclusion
criteria
were:
age
18
years,
and
registration
2
months
before
the
running
event.
The
digital
baseline
questionnaire
obtained
at
registration
consisted
of
demographics,
training
characteristics,
previous
participation
in
events,
lifestyle
and
previous
running-related
injuries.
All
participants
received
3
follow-up
questionnaires
up
to
1
month
after
the
running
event
with
self-reported
AT
as
primary
outcome
measure.
To
study
the
relationship
between
baseline
variables
and
AT
onset,
multivariable
logistic
regression
analyses
were
performed.
Results:
In
total,
2378
runners
were
included,
of
which
1929
completed
1
follow-up
questionnaire,
and
100
(5.2%,
95%CI
[4.2;6.2])
developed
AT.
Runners
registered
for
a
marathon
(7.4%)
had
the
highest
incidence
of
AT.
Risk
factors
for
developing
AT
were
use
of
a
training
schedule
(odds
ratio
(OR)
=
1.8
(95%Confidence
Interval(CI)[1.1;3.0])),
use
of
sport
compression
socks
((OR
=
1.7,
95%CI[1.0;2.8])
and
AT
in
the
previous
12
months
(OR
=
6.3,
95%CI[3.9;10.0]).
None
of
the
demographic,
lifestyle
or
training-related
factors
were
associated
with
the
onset
of
AT.
Conclusion:
One
in
twenty
recreational
runners
develop
AT.
AT
in
the
preceding
12
months
is
the
strongest
risk
factor
for
having
AT
symptoms.
Using
a
training
schedule
or
sport
compression
socks
increases
the
risk
of
developing
AT
and
this
should
be
discouraged
in
a
comparable
running
population.
Trial
registration
number:
The
Netherlands
Trial
Register
(ID
number:
NL5843).
©
2019
Published
by
Elsevier
Ltd
on
behalf
of
Sports
Medicine
Australia.
Practical
implications
Achilles
tendinopathy
is
a
serious
problem,
as
it
occurs
in
one
in
twenty
runners.
Marathon
runners
have
the
highest
incidence
of
Achilles
tendinopathy,
with
an
incidence
of
7.4%.
Previous
Achilles
tendinopathy
is
the
strongest
risk
factor
for
hav-
ing
(recurrent)
symptoms,
so
runners
with
a
history
of
Achilles
tendinopathy
should
be
regarded
as
high-risk.
Corresponding
author.
E-mail
address:
i.lagas@erasmusmc.nl
(I.F.
Lagas).
The
use
of
a
training
schedule
and
use
of
sport
compression
socks
should
not
be
encouraged
in
a
high-risk
population,
as
they
may
increase
the
risk
of
developing
Achilles
tendinopathy.
Other
possible
risk
factors,
such
as
alcohol
use,
do
not
increase
the
risk
of
developing
Achilles
tendinopathy.
1.
Introduction
Achilles
tendinopathy
(AT)
is
a
tendon
disorder
consisting
of
pain,
swelling
and
impaired
performance,
and
can
cause
pro-
longed
absence
from
health-promoting
activities.1,2 An
increase
in
physical
activity
level
is
often
thought
to
be
associated
with
the
development
of
tendinopathy,
which
ranges
from
a
reactive
to
a
chronic
state.3Reactive
AT
is
considered
to
be
caused
by
increased
cell
proliferation
with
an
increase
in
water-attracting
https://doi.org/10.1016/j.jsams.2019.12.013
1440-2440/©
2019
Published
by
Elsevier
Ltd
on
behalf
of
Sports
Medicine
Australia.
Please
cite
this
article
in
press
as:
Lagas
IF,
et
al.Incidence
of
Achilles
tendinopathy
and
associated
risk
factors
in
recreational
runners:
A
large
prospective
cohort
study.
J
Sci
Med
Sport
(2019),
https://doi.org/10.1016/j.jsams.2019.12.013
ARTICLE IN PRESS
G Model
JSAMS-2222;
No.
of
Pages
5
2
I.F.
