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Experimental pain in the groin may refer into the lower abdomen: Implications to clinical assessments

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Objectives: To investigate the effects of experimental adductor pain on the pain referral pattern, mechanical sensitivity and muscle activity during common clinical tests. Design: Repeated-measures design. Methods: In two separate sessions, 15 healthy males received a hypertonic (painful) and isotonic (control) saline injection to either the adductor longus (AL) tendon to produce experimental groin pain or into the rectus femoris (RF) tendon as a painful control. Pain intensity was recorded on a visual analogue scale (VAS) with pain distribution indicated on body maps. Pressure pain thresholds (PPT) were assessed bilaterally in the groin area. Electromyography (EMG) of relevant muscles was recorded during six provocation tests. PPT and EMG assessment were measured before, during and after experimental pain. Results: Hypertonic saline induced higher VAS scores than isotonic saline (p<0.001), and a local pain distribution in 80% of participants. A proximal pain referral to the lower abdominal region in 33% (AL) and 7% (RF) of participants. Experimental pain (AL and RF) did not significantly alter PPT values or the EMG amplitude in groin or trunk muscles during provocation tests when forces were matched with baseline. Conclusions: This study demonstrates that AL tendon pain was distributed locally in the majority of participants but may refer to the lower abdomen. Experimental adductor pain did not significantly alter the mechanical sensitivity or muscle activity patterns.
Content may be subject to copyright.
Journal
of
Science
and
Medicine
in
Sport
20
(2017)
904–909
Contents
lists
available
at
ScienceDirect
Journal
of
Science
and
Medicine
in
Sport
journal
h
om
epa
ge:
www.elsevier.com/locate/jsams
Original
research
Experimental
pain
in
the
groin
may
refer
into
the
lower
abdomen:
Implications
to
clinical
assessments
M.K.
Drewb,c,
T.S.
Palssona,
R.P.
Hirataa,
M.
Izumia,d,
G.
Lovelle,
M.
Welvaertf,
P.
Chiarellib,
P.G.
Osmotherlyb,
T.
Graven-Nielsena,,1
aCenter
for
Neuroplasticity
and
Pain
(CNAP),
SMI,
Department
of
Health
Science
and
Technology,
Faculty
of
Medicine,
Aalborg
University,
Denmark
bSchool
of
Health
Sciences,
Faculty
of
Health
and
Medicine,
University
of
Newcastle,
Australia
cDepartment
of
Physical
Therapies,
Australian
Institute
of
Sport,
Australia
dDepartment
of
Orthopedic
Surgery,
Kochi
University,
Japan
eDepartment
of
Sports
Medicine,
Australian
Institute
of
Sport,
Australia
fUCRISE,
University
of
Canberra,
Australia
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
15
September
2016
Received
in
revised
form
14
February
2017
Accepted
16
April
2017
Available
online
21
April
2017
Keywords:
Athlete
EMG
Pressure
pain
sensitivity
Adductor
longus
tendon
Rectus
femoris
tendon
a
b
s
t
r
a
c
t
Objectives:
To
investigate
the
effects
of
experimental
adductor
pain
on
the
pain
referral
pattern,
mechan-
ical
sensitivity
and
muscle
activity
during
common
clinical
tests.
Design:
Repeated-measures
design.
Methods:
In
two
separate
sessions,
15
healthy
males
received
a
hypertonic
(painful)
and
isotonic
(con-
trol)
saline
injection
to
either
the
adductor
longus
(AL)
tendon
to
produce
experimental
groin
pain
or
into
the
rectus
femoris
(RF)
tendon
as
a
painful
control.
Pain
intensity
was
recorded
on
a
visual
ana-
logue
scale
(VAS)
with
pain
distribution
indicated
on
body
maps.
Pressure
pain
thresholds
(PPT)
were
assessed
bilaterally
in
the
groin
area.
Electromyography
(EMG)
of
relevant
muscles
was
recorded
during
six
provocation
tests.
PPT
and
EMG
assessment
were
measured
before,
during
and
after
experimental
pain.
Results:
Hypertonic
saline
induced
higher
VAS
scores
than
isotonic
saline
(p
<
0.001),
and
a
local
pain
distribution
in
80%
of
participants.
A
proximal
pain
referral
to
the
lower
abdominal
region
in
33%
(AL)
and
7%
(RF)
of
participants.
Experimental
pain
(AL
and
RF)
did
not
significantly
alter
PPT
values
or
the
EMG
amplitude
in
groin
or
trunk
muscles
during
provocation
tests
when
forces
were
matched
with
baseline.
Conclusions:
This
study
demonstrates
that
AL
tendon
pain
was
distributed
locally
in
the
majority
of
par-
ticipants
but
may
refer
to
the
lower
abdomen.
