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Journal
of
Orthopaedic Research
991-103 Raven Press,
Ltd.,
New York
1991 Orthopaedic Research Society
Electrom yograph ic Activity of the Abdom inal and
Low
Back Musculature During the Generation of Isometric and
Dy nam ic Axial Trun k Torq ue: Implications fo r
Lumbar Mechanics
Stuart
M .
McGill
Occup ational Biomechanics Labo ratory, Department of Kinesiology, University of Waterloo,
Waterloo, Ontario, Canada
Summary: This study focused on the electromyographic activity of the trunk
musculature, given the well-documented link between occupational twisting
and the increased incidence
of
low back pain. Ten men and 15 women volun-
teered for this study, in which several aspects of muscle activity w ere exam -
ined. The first aspect assessed the myoelectric relationships during isometric
exertions. T here was great variability in this relationship betw een muscles an d
betw een subjects. Furt her, the m yoelectric activity levels (normalized
to
max-
imal electrical activity) obtained from nontwist activities were not maximal
desp ite maximal efforts to gene rate axial torqu e (e.g., rectus abdom inis, max-
imum voluntary contraction; 22% external oblique, 52%; internal oblique,
5 5 ;
latissimus dorsi,
74%;
upper erector spinae [T9], 61%; lower erector
spinae [L3], 33%). In the second aspect of the study, muscle activity was
examined during dynamic axial twist trials conducted at
a
velocity of 30 and
60°/s. The latissimus dorsi and external oblique appeared to be strongly in-
volved in the generation of axial torqu e throughout the twist range and activity
in the up per erecto r spinae displayed
a
strong link with axial torque a nd di-
rection of twist, even though they have no mechanical potential to contribute
axial torque, suggesting a stabilization role. The third aspect of the study
was comprised of the formulation
of
a model consisting
of a
three-dimensional
pelvis, rib cage, and lumbar vertebrae and driven from kinematic measures of
axial twist and muscle electromyograms. The relatively low letels of normal-
ized myoelectric activity during maximal twisting efforts coupled with large
levels of agonistTantagonist cocontraction caused the model to severely un-
derp redict measured torq ues (e.g., 14 Nm predicted for 91 Nm measured).
Such dom inant coactivity suggests tha t stabilization of the join ts during twist-
ing is far more imp ortant to th e lumbar sp ine than production of large levels of
axial torque.
Key Words:
Low back mechanics-Lumbar electromyography-
Trunk twisting.
The key to unde rstanding the link between occu-
pational twisting a nd th e incidence of low bac k pain
is to elucidate the mechanisms that contribute to
twisting injury. A xial trunk twisting and th e gener-
Received June IS, 1989;accepted March 20, 1990.
Address correspondence and reprint requests
to
Dr.
S.
McGill, Occupational Biomechanics Laboratory, Department of
Kinesiology, University
of
Waterloo, Waterloo, Ontario, Can-
ada
N2L
3G1.
ation of axial torsion are the result of muscular
force. These sam e muscle forces, howeve r, impose
a tremendous am ount of s tress on the lum bar inter-
vertebral join t du e to their relatively small mechan-
ical moment arm . For this reason, w e report an in-
vestigation, consisting of several experiments, into
the m uscular response during trunk twisting. Th ere
are several features that m ake analysis of the mus-
culature during twisting interesting and unique. For
91
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92
S.
M .
McGILL
example, there is no trunk muscle specifically de-
signed to produce axial rotation because the gener-
ation of axial torque is coupled with the production
of either lateral bending or sagittal plane torque, or
both. While electromyographic investigations of
muscle activity have been performed during the
generation of isometric torque
(17),
the muscular
response throughout the twisting range of motion
remains poorly understood. Electromyography pro-
vides information, within certain limitations, about
the neural drive to various components of the mus-
culature. In addition, such information on activa-
tion, combined with geometric modeling of the mus-
culoskeletal tissues, constitutes a powerful tool to
increase functional understanding of the twisting
mechanism of the trunk and related injury.
The acquisition of electromyographic signals is
difficult, and the raw, unprocessed form of the sig-
nal possesses little information content. Often the
raw signal is rectified and low pass filtered (linear
envelope) (26) to forge a link between the raw mus-
cle activation signal and force production. To fur-
ther enhance physiologic interpretation, the signal
is often normalized to, or expressed as a percentage
of, the maximum electrical activity (MEA) obtained
during a static maximum voluntary contraction
(MVC). However, whereas voluntary efforts that
produce maximum myoelectric signals from some
muscles have been published (27), the method to
achieve maximum signals for normalization from
the trunk musculature remains to be established.
This is a critical issue for those models of the low
back that use myoelectric signals as neural drive to
the modeled muscles. This issue constitutes the first
aspect of this study.
The remaining aspects reported in this study deal
with muscle activation during twisting. The rela-
tionship between torque and myoelectric signal am-
plitude in the trunk has been addressed by several
researchers. Both Stokes et al. (23) and Vink et al.
(24) demonstrated linear and curvilinear electro-
myogram to isometric extensor torque relation-
ships, whereas Seroussi and Pope (21) demon-
strated a linear relationship for isometric holds of
extension efforts 2 = 0.96) and for the difference
between left and right erector spinae myoelectric
activity and the lateral bending torque 3 0.95).
