Journal
of
Physiology
(1990),
422,
pp.
55-65
55
With
5
figures
Printed
in
Great
Britain
EFFECTS
OF
IMMOBILIZATION
ON
CONTRACTILE
PROPERTIES,
RECRUITMENT
AND
FIRING
RATES
OF
HUMAN
MOTOR
UNITS
BY
JACQUES
DUCHATEAU
AND
KARL
HAINAUT
From
the
Laboratory
of
Biology
and
the
Brain
Research
Unit,
University
of
Brussels,
28
Avenue
P.
Heiger,
1050
Brussels,
Belgium
(Received
31
May
1989)
SUMMARY
1.
The
contractile
properties,
recruitment
and
firing
rates
of
motor
units
from
the
human
adductor
pollicis
and
the
first
dorsal
interosseous
were
studied
during
voluntary
isometric
contractions
after
6-8
weeks'
immobilization
of
the
corre-
sponding
limbs.
2.
In
both
muscles,
motor
units
of
different
force
thresholds
showed
a
proportionally
identical
twitch
tension
decrease
and
slowing
of
their
time
course
after
immobilization.
3.
When
expressed
as
a
percentage
of
the
maximal
voluntary
contraction,
more
high-threshold
motor
units
were
recorded
in
disused
muscles
than
in
control
muscles,
but
the
order
of
recruitment
was
maintained.
4.
The
motor
unit
firing
rate
at
recruitment
was
identical
in
control
and
disused
muscles,
but
the
maximal
firing
rate
decreased
in
all
motor
units
after
immobil-
ization.
This
decrease
of
the
maximal
firing
rate
was
greater
in
motor
units
of
lower
threshold
than
in
those
of
higher
threshold.
5.
The
results
further
document
motoneuronal
plasticity
in
human
muscles
of
different
fibre
type
composition.
INTRODUCTION
The
immobilization
of
skeletal
muscles
induces
functional
alterations
which
are
associated
with
morphological,
biochemical
and
neurophysiological
changes
(Booth,
1982;
Appell,
1986).
In
animals,
experimental
data
indicate
that
disuse
is
not
only
associated
with
decreased
muscle
force
generating
capacity
(Witzmann,
Kim
&
Fitts,
1982;
St
Pierre
&
Gardiner,
1985),
but
also
with
alterations
of
the
electromyographic
(EMG)
activity
(Fudema,
Fizzell
&
Nelson,
1961;
Fournier,
Roy,
Perham,
Simard
&
Edgerton,
1983).
This
last
observation
suggests
that
the
muscle
membrane
electrical
activity
is
reduced
and/or
that
neural
changes
are
present.
These
last
changes
could
result
from
impaired
central
drive
and/or
reduction
of
proprioceptive
afferents
on
the
motoneurons.
Possible
neural
adaptations
to
disuse
are
difficult
to
approach
experimentally
in
animal
preparations,
but
they
have
been
proposed
on
the
basis
of
experiments
performed
in
humans
(Fuglsang-Frederiksen
&
Scheel,
1978;
Sale,
McComas,
MacDougall
&
Upton,
1982).
In
a
previous
paper
(Duchateau
&
Hainaut,
1987),
we
compared
maximal
voluntary
contractions
MS
7717
J.
DUCHATEAU
AND
K.
HA
INA
UT
(MVCs)
and
electrically
evoked
contractions
of
the
human
adductor
pollicis
and
concluded
that
the
functional
alterations
observed
during
immobilization
resulted
from
changes
in
the
peripheral
processes
associated
with
the
contraction
and
also
from
changes
in
central
and/or
peripheral
afferents
on
the
motoneurons.
The
effects
of
immobilization
on
muscle
fibres
of
different
size
have
been
estimated
on
the
basis
of
histochemical
observations
(Edstr6m,
1970;
Sargeant,
Davies,
Edwards,
Maunder
&
Young,
1977;
MacDougall,
Elder,
Sale,
Moroz
&
Sutton,
1980),
but
the
contractile
adaptation
of
muscles
of
different
fibre
type
composition
to
disuse,
and
a
possible
selective
effect
on
type
I
and
II
fibres,
remains
an
exciting
question
(Stokes
&
Young,
1984).