Experimental
adductor
pain
did
not
significantly
alter
the
mechanical
sensitivity
or
muscle
activity
patterns.
©
2017
Sports
Medicine
Australia.
Published
by
Elsevier
Ltd.
All
rights
reserved.
1.
Introduction
The
prevalence
of
hip
and
groin
pain
in
athletes
is
generally
high
with
a
career
prevalence
of
45%
reported
in
professional
Australian
football
players1and
a
high
incidence
in
sports
such
as
football2
and
ice
hockey.3Adductor-related
groin
pain
is
characterised
as
pain
on
resisted
adduction
and
pain
on
palpation
of
the
adduc-
tor
longus
muscle.4In
contrast,
abdominal
symptoms
present
with
Corresponding
author
at:
Center
for
Neuroplasticity
and
Pain
(CNAP)
SMI,
Department
of
Health
Science
and
Technology
Faculty
of
Medicine
Aalborg
Uni-
versity,
Fredrik
Bajers
Vej
7D-3
9220
Aalborg
E,
Denmark.
Tel:
+45
9940
9832;
fax:
+45
9815
4008.
E-mail
address:
tgn@hst.aau.dk
(T.
Graven-Nielsen).
1http://www.cnap.hst.aau.dk/.
pain
on
resisted
trunk
flexion
and
pain
on
palpation
of
the
rectus
abdominis
distal
enthesis.5Yet
characteristics
of
groin
pain
per
se
are
poorly
understood
with
few
reports
of
pain
referral
patterns
and
clinical
symptomatology.
Pain
referral
patterns
are
typically
semi-
(referring
distally)
or
bi-directional
(referring
both
distally
and
proximally)
with
referred
pain
distributions
extending
to
neigh-
bouring
vertebral
segments
that
are
supplying
the
painful
muscle
or
tendon.6Clinically,
pain
in
both
the
adductor
and
abdominal
area
is
associated
with
longer
recovery
times
compared
to
a
sin-
gle
site.7The
role
of
pain
referral
patterns
has
not
previously
been
examined
and
may
present
a
plausible
alternate
hypothesis
to
co-
existing
pain
locations5,8,9 in
this
region.
That
is,
abdominal
pain
may
present
clinically
as
a
result
of
referred
pain
from
the
adduc-
tor
region.
If
this
is
true,
it
challenges
using
pain
location
alone
as
http://dx.doi.org/10.1016/j.jsams.2017.04.007
1440-2440/©
2017
Sports
Medicine
Australia.
Published
by
Elsevier
Ltd.
All
rights
reserved.
M.K.
Drew
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
20
(2017)
904–909
905
diagnostic
criteria
in
either
classifying
patients
into
entities
or
to
specific
pathoanatomical
tissue
diagnoses.
Electromyographic
(EMG)
muscle
activity
has
been
shown
to
be
significantly
reduced
in
m.
adductor
longus,
m.
pectineus,
and
m.
gracilis,
in
patients
with
a
history
of
groin
pain
during
clinical
tests
when
compared
to
healthy
activity-matched-controls.11 Such
changes
occur
soon
after
the
initiating
painful
event.12 Given
the
complex
relationship
between
muscle
and
fascial
structures
in
the
groin
and
abdominal
region,
this
possible
reduction
in
muscle
activ-
ity
could
shift
the
balance
of
the
forces
between
the
adductor
and
abdominal
muscles
thus
influencing
performance
during
diagnos-
tic
testing.
If
muscle
activation
patterns
change,
it
may
be
possible
to
maintain
the
same
force
output
despite
the
existence
of
a
painful
condition
as
shown
in
other
pain
states.13,14 This
may
have
clinical
implications
with
regards
to
the
interpretation
of
clinical
diagnos-
tic
tests
due
to
alterations
in
muscle
activity
and
also
the
transition
from
acute
into
long-standing
groin
pain.15
Experimental
pain
caused
by
injection
of
hypertonic
saline
into
tendons
in
healthy
participants
has
been
shown
to
cause
increased
trunk
muscle
activity,16,17 large
pain
referral
patterns,16,18 regional
hyperalgesia,16,18,19 and
facilitated
response
to
clinical
orthopaedic
tests
for
the
hips
and
pelvic
girdle.16,18,19 Therefore,
a
hypertonic
saline
model
may
provide
insights
into
the
effect
of
pain
in
the
groin
region
on
the
muscle
activity,
mechanical
sensitivity,
and
referral
patterns.