Pope et al. (18) demonstrated curvilinear relation-
ships between the rectus abdominis, abdominal ob-
liques, and erector spinae during the production of
isometric axial torque. However, several issues re-
main unanswered regarding the monitoring of mus-
cle activity during both isometric efforts and dy-
namic twists. For example, are there individual
dif-
ferences in muscle recruitment patterns indicative
of function, and which muscles should be moni-
tored for activity in order to obtain reasonable doc-
umentation of the torsional-twisting mechanism of
the lumbar spine?
Several studies have attempted to identify muscle
function using various forms of the electromyogram
during twisting. Jonsson 6 ) noted that the erector
spinae was active during twisting and speculated
that such contractions were for torque production
and/or to provide a stabilizing component to the
spine. Morris et al. (15) documented similar coacti-
vation in erector spinae under axial twisting condi-
tions. Pope et al. (17) summarized that the issue of
relative activity of the trunk musculature during
twisting is unclear in the literature, but agreed that
the observed cocontractions in right and left side
muscles must contribute to spine stabilization. Cer-
tainly if large discrepancies in muscle activation and
cocontraction patterns are observed between sub-
jects, it is unlikely that a single equivalent twisting-
axial torque muscle could be found to represent
muscular contributions for use in simple models in-
tended to estimate occupational joint loads.
Given the several contentious issues presented in
this introduction, the purposes of the present study
were (a) to find a method to obtain the maximum
myoelectric signal amplitude for normalization of
the trunk musculature, (b) to examine the myoelec-
tric activity-axial torque relationship of various
trunk muscles, and
(c)
to combine myoelectric sig-
nal information with an analytical model in an effort
to increase insight into muscular axial torque pro-
duction.
Thus, this paper reports
a
series of experiments
performed on the same group of subjects in an at-
tempt to address these issues. Each subject per-
formed the experiments in a single session so that
the electrodes were applied once and all instrumen-
tation settings remained constant.
METHODS
Subjects
Ten men and five women were recruited from a
university student population (see Table 1 for sub-
ject data). An additional man participated in the
MVC portion of the study for a total of 1 1 men. All
J
Orthop
Res, Vol. 9, N o . 1, 1991
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EMG ACTIVITY OF TRUNK MUSCLES DU RING TWISTING 93
TABLE
1.
Subject
data
Men Women
meadSD meadSD
N
10
5
Age
(yr)
2 1.218.5 2411.9
Height (cm)
173
m . 7
15914.2
Weight (kg)
71.417.7 53.316.7
subjects were healthy and had not experienced low
back pain for at least 1 year.
Instrumentation
Six
pairs
of
bipolar, AgAgCl surface electromyo-
gram (EMG) electrodes were attached to the skin
on the right side of the body: rectus abdominis,
3
cm lateral to the umbilicus; external oblique, ap-
proximately
15
cm lateral to the umbilicus; internal
oblique, below the external oblique electrodes and
just superior to the inguinal ligament; latissimus
dorsi, lateral to T9 over the muscle belly; thoracic
erector spinae,
5 cm
lateral to T9 spinous process,
and lumbar erector spinae,
3
cm lateral to
L3
spinous process. These electrode locations and ar-
rangements have been shown to best represent the
differential muscle activity patterns and minimize
signal cross-talk between electrode pairs during
bending and twisting tasks (8). In addition, Vink et
al.
(23)
quantified cross-talk using
12
pairs of bipo-
lar surface electrodes over the erector spinae group
during isometric contractions of
10, 20, 30,
40
50,
60,
70,
80,
90, and
100%
MVC. Using the cross-
correlation coefficient function, they demonstrated
that the absolute maximum in the correlation coef-
ficient was less than 0.30 (or about 10% of common
signal) when electrode pairs were placed more than
30 mm apart. They concluded that, even at the
small distance of 30 mm between electrode pairs,
myoelectric signals are specific and optimize selec-
tive recording of localized muscle activity in the
erector spinae. The distance between the electrode
pairs reported in this study were always greater
than 100 mm.
All raw myoelectric signals were prefiltered to
produce a band width of
10-500
Hz and were am-
plified with a differential amplifier (common mode
rejection ratio of
80
dB at
60
Hz) to produce signals
of approximately
*
4 V.
A torsional dynamometer for the trunk was fab-
ricated by removing the measurement head from a
Cybex
I1
commercial isokinetic dynamometer. The
head was mounted on the wall approximately
2.5
m
above the floor with the sensing axle oriented ver-
tically but pointing downward. A telescopic shaft of
approximately
0.6
m in length was fixed to the Cy-
bex axle with a universal joint. An adjustable jig
harness was clamped to the upper torso at the
shoulder level of the subject and connected to the
telescopic shaft with a second universal joint. Han-
dles were attached to the front of the shoulder jig
harness to provide additional stability and to stan-
dardize arm position.
The anterior-superior iliac spines of the pelvis
were fitted to a rigid fixator and secured by an ad-
justable belt around the waist of the standing sub-
ject. This prevented significant pelvic motion
throughout the experiments. Both the pelvic and
upper trunk harness arrangements were fully ad-
justable for variations in height and girth via the
telescopic construction. The subjects could apply
torque directly for the trunk to the Cybex measure-
ment head, which eliminated a weak link in the
transmission of torque. The use of two universal
joints in the measurement linkage enabled free
movement of the trunk in all directions except axial
rotation. A photograph of a subject and dynamom-
eter is shown in Fig. 1.
