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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



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



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

J Orthop Res, Vol . 9,No 1991



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



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

J Orthop

Res, Vol . 9.N o . I , 1991



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



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



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 ,

Vol. 9, No. l , 1991



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 ,

Vol. 9 N o . 1 1991



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



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



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



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.

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J Orthop Res,

Vol.

9 , N o . I 1991

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