While
many
studies
have
focused
on
the
diagnosis
of
groin
pain
in
athletes,
little
is
understood
about
the
effect
of
pain
itself
on
the
muscle
activation
during
the
diagnostic
tests,
pain
referral
pat-
terns,
and
mechanical
sensitivity,
all
of
which
are
recommended
diagnostic
criteria.4This
study
aimed
to
examine
three
hypothe-
ses
surrounding
experimental
pain
at
the
proximal
insertion
of
the
adductor
longus:
1.
the
pain
experienced
can
radiate
superior
to
the
pubic
crest.
2.
The
pain
experienced
causes
alteration
of
EMG
muscle
activity
patterns.
3.
The
pain
experienced
produces
local
deep
tissue
hyperalgesia.
2.
Methods
Fifteen
healthy
male
participants
were
included
for
this
study
(mean
±
SD;
age,
26.9
±
3.4
years;
height,
183.9
±
5.4
cm;
weight,
81.5
±
7.1
kg).
Inclusion
criteria
were
1)
no
current
or
previous
hip,
groin,
or
lumbar
region
injuries;
2)
no
signs
of
neurological
disor-
ders
or
rheumatologic
diseases
which
could
affect
the
outcome
of
the
experimental
procedure;
3)
no
reported
medication
use
either
on
enrolment
or
on
a
regular
basis;
4)
currently
participating
in
reg-
ular
exercise
or
sport
of
total
duration
of
greater
than
or
equal
to
2.5
h
a
week.
Exclusion
criteria
were
current
injury,
any
history
of
pain
or
injury
in
the
hip,
groin,
lower
abdominal
or
lumbar
regions,
a
history
of
lower
limb
injury
in
the
previous
2
years,
usage
of
cannabis,
opioids
or
other
drugs,
current
use
of
pain
medication,
previous
neurologic,
musculoskeletal
or
mental
illnesses,
or
lack
of
ability
to
cooperate.
Participants
were
given
a
detailed
verbal
and
written
explanation
of
the
experimental
procedure.
All
participants
provided
written
informed
consent.
The
study
was
approved
by
the
Danish
Regional
Ethics
Committee
(N-20130036)
and
conducted
in
accordance
with
the
Helsinki
Declaration.
The
experiment
had
a
randomized,
single-blinded,
balanced-
crossover,
repeated-measures
design
conducted
in
two
sessions
within
one
week.
Randomisation
was
achieved
through
the
selec-
tion
of
one
of
16
identical
envelopes
by
an
experimenter
(blinded
to
the
injector
and
experimenters)
containing
one
of
all
16
pos-
sible
order
combinations
of
injection
site,
side,
and
injection
site.
Blinding
was
achieved
through
unlabelled,
identical
pre-prepared
syringes
prior
to
the
experimenters
entering
the
room.
The
par-
ticipants
were
not
advised
of
the
order
of
injections
at
any
stage
throughout
the
procedure.20 Experimental
groin
pain
and
a
painful
control
condition
outside
the
groin
area
were
evaluated.
Clinical
provocation
tests
with
recordings
of
the
muscle
activity
and
assess-
ment
of
the
pressure
pain
sensitivity
were
administered
at
baseline,
during
and
after
(post-pain)
experimental
pain
with
participant
lying
supine
on
a
plinth.
Prior
to
baseline
testing,
all
participants
were
familiarised
with
the
experimental
procedure
and
confirmed
to
be
pain-free
prior
to
commencing
the
study.
The
post-pain
state
was
defined
as
five
minutes
after
the
cessation
of
experimental
pain.
The
participants
participated
in
two
sessions
and
received
one
hypertonic
and
one
isotonic
saline
injection
each
session,
one
in
each
side
of
the
same
site
(AL
or
RF)
during
each
session.
The
alter-
nate
site
was
injected
in
the
following
session.
The
order
of
the
saline
type
(hypertonic
or
isotonic)
and
site
(AL
or
RF)
and
side
(left
or
right)
was
randomised
in
a
balanced
way.
Groin
pain
was
induced
by
injecting
sterile
hypertonic
saline
(1
ml,
5.8%)
into
the
adductor
longus
(AL)
tendon
with
isotonic
saline
(1
ml,
0.9%)
injected
as
a
non-painful
control
into
the
same
anatomical
site
on
the
contralat-
eral
side
within
the
same
session.
As
a
positive
(painful)
control
injection
outside
the
groin
area,
the
proximal
tendon
of
the
long
head
of
the
rectus
femoris
(RF)
muscle
was
injected
in
a
sepa-
rate
session.
The
same
volume
of
hypertonic
or
isotonic
saline
was
injected
into
the
control
site
as
designated
by
the
randomisation.
Participants
and
injector
were
blinded
to
saline
type
administered.
All
injections
were
given
by
an
orthopaedic
surgeon
(MI).
