CLINICAL CONSIDERATIONS IN THE USE OF SURFACE

ELECTROMYOGRAPHY: THREE EXPERIMENTAL STUDIES

Gregory J. Lehman, MSca

ABSTRACT

Background: The validity and applicability of erector spinae electromyogram (EMG) use in clinical
practice is questionable. Differences in the amplitude of the EMG signal between populations with low
back pain (LBP) and controls have been hypothesized but not sufficiently documented.
Objective: To examine issues of EMG asymmetry and repeatability in populations with LBP and
populations without LBP in 3 separate studies.
Study Design: Three separate experimental research studies.
Methods: Study 1 examined the intraclass correlation of the EMG signal of the paraspinal musculature
at L3 on 3 separate days during quiet stance with use of 3 different normalization techniques: percent
maximum voluntary contraction, percent submaximal voluntary contraction, and percent averaged
submaximal contractions. Study 2 used a population with chronic LBP to compare the bilateral
asymmetry of paraspinal musculature in segments exhibiting pain with segments not exhibiting pain
during quiet stance. A 1-way analysis of variance was used to compare differences between asymmetry
scores in the pain and nonpain groups. In study 3, persons with LBP and persons in the control group had
the dynamic asymmetry of paraspinal musculature compared during forward bending. A cross-correlation
coefficient assessing bilateral muscle activity was calculated for each subject and differences between
groups were evaluated using a 1-way analysis of variance.
Results: Study 1: Subjects showed excellent repeatability (ICC ⬎ 0.75) regardless of the normalization
technique. Study 2: During quiet stance no differences were found in bilateral asymmetry between painful
and nonpainful motion segments. Study 3: No differences existed in bilateral dynamic symmetry for the
upper erector spinae; however, differences between groups did exist for the lower erector spinae.
Conclusion: Repeatability of the EMG signal during quiet stance is acceptable; however, bilateral
asymmetry may not be a definitive indicator of dysfunction. (J Manipulative Physiol Ther 2002;25:293-9)
Key Indexing Terms: Electromyography; Erector Spinae Muscles; Low Back Pain

INTRODUCTION
he use of the erector spinae electromyogram (EMG) ers, a general consensus on the use of surface EMG in clinical

T signal has been researched in an attempt to dis-

cern differences between those with low back
injury and those without injury. Despite the work of research-
practice is lacking. Questions regarding its validity and useful-
ness exist, yet its use is often embraced by many without
reserve. In contrast to this willful acceptance is an attitude
that quickly shuns its use without fully exhausting the EMG
a
Research Associate, Graduate Studies and Research Department, signals’ potential for clinical decision making. The purpose
Canadian Memorial Chiropractic College, Toronto, Ontario, Canada. of this article is to add to the EMG knowledge base by
Funded by the Natural Sciences and Engineering Research addressing the 3 following questions:
Council (NSERC).
Submit reprint requests to: Gregory Lehman, DC, Canadian
1. Can normalization tasks improve the repeatability of
Memorial Chiropractic College, 1900 Bayview Ave, Toronto, EMG values during quiet standing?
Ontario, M4G 3E6, Canada (e-mail: gregorylehman@hotmail. 2. Is the paraspinal EMG activity at a painful motion
com). segment more asymmetrical than at a nonpainful mo-
Paper submitted February 13, 2001; in revised form March 15, tion segment in the same person?
2001.
Copyright © 2002 by JMPT. 3. Is the bilateral paraspinal dynamic EMG activation
0161-4754/2002/$35.00 ⫹ 0 76/1/124423 profile more asymmetric in a population with low
doi:10.1067/mmt.2002.121410 back pain (LBP) than in an asymptomatic population?