Lagas
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
xxx
(2019)
xxx–xxx
glycosaminoglycans.3Chronic
AT
is
characterised
by
tissue
degen-
eration
with
structural
collagen
changed
and
a
long
recovery
time
and
the
challenge
of
finding
successful
treatment
options.4This
emphasizes
that
developing
a
prevention
strategy
in
an
‘at
risk’
population
is
a
priority.
The
first
step
towards
a
prevention
strategy
is
to
establish
the
extent
of
the
problem
by
reporting
the
injury
incidence.5The
inci-
dence
of
tendinopathy
is
dependent
on
the
population
examined,
as
it
is
mainly
described
in
the
general,
working
and
sporting
population.6For
example,
in
elite
runners
the
cumulative
inci-
dence
of
AT
is
as
high
as
52%.7Running
grows
in
popularity
it
is
estimated
that
around
50
million
people
in
Europe
(12%
of
inhabitants
age
15–80
years)
run
on
a
regular
basis.8Runners
have
a
high
risk
to
develop
an
injury,
with
6.1
running-related
injuries
per
1000
running
hours.9AT
is
one
of
the
most
frequent
reported
injuries
in
runners,
with
incidence
rates
varying
from
3.5%
to
8.3%.10,13,14 Unfortunately,
these
studies
mostly
consist
of
relatively
small
sample
sizes,
with
a
high
variability
in
running
pop-
ulations
(for
example;
recreational
versus
elite
runners,
injuries
with
self-referral
versus
practitioner
referred,
or
young
versus
old
running
athletes).10,13,14
The
second
step
in
developing
a
prevention
strategy
is
to
deter-
mine
risk
factors.5Risk
factors
can
be
categorised
as
modifiable
(e.g.
alcohol
use,
running
distance)
and
non-modifiable
(e.g.
sex,
age).
Modifiable
risk
factors
can
be
used
for
developing
a
preven-
tion
strategy.
A
recent
systematic
review
showed
limited
evidence
for
9
risk
factors
associated
with
AT
onset
in
diverse
populations.15
Modifiable
factors
were
moderate
alcohol
consumption,
ofloxacin
use
and
a
reduced
plantar
flexor
strength.15 A
limitation
of
this
systematic
review
was
the
fact
that
these
factors
were
assessed
in
many
different
populations,
including
runners
ranging
from
novice
to
elite
running
experience.15 This
questions
the
generalizability
of
these
associations
for
specific
groups
of
athletes.
Another
limitation
in
current
literature
is
the
small
numbers
of
injured
participants
(injury
events)
reported
in
the
specific
studies,
as
at
least
20–50
injury
cases
are
needed
to
detect
strong
to
moderate
associations.16
Consequently,
current
evidence
for
risk
factors
associated
with
AT
in
runners
is
limited.
We
conducted
a
large
prospective
study
with
the
primary
aim
to
determine
the
incidence
of
AT
in
recreational
runners
and
with
the
secondary
aim
to
determine
risk
factors
for
AT.
2.
Methods
This
study
is
part
of
the
INSPIRE
trial
(INntervention
Study
on
Prevention
of
Injuries
in
Runners
at
Erasmus
MC)2and
was
approved
by
the
Medical
Ethics
Committee
of
the
Erasmus
MC
University
Medical
Centre
Rotterdam,
The
Netherlands
(MEC-
2016-292).
The
trial
is
registered
in
the
Netherlands
Trial
Register
(NTR
number:
NL5843).
Runners
of
18
years
or
older
signing
up
for
one
of
three
large
running
events
(5–42.2
km)
in
the
Netherlands
were
asked
to
par-
ticipate
in
this
study.
Recruitment
was
from
October
2016
until
April
2017.
Runners
were
excluded
if
they
(1)
did
not
have
email
access,
(2)
were
not
familiar
with
the
Dutch
language
or
(3)
regis-
tered
within
two
months
before
the
running
event.
All
runners
were
asked
to
complete
online
questionnaires
on
four
time
points:
at
baseline
(2
months
before
the
running
event),
2
weeks
before
the
running
event,
1
day
after
the
running
event
and
1
month
after
the
running
event.
The
questions
in
the
question-
naires
were
based
on
existing
literature
on
risk
factors
for
running
related
injuries.2The
baseline
questionnaire
was
divided
in
four
different
sections:
(1)
demographics,
(2)
training
characteristics,
(3)
lifestyle
and
(4)
running-related
injuries
in
the
previous
12
months
(Supplementary
file
1).