After
a
standard
disinfection
protocol,
the
injections
were
given
over
the
duration
of
approximately
10
s
using
a
2-ml
plastic
syringe
with
a
disposable
needle
(27G).
Pre-defined
anatomical
landmarks
for
injection
sites
for
AL
and
RF
tendons
were
utilised.
The
location,
depth
and
alignment
of
all
injection
sites
were
confirmed
by
real
time
ultrasound
(US)
imaging
(Acuson
128XP10,
NativeTM).
The
AL
tendon
was
identified
using
a
method
previously
described.18 Both
the
AL
and
RF
injections
positions
followed
a
previously
published
protocol
(Supplement
1).20
The
pain
intensity
produced
by
hypertonic
saline
injections
was
assessed
on
a
10
cm
electronic
visual
analogue
scale
(VAS)
which
could
be
adjusted
by
using
an
external
handheld
slider.
The
VAS
was
anchored
with
‘no
pain’
and
‘maximum
pain’,
0
cm
and
10
cm,
respectively.
A
continuous
recording
(sample
frequency
of
20
Hz)
of
the
VAS
signal
was
made
after
each
injection
until
all
pain
had
subsided.
For
analysis,
the
area
under
VAS-time
curve
(VAS
area)
and
VAS-peak
were
extracted.
The
quality
of
pain
was
assessed
once
the
pain
had
subsided.
Participants
were
allowed
to
answer
using
either
the
Danish21 or
English22 version
of
the
McGill
Pain
Questionnaire
based
upon
their
language
preference.
The
Danish
results
were
converted
to
the
English
equivalent
for
analysis.
Participants
were
asked
to
mark
their
pain
distribution
by
filling
in
a
standard
body
chart.
Body
areas
were
divided
into
groin
regions
by
using
the
“Groin
Triangle”.23 The
groin
triangle
is
defined
as
the
triangle
created
by
the
three
land-
marks:
the
anterior
superior
iliac
spine
(ASIS),
pubic
tubercle
and
the
median
point
between
the
ASIS
and
the
superior
pole
of
the
patella
in
the
anterior
coronal
plane
(‘3G
point’).24 Local
pain
was
defined
as
pain
experienced
only
at
the
injection
site
and
related
“Groin
Triangle”
segment
while
referred
pain
was
defined
as
any
pain
felt
outside
the
segment
containing
the
injection
site.
The
body
regions
were
analysed
by
registering
the
frequency
of
pain
experienced
in
the
region
for
all
four
injections.
Pressure
pain
thresholds
(PPTs)
were
assessed
at
regional
and
distant
sites
using
a
handheld
pressure
algometer
(Somedic,
Sweden)
with
a
1
cm2probe
and
using
a
30
kPa/s
ramp.
The
four
bilateral
assessment
sites
were
the
AL
tendon
injection
site,
the
RF
tendon
injection
site,
the
anterior
surface
of
the
superior
pubic
rami
(PB),
and
the
tibialis
anterior
(TA)
muscle,
measured
as
the
906
M.K.
Drew
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
20
(2017)
904–909
proximal
site
1/3
the
distance
from
the
lateral
joint
line
of
the
knee
to
the
inferior
aspect
of
the
lateral
malleolus.
Each
measure-
ment
was
recorded
three
times
at
baseline
with
two
measurements
recorded
during
pain
and
post-pain
to
ensure
all
testing
could
be
completed
within
the
short-lasting
window
of
saline-induced
pain.
The
average
of
the
measurements
was
used
for
statistical
analysis.
PPT
measurement
was
ceased
at
1200
kPa
to
avoid
sensitisation
after
repeated
assessments.
A
battery
of
six
pain
provocation
tests
(Supplement
2)
was
employed
with
all
tests
performed
by
a
single
clinically-trained
experimenter
(MD).
All
participants
were
confirmed
to
be
pain-free
on
all
tests
prior
commencing
the
study.
The
tests
administered
were
as
previously
published:20 1)
bilateral
adduction
(squeeze)
test
with
hips
at
0resisted
at
the
ankles25 2)
a
bilateral
squeeze
test11 with
hips
flexed
at
453)
a
bilateral
squeeze
test11 with
hips
flexed
to
904)
resisted
abdominal
crunch25 5)
resisted
oblique
crunch,
one
side
at
a
time.25 The
force
of
contraction
was
measured
using
a
hand-held
dynamometer
(MicroFET2,
Hoggan
Health
Indus-
tries,
USA)
at
baseline,
during-pain
and
post-pain.
The
reliability
of
the
0adduction
test
is
high
(ICC
=
0.97,
minimal
detectable
change
(%)
=
6.6).26 Verbal
encouragement
by
the
assessor
was
given
to
ensure
force
output
remained
constant
for
each
repetition
(within
10%
of
baseline
measures).