293
294 Lehman Journal of Manipulative and Physiological Therapeutics
Clinical Use of PMG June 2002

Study 1 addresses issues of between-day repeatability of all influence measured EMG activity level. Nonhomogene-
raw and normalized EMG signals during quiet standing. ity in these factors between sides of the body may relegate
Although well-researched, conflicting results have been asymmetry in measured EMG activity to be the norm even
shown. Dynamic tasks have shown moderate to high repeat- though it is possible that bilateral muscles are contracting at
ability,1,2,17 whereas evaluation of static tasks have demon- equal intensities. Because of nonhomogeneity in the preced-
strated poor repeatability4 or excellent repeatability,5 de- ing factors we may be unable to adequately measure it.
pending on the study. These inconsistencies in static tasks Timing or activation profiles (linear envelope) of EMG
motivated study 1; various normalization techniques were activity is not influenced by these factors. One can compare
employed in an attempt to improve the reproducibility of the the shape of the EMG profile between bilateral muscle
EMG signal during quiet stance across days. groups. In a small study (n ⫽ 6), Grabiner11 demonstrated
The remaining 2 studies address the issue of asymmetry differences in bilateral coupling of the EMG waveform in
between bilateral erector spinae muscle groups during sym- persons with LBP and control groups during isometric ex-
metrical tasks. EMG asymmetry is often considered to be a ertions at different loads. However, the statistical test used
sign of dysfunction, although support for this belief is not to assess asymmetry compared the phasic properties of the
fully supported by the literature. Table 1 presents a brief EMG signal with the amplitude characteristics; this may
overview of the studies investigating EMG amplitude and detract from the conclusion of the study based on the
asymmetry differences between populations with LBP and limitations in comparing amplitudes because of factors dis-
pain-free controls. The studies appear to suggest that dif- cussed earlier. It is possible to assess the phasic qualities of
ferences exist between the groups during dynamic flexion the EMG signal by using a simple cross-correlation func-
tasks at peak flexion (ie, those with LBP typically lack the tion.
flexion relaxation phenomenon) and between the ratio of In an attempt to explore all concepts of asymmetry,
activity during forward flexion and re-extension. However, studies 2 and 3 address 2 asymmetry concepts. Study 2
EMG activity during other dynamic tasks and static tasks explores whether a spinal segment that exhibits pain and
does not appear to be statistically different between the 2 tenderness has more amplitude asymmetry than a segment
groups. However, a general theme seems to be a trend for within the same person who is free of pain and tenderness.
increased activation in the group with LBP. The concept of This study was motivated by the use of scanning EMG
EMG amplitude symmetry between left and right erector equipment that is often used to direct treatment of perceived
spinae has also been investigated. Those who promote the problematic segments because of their EMG asymmetry.
use of scanning EMG suggest that left/right differences in Study 3 evaluates the bilateral asymmetry of the erector
EMG activation level are indicative of pathology and there- spinae linear envelope during a dynamic forward flexion
fore are suggestive of a biomechanical lesion. The research movement using a cross-correlation function. This function
to lend credence to this idea is mixed and sparse. Four large only addresses issues of phasic imbalance but not amplitude
studies4,6-8 have compared left/right EMG activity between differences between the signals. The methods and results
populations with LBP and populations without LBP during will be presented together for each study followed by a
static and dynamic tasks and demonstrated no differences general discussion involving all 3 studies.
between the 2 groups. Conversely, 2 smaller studies have
shown contrary results. Aleksiev9 found differences in sym- METHODS
metry between groups with LBP and control groups during
various levels of static contraction but no differences were Three separate studies with different study protocols and
found during rest. Similar results were found by Leach et different subject populations were conducted. The follow-
al10 in the lower erector spinae. Although the research on ing sections will present the 3 different methods for each of
asymmetry appears mixed, the more robust studies appear to the separate experiments.
indicate that amplitude asymmetry is not a distinguishing
feature of low-back injured patients. However, none of Study 1: Investigating Normalization
these studies specifically directed the location of electrode Patient characteristics. Eight subjects were recruited with no
placement to the level of a spinal lesion (indicated by pain) history of LBP within the past 6 months (5 men, 3 women;
to adequately assess the idea that problematic segments are age ⫽ 24.6 ⫾ 2.3 years, height ⫽ 175.5 ⫾ 10.3 cm, and
inherently more asymmetrical than asymptomatic spinal weight 73.2 kg ⫾ 12.15 kg). All read and signed informa-
levels in controls or within the subject’s own spinal column. tion and consent forms approved by the University of Wa-
The reason the majority of studies to date have not terloo Office of Research Ethics.
demonstrated differences in symmetry between populations Study protocol. The between-day repeatability of the average
may be a result of many factors that modulate measured EMG activity during quiet standing of the erector spinae
EMG activity level that are not related to the level of neural muscle at the L3 vertebrae was assessed on 3 different days.
drive. Electrode placement, skin temperature, moisture, cu- Day 1 and day 2 were 1 day apart, whereas day 3 was 6 days
taneous fat distribution, and muscle-fiber type and size can after day 2. During a single test session, bilateral EMG
Journal of Manipulative and Physiological Therapeutics Lehman 295
Volume 25, Number 5 Clinical Use of PMG