Tasks
There were three tasks to examine myoelectric
activity reported in this paper.
MVC Trials
The first objective was to select a method of ex-
ertion that would consistently produce the largest
amplitudes of myoelectric activity from selected
trunk muscles in order to provide a basis for nor-
malization.
Four
basic isometric restraint strategies
were used in which subjects attempted to produce
maximum muscle activity (Fig.
2).
Three trials of
each strategy were performed. The first strategy
consisted of the subject starting in a bent-knee sit-
up posture with the feet restrained by a strap in an
attempt to recruit the abdominals. Hands were
placed behind the head and the trunk formed an
angle with the horizontal of approximately
30 .
An
assistant provided a matching resistance to the
shoulders during a maximum sit-up effort. The sec-
ond method consisted of subjects leaning over the
edge of the test bench with the legs restrained.
While lying on the back supine, a flexor effort was
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94
S . M . M c G I L L
FIG. 1. A subject posi t ioned in the twist ing j ig w here axia l
torqu e and twisted posi t ion is measured by the cybex head
mo unted overhead and EMG is recorded from rectus abdo-
min is, external and intern al obliqu e, latissimus dorsi, and
upper (T9) and lower L3) rector spinae from the r ight s ide
of the trunk.
performed; while lying on the side, a lateral bend;
while lying on the stomach prone, an extensor ef-
fort. The third strategy entailed maximum isometric
exertions while standing in a restraint jig. The pelvis
was fixated with a strap against a padded bar, and
flexor, lateral bend, and extensor efforts were per-
formed against a strap around the chest. The fourth
strategy was to examine muscle activity during the
maximum isometric twists, and if that activity ex-
ceeded the activity observed during any of the
MVC strategies, it was taken as
100%
MVC for that
particular muscle.
Static Axial Twisting Efforts
Subjects performed three trials each of maximum
isometric axial twisting efforts both in the clockwise
(cw) and counterclockwise (ccw) direction with the
trunk at 0 and prerotated +
30
and -
0
for a total
of 18 trials. Subjects were instructed to slowly build
up to maximum twisting effort, peaking at about
4
seconds.
Dynamic Twisting Efforts
Maximum effort dynamic twists were collected at
30 ls
and
60'1s
in both the cw and ccw directions.
Three trials of each condition were performed for a
total of
12
dynamic trials.
Data Reduction
Myoelectric, axial torque, and twist position sig-
nals were AID converted
(12
bit resolution) at
1,000
Hz. The sample rate of 1,000 Hz was shown by
Lafortune (7) to have no effect on amplitude domain
processing, as was done in this study, and minimal
effect on frequency domain information. Indeed, he
found that the mean power frequency of raw myo-
electric signals sampled at
8,192, 4,096,
and
1,024
Hz was uneffected (140.8, 140.3, and 140.2 Hz, re-
spectively). However, when the signal was sampled
at
512
Hz, a significant decrease was noted in the
mean power frequency
(131.6
Hz).
The myoelectric signals were full wave rectified
and low pass filtered (single pass, Butterworth) at a
cutoff frequency of 3 Hz, and then normalized to
the maximum activity observed during the MVC
trials. The cutoff frequency of 3 Hz was chosen in
the following way. Olney and Winter
(16)
reported
the frequency response of the rectus femoris to be
between
1.0
and
2.8
Hz during walking, whereas
Milner-Brown et al. (13) reported approximately
3
Hz in the first dorsal interosseous. In addition, the
3 Hz cutoff produced an impulse response (time to
peak) of
53
ms which is compatible with the
30-90
m s contraction times reported by Buchthal and
Schmalbruch (1).
Torsion Model of the Trunk
Anatomical Description
A three-dimensional skeleton consisting of a rigid
pelvis, rib cage, and the five intervening lumbar
vertebrae was constructed using radiologic archives
of 50th percentile males. Attachments of muscles
and ligaments were observed on cadaveric speci-
J
Orthop Res , Vol . 9 , N o . 1 1991
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EMG A CTIVITY OF T R U N K MUSCLES DUR ING TWISTING
95
A
sit
up
FIG.
2.
Four methods were evaluated
to
produce the m aximum ampl i tude o f
“ s i t - u p ” e f f o r t
(A);
exer t ions whi le
B);
standing exert ion against an up-
per t runk be l t C); and maximum tw is t -
ing e f for t
D).
EMG for norm al izat ion: The restrained
h nging
hanging over the edge of the test table
@-
/ f l ex
@
mens in the anatomy laboratory and located on the
appropriate bony surface. These locations were dig-
itized and stored in computer memory. The cross-
sectional area of 50 selected muscle slips that cross
the
L4/L5
joint was measured from multiple CT
scans of 13 men and these anatomic areas were con-
verted to physiologic areas (force-producing areas)
by correcting for the cosines of those muscles that
did not present a true transverse section to the
plane of the scanner gantry
(1 1)
and taking into ac-
count fiber-tendon architecture.
Kinematics
The axial torque potential of the musculature was
modeled throughout the twisting range but only
compared with the measured torque values at
0”.