The
skin
at
each
assessment
site
was
shaved,
abraded
and
cleaned
with
alcohol
in
accordance
with
the
SENIAM
guidelines.27
Disposable
electrodes
(Ambu®,
Neuroline
720,
Denmark)
were
mounted
bilaterally
with
an
inter-electrode
distance
of
20
mm
in
a
bipolar
configuration
at
the
m.
tensor
fascia
latae
(TFL),
the
m.
adductor
longus
(AL),
m.
rectus
abdominis
(RA),
and
m.
external
obliques
(EO).11,28 A
ground
electrode
was
placed
on
the
right
wrist.
The
EMG
signal
from
the
AL
muscle
was
used
as
reference
to
deter-
mine
the
time
window
for
the
amplitude
analysis
(from
onset
to
offset)29 where
the
root-mean-square
(RMS)
value
was
extracted
for
all
muscles
during
all
six
tests
for
the
middle
epoch
defined
as
middle
third
of
the
period
between
onset
and
offset
(see
Supple-
ment
1
for
extended
methodology).
The
RMS
value
represents
the
muscle
activity
of
the
muscle.
The
onsets
and
offsets
were
auto-
matically
detected
based
on
the
AL
muscle
EMG
data
as
previously
described
in
detail
by
Santello
and
McDonagh.29 All
onset/offset
detections
were
confirmed
by
visual
inspection
at
each
time
point.
No
manual
correction
of
the
data
was
required.
Onsets
and
offsets
were
not
analysed
as
the
research
question
investigated
related
to
maximal
muscle
activity
pre-,
during
and
post-experimental
pain
conditions
rather
than
changes
in
the
order
of
activation
as
a
result
of
pain.
Filtered
EMG
data
was
utilised
for
analysis
however
fil-
ter
and
normalised
data
to
baseline
measures
is
reported
in
the
supplements
for
the
ease
of
interpretation
clinically.
All
data
was
assessed
for
normality
using
the
Kolmogorov–Smirnov
test.
Means
and
standard
deviations
(SD)
are
presented
for
parametric
data.
All
statistical
analyses
were
performed
using
Stata
13
IC
unless
indicated
(StataCorp,
USA).
An
a
priori
estimate
of
group
size
indicated
15
participants
were
required
(estimated
20%
difference
in
effect
parameters;
˛
=
5%;
ˇ
=
20%;
coefficient
of
variance
=
25%).
The
VAS
area
was
analysed
with
an
analysis
of
variance
(ANOVA)
with
muscle
(AL
and
RF)
and
injection
(hypertonic
and
isotonic)
as
independent
factors.
To
assess
the
relationship
of
PPTs
and
the
injection
site,
side
and
injection
type,
a
linear
mixed-effect
model
(restricted
maximum
likelihood
[REML]
regression)
was
fitted
with
PPT
site
(AL,
pubic
bone,
RF,
and
tibialis
anterior),
injection
type
(hypertonic
and
isotonic),
side
(ipsi-
or
contralateral)
and
injection
site
(RF
and
AL)
and
time
(baseline,
during
or
post)
and
their
interactions
as
fixed-effects.
For
analysis,
filtered
EMG
data
was
utilised
to
assess
the
relationship
between
mean
RMS-EMG
of
each
clinical
test
and
the
effects
of
injection
type
(isotonic
and
hypertonic),
time
point
(baseline,
during,
post-pain),
each
muscle
(AL,
TFL,
EO,
RA),
injection
site
(AL
and
RF)
and
side
(ipsilateral
and
contralateral)
and
their
interactions
with
a
random
effect
for
participant
in
a
General
Linear
Mixed
Model
using
the
R
package
lme4
(R
Core
Team,
2016).30 This
approach
can
handle
missing
data
which
created
an
unbalanced
design.31 Means
were
analysed
post-hoc
to
explain
significant
effects.
Bonferroni
correction
was
applied
where
multiple
post-hoc
analyses
were
undertaken.
Significance
was
set
at
p
<
0.05
for
all
statistical
tests.
3.
Results
The
VAS
area
after
hypertonic
saline
injected
into
the
AL
(13112
±
11147
mm
s)
and
RF
(12110
±
8829
mm
s)
tendons
were
higher
compared
with
isotonic
saline
(AL:
206
±
405
mm
s;
RF:
815
±
2037
mm
s;
ANOVA:
F(2,53)
=
20.05,
p
<
0.001).
The
VAS-
peaks
reported
for
each
test
condition
were
AL
isotonic
(2
±
4
mm),
AL
hypertonic
(22
±
12
mm),
RF
isotonic
(4
±
7
mm),
and
RF
hyper-
tonic
(22
±
12
mm).