Table 1. Ability to discriminate populations based on EMG amplitude during static and dynamic tasks

Authors No. of Subjects Methods Major findings

Kravitz, Moore, and LBP ⫽ 22 Evaluated EMG activity during No differences between groups. Trends for
Glavos15 Control ⫽ 17 prone lying increased elevation in LBP

Sherman8 Control ⫽ 15 Tasks included sitting, standing, No left/right asymmetry differences between
History of pain ⫽ 28 bending and rising. groups, no single aspect of EMG had
Experiencing pain ⫽ 83 predictive value.
Large variability in raw values.
Trends existed suggesting pain sufferers had
higher EMG levels.

Miller16 LBP ⫽ 11 Performed sitting and standing No differences in EMG amplitude between
Control ⫽ 11 trials groups

Nouwen7 LBP ⫽ 20 Performed flexion, extension, Loss of FRP in LBP group

LBP ⫽ 20 lateral bend and twist Both groups showed bilateral asymmetry but no
difference between groups.

Ahern et al.4 LBP ⫽ 20 Performed flexion/extension, Static stance correctly classified 59% of subjects
Control ⫽ 20 rotation and stance with no difference found in EMG amplitude
or bilateral symmetry between groups.
Using lumbar ROM, EMG activity during peak
flexion, EMG activity during flexion correctly
classified 89% of subjects.

Triano et al3 Loss of FRP in 39/40 subjects

Peach and McGill 17 Loss of FRP

Arena et al 19 Control ⫽ 29 Evaluated EMG levels during Discriminant function analysis correctly
Spondyloarthritis ⫽ 19 sitting, standing, bending and classified 52.4% of subjects.
Disk ⫽ 52 rising in various populations ANOVA analysis found no differences in
Nonspecific ⫽ 66 of LBP patients bilateral asymmetry between groups, controls
Combined ⫽ 17 had less activity during stance than all other
Other ⫽ 24 groups.
Different groups showed differences in EMG
activity levels depending on movement task.

Sihvonen et al2 Controls ⫽ 25 Performed forward bending, Did not quantify differences between groups
LBP ⫽ 87 (52 had no extension and stance during stance.
pain during testing) 41% of all patients lacked FRP (31 total)
Only 10% of pain free showed no FRP.
Patients differed from controls in ratio of EMG
activity found during flexion and extension.

Alexiev9 LBP ⫽ 40 EMG collected during static No significant bilateral differences found at 0%
Control ⫽ 40 exertions of 50%, 30% and 0% in either group.
of maximum voluntary Significant differences in bilateral EMG at 30%,
contraction. 50% & 100% MVC in LBP group but not in
controls.

Leach, Owens, and LBP ⫽ 10 Collected paraspinal EMG Significant differences between groups in ratio
Giesen10 non-LBP ⫽ 6 during, stance, forward of thoracic ES activity and lumbar ES activity
flexion-extension and left/right asymmetry at lower ES.
Loss of FRP in LBP group

Watson18 Control ⫽ 2 Performed forward flexion and No differences between groups during stance.
LBP ⫽ 70 extension. Developed a Patients had a significantly higher activity
flexion/relaxation ratio during the peak flexion posture and
comparing peak activity during significantly lower flexion/relaxation ratio.
flexion with that found during Discriminant analysis revealed sensitivity of
maximally flexed position. 75% and sensitivity of 93%.