The
baseline
questionnaire
also
inquired
if
the
runner
still
had
symptoms
of
a
running-related
injury.
All
runners
received
all
follow-up
questionnaires,
regardless
of
injury
status.
Follow-up
questionnaires
consisted
of
questions
about
the
status
of
previous
reported
running-related
injuries.
The
next
section
of
the
questionnaire
handled
information
about
new
running-related
injuries.
Runners
were
included
in
data-analysis
if
they
completed
one
or
more
follow-up
questionnaires.
The
primary
outcome
measure
was
the
incidence
of
self-
reported
AT
during
the
follow-up
period.
Runners
were
asymp-
tomatic
at
baseline
and
reported
AT
in
the
section
about
new
running-related
injuries
in
one
of
the
follow-up
questionnaires.
AT
was
defined
as
an
injury
of
the
Achilles
tendon
caused
by
running,
and
when
one
or
more
of
the
following
criteria
were
met:
(1)
the
injury
causes
a
reduction
in
running
distance,
frequency,
speed
or
duration
for
at
least
1
week,
or
(2)
the
injury
leads
to
an
appoint-
ment
with
a
doctor
and/or
physiotherapist
or
(3)
medication
is
necessary
to
reduce
symptoms
(Supplementary
file
1).
SPSS
software
(V.24.0.0.1;
SPSS,
Chicago,
Illinois,
USA)
was
used
for
statistical
analysis.
We
used
a
Shapiro
Wilk
test
for
nor-
mality.
We
assumed
normal
distribution
of
the
data
if
W
>
0.90.
To
evaluate
differences
between
responders
and
non-responders,
baseline
characteristics
of
included
runners
and
runners
who
did
not
complete
any
follow-up
questionnaire
were
compared
using
an
independent
sample
t-test
(normally
distribution)
or
Mann-
Whitney
U
test
(not
normally
distributed).
Categorical
variables
were
analysed
using
a
chi
square
test.
The
incidence
of
AT
(pri-
mary
aim)
was
calculated
by
dividing
the
total
number
of
included
runners
with
the
number
of
runners
that
reported
AT.
Incidence
of
AT
per
time
frame
was
calculated
by
dividing
the
number
of
AT
developed
during
that
time
frame
by
the
mean
days
between
two
questionnaires.
The
incidence
of
AT
per
time
frame
is
presented
as
number
of
patients
developing
AT
per
day.
Risk
factors
for
developing
AT
(secondary
aim)
were
identified
using
a
multivariable
logistic
regression
analysis
[ENTER
model].
We
assessed
the
relationship
between
an
event
(onset
of
self-
reported
AT)
and
the
following
variables:
sex,
age,
Body
Mass
Index
(BMI),
units
of
alcohol
per
week,
running
experience,
running
dis-
tance
per
week,
use
of
a
training
schedule,
use
of
sport
compression
socks,
use
of
insoles,
number
of
running
shoes
per
year,
land-
ing
type,
running
80%
on
paved
road
and
AT
in
the
previous
12
months.
Results
were
presented
as
odds
ratio
(OR)
with
95%
con-
fidence
interval(CI).
A
p-value
<0.05
was
considered
statistically
significant.
3.
Results
A
total
of
2378
runners
were
included
in
the
INSPIRE
trial.
Of
these
runners,
1929
(81.1%)
completed
one
or
more
follow-up
questionnaires
with
a
mean
follow-up
(standard
deviation,
SD)
of
20.5
(7.0)
weeks,
and
were
therefore
included
in
the
current
study
(Table
1).
We
found
a
number
of
statistical
differences
between
the
included
runners
and
the
runners
who
did
not
complete
any
follow-up
questionnaire
(Supplementary
file
2).
Of
the
1929
included
runners,
100
runners
reported
the
onset
of
AT
(5.2%
(95%CI
[4.2;6.2]).
The
included
runners
were
mostly
male
(52.9%),
were
an
average
age
of
41.9
(SD
12.1)
years
old
and
ran
a
median
of
18.0
km
(interquartile
range
(IQR)
20.0)
per
week.