Muscle lengths and unit vectors to describe lines of
action were calculated from three-dimensional ab-
solute coordinates of origin and insertion.
Kinetics
Maximum muscle forces were estimated by mul-
tiplying the physiologic cross-sectional area by a
value of force production per cross-sectional area
-
according to equation 1
standing
D
twist
MS
MSMEA
F, =
~
x
CS
x K ,
where
F, =
muscle force (N), K
=
force produc-
tion per cross-section (N/cm2), CS
=
muscle cross-
sectional area (cm2), MS
=
myoelectric signal, and
MS,,A =
maximum myoelectric signal amplitude
observed during normalization contraction. Previ-
ous experiments suggested that the lumbar muscu-
lature produced force as a function of cross-section
at approximately 35 N/cm2 to 50 N/cm2 (10). These
values were well within the range reported in the
literature
(3).
Muscle forces were estimated in this
study assuming a force potential of 35 N/cm2. The
axial torque potential of each muscle was then
cal-
culated from the
3-D
triple scalar product of muscle
force about the twisting axis of L4/L5. Estimates of
total torque were obtained from the sum of the ag-
onist muscle forces. This process is described in the
matrix below.
where: M = moment created by muscle about the
axis of axial twist, rxyz= absolute
X, Y Z
compo-
nents of the distance between the muscle line of
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Res, Vol . 9.N o . I , 1991
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96
S.
M . McGILL
action and the joint center of rotation. Fxyz= the
component of the muscle force in the
X , Y,
Z di-
rection, and UXyfhe unit vector describing the di-
rection of the twisting axis.
RESULTS
The results are arranged under five subheadings
in accordance with the experiments conducted dur-
ing this study of myoelectric activity during twisting
efforts.
Selection
of
a Method to Obtain Maximum Muscle
Activity for Myoelectric Signal Normalization
The lower erector spinae was the only electrode
location in which maximum activity was consis-
tently obtained by the same method in all subjects:
hanging over the edge of the test table in a prone
posture and extending upward against resistance.
Whereas hanging over the edge of the test table
facing upward and flexing against resistance proved
to be the superior method for obtaining maximum
activity in rectus abdomis, no other muscle demon-
strated consistent trends in the choice of the
method for obtaining maximum myoelectric activ-
ity. Perhaps this indicates differences in the way in
which individuals recruit the trunk musculature to
support both simple and coupled torques. The re-
sults for each muscle location are shown in Table 2
and the test postures are shown in Fig.
2.
Muscle Activity During Isometric Exertions
Mean muscle activity on the right side of the body
at the time of peak isometric torque generation is
shown in Table
3.
Peak muscle activity was not
maximal, given that these trials were maximum
twisting efforts. For example, the greatest activity
observed during any isometric exertion was rectus
abdominis,
22%
MVC; external oblique,
52%;
inter-
nal oblique,
55 ;
latissimus dorsi, 74%; upper erec-
tor spinae,
61%;
and lower erector spinae,
33%.
It
appears that relative activity within a given muscle
is associated more with direction of rotation rather
than with starting position. This is consistent with
the fiber direction of an agonist muscle on one side,
which produces twist, versus its antagonistic coun-
terpart on the other side of the body which opposes
the effort and has a lower activation level. Large
differences in activity between right and left sides
during isometric exertions at the
0
position were
observed in the obliques
(28-50%
MVC external,
55-16%
MVC internal), latissimus dorsi (7415%
MVC), and the upper erector spinae
(56-12%
MVC). Small differences were noted between sides
in rectus abdominals and the lower erector spinae.
Myoelectric Signal to Isometric Axial
Torque Relationship
No
consistent linear or nonlinear EMG-torque
relationship was observed between subjects or
within trials of subjects. Although myoelectric sig-
nal amplitude generally increased with an increase
in torque, the ramp loading did not result in a
smooth progression
of
the myoelectric signal as
can be seen in four sample trials shown in Fig.
3.
However, examination of paired muscle activity
showed some relationship with direction of twist
effort. For example, the right latissimus dorsi dem-
TABLE
2.
Method that produced
m ximum
EMG at each electrode site
Sex
Muscle (11
M ,
5
F)
Method
Rectus abdominis M
F
External oblique M
F
Internal oblique
M
F
Latissimus dorsi
M
F
F
F
Upper erector spinae
M
Lower erector spinae M
9
hang-flex,
1
sit-up,
1
stand-flex
4
hang-flex,
1
twist 30 cw
5
sit-ups, 2 hang-lat bends, 2 hang-flex,
1
stand-flex,
1
twist 30 cw
3 hang-lat bends, hang-flex,
1
sit-up
3
stand-lat bends,
3
twists 30 cw,
2
twists
0
cw, twist
30
cw,
1
hang-lat bend,
2
hang-lat b ends,
1
hang-flex,
1
twist
30
cw,
1
twist
0
cw
3
twists
30
cw,
2
twists
30
cw, 2 stand-lat bends,
1
hang-lat bend,
1
twist 0 cw,
1
2
twists 30 cw,
2
stand-lat bends,
1
twist 30 cw
4
hang-extend,
4
twists
30
cw,
1
twist
30
cw,
1
stand-lat bend
4
hang-extend,
1
twist 30 cw
1 1
hang-extend
5
hang-extend
sit-up
twist 0 ccw,
1
stand-flex
See Fig.