This
paper
examines
the
effects
of
the
immobilization
in
two
human
muscles
of
different
fibre
type
composition
(Johnson,
Polgar,
Weightman
&
Appleton,
1973),
the
adductor
pollicis
and
first
dorsal
interosseous
(FDI),
on
motor
unit:
(1)
contractile
properties;
(2)
order
of
recruitment;
(3)
firing
rate
at
recruitment
and
maximal
firing
rate.
METHODS
The
effects
of
6-8
weeks'
immobilization
were
investigated
in
the
adductor
pollicis
of
three
subjects
and
in
the
FDI
of
two
subjects.
The
subjects
(three
male
students
in
physical
education,
one
male
physical
education
teacher
and
one
female
subject)
were
20-42
years
old
and
all
were
well
accustomed
to
maximal
voluntary
contractions.
Disuse
of
the
adductor
pollicis
was
achieved
in
two
patients
after
forearm
fracture
and
in
one
subject
without
fracture,
by
immobilization
of
the
thumb
at
45
deg
abduction
with
a
plaster
cast
(Duchateau
&
Hainaut,
1987).
For
the
FDI,
the
first
three
fingers
of
the
hand
were
fixed
together
with
an
aluminium
splint
after
injury
of
the
first
interphalangeal
joint
of
the
third
finger.
In
the
four
patients,
electrical
and
mechanical
properties
of
the
disused
muscle
were
compared
with
that
of
the
contralateral
muscle.
In
the
subject
who
volunteered
to be
immobilized,
the
adductor
pollicis
of
the
same
hand
was
compared
before
and
after
immobilization.
These
investigations
were
approved
by
the
Ethical
Committee
of
the
University
of
Brussels
and
all
subjects
gave
their
informed
consent
to
participate
in
this
study.
EMO
recording
Motor
unit
action
potentials
were
recorded
by
selective
electrodes
made
of
a
40
,m
diamel-
coated
Nichrome
wire
inserted
into
the
muscle
by
an
hypodermic
needle
(Desmedt
&
Godaux,
1977).
Motor
unit
action
potentials
were
amplified
by
a
differential
preamplifier
and
filtered
(100
Hz-10
kHz)
before
being
displayed
on
a
Tektronix
565
oscilloscope.
The
signal
was
then
stored
on
a
Hewlett-Packard
4-channel
FM
magnetic
tape-recorder
operated
at
15
in
s-1.
For
the
FDI,
the
electrode
was
inserted
in
the
mid-part
of
the
belly
of
the
muscle,
whereas
for
the
adductor
pollicis
the
needle
was
inserted
through
the
palmar
skin
of
the
hand,
in
the
mid-line
between
the
first
and
second
metacarpal
joints,
approximately
0-5
cm
distally
to
the
border
of
the
opponens
pollicis.
For
each
subject,
the
needle
was
reinserted
in
at
least
three
separate
locations.
For
a
given
site,
different
depths
and
angles
were
explored
in
order
to
obtain
action
potentials
from
different
motor
units.
In
order
to
check
possible
motor
unit
synchronization
(Milner-Brown,
Stein
&
Yemm,
1973a),
surface
EMG
activity
was
recorded
by
means
of
two
surface
electrodes,
one
placed
at
the
motor
point
of
the
muscle,
the
other
on
the
distal
tendon.
The
EMG
response
was
amplified,
filtered
(10
Hz-1
kHz)
and
full-wave
rectified.
Synchronized
data
were
not
considered.
Force
recording
The
isometric
force
of
the
adductor
pollicis
and
FDI
muscles
was
measured
with
a
strain-gauge
transducer
(Philipps
PR
9212;
compliance
7-6
#tm
N-1;
resonance
frequency
600
Hz).
The
analog
signal
from
the
transducer
was:
(1)
low-gain
DC
amplified
and
unfiltered;
(2)
high-gain
AC
amplified
after
filtering
out
low-frequency
fluctuations
(1-100
Hz)
by
a
3A9
plug-in
Tekronix
amplifier.
For
the
adductor
pollicis
the
first
phalanx
of
the
thumb
was
connected
to
the
transducer
56
MOTOR
UNIT
BEHAVIOUR
AFTER
IMMOBILIZATION
via
an
inextensible
steel
cable
so
that
the
hand
and
the
thumb
were
in
the
same
plane.