The
three
most
common
words
to
describe
the
sensation
after
the
AL
tendon
hypertonic
injections
were
“annoy-
ing”
(33%
of
participants),
“tugging”
(27%)
and
“pressing”
(27%)
whereas
the
three
most
common
descriptions
after
the
RF
tendon
hypertonic
injections
“tight”
(47%),
“pressing”
(33%),
“annoying”
(27%)
for
RF
tendon.
Hypertonic
saline-induced
pain
in
the
AL
tendon
primarily
demonstrated
a
local
pattern
of
pain
where
it
was
mainly
per-
ceived
within
and
medial
to
the
“Groin
Triangle”
but
also
in
the
lower
abdominal
region
(Fig.
1,
Table
1).
Injections
of
hypertonic
saline
into
the
RF
tendon
primarily
caused
pain
experienced
within
the
triangle
and
the
anterior
and
lateral
thigh
indicating
a
larger
pain
referral
pattern.
During
isotonic
saline
injections
into
the
RF
tendon,
11
participants
drew
the
pain
on
the
anterior
thigh.
Pain
in
the
contralateral
side
to
the
injection
was
also
reported
in
one
participant
in
three
areas
(Supplementary
3)
after
the
hypertonic
injection
into
the
RF
tendon.
No
participants
reported
pain
on
the
contralateral
side
with
an
absence
of
pain
in
the
ipsilateral
injec-
tion
side.
Therefore,
these
reports
should
be
considered
as
bilateral
pain
distributions.
PPT
values
did
not
significantly
change
across
time
periods
under
any
conditions.
Significant
fixed
effects
were
observed
for
the
RF
(REML:
Coeff
=
362.5,
95%CI
265.8–564.2,
p
<
0.001)
and
TA
sites
(REML:
Coeff
=
469.8,
95%CI
374.8–561.8,
p
<
0.001)
indicating
that
the
TA
and
RF
sites
were
generally
higher
than
the
adductor
and
pubic
sites.
However,
no
significant
fixed
effects
or
interac-
tions
were
observed
with
the
inclusion
of
time
(p
=
0.27–0.99).
As
time
was
not
a
significant
fixed
effect,
this
can
be
interpreted
as
the
PPT
values
were
not
significantly
influenced
by
experimental
pain
conditions.
The
distributions
of
PPT
values
across
the
experimental
conditions
and
time
points
are
presented
in
Fig.
2.
The
magnitude
of
the
muscle
activity
did
not
change
signifi-
cantly
across
time
periods
under
any
conditions
when
compared
to
baseline
conditions.
Normalised
RMS-EMG
for
the
“during”
and
“post”
conditions
are
presented
in
Supplementary
Tables
1–4.
A
five-way
interaction
between
clinical
test,
injection
type,
muscle,
injection
site
and
side
was
observed
(F(15,7771)
=
8.68,
p
<
0.001)
however
time
was
not
a
significant
fixed
effect
in
the
model
or
any
interactions.
As
time
was
not
a
significant
fixed
effect
it
can
be
interpreted
as
the
muscle
activation
patterns
of
the
four
mus-
cles
varied
across
the
clinical
tests,
injection
type
and
site,
and
side
when
compared
to
each
other
yet
were
not
significantly
uninflu-
enced
by
the
experimental
pain.
Therefore,
no
post-hoc
analyses
were
performed.
4.
Discussion
This
is
the
first
study
to
report
the
muscle
activation
pat-
tern
involved
in
commonly
used
clinical
tests
for
groin
pain
and
M.K.
Drew
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
20
(2017)
904–909
907
Table
1
Frequency
of
pain
relative
to
the
“Groin
Triangle”
following
injections
of
hypertonic
and
isotonic
saline
into
the
adductor
longus
and
rectus
femoris
tendons.
Adductor
longus
tendon
Rectus
femoris
tendon
Isotonic
saline
Hypertonic
saline
Isotonic
saline
Hypertonic
saline
Ipsilateral
Contralateral
Ipsilateral
Contralateral
Ipsilateral
Contralateral
Ipsilateral
Contralateral
“Groin
Triangle”
Within
the
triangle
3
(20)
0
12
(80%)
0
4
(27%)
1
(7%)
15
(100%)
2
(13%)
Lateral
to
the
triangle
0
0
1
(7%)
0
0
0
2
(13%)
0
Medial
to
the
triangle
7
(47)
0
12
(80%)
0
0
0
0
0
Superior
to
the
triangle 0
0
5
(33%) 0
0
0
1
(7%)
0
Other
areas
Greater
trochanter
0
0
0
0
0
0
0
2
(13%)
Anterior
thigh
0
0
1
(7%)
0
2
(13%)
0
5
(33%)
2
(13%)
Lateral
thigh
0
0
0
0
0
0
4
(27%)
1
(7%)
Knee
0
0
0
0
0
0
0
1
(7%)
Lower
leg
0
0
0
0
0
0
1
(7%)
2
(13%)
Foot
0
0
0
0
0
0
0
0
Contralateral/ipsilateral
relative
to
the
side
of
injection;
frequencies
reported
as
number
of
responses
(percentage).