1
for explanation of exertion methods.
J Orthop
Res
Vol.
9
No.
,
1991
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EMG ACTIVITY OF TR UNK MUSCLE S DURIN G TWISTING 97
TABLE 3 .
Peak torque and right side muscle activity expressed
as
a MVC during isometric exertions
Peak Upper Lower
torque Rectus External Internal Latissimus erector erector
(N.m) abdominus oblique oblique dorsi spinae spinae
0
cw
M 87 (17) 16 (1 1) 28 (10)
55
(21) 74 (13) 56 (15) 33 (11)
F
39 (7)
14 (11)
21 (8)
37 (22)
64
12) 46 (19) 23 (10)
M 95 (16) 15 (11)
50 (13)
16 (12) 15 (10) 12 (12) 26 (14)
0 ccw
F
41 (11) 21 (24) 32 (10) 16 (10)
17 (11)
7
5)
8
(4)
M
62 (14)
10 ( 5 ) 27 (13)
51 (20) 66 (17) 61 (17) 33 (15)
F
25
(8)
11 (10)
20 (9) 31 (18)
66 (19) 50 (23) 26 (11)
M 96 (18)
19 (16)
44
(18)
15 (11)
11
(7)
12 (14) 16 (7)
F 44 ( 5 )
16 (15)
28 (12) 14 (4)
10
5 )
5 (3) 4 (2)
M
102 (18) 21 (13)
26 (10)
51 (18)
64
16) 49 (17) 20 (12)
F 44 (7)
22 (26)
16 (6) 31 (16)
53 (19)
41 (24) 16
(7)
M 65 (17) 12 (10)
52 (18) 16 (15) 21 (14) 13 (15) 26 (14)
F
29
(8)
12 (13)
37 (14)
14 (7)
23 (16)
5
(4) 10 (9)
+
30
cwa
+
30
ccw
-
30
cw
-
0
ccw
Values are the mean (males,
10
subjects
x 3
trials for
30
observations; females,
5
subjects
X 3
trials for
15
observations) and standard
a +
30
corresponds to a prerotated twist that results from a cw twist.
deviation.
onstrated a strong relationship with torque in the cw
direction but not in the ccw direction. Similar
paired differences may be observed in the obliques
and in the upper erector spinae, suggesting greater
involvement of these muscles in the twisting mech-
anism.
Myoelectric Signals During Dynamic
Torque Production
The mean myoelectric signals (normalized to
100% MVC) over the range of twist for the
10
men
performing maximal efforts are shown in Fig.
4.
The
electrodes were placed on the right side of the body.
It was interesting to observe that although subjects
were requested to produce a maximum twisting ef-
fort, rarely did any muscle exhibit activation levels
that exceeded 50% MVC. For ccw efforts the right
external oblique was the most active muscle, with
peak activation of approximately
50%
MVC occur-
ring at a position of
-20 ,
where peak torque was
produced prior to the 0 position. During cw ro-
tations, the abdominal obliques (internal and exter-
nal) demonstrated a large initial burst of activity but
leveled off at approximately
45%
MVC (internal)
and 25 MVC (external) during 60 /s rotation. The
right upper erector spinae and right latissimus dorsi
demonstrated the largest constant levels of activity
throughout the range of cw rotation. However, cor-
responding low levels of activity were observed in
these muscles during ccw rotation, suggesting that
the erector spinae and latissimus dorsi must be in-
volved in the twisting mechanism, although not spe-
cifically the generation of torque.
Model Results
The normalized myoelectric signals averaged
across the subjects, obtained during the maximum
effort isometric cw twist with the trunk in the neu-
tral position, was input to the trunk model. First,
the myoelectric signal was normalized to the level
of maximum activity that was observed in any of
the test postures that were shown in Fig.
2.
Whereas 91 N.m of axial torque was measured from
the subjects, model output predicted that
26
N.m of
ccw torque was produced by the antagonist muscu-
lature and 40 N.m of cw torque was produced by
the agonists for a net of only
14
N.m cw torque.
These low levels of predicted torque were owing to
the relatively low levels of normalized myoelectric
signal used to estimate the force levels in the vari-
ous muscle slips. Furthermore, in addition to rela-
tively low activation levels, the coactivity observed
in pairs of muscles (i.e., those on the right side of
the trunk and its counterpart on the left side) pro-
duced antagonistic torque, which is subtracted from
the agonist torque, resulting in a reduced, exter-
J
Orthop
Res
Vol .
9 ,
No.
,
1991
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S .
M .
McGILL
8
A
5 0
2 5
7 5
5 0
25
*
xternal oblique
0 2 5 5 0
Torque (N-m)
& Tho:ocic
erector
spinoe
75
2 5 5 0
2 5 5 0
7 5
5
25
5 0
2 5
nternal
oblique
V
j
2 5 50
t
Lumbar
erec tor
spmoe
P
2 5 5 0
FIG.