The
forearm
and
the
hand
were
placed
in
a
clothed
Perspex
holder
in
a
supinated
position
and
strapped
without
interfering
with
the
normal
blood
circulation.
For
the
FDI,
the
palm
of
the
hand
rested
on
a
moulded
Perspex
plate.
The
last
three
fingers
were
fixed
on
the
plate
with
a
thick
rubber
band.
The
thumb
was
immobilized
in
abduction
and
extension
by
another
Perspex
plate
covered
with
soft
rubber
and
the
first
phalanx
of
the
index
finger
was
connected
to
the
transducer.
In
these
conditions,
the
muscle
contraction
was
nearly
isometric.
It
would
obviously
be
impossible
to
identify
and
test
the
same
motor
unit
in
tests
carried
out
after
a
6-8
week
interval.
Therefore
recruitment
thresholds
were
normalized
as
a
percentage
of
MVC
in
order
to
compare
motor
units
that
are
recruited
at
the
same
fractional
force
relative
to
MVC
in
control
and
disused
muscles.
Maximum
voluntary
contraction
was
determined
during
three
contractions
of
4-5
s
duration
separated
by
3
min.
The
largest
of
the
three
contractions
was
considered
as
the
MVC.
During
the
contractions,
the
subjects
were
verbally
encouraged
and
a
visual
feedback
was
provided.
Experimental
procedure
and
data
proce8sing
Once
a
single
motor
unit
action
potential
was
isolated,
the
twitch
force
of
the
motor
unit
was
measured
using
the
spike-triggered
averaging
method
(see
Milner-Brown
et
al.
1973a).
Briefly,
the
method
consists
of
triggering
the
sweep
of
an
averager
(Nicolet,
4094c)
with
the
motor
unit
action
potential
during
a
steady
contraction
and
recording
the
evoked
isometric
force.
It
is
then
possible
to
extract
the
single
motor
unit
contribution
from
the
whole
mechanical
force.
Since
a
low
steady
motor
unit
firing
rate
is
necessary
to
avoid
mechanical
summation
(Calancie
&
Bawa,
1986)
subjects
were
provided
with
visual
and
auditory
feedback.
Moreover,
a
rate
limiter
was
used
to
average
mechanical
responses
from
the
same
motor
unit.
These
were
separated
from
each
other
by
at
least
100
ms.
Thereafter,
the
subject
was
required
to
make
3-5
slow
isometric
ramp
contractions
of
5
%
MVC
s-I
while
following
a
target
on
an
oscilloscope
screen
(Hewlett-Packard,
1201B).
A
rest
of
5-10
min
was
allowed
between
two
successive
recordings.
For
each
ramp
contraction,
we
determined
the
motor
unit
recruitment
threshold
as
the
level
of
force
at
which
the
motor
unit
action
potential
was
first
recruited.
The
instantaneous
motor
unit
firing
rate
during
the
steady
and
ramp
contractions
was
also
measured
off-line.
The
raw
data
stored
on
magnetic
tape
(Hewlett-
Packard,
3960)
were
passed
through
a
window
discriminator
and
the
instantaneous
motor
unit
firing
rate
was
calculated
by
an
Apple
II
computer
after
analog-to-digital
conversion.
In
cases
where
accurate
triggering
of
the
window
discriminator
was
not
possible,
the
EMG
was
photographed
on
Kodak
35
mm
film
and
the
firing
rate
was
determined
by
measuring
the
time
between
successive
spikes.
In
all
experiments,
cutaneous
temperature
was
continuously
controlled
and
maintained
at
35
+
0
5
°C
with
infra-red
light.
RESULTS
Contractile
properties
After
6-8
weeks'
immobilization,
the
comparison
between
the
distribution
of
the
twitch
tensions
recorded
from
control
and
disused
adductor
pollicis
and
FDI
shows
a
shift
towards
a
larger
number
of
small
tension
motor
units
(Fig.
1).
The
means
are
summarized
in
Table
1.
In
both
muscles,
the
comparison
of
the
distribution
of
the
motor
unit
contraction
times
indicates
a
mean
increase
of
16%
(range
13-17%)
in
the
adductor
pollicis
and
of
13%
(10-15%)
in
the
FDI.
Moreover,
immobilization
also
shifts
the
motor
unit
twitch
time-to-half-relaxation
towards
larger
values
of
12%
(6-16%)
and
13%
(9-17%)
in
the
adductor
pollicis
and
FDI
respectively
(Table
1).