The
incidence
of
AT
increased
with
increasing
event
distance
from
4.0%
when
running
10
km
to
7.4%
when
running
a
full
marathon
(Table
2).
The
incidence
of
AT
was
higher
in
runners
who
registered
for
a
marathon
compared
to
other
distances
(OR
1.7,
p
=
0.014).
The
incidence
of
AT
was
low
in
the
period
from
registration
up
to
2
weeks
before
the
running
event
(0.5
AT
per
day),
increased
in
the
period
from
2
weeks
before
until
1
day
after
the
event
(1.9
AT
per
Please
cite
this
article
in
press
as:
Lagas
IF,
et
al.Incidence
of
Achilles
tendinopathy
and
associated
risk
factors
in
recreational
runners:
A
large
prospective
cohort
study.
J
Sci
Med
Sport
(2019),
https://doi.org/10.1016/j.jsams.2019.12.013
ARTICLE IN PRESS
G Model
JSAMS-2222;
No.
of
Pages
5
I.F.
Lagas
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
xxx
(2019)
xxx–xxx
3
Table
1
Baseline
characteristics
of
included
runners.
Included
runners
N%/Mean
(SD)/Median;
IQR
N
1929
Demographics
Sex
(male)
52.9%
Age
(years) 41.9
(12.1)
BMI
(kg/m2)
23.6
(2.9)
Lifestyle
Alcohol
use
(units
per
week)
3.0
;
5.0
Running
event
5
or
7.5
km
5.8%
10
km 38.6%
21.1
km 31.1%
42.2
km
24.6%
Training
Running
experience
(years)
4.0;
6.0
Running
distance
per
week
(km)
18.0;
20.0
Use
of
training
schedule
62.3%
Use
of
sport
compression
socks
15.6%
Use
of
insoles
21.9%
Number
of
running
shoes
per
year 2.0;
1.0
Landing
type
Hind-
or
midfoot
60.5%
Forefoot
18.0%
Running
80%
on
paved
road
77.3%
Injuries
Injury
in
the
previous
12
months
51.5%
Achilles
tendinopathy
in
the
previous
12
months 8.2%
Completed
follow-up
questionnaire
Completed
FU
questionnaire
1
91.8%
Completed
FU
questionnaire
2
90.6%
Completed
FU
questionnaire
3
82.4%
Completed
all
FU
questionnaires 74.0%
Mean
number
of
FU
questionnaires
completed
(1–3)2.7
(0.6)
SD
=
Standard
Deviation;
IQR
=
inter
quartile
range;
BMI
=
Body
Mass
Index;
FU
=
follow-up.
day)
and
lowered
in
the
period
of
1
day
after
until
1
month
after
the
event
(1.0
AT
per
day)
(Table
2).
Risk
factors
for
AT
were
presence
of
AT
in
the
previous
12
months
(OR
6.3,
95%CI[3.9;10.0]),
use
of
a
training
schedule
(OR
1.8,
95%CI[1.1;3.0])
and
use
of
sports
compression
socks
(OR
1.7,
95%CI[1.0;2.8])
(Table
3).
4.
Discussion
This
is
the
first
large
prospective
cohort
study
in
recreational
runners
reporting
the
incidence
of
AT
and
the
risk
factors
for
devel-
oping
AT.
We
found
an
overall
AT
incidence
in
runners
of
5.2%
with
the
highest
incidence
in
the
subgroup
of
runners
registered
for
a
marathon
(7.4%).
In
the
two-week
period
before
up
to
1
day
after
the
running
event,
onset
of
AT
peaked
to
1.9
developed
AT
per
day.
Presence
of
AT
in
the
previous
12
months
was
the
strongest
risk
factor
for
having
(recurrent)
AT
symptoms.
The
use
of
a
training
schedule
and
sport
compression
socks
also
increased
the
risk
of
developing
AT.
Other
demographics,
lifestyle-
or
training-related
factors
at
baseline
were
not
identified
as
risk
factors
for
AT.
These
findings
are
relevant
for
sports
medicine
healthcare
providers,
as
information
about
incidence
rates
of
specific
injuries
in
specific
sports
increases
awareness
of
important
problems
within
this
field.