3. EMG-torque relationships for one subject (cw [A] and ccw [B]) and for another (cw [C] and ccw [D]). No consistent linear
or curvilinear relationships were observed. However, there is a relationship between muscle activity and the direc tion of twist
effort. (Continued)
nally measured, net torque. To address normaliza-
tion concerns,a second normalization strategy was
adopted and tested. This involved normalizing the
average myoelectric signals to the maximum myo-
electric signal amplitude observed only during max-
imal isometric twisting efforts in the neutral upright
standing posture. In other words, the myoelectric
signal amplitude produced during a twist was nor-
malized to only a twisting effort. The model pre-
dicted that antagonists produced
58
N.m, whereas
the agonists produced
86
N.m
for
a net torque of 38
N.m. Some individual muscle forces are shown in
Table
4
for both normalization strategies.
DISCUSSION
The task of finding a method that consistently
produced maximum myoelectric signals for all sub-
jects
proved difficult. While motivational factors
have been identified to alter torque measurements
during MVC efforts
( 5 )
the subjects in this study
were well aware of the importance to put forth a
maximum effort. In addition, it was felt that fatigue
was not a factor during the MVC tests due to the
brief duration of contraction effort. Hence, the in-
consistency
of
results demonstrated the importance
of attempting several exertion tasks when
a
mea-
J Orthop
R es ,
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EMG ACTIVITY
OF
TRUNK MUSCLES DURING TWISTING
-8-
Rect u s ob d orn t n ls
E
60
4
20
-- t
External oblique
rx
f
rt
0
2 5 5 0
40
2 0
C
4 0
--t L o t i s s i r n u s dorsi
2
-+- Thoroctc
erec t or spmoe
2 5
5 0
2 5
5
FIG. 39
sure of maximum myoelectrical signal amplitude is
desired from the trunk musculature, i.e., there ap-
pears to be no single method that is best for all
subjects. Further, it is clear that commonly used
isometric extensor exertions in the standing posture
were inferior to other methods of obtaining maxi-
mum myoelectric activity; specifically, hanging
over the table edge and extending.
Observations of low levels of myoelectric activity
during maximal isometric twisting efforts may at
first appear perplexing but also have been noted by
Portnoy and Morin (19) and Carsloo (2). In this
study, the various methods used to obtain maximal
myoelectric activity increased the probability of full
muscle activation. In addition, expression of activ-
ity level as a percentage of MVC during twisting
tasks tends to result in lower levels of activation.
4
2 0
0
69
4 0
2
0
-
n t ern of ob l iq u e
99
B
0
2 5 5
$*
0 2 5 5 0
Nonetheless, our low values of activation were sim-
ilar to those reported by Miller and Schultz (12) for
maximal twists (for example, they reported 13%
MVC, rectus abdominis; 18%, erector spinae at L3
for the right side during cw efforts; and 20% MVC,
rectus abdominis; 26%, erector spinae at
L3
for ccw
efforts). Perhaps these low levels
of
activity indi-
cate the presence of some form of inhibitory mech-
anism that serves a protective function to the spinal
tissues under stresses that are generated during ax-
ial torsion efforts. Perhaps these trunk muscles are
required to balance flexion-extension and lateral
bending moments and thus are limited in their con-
tributions to axial torque. It was suggested by
Schultz et al. (20) that some muscles function to
counterbalance flexion and lateral bending mo-
ments that are produced by the primary axial torque
J Orthop
R e s ,
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100
S. M .
McGILL
50
0
Q
2 5
5
75
Torque (N -m)
75
t
-++ E x l e r n o l oblique
J
t
2 5 50
7 :
0
5 7:
2 5
0
2 5 5
75
25
50 75
FIG.
3C
generators such that maximum activation would not
be expected from these stabilizing muscles.
The myoelectric signal-torque relationship pro-
duced by our subjects during axial torsion efforts
were not as smooth as those published by Stokes et
al. (22) during isometric extension efforts. How-
ever, the extensor musculature has the primary
function of supporting extensor moments, whereas
no particular trunk muscle has a specific role to
support axial torque. Perhaps the increased vari-
ance in the myoelectric signal-torque relationship
indicated by the “unsmooth” lines in Fig.
3
during
the production
of
axial torque is indicative of a sys-
tem that has freedom in the choice of muscle re-
cruitment in order to generate axial torque. On the
other hand, to specifically generate extension, there
is
no choice but to recruit the extensors. Examples
of both linear (9) and curvilinear
25)
myoelectric
signal-torque relationships have appeared in the lit-
erature for various joints and muscles. Our data
show that both linear and curvilinear relationships
were produced in the trunk musculature, although
there was no consistency in either relationship be-
tween subjects or between muscles within a sub-
0
75
5 50
ject. However, because the relationships were
formed between an individual muscle and a mea-
sure of net axial torque that is the function of all
agonist and antagonistic muscles, speculation as to
the force output of an individual muscle
is
limited.
Nonetheless, such muscle activity may be indica-
tive of axial torque contribution or alternatively
may function to increase stability while other mus-
cles supply torque. Because the thoracic erector
spinae (longissimus thoracis pars thoracis and ilio-
costalis lumborum pars thoracis) have only very mi-
nor potential to contribute to lumbar axial torque
(as can be seen from the model results in Table 4)
but nonetheless demonstrate a strong link with axial
torque, they may be a prime candidate to function
in a stabilizing role. Furthermore, they can function
in this role over the entire lumbar spine because of
their thoracic insertion and extensor origin on the
sacrum and posterior aspects of the ilium. As such,
the stabilized lumbar joints form a more rigid struc-
ture upon which the primary twisting muscles can
produce greater amounts of torque.