One
of
the
three
subjects
immobilized
by
a
plaster
cast
had
no
fracture,
but
the
findings
were
the
same:
the
motor
unit
twitch
force
decreased
by
38
%
compared
to
41
and
42
%
in
the
two
patients,
the
contraction
time
increased
by
13%
compared
to
17
and
16%,
and
the
half-relaxation
time
increased
by
14%
compared
to
16
and
57
J.
DUCHATEAU
AND
K.
HAINA
UT
16
%.
In
this
subject,
the
MVCs
of
the
adductor
pollicis
of
the
left
hand
and
the
right
hand
were
90
3
and
92-8
N
respectively.
Therefore
the
control
values
obtained
from
the
same
muscle
in
this
subject
were
pooled
with
control
values
obtained
from
the
contralateral
muscle
in
the
two
patients.
30-
v
AP
AP
25
2
.....|....
v
15-
°~
30
FDI
*FDI
25-
...
L20
..
i
15
*
.......
'.'
~~~~~~~~~~~~~~~~~~~~~~~~~..............
..'I
.
5 i
............................~
~
~
~
~
~
~~~~...................
o~~~~~~~~~~~~~~~~~~~~~~~~~~~.
0
40
80
120 160
200
240
20
30
40
50
60
70
80
90
Twitch
force
(inN)
Contraction
time
(ins)
Fig.
1.
Histograms
showing
the
distribution
of
motor
units,
twitch
force
and
contraction
time
in
control
E
and
after
immobilization
LI
in
adductor
pollicis
(AP)
and
FDJ
for
all
subjects.
The
arrows
indicate
the
mean
of
each
distribution.
The
distributions
of
the
twitch
forces
are
significantly
different
(Kolmogorov
..mirnov
two-sample
test)
at
P
<
001
and
P
<
005
respectively
in
adductor
pollicis
and
FDL.
The
distribution
of
the
contraction
time
is
significantly
different
at
P
<
0-02
and
P
<
0-06
respectively
in
adductor
pollicis
and
FDI.
Order
of
recruitment
Figure
2A
illustrates
in
FDJ
of
one
subject
the
relationship
between
the
recruitment
threshold,
expressed
as
a
percentage
of
the
MVC,
and
the
motor
unit
twitch
tension
in
the
control
muscle
versus
the
immobilized
muscle.
This
figure
suggests
a
similar
effect
on
the
twitch
tension
of
motor
units
at
different
thresholds
and
also
indicates
that
the
size
principle
(Henneman,
Somjen
&
Carpenter,
1965)
does
not
change
in
disused
muscles.
These
results
are
illustrated
in
one
subject
but
were
recorded
from
all
subjects.
In
the
five
subjects
the
linear
correlation
coefficients
of
these
relations
ranged
from
O
7O
to
O
93
in
control
and
from
O
74
to
0-91
in
disused
muscles.
In
these
subjects,
the
slopes
of
the
linear
regression
in
control
and
disused
muscles
were
respectively
6e3
and
2*6,
5*5
and
2*9,
4
0
and
2v3
in
adductor
pollicis
and
58
MOTOR
UNIT
BEHAVIOUR
AFTER
IMMOBILIZATION
6-0
and
1-3,
7-2
and
3-1
in
FDI.
Figure
2B-E
illustrates
in
the
same
subject
as
Fig.
2A
the
twitch
electrical
and
mechanical
responses
of
motor
units
recruited
at
different
force
thresholds
in
control
(B
and
D)
and
disused
(C
and
E)
FDI.
The
MVC
of
this
subject
in
the
control
and
the
muscle
immobilized
for
8
weeks
was
57-1
and
21-0
N
respectively.
TABLE
1.
Motor
unit
contractile
properties,
recruitment
threshold
and
firing
rates
for
all
subjects
Half-
Firing
Twitch
Contraction
relaxation
Recruitment
rate
at
Maximal
force
time
time
threshold
recruitment
firing
rate
(mN)
(ms)
(ms)
(%
MVC)
(Hz)
(Hz)
Adductor
pollicis
Control
70+42
43+10
35+10
6-6+5-9
6-6+16-1
22-6+7-4
(57)
(57)
(57)
(55)
(55)
(55)
Immobilized
41+30
50+11
39+11
15-4+13-7
6-2+15-3
13-1+3-7
(57)
(57)
(57)
(56)
(56)
(51)
P
0-01
0-02
0-05
0003
0-4
(n.s.)