Knowledge
of
risk
factors
aid
in
development
of
effective
preventive
intervention
programs.
A
study
by
Hirschmüller
et
al.17 reported
a
7.5%
incidence
of
AT
in
long-distance
runners
after
a
follow-up
of
1
year.
Two
major
differences
are
that
runners
in
the
study
by
Hirschmüller
et
al.17
ran
twice
as
many
kilometres
per
week
(35.3
versus
19.9
km)
and
that
they
had
twice
as
much
running
experience
(12.7
versus
6.5
years)
than
runners
included
in
our
study.
However,
when
compar-
ing
the
incidence
of
AT
in
marathon
runners
in
our
study
with
the
long-distance
runners
included
by
Hirschmüller,
results
are
com-
parable
(7.4%
versus
7.5%,
respectively).
Another
study
by
McKean
et
al.10 divided
runners
in
masters
(age
40
years)
and
younger
run-
ners
(age
<40
years).
Master
runners
had
more
AT
than
younger
runners
(6.2%
versus
3.5%).
This
is
conflicting
with
our
data,
as
we
found
no
correlation
with
developing
AT
and
age
or
running
distance
per
week.
One
possible
explanation
could
be
that
mas-
ter
runners
report
to
run
2.5
times
as
many
kilometres
per
week
than
our
included
runners,10 which
is
a
large
difference.
Lysholm
et
al.11 reported
a
8.3%
incidence
of
AT
in
a
mixed
group
of
sprinters,
middle-distance
runners
and
marathon
runners.
While
it
is
impor-
tant
to
report
incidence
rates
in
specific
groups
of
athletes,
it
can
be
even
more
valuable
to
subdivide
the
incidence
rates
of
specific
groups
of
running
athletes
as
a
previous
systematic
review
showed
a
large
variability
in
running-related
injuries
among
runners.18 Our
study
adds
value
by
reporting
incidence
rates
in
recreational
run-
ners
including
all
running
event
distances
and
divided
by
running
event
distance.
Our
results
show
that
AT
incidence
is
higher
in
marathon
runners
compared
to
smaller
distances.
Therefore,
devel-
opment
of
prevention
strategies
seems
especially
relevant
for
this
target
group.
The
strongest
risk
factor
for
having
(recurrent)
AT
symptoms
was
the
presence
of
AT
in
the
previous
12
months.
Multiple
other
studies
identified
a
previous
injury
as
a
risk
factor
for
a
new
injury.2,17,18 This
suggests
that
certain
individuals
have
a
combination
of
unfavourable
inherited
or
biomechanical
charac-
teristics
which
predispose
them
for
developing
recurrent
AT.
One
unfavourable
characteristic
might
be
muscle
strength,
as
persons
with
a
lower
plantar
flexor
strength
have
higher
risk
of
developing
an
Achilles
tendon
injury.19 Furthermore,
insufficient
healing
of
the
AT,
perhaps
as
a
result
of
inadequate
rehabilitation
or
inappropriate
self-management,
could
also
result
in
increased
injury
risk.20 For
instance,
a
premature
return
to
sports
after
a
previous
AT
might
play
a
role
in
having
(recurrent)
AT
symptoms.
Objective
training
load
measures
of
patients
recovering
from
AT
would
be
needed
to
test
this
hypothesis.
A
training-related
risk
factor
for
AT
in
a
previous
study
was
train-
ing
in
cold
weather.15 Other
training-related
risk
factors
have
not
been
reported
in
literature.
We
identified
two
training-related
risk
Table
2
Incidence
of
Achilles
tendinopathy
per
running
distance.
Runners
developing
AT
Runners
without
AT
Incidence
of
AT
%
(95%CI)
N
100
1829
5.2%
(4.2;6.2)
Event
distance
5
or
7.5
km
5
107
4.5%
(0.6;8.4)
10
or
10.55
km
(quarter
marathon)
30
715
4.0%
(2.6;5.4)
21.1
km
(half
marathon)
30
571
5.0%
(3.2;6.7)
42.2
km
(marathon)
35
440
7.4%
(5.0;9.7)
*Statistically
significant
difference
(p-value
<
0.05).