The model output provided some insight into
muscular contribution to axial torque production.
J
Orthop
Res,
Vol . 9 , N o .
1 ,
1991
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EMG ACTIVITY OF TRUN K MUSCLES DU RING TWISTING
101
D
0
0 2 5
5
75
I
Torque (14-rn)
2 5 5 75
\ 00
FIG. D
Using the different myoelectric signal normalization
strategies-one to normalize to the maximum
amount of activity observed in any trunk exertion
effort (Shown in Fig. 2) and the other to normalize
to that activity observed during an actual maximal
isometric twist exertion (20)-the resulting net
torque predictions were quite low
(14
and
38
Nm,
respectively;
91
Nm was the actual measurement).
Perhaps the model musculature was underestimated
during our computed tomography (CT) scan and ca-
daver studies, although this was unlikely because of
the multiple scans and consideration of fiber pen-
nation and fiber-tendon architecture. Perhaps the
model's assumption that muscle produces force as a
function of cross-sectional area at 35
N
m
is
in-
correct. If this force production potential were in-
creased to 228 N/cm2 for normalization strategy
(Table 4, A) and 84 N/cm2 for strategy (Table 4, B),
then model predictions would match the measured
torque. Although values of force production of 98
N/cm2 were suggested by Fick
(4)
and
90
N/cm2 by
Morris
(14)
in muscles that act across the elbow and
knee, our previous work suggests that these values
are far too high for the trunk musculature
(10).
Al-
though satisfying the axial torque requirement, such
large forces also would generate an unrealistic ex-
tensor moment because of the large area of the lum-
bar and thoracic components of longissimus thora-
- cis and iliocostalis lumborum. For this reason it
would appear that normalization to the maximum
2 0 .
0
0 2 5 5 7 5 100
signal amplitude obtained-regardless of the iso-
metric position-is necessary to satisfy the net
torque requirements about the three orthopaedic
axes in the low back. Perhaps the model is anatom-
ically incorrect. However, the vector cosines and
moment arms of all muscles were checked in three
dimensions with CT scan and cadaveric data. It is
doubtful that anatomical inaccuracies would ac-
count for the discrepancies in measured and mod-
eled torque. Such a large underprediction of net
torque suggests that other factors, as yet unknown,
influence axial torque production that are not incor-
porated into the model at this stage of model devel-
opment.
__.__
____
CONCLUSION
Since subjects exhibited variability in the posture
to obtain maximum muscle activity, it appears that
the use of several maximum exertion postures,
rather than a single posture, should be encouraged
when seeking maximum activity for myoelectric
signal normalization.
Myoelectric signal-axial torque relationships are
not always linear or nonlinear between muscles or
between subjects, although it should be noted that
the torque contribution of
a
single muscle cannot be
partitioned from other contributors during produc-
tion of axial torque. Correspondingly, differences
between the right and left upper erector spinae dur-
ing twisting in the cw and ccw directions, coupled
J
Orthop Res,
Val.
9,
N o . 1
1991
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102
S .
M . M c G I L L
C,D 80
6
4o
2
0
80
TORQUE + ORQUE
f RA
A
6
-+ O-% EO
i 10
8
0
D 7 C - LD
-
ES
LES
-
-
0 I
TABLE
4.
Model predictions
of muscle
force
Normalization (A) Normalization (B)
Activation Force Torque Activation Force Torque
(%
MVC)
(N)
(N.m)
(%
MVC) (N) (N.m)uscle
Rectus abdominis
R
L
R
L
R
L
R
L
R
L
R
L
External oblique
Internal oblique
Latissimus dorsi
Upper erector spinae
Lower erector spinaeb
16
15
56
53
1
-1
100
83
350
29 1
4
-3
28
50
112
200
8
-
15
50
100
200
400
15
- 0
55
16
180
52
-
12
3
100
44
230
144
-21
9
74
15
104
21
-4
1
100
23
140
32
-6
1
56
12
353
76
-1
0
100
20
100
70
630
126
2825
1978
-2
0
33
26
932
733
3
2
-11
9
a
For a maximal isometric twisting effort at 0 of twist, assuming two strategies
for
EMG normalization: (A) normalize to maximum
R, right; L, left.
a Upper erector spinae is comprised of the thoracic portions of longissimus thoracis and iliocostalis lumborum.
activity in any posture and
(B)
normalize to maximum activity observed in
an
isometric twisting effort.
Lower erector spinae is comprised of the pars lumborum components of longissimus thoracis and iliocostalis lumborum and
multifidus.
J Orthop
R e s ,
Vo l .
9,
N o .
1, 1991
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EMG ACTIVITY OF TRUN K MUS CLES DURING TWISTING
103
with the fact that they have no real potential to
contribute axial torque, suggests that this group of
muscles performs a balancing and stabilizing role
while other muscles generate axial torque.
Muscle activity during maximum twisting efforts
is much lower than the activity during other activ-
ities, suggesting that either the trunk musculature is
inhibited during twisting efforts or that it is respon-
sible for maintaining equilibrium about all axes and
therefore cannot fully activate just to supply axial
torque.