0-001
FDI
Control
62+46
39+10
30+9
8-6+6-9
6-5+13-6
31-0+8-9
(44)
(44)
(44)
(43)
(43)
(39)
Immobilized
39+27
44+12
34+8
19-2+11-8
6-1+11-8
19-0+4-9
(41)
(41)
(41)
(41)
(41)
(38)
P
0-05
0-06
0-05
0-001
0-2
(n.s.)
0-001
Values
are
means+
S.D.
The
number
of
motor
units
is
indicated
in
parentheses
as
well
as
the
level
of
significance
(P
value).
The
comparison,
in
control
and
disused
muscles,
of
the
recorded
motor
units'
distribution
in
relation
to
their
recruitment
thresholds
(Fig.
3)
shows
a
larger
number
of
higher-threshold
motor
units
after
immobilization.
When
expressed
as
a
percentage
of
the
MVC,
the
means
were
significantly
increased
by
133%
in
the
adductor
pollicis
and
by
123%
in
the
FDI
(see
Table
1).
Electrical
activity
anddfiring
rate8
The
analysis
of
the
motor
units'
action
potentials
after
disuse
indicates
an
overall
decrease
of
the
peak-to-peak
amplitude
(Fig.
2B-E).
Although
this
parameter
is
influenced
by
the
distance
which
separates
the
electrode
from
the
fibres,
it
always
showed
a
very
large
decrease
of the
order
of
40-50
%
when
measured
in
98
motor
units
after
immobilization
and
compared
with
101
motor
units
in
the
control.
In
both
muscles
the
immobilization
did
not
significantly
change
the
motor
units'
discharge
rate at
recruitment,
whereas
the
maximal
firing
rate
was
significantly
reduced
(Fig.
4).
The
mean
of
the
distribution
of the
maximal
firing
rate
was
reduced
in
adductor
pollicis
and
FDI
by
42
and
39%
respectively.
The
subject
without
fracture
responded
like
the
patients
and
showed
a
mean
reduction
of
43
%
compared
to
44
and
38
%
in
the
two
patients.
Moreover,
in
control
muscles
the
motor
unit
fired
continuously
during
the
MVC,
whereas
in
disused
muscles
short
periodical
interruptions
were
observed,
before
the
motor
unit
spontaneously
started
firing
again
at
the
same
frequency
as
before
the
interruption.
59
60
J.
DUCHA
TEA
U
AND
K.
HAINA
UT
A
0
8
C
150
X
Recrit
onttresold
%MC
0m
.1oo
00
~~~~D
E
0
1
mV
2otor
1nits)
an
Ms
(R
=
0
74;
P
<
0
001)after
immobilization
l
oins
0
0
50~~~~~~~~~~~~~Is
mN
0
10
20
30
40
50.........
1...........
Recruitment
threshold
%m
MVC)
30
ms
Fig.
2.
A,
twitch
force
of
motor
units
from
FDI
of
one
subject
in
control
(twenty-four
motor
units)
and
after
8
weeks'
immobilization
(twenty-five
motor
units)
plotted
as
a
function
of
the
recruitment
threshold
(expressed
as
a
percentage
of
MVC).
The
linear
regression
is
y
=
6-02x+
12-0
(R
=
0-89;
P
<
0-001)
in
control
and
y
=
1P34x+4-3
(R
=
0-74;
P
<
0-00t)
after
immobilization.
The
slopes
of
the
regression
lines
before
and
after
immobilization
are
significantly
different
(P
<
0-01).
B-E,
motor
unit
action
potential
and
the
corresponding
isometric
twitch
of
single
motor
units
extracted
by
spike-
triggered
averaging
(64
sweeps
in
B
and
C;
32
sweeps
in
D
and
E).
Motor
units
in
B
and
D
are
control
twitches
whereas
those
in
C
and
E
are
recorded
after
immobilization.
Motor
units
in
B
and
C
are
recruited
at
low
force
thresholds
(4-0%
and
4-5%
of
MVC
respectively);
those
in
D
and
E
are
recruited
at
higher
force
thresholds
(26
%
and
25
%
of
MVC
respectively).