AT
=
Achilles
tendinopathy;
SD
=
Standard
deviation;
IQR
=
inter
quartile
range;
CI
=
confidence
interval.
Please
cite
this
article
in
press
as:
Lagas
IF,
et
al.Incidence
of
Achilles
tendinopathy
and
associated
risk
factors
in
recreational
runners:
A
large
prospective
cohort
study.
J
Sci
Med
Sport
(2019),
https://doi.org/10.1016/j.jsams.2019.12.013
ARTICLE IN PRESS
G Model
JSAMS-2222;
No.
of
Pages
5
4
I.F.
Lagas
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
xxx
(2019)
xxx–xxx
Table
3
Risk
factors
for
AT
in
runners.
Runners
developing
AT
Runners
without
AT
Multivariable
analysis
N
%/Mean
(SD)/median;
IQR
N
%/Mean
(SD)/median;
IQR
OR
(95%CI)
N
100
1829
Demographics
Sex
(male) 67.0%
52.1%
1.39
(0.85;2.27)
Age
(years) 45.0
(10.6)
41.7
(12.1)
1.02
(1.00;1.04)
BMI
(kg/m2)
23.8
(3.3)
23.6
(2.8)
1.00
(0.92;1.09)
Lifestyle
Alcohol
use
(units
per
week)
2.0
;
5.0
3.0
;
5.0
0.99
(0.95;1.04)
Training
Running
experience
(years) 4.6;
7.6 4.0;
6.0 1.00
(0.97;1.02)
Running
distance
per
week
(km) 20.0;
20.0 17.0;
21.0 1.00
(0.99;1.02)
Use
of
training
schedule
(yes)
77.0%
61.5%
1.82
(1.10;3.01)*
Use
of
sport
compression
socks
(yes)
27.0%
14.9%
1.68
(1.03;2.75)*
Use
of
insoles
(yes)
21.0%
21.9%
0.73
(0.43;1.23)
Number
of
running
shoes
per
year
2.0;1.0
2.0;1.0
1.07
(0.86;1.33)
Landing
type
60.4%
Hind-
or
midfoot
(yes)
63.0%
17.7%
1.14
(0.61;2.12)
Forefoot
(yes)
23.0%
1.29
(0.62;2.69)
Running
80%
on
paved
road
(yes)
81.0%
77.0%
1.32
(0.77;2.28)
Previous
injuries
AT
in
the
previous
12
months
(yes)
35.0%
6.7%
6.25
(3.90;10.00)*
AT
=
Achilles
tendinopathy;
SD
=
standard
deviation;
IQR
=
inter
quartile
range;
OR
=
odds
ratio;
CI
=
confidence
interval;
BMI
=
body
mass
index.
*Statistically
significant
difference
(p-value<0.05).
factors
that
were
associated
with
developing
AT:
use
of
a
train-
ing
schedule
and
use
of
sport
compression
socks.
Use
of
a
training
schedule
was
included
in
the
analysis
with
the
hypothesis
that
it
might
be
a
protective
factor
for
developing
AT.
A
training
sched-
ule
could
help
prevent
an
imbalance
of
acute
(level
of
fatigue)
to
chronic
(level
of
fitness)
training
load
by
aiding
the
runner
to
progress
training
load
gradually.21 Since
it
has
been
demonstrated
that
a
peak
in
training
load
per
week,
compared
to
the
average
training
load
of
that
month
leads
to
an
increased
risk
of
injury,21
we
did
not
expect
that
use
of
a
training
schedule
would
be
associ-
ated
with
a
higher
AT
injury
risk.
This
is
the
first
time
that
this
factor
is
explored
in
a
running
population
with
AT
as
outcome.
One
expla-
nation
for
this
finding
could
be
that
runners
were
more
likely
to
use
a
training
schedule
when
they
are
more
prone
to
injury
throughout
their
running
career.
Another
explanation
might
be
that
runners
are
too
focussed
on
pursuing
their
schedule,
rather
than
paying
atten-
tion
to
the
onset
of
pain
which
may
precede
injuries
that
eventually
can
result
in
reduction
or
cessation
of
running
activity.
Another
unexpected
risk
factor
for
AT
was
the
use
of
sport
com-
pression
socks.