Fig.
1.
Pain
distributions
of
the
adductor
longus
are
indicated
on
the
body
chart’s
right
side.
mechanical
sensitivity
of
the
lower
limb
in
an
experimental
pain
model.
This
study
aimed
to
examine
three
hypotheses
surrounding
experimental
pain
at
the
proximal
insertion
of
the
adductor
longus.
The
results
of
this
study
support
the
hypothesis
that
experimental
pain
in
the
proximal
adductor
longus
can
proximally
refer
to
the
lower
abdomen
and
may
explain
why
pain
can
be
experienced
in
both
locations
clinically.
This
study
fails
to
provide
evidence
that
experimental
pain
in
the
AL
alters
the
muscle
activity
and
produces
local
or
widespread
deep
tissue
hyperalgesia.
These
findings
have
implications
for
clinical
assessment
particularly
related
to
diagnos-
tic
or
classification
criteria
which
rely
on
pain
referral
patterns
as
they
can
be
influenced
by
region
structures.
The
AL
tendon
produced
a
local
pain
distribution
contained
mainly
medial
to
and
within
the
“Groin
Triangle”.
Moreover,
in
33%
of
participants
the
tendon
of
adductor
longus
was
capable
of
pro-
voking
proximal
referral
into
the
lower
abdominal
region.
This
has
clinical
relevance
as
it
is
commonly
reported
in
the
literature
that
multiple
pathologies
or
clinical
entities
exist
in
athletes
with
groin
pain.5Experimentally-induced
AL
tendon
pain
is
capable
of
pro-
ducing
false
positive
test
results
with
abdominal
manoeuvres.20
Therefore,
comprehensive
clinical
assessment
is
required
to
rule
out
involvement
of
AL
tendon
when
pain
in
the
lower
abdomen
is
present
particularly
when
coexisting
with
pain
in
the
upper
inner
thigh.
The
results
of
experimental
pain
models20 indicate
that
45
and
90adduction
tests
have
the
best
negative
likelihood
ratio,
suggesting
their
utility
to
rule
out
adductor
longus
as
a
potential
source
of
nociception.
The
positive
control
condition
(experimental
RF
tendon
pain)
produced
a
greater
distribution
of
pain
covering
the
regions
within,
lateral
to
and
superior
to
the
groin
triangle
although
no
pain
was
reported
medial
to
the
triangle.
Bilateral
leg
pain
distribution
was
produced
in
one
participant
under
the
RF
ten-
don
hypertonic
and
isotonic
saline
conditions.
This
represents
an
unusual
pain
referral
pattern
that
is
not
typically
observed
clinically
and
may
be
related
to
individual
characteristics
of
the
participant.
In
the
present
study,
pain
induced
in
adductor
and
thigh
regions
was
unable
to
alter
the
mechanical
sensitivity.
Primary
mechani-
cal
hyperalgesia
of
the
adductor
longus
tendon
has
been
reported
in
Australian
football
players
currently
experiencing
groin
pain.1
This
indicates
the
hypertonic
saline
tendon
pain
model
may
not
replicate
the
clinical
pain
presentations
of
groin
region.
Proximal
(secondary)
hyperalgesia
has
been
hypothesised
to
be
explained
by
amplification
of
central
pain
mechanisms.32 No
change
was
observed
at
the
pubic
bone
or
distally
on
either
sides
which
con-
curs
with
clinical
pain
studies
of
the
groin
region.1The
diagnostic
criteria
for
adductor-related
groin
pain
are
pain
on
resisted
adduc-
tion
tests
with
tenderness
(mechanical
sensitivity)
on
palpation.4
908
M.K.
Drew
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
20
(2017)
904–909
Fig.
2.
Distribution
of
the
pressure
pain
thresholds
at
baseline,
during
pain
and
post-pain
across
injection
types
and
sites
represented
as
a
box-plot.
In
acute
groin
injuries,
palpation
(mechanical
sensitivity)
has
the
greatest
diagnostic
capacity
to
predict
MRI
findings.33 In
the
present
study,
no
changes
were
observed
at
the
site
of
the
injection
or
on
the
pubic
bone
PTTs
under
the
AL
or
RF
ipsilateral
hypertonic
saline-induced
pain
indicating
secondary
mechanical
hyperalge-
sia
is
less
of
a
concern
for
this
site.