50
AP
FDI
Ch
40
0
0
E
30.
V
0
0B
20
a)
C10
0)
0
0
10
20
30
40
50
0
10
20
30
40
50
Recruitment
threshold
(%
MVC)
Fig.
3.
Histograms
showing
the
distribution
of
the
recruitment
thresholds
(expressed
as
a
percentage
of
MVC)
in
control
O
and
after
immobilization
:
in
adductor
pollicis
(AP)
and
FDI
for
all
subjects.
The
arrows
indicate
the
mean
of
each
distribution.
The
distributions
are
significantly
different
(Kolmogorov-Smirnov
two-sample
test)
at
P
<
0003
and
P
<
0
001
respectively
in
adductor
pollicis
and
FDI.
MOTOR
UNIT
BEHAVIOUR
AFTER
IMMOBILIZATION
The
comparison,
in
control
and
disused
muscles,
of
the
difference
between
the
maximal
firing
rate
and
the
firing
rate at
recruitment
(A
firing
rate)
is
illustrated
in
Fig.
5
for
motor
units
of
different
recruitment
thresholds.
In
control
muscles,
A
firing
rate
decreased
with
increasing
recruitment
threshold.
In
disused
muscles
it
also
40
AP
30
+
20
+
10i
o0
l l
l
20
10
FDI
0
2
4
6
8
10
12
Firing
at
recruitment
AP
FDI
0
10
20 30
40
Maximal
firing
rate
(Hz)
Fig.
4.
Histograms
showing
the
distribution
of
the
firing
rate
at
recruitment
and
the
maximal
firing
rate
in
control
O]
and
after
immobilization
El
in
adductor
pollicis
(AP)
and
FDI
for
all
subjects.
The
arrows
indicate
the
mean
of
each
distribution.
No
significant
differences
were
recorded
for
the
firing
rate
at
recruitment
before
and
after
immobilization
for
both
muscles,
whereas
the
distributions
of
the
maximal
firing
rates
are
significantly
different
(Kolmogorov-Smirnov
two-sample
test)
at
P
<
0001
for
adductor
pollicis
and
FDI.
decreased
with
increasing
threshold,
but
the
slope
of
the
regression
lines
was
significantly
different
from
that
for
control
muscles.
No
significant
difference
in
the
decrease
in
A
firing
rate
was
observed
in
disused
adductor
pollicis
and
FDI
when
motor
units
of
proportionally
similar
force
thresholds
were
compared
(Fig.
5).
DISCUSSION
In
the
present
investigation
the
most
striking
change
of
the
motor
unit
behaviour
after
immobilization
is
the
marked
reduction
in
maximal
firing
rate.
After
6-8
weeks
of
immobilization,
pain
could
inhibit
motoneurons
during
MVCs
(Stokes
&
Young,
1984)
and
so
explain
this
reduction
in
firing
rate
and
the
shift
towards
higher
(n
0
.1
o
0
E
0
0
Cu
0
0
0~
40T
30
50
61
J.
DUCHATEAU
AND
K.
HA
INA
UT
recruitment
threshold.
However,
during
the
experiments
none
of
the
subjects
reported
such
discomfort
during
MVC
and
the
decrease
in
firing
rate
was
also
observed
in
all
five
subjects
during
submaximal
voluntary
contractions.
Moreover,
the
subject
without
fracture
responded
to
immobilization
like
the
patients.
Thus
the
40
~
AP
FDI
0
0
0
0
30
000
N
000
0$O
0
0
o%o
040
0
20
o
0
.
0
00
N.
0
No
0
@
0
0
N0
20-
000
0
0
0
0NK00
0
0
*
0
0
030
0
0
o
0
20
0
*
0~~~~~~~~~~~~~o
0~O
et
0
0
0
0
0
0~~~~~~~~~
0
10
20
30
40
50
0
10
20
30
40
50
Recruitment
threshold
(%
MVC)
Fig.
5.
Difference
between
the
maximal
firing
rates
and
the
firing
rate
at
recruitment
(A
firing
rate)
in
control
(0)
and
after
immobilization
(1)
for
all
of
our
subjects,
plotted
as
a
function
of
the
recruitment
threshold.