Sport
compression
socks
are
thought
to
improve
the
venous
return,
which
reduces
venous
stasis
the
lower
leg.22
This
corresponds
with
an
increased
arterial
perfusion
and
deeper
tissue
oxygenation,23 which
in
turn
may
eventually
lead
to
a
decreased
muscle
soreness
and
lower
likelihood
of
hypoxia-
induced
injuries.24 Contrary
to
this
hypothesis,
we
found
the
use
of
sport
compression
socks
to
be
a
risk
factor
for
AT.
The
following
theories
might
explain
this
finding.
First,
it
could
be
that
run-
ners
started
wearing
sport
compression
socks
because
they
were
more
prone
to
injuries
throughout
their
running
career.
Second,
one
could
hypothesise
that
the
use
of
ankle-length
compression
socks
causes
increased
pressure
on
the
Achilles
tendon.
As
com-
pressive
forces
are
thought
to
have
an
important
role
in
insertional
tendinopathies,
this
could
be
a
potential
mechanism
of
develop-
ing
AT.25 We
did
not
ask
which
level
of
compression
or
height
of
the
sport
compression
socks
were
used,
and
there
is
no
research
performed
on
the
level
of
compression
or
height
of
sport
com-
pression
socks
in
relation
to
injury
incidence.
This
leads
us
to
the
third
interesting
theory,
which
is
that
sport
compression
socks
cause
restriction
of
total
blood
volume
and
oxygen
uptake,
and
this
repeated
restriction
can
eventually
lead
to
hypoxia
and
eventually
result
in
AT.
Studies
showed
a
reduction
in
total
blood
volume
and
peak
oxygen
uptake
when
wearing
sport
compression
socks.26,27
This
could
potentially
lead
to
hypoxic
degeneration,
which
is
one
of
the
mentioned
histopathological
features
of
AT.28
Moderate
alcohol
use
is
suggested
as
a
risk
factor
for
develop-
ing
AT
with
limited
evidence
(OR1.33,
95%CI[1.00;1.76])
in
military
personnel.15,29 It
is
hypothesised
that
alcohol
consumption
is
asso-
ciated
with
risky
behaviour
and
that
it
affects
metabolic
factors
predisposing
for
AT.30 Contrary
to
Owens
et
al,29 we
did
not
find
a
relation
between
units
of
alcohol
per
week
and
AT
onset.
Our
dif-
ferent
results
can
be
explained
by
the
difference
in
runner
sample
and
the
classification
of
alcohol
use.
Owens
et
al.29 defined
moder-
ate
alcohol
use
as
7–13
drinks
per
week
for
men
and
4–6
drinks
per
week
for
women,
while
we
analyzed
alcohol
in
units
per
week.29
Using
numerical
data
leads
to
no
data
reduction,
compared
to
using
categorical
data.
A
major
strength
of
our
study
is
the
fact
that
we
were
able
to
include
a
very
large
cohort
of
recreational
runners.
We
did
not
select
specific
runners
based
on
age,
experience
or
running
dis-
tance
in
our
analysis
to
be
able
to
represent
the
general
running
population.
This
increases
the
generalizability
of
our
results
to
the
general
running
population.
All
included
runners
were
asked
whether
they
experienced
an
injury
through
online
questionnaires.
With
this
approach,
we
were
able
to
reach
a
large
part
of
the
tar-
get
population
and
not
only
the
runners
with
AT
who
presented
to
a
healthcare
provider.
Another
advantage
of
this
large
cohort
is
the
fact
that
we
had
a
high
likelihood
to
identify
risk
factors
for
AT.
As
our
study
reported
100
cases,
we
were
able
to
detect
even
moderate
associations.16
There
are
some
limitations
of
our
study.
First,
we
used
online
questionnaires
to
inquire
about
potential
injuries.
With
this
approach
of
self-reported
injuries,
it
remains
uncertain
whether
the
diagnosis
of
AT
is
correct.
Recent
studies
showed
that
pain
can
be
located
adequately
by
patients.31,32 To
increase
the
likelihood
that
the
reported
injury
was
indeed
an
AT,
we
used
a
very
strict
criteria
as
definition
for
injury.