Therefore,
hyperalgesia
of
the
pubic
bone
may
represent
local
mechanical
hyperalgesia
rather
than
regional/widespread
pain
and
as
such
may
be
implicated
as
a
nociceptive
driver.
Clinically,
mechanical
sensitivity
(tenderness
on
palpation)
at
the
pubic
enthesis
may
represent
local
nociception
rather
than
a
consequence
of
adductor
tendon
pain
(as
in
the
case
of
secondary
hyperalgesia).
Confirmation
in
the
clinical
setting
is
warranted
however.
The
magnitude
of
muscle
activity
in
the
region
during
the
painful
condition
was
not
statistically
significantly
different
from
the
baseline
condition.
This
is
hypothesised
to
be
due
to
the
study
design
in
which
force
was
maintained
equal
to
baseline
measures.
This
indicates
that
irrespective
of
pain
in
the
region,
the
motor
cortex
may
allow
for
the
task
to
be
completed
with
equal
force
production.
The
0adduction
test
has
been
suggested
to
be
diagnostically
superior
to
identify
experimentally-induced,
adductor-related
pain.20 However,
the
results
of
this
paper
indicate
that
changes
in
muscle
activation
less
likely
to
be
associated
with
the
diagnostic
capabilities
reported.
Again,
this
hypothesis
should
be
tested
in
clinical
populations.
This
study
allowed
the
evaluation
of
the
outcome
measures
under
controlled
conditions.
This
removes
the
complications
of
multiple
pathologies
detected
on
clinical
assessment5and
imaging8in
athletes
with
groin
pain.
Nonetheless,
pain
generated
from
experimental
models
differs
from
clinical
pain18 and
replica-
tion
of
the
results
in
clinical
populations
is
warranted
as
previously
indicated.
In
the
analysis
of
PPT
and
EMG
data,
a
unified
linear
mixed
model
was
chosen
given
it
ability
to
account
for
the
char-
acteristics
of
the
data
and
to
reduce
the
Type
I
error
associated
with
multiple
sub-grouping
analyses.
The
lack
of
positive
findings
observed
may
be
potentially
explained
by
lower
power
however
this
is
offset
by
the
degrees
of
freedom
created
by
every
participant
undertaking
each
component
of
the
study.
Significant
variability
in
the
data
was
observed
in
the
PPT
and
the
level
of
pain
(VAS)
measures
across
participants.
This
variability
reduced
the
ability
to
obtain
significant
effects;
an
increase
in
sample
size
is
unlikely
to
alter
the
results
and
are
likely
to
represent
the
individual
nature
of
the
response
to
pain.
Post-hoc
power
analyses
are
therefore
not
indicated.34
5.
Conclusion
This
study
has
shown
that
pain
arising
from
the
adductor
longus
tendon
is
locally
distributed
in
the
majority
(80%)
but
capable
of
producing
pain
superior
to
the
pubic
crest
in
33%
of
participants.
PPTs
were
not
altered
by
experimental
pain
induced
by
hyper-
tonic
saline.
An
alteration
of
the
magnitude
of
EMG
activity
of
the
adductor
longus,
tensor
fascia
latae,
rectus
abdominis
and
exter-
nal
obliques
was
not
detected
under
experimental
pain
conditions
when
force
was
matched
to
baseline
measures.
Therefore,
diagnos-
tic
criteria
based
on
pain
distribution
alone
may
be
influenced
by
pain
itself
in
the
region
and
may
not
represent
tissue
pathology
or
multiple
clinical
entities
of
groin
pain.
Practical
implications
The
adductor
longus
tendon
has
a
local
pattern
of
pain
distribu-
tion
however
can
refer
proximally
to
the
lower
abdominal
region.
Diagnostic
criteria
based
on
pain
distribution
are
potentially
influenced
by
pain
itself
in
the
region
and
may
not
represent
tissue
pathology.
Funding
sources
This
study
received
(non-grant)
financial
support
through
the
University
of
Newcastle,
Australia.
Ethical
approval
Danish
Regional
Ethics
Committee
(N-20130036).
M.K.
Drew
et
al.
/
Journal
of
Science
and
Medicine
in
Sport
20
(2017)
904–909
909
Acknowledgements
The
authors
would
like
to
thank
Dr
XXX
XXXX
and
Prof.
XXX
XXXX
for
their
assistance
with
the
statistical
analyses.
This
study
received
(non-grant)
funding
from
the
University
of
XXX
and
the
XXX.
Appendix
A.
Supplementary
data
Supplementary
data
associated
with
this
article
can
be
found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.jsams.2017.04.
007.
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