In
adductor
pollicis
(AP),
the
linear
regressions
are
respectively
y=
-048x+2141
(R=
-0-51;
P<0001)
and
y=
-006x+19-2
(R=
-
0-29;
P
<
0-05)
in
control
and
after
immobilization.
In
FDI,
the
linear
regression
are
respectively
y
=
-
048x
+28-4
(R
=
-
0-64;
P
<
0-00
1)
and
y
=
-
0-I
Ix
+
14-3
(R
=
-0
39;
P
<
0-05)
in
control
and
after
immobilization.
The
comparison
of
the
slope
of
the
regression
lines
in
the
two
conditions
is
significantly
different
in
adductor
pollicis
(P
<
0-001)
and
in
FDI
(P
<
0
01).
change
in
motor
unit
behaviour
reflects
a
neural
adaptation
to
disuse
and
should
not
be
related
to
the
fracture
or
the
injury.
In
the
patients,
the
contralateral
muscle
served
as
control
whereas
in
the
subject
without
fracture
the
same
muscle
was
tested
before
and
after
immobilization.
In
this
subject
the
difference
in
control
MVC
recorded
from
the
two
adductor
pollicis
was
less
than
3%
and
the
motor
unit
behaviour
was
not
different.
Therefore
control
values
of
all
three
adductor
pollicis
were
pooled.
The
finding
that
the
maximal
firing
rate
decreases
after
immobilization
without
change
in
the
firing
rate
at
recruitment
indicates
that
the
motor
unit
frequency
modulation
is
narrowed
in
disused
muscles.
This
point
is
illustrated
by
Fig.
5
which
also
shows
that
the
decrease
in
A
firing
rate
after
immobilization
is
larger
in
motor
units
of
low
recruitment
threshold
and
large
frequency
modulation
as
compared
to
motor
units
of
higher
threshold
and
lower
frequency
modulation.
Decrease
in
maximal
firing
rate
could
be
explained
by
changes
in
proprioceptive
afferents
on
the
motoneurons
(Mayer,
Burke,
Toop,
Hodgson,
Kanda
&
Walmsley,
1981)
and/or
62
MOTOR
UNIT
BEHA
VIOUR
AFTER
IMMOBILIZATION
reduced
ability
to
activate
motor
units
(Fuglsang-Frederiksen
&
Scheel,
1978;
Sale
et
al.
1982).
This
last
point
was
suggested
by
the
finding
after
immobilization
of
a
smaller
reflex
potentiation
which
is
closely
controlled
by
the
central
drive
(Upton,
McComas
&
Sica,
1971).
The
decrease
in
the
motor
unit
twitch
force
observed
in
these
experiments
after
immobilization
is
coherent
with
previous
results
which
showed
a
decrease
of
the
maximal
force
during
voluntary
contractions
(Sale
et
al.
1982)
and
electrically
evoked
contractions
(White
&
Davies,
1984;
Davies,
Rutherford
&
Thomas,
1987;
Duchateau
&
Hainaut,
1987).
The
finding
in
all
five
subjects
of
a
proportionally
identical
decrease
in
twitch
force
in
motor
units
of
different
force
thresholds
does
not
support
the
proposition
that
red
fibres
are
more
affected
by
reduced
use
than
white
fibres
(Edstr6m,
1970).
Our
results
are
in
agreement
with
those of
Sargeant
et
al.
(1977)
and
MacDougall
et
al.
(1980)
and
indicate
that
immobilization
has
the
same
effect
on
motor
units
of
different
force
thresholds.
Under
our
experimen
al
conditions
the
subject
performed
sustained
isometric
contractions
as
large
as
50%
of
the
maximum
in
control
and
70
%
after
immobilization.
Thus
it
is
assumed
that
nearly
all
motor
units
were
recruited
(Kukulka
&
Clamann,
1981;
De
Luca,
Lefever,
McCue
&
Xenakis,
1982)
and
that
the
larger
motor
units
were
also
tested
in
disused
muscles.
The
slowing
of
the
motor
unit
twitch
time
course
is
in
line
with
previous
observations
of
a
slower
muscle
twitch
time
course
after
immobilization
(Sale
et
al.
1982;
Davies
et
al.
1987;
Duchateau
&
Hainaut,
1987).