Another
limitation
is
the
loss
to
follow-up
rate
in
our
study.
Of
the
included
runners,
74%
completed
all
follow-up
question-
naires.
The
included
runners
had
some
differences
compared
to
runners
who
did
not
complete
any
questionnaire,
which
might
indicate
selection
bias.
However,
there
were
no
clinically
relevant
differences,
as
all
differences
were
very
small
and
probably
statis-
Please
cite
this
article
in
press
as:
Lagas
IF,
et
al.Incidence
of
Achilles
tendinopathy
and
associated
risk
factors
in
recreational
runners:
A
large
prospective
cohort
study.
J
Sci
Med
Sport
(2019),
https://doi.org/10.1016/j.jsams.2019.12.013
ARTICLE IN PRESS
G Model
JSAMS-2222;
No.
of
Pages
5
I.F.
Lagas
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
xxx
(2019)
xxx–xxx
5
tically
significant
as
a
result
of
the
large
sample
size.
This
increases
the
likelihood
that
the
responders
were
comparable
to
the
non-
responders.
Furthermore,
the
questionnaire
in
this
study
was
not
validated.
This
could
have
led
to
inaccurate
answers.
For
exam-
ple,
we
asked
runners
to
describe
their
landing
type.
As
we
did
not
use
video
analysis
to
affirm
their
choice,
it
could
be
that
run-
ners
had
a
different
landing
type
than
thought.33,34 However,
most
questions
are
straightforward
to
answer
and
not
susceptible
for
interpretation
(e.g.
sex,
running
experience,
running
shoes
etc.).
A
last
limitation
could
be
that
we
surpassed
the
one
in
ten
rule,
as
we
analysed
thirteen
variables
in
the
multivariable
logistic
regres-
sion
analysis.
We
included
these
variables
as
they
were
identified
by
previous
studies
as
potential
risk
factors
or
hypothesised
to
be
risk
factors.15 We
used
cross-validation
to
verify
this
outcome.
As
this
analysis
showed
similar
outcome
to
our
statistical
analysis,
we
ruled
out
potential
bias
by
including
more
than
ten
variables.
The
outcome
of
our
study
provides
more
insight
in
the
inci-
dence
and
risk
factors
for
AT
in
recreational
runners.
Studies
on
prevention
of
AT
in
runners
should
be
focussed
on
marathon
run-
ners,
as
the
incidence
is
highest
in
this
subgroup.
We
recommend
to
focus
future
research
on
modifiable
risk
factors
as
these
are
promis-
ing
for
designing
new
effective
prevention
programs.
The
finding
that
use
of
sport
compression
socks
is
a
modifiable
risk
factor
for
AT
warrants
further
investigation.
We
suggest
non-invasive
blood
flow
measurements
in
runners
wearing
sport
compression
socks
to
analyse
why
sport
compression
socks
lead
to
development
of
AT.
For
use
of
a
training
schedule,
further
research
should
be
conducted
aimed
at
the
correlation
between
the
progression
of
actual
training
load
and
the
development
of
AT.
5.
Conclusion
The
incidence
of
AT
in
the
recreational
running
population
is
5.2%
and
this
incidence
rate
is
especially
high
in
the
runners
prepar-
ing
for
a
marathon
(7.4%).
AT
in
the
previous
12
months
was
the
strongest
risk
factor
for
having
(recurrent)
AT
symptoms.
Use
of
a
training
schedule
and
use
of
sport
compression
socks
are
two
newly
discovered
risk
factors
for
developing
AT.
Contrary
to
popu-
lar
belief,
often
suggested
demographics-related,
lifestyle-related
and
training-related
risk
factors
did
not
influence
the
risk
of
devel-
oping
AT.
Acknowledgements
Our
thanks
go
out
to
the
Vereniging
voor
Sportgeneeskunde
(VSG)
and
Golazo
Sports
for
the
collaboration.
We
are
grateful
for
the
participation
of
all
runners
in
the
INSPIRE
trial.
Appendix
A.
Supplementary
data
Supplementary
material
related
to
this
article
can
be
found,
in
the
online
version,
at
doi:https://doi.org/10.1016/j.jsams.2019.12.
013.
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