Modeling efforts to predict axial torque from
my-
oelectric signals demonstrated the large counterpro-
ductive contributions of the cocontracting antago-
nistic musculature. Such dominant coactivity sug-
gests that stabilization of the lumbar joints is
paramount because it is achieved at the expense of
axial torque production.
Acknowledgment:
This work
was
funded
by
the Natural
Sciencesand Engineering Resea rch Council, Canada, a nd
the assistance of Sheri Lynn
Kane
during
data
collection
is appreciated.
REFERENCES
1. Buchthal F, Schmalbruch H: Contraction times and fibre
types in intact human muscle. Acta Physiol Scand 79:435-
52, 1970
2. Carlsoo S : The static muscle load in different work posi-
tions: An electomyographic study. Ergonomics 4:193, 1961
3. Farfan H: Mechanical disorders of the low back. Philadel-
phia: Lea & Febiger, 1973
4 . Fick R Handbuck der Antomie und Mechanik der Gelenke
unter, Berucksichtigung der bewegenden muskeln. Jena,
East Germany: Fisher,
1910
5 .
Ikai M, Steinhaus AH: Some factors modifying expression
of human strength
J
App l Physiol 16:157-6 3, 1961
6 . Jonsson B: The lumbar part of the erector spinae muscle: a
technique for electromyographic studies of the function of
its individual muscles [Thesis] Gothenburg, Sweden: Eland-
ers, 1970
7 . Lafortune D: The variability of EMG amplitude and fre-
quency measures obtained from selected trunk musculature
during sagittal plane and twisting lifts [M.Sc. thesis]. Water-
loo, Canada: University of Waterloo,
1988
8 . Lafortune D, Norman RW, McGill SM: Ensemble average
linear enveloped EMGs during lifting.In Proceedings
of
the
biannual conference of the Canadian society fo r biomechan-
ics. Ottawa, Canada, August 1988:92-3
9 .
Lippold OCS: The relation between integrated action poten-
tials in a human muscle and its isometric tension. J Physiol
Lond) 117:492499, 1952
10. McGill SM, Norman RW: Partioning of the L4/L5 dynamic
moment into disc, ligamentous, and muscular components
during liiting. Spine 11:666-78, 1986
1 1 . McGill SM, Patt N, Norman RW: Measurement of the trunk
musculature of active males using CT scan radiography: im-
plications for force and moment generating capacity about
the L4/L5 joint. Biomech 21 :32 941 , 1988
12. Miller JAA, Schultz AB: Biomechanics of the human spine
and trunk. In: Pandolf KB, ed. Exercise and sports sciences
reviews, vol Zb. New York: MacMillan, 1988
13. Milner-Brown HS, Stein RB, Yemm R: The contractile
properties of human motor units during voluntary isometric
contractions. Physiol228:285-306, 1973
14.
Moms CB: The measurement of the strength of muscle rel-
ative to the cross-section. Research
Q
20:295-303, 1949
15. Moms JM, Benner G, Lucas JB: An electromyographic
study of the intrinsic muscles of the back in man. J Anat 96:
16. Olney SJ, Winter DA: Predictions of knee and ankle mo-
ments of force in walking from EMG and kinematic data. J
Biomech 189-20, 1985
17. Pope MH, Andersson GBJ, Broman
H ,
Svensson M, Zet-
terberg C: Electromyographic studies of the lumbar trunk
musculature during the development of axial torques. J Or-
thop R es 4:28 8-97, 1986
18. Pope MH, Svensson M, Andersson GBJ, Broman H, Zet-
terberg C: The role of prerotation of the trunk in axial twist-
ing efforts. Spine 12:1041-5, 1987
19. Portnoy H, Morin F: Electromyographic study of postural
muscles in various positions and movements.
A m
J
Physiol
186:122, 1956
20. Schultz A, Haderspeck K , Warwick D, Portillo D: Use of
lumbar trunk muscles in isometric performance of mechan-
ically complex standing tasks.
J Orth op Re s 1:77-91, 1983
21. Seroussi RE, Pope MH: The relationship between trunk
muscle electromyography and liiting moments in the sagittal
and frontal planes. J Biomech 2 0:1 354 6, 1987
22. Stokes IAF, Rush S, Moffroid M, Johnson GB, Haugh LD:
Trunk extensor EMG-torque relationship. Spine 12:770-6,
1987
23.
Vink P, Daanen HAM, Verbout
AJ:
Specifcity of surface
EMG on the intrinsic lumbar back muscles 1Ph.D. thesis].
Leiden, The Netherlands: University of Leiden,
1989
24. Vink P, vander Velde EA, Verbout AJ: A functional subdi-
vision of the lumbar extensor musculature: recruitment pat-
terns and force-RA-EMG relationships under isometric con-
ditions.
Electomyog r Clin Neurophysiol28 :517-25, 1988
25. Vredenbregt J, Rau G: Surface electromyography in relation
to force, muscle length and endurance. In: Desmedt JE, ed.
New developments in electromyography an d clinical neuro-
physiology. Basel, Switzerland: Karger, 1973
26.
Winter DA:
Biomechanics
of
human movement.
Toronto:
Wiley, 1979
27. Yang JG, Winter DA: Electromyography reliability in max-
imal and submaximal isometric contractions. Arch Phys
Me d Rehabil64:417-20, 1983
509-20, 1962
J Orthop Res,
Vol.
9 , N o . I 1991