Although
the
analysis
of
the
motor
unit
action
potential
amplitude
is
of
limited
value
(Bellemare,
Woods,
Johansson
&
Bigland-Ritchie,
1983),
our
results
are
coherent
with
previous
observations
of
the
effects
of
immobilization
on
the
muscle
twitch
action
potential
recorded
via
surface
electrodes
and
voluntary
EMG
recorded
with
a
coaxial
needle
(Fuglsang-Frederiksen
&
Scheel,
1978).
After
immobilization
the
amplitude
of
the
motor
unit
action
potential
is
smaller
because
fibre
atrophy
is
present
(Fudena
et
al.
1961)
and
the
motor
unit
cannot
fire
consistently
throughout
the
MVC.
Thus
the
EMG
is
not
only
reduced
in
amplitude
but
silent
periods
appear
(Duchateau
&
Hainaut,
1987,
Fig.
2).
The
observed
shift
of
the
motor
units'
distribution
towards
higher
muscle
force
thresholds
could
be
explained
by
the
fact
that
after
immobilization
it
is
technically
easier
to
record
motor
units
of
larger
threshold
because
the
maximal
firing
frequency
is
decreased
and
the
interference
of
motor
units
is
smaller.
Our
interpretation
is
that
in
disused
muscles
a
larger
number
of
motor
units
is
needed
to
develop
a
submaximal
force
of
contraction
because
all
motor
units
have
lost
a
part
of
their
contractile
tension.
This
point
of
the
discussion
is
coherent
with
the
finding
that
after
exercise
training
the
contractile
tension
of
the
motor
units
is
increased
and
fewer
high-
threshold
motor
units
are
recruited
during
submaximal
voluntary
contractions
(Hainaut,
Duchateau
&
Desmedt,
1981,
Figs
3
and
4).
Moreover,
the
observation
in
all
subjects
of
a
positive
correlation
between
the
recruitment
threshold
and
the
twitch
tension
of
the
motor
units
after
immobilization
extends
Henneman's
size
principle
to
disused
human
muscles.
This
concept,
which
was
originally
proposed
in
the
decerebrated
cat
(Henneman
et
al.
1965),
has
been
confirmed
in
humans
during
isometric
and
dynamic
contractions
(Milner-Brown,
Stein
&
Yemm,
1973b;
Stephens
&
Usherwood,
1977;
Desmedt
&
Godaux,
1977)
and
more
recently
after
3
months'
exercise
training
(Hainaut
et
al.
1981).
63
J.
DUCHATEAU
AND
K.
HA
INA
UT
It
is
concluded
that
immobilization
not
only
alters
the
peripheral
electrical
and
mechanical
processes
of
the
muscle
contraction,
but
also
similarly
changes
the
motor
units'
behaviour
in
human
adductor
pollicis
and
FDI.
In
both
muscles
the
motor
unit
frequency
modulation
is
narrowed
by
a
decrease
in
the
maximal
firing
rate,
which
appears
to
be
larger
in
motor
units
of
low
recruitment
threshold,
although
all
motor
units
show
similar
contractile
adaptations
to
immobilization.
This
work
was
supported
by
the
Fonds
National
de
la
Recherche
Scientifique
of
Belgium,
the
Conseil
de
la
Recherche
of
the
University
of
Brussels
and
the
Reckitt
and
Colman
Foundation.
The
authors
thank
Miss
A.
Deisser
and
Miss
L.
de
Montigny
for
assistance
in
the
preparation
of the
manuscript.
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H.
J.
(1986).
Skeletal
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atrophy
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immobilization.
International
Journal
of
Sport8Medicine
7,
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BELLEMARE,
F.,
WOODS,
J.
J.,
JOHANSSON,
R.
&
BIGLAND-RITCHIE,
B.
(1983).
Motor
unit
discharge
rates
in
maximal
voluntary
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of
three
human
muscles.
Journal
of
Neurophysiology
50,
1380-1392.
BOOTH,
F.
W.
(1982).
Effect
of
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1113-1118.
CALANCIE,
B.
&
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P.
(1986).
Limitations
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T.
M.,
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C.
&
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0.
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of
Applied
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56,
306-312.
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LUCA,
C.
J.,
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P.
(1982).
Behaviour
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in
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varying
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J.
E.
&
GODAUX,
E.
(1977).
Ballistic
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Physiology
264,
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&
HAINAUT,
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Electrical
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L.
(1970).
Selective
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R.
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