Grabiner, Koh, and LBP ⫽ 7 EMG activity recorded during Significant differences were found between
Ghazawi11 Control ⫽ 6 isometric exertion at 2 trunk groups in bilateral coupling.
angles

FRP, Flexion relaxation phenomenon; ES, erector spinae; ROM, range of motion.
296 Lehman Journal of Manipulative and Physiological Therapeutics
Clinical Use of PMG June 2002

activity was conducted during 5 trials of quiet standing. whereas a transformed score of –1 indicates the left mus-
Each quiet-standing trial lasted 5 seconds and each trial was cular activity was twice as great as the right. After all
preceded by 5 seconds of stepping forward and backward. transformed scores were calculated, the absolute value of
Disposable bipolar Ag-AgCl disc surface electrodes with each score was used to find the average ratio for the group.
a diameter of 1 cm were adhered bilaterally over the erector To find the transformed score, the right erector spinae
spinae musculature. Raw EMG was band-pass filtered (10 activity was first divided by the left erector spinae activity.
and 500 Hz) and analogue-to-digital (A/D)– converted at If the ratio was ⬎1, then 1 was subtracted from the ratio to
1024 Hz. The signal was then full-wave rectified, and low- give the transformed score. If the ratio was ⬍1, then the
pass filtered (Butterworth filter cut-off frequency of 2.5Hz) following formula was used: (1/ratio – 1) ⫻ (–1).
to produce a linear envelope. The signals were then nor-
malized to the amplitude obtained during the submaximal Study 2: Asymmetry in Paraspinal EMG Amplitude During Quiet Stance
reference task and the peak amplitude obtained during the Patients. Seventeen subjects with back pain (14 with LBP,
maximum voluntary contraction (MVC) tasks. 3 with thoracic pain below T4) were referred from the
Normalization tasks procedure. The different normalization pro- University of Waterloo-Canadian Memorial Chiropractic
cedures occurred on the same task session as the quiet- College (UW-CMCC) Research Clinic. Nine subjects were
standing trials. The MVC tasks occurred before the stance male (height ⫽ 171 ⫾ 5 cm. weight ⫽ 73.5 ⫾ 8.04 kg, age
trials, and the 4 trials of the submaximal task occurred after ⫽ 26.6 ⫾ 13 years) and 8 subjects were female (height ⫽
the tasks. The MVC required the subject to lie prone on a 164.25 ⫾ 7.5 cm, weight ⫽ 57.6 ⫾ 6.4 kg, age ⫽ 20.1 ⫾
bench with his or her trunk over the ledge while the legs 1.5 years). All read and signed information and consent
were strapped to the table. The subject was then required to forms approved by the University of Waterloo Office of
perform a maximum extension effort against the resistance Research Ethics.
of 1 or 2 experimenters. Two MVCs were performed with 3 Task protocol. Subjects were examined by the clinic chiro-
minutes of rest between each exertion. The maximum ac- practor. A segment exhibiting pain and a therapist-per-
tivity found was used as the reference amount. The sub- ceived motion restriction was identified by manual palpa-
maximal task required the subject to hold a 5-kg weight tion. We recognized the concerns regarding the reliability of
suspended from 45-cm chain and bar 1 cm off the floor motion palpation; therefore pain and local tenderness were
while maintaining a lordotic back with a slight bend at the the deciding factor in determining the site for electrode
knee and a greater bend at the hips in a standing position. placement. Disposable bipolar Ag-AgCl disc surface elec-
The EMG signals were processed in the same manner as trodes with a diameter of 1 cm were adhered bilaterally over
those from the standing trials. the erector spinae musculature at the painful segment level
Normalization techniques. The raw data during stance was nor- and at a nonpainful and nonrestricted segment, 5 segments
malized using 3 different techniques. The data was first superior or inferior to the painful segment. Typically, the
expressed as a percent of the maximum activity found nonpainful segment was superior to the painful segment.
during the maximum exertion (% MVC). The second nor- Only when the painful and restricted segment was above T7
malization technique expressed the data as a percent of the was a nonpainful segment located inferiorly (3 occurrenc-
peak activity found during one submaximal reference task. es). EMG signals were collected for 20 seconds during quiet
The third normalization technique expressed the raw stance standing 2 times and 4 minutes apart.
data as a percent of average EMG activity during the four EMG processing. Raw EMG was band-pass filtered (10 and
submaximal tasks. It should be stressed that the same-day 500 Hz) and A/D converted at 1024 Hz. The amplifier had
stance data was normalized to the same-day normalization a CMRR of 10,000:1. The signal was then full-wave recti-
task data. fied, and low-pass filtered (filter cut-off frequency of 2.5Hz)
Data analysis. A 1-way random-effects model calculated the to produce a linear envelope.
single measure intraclass correlation coefficient (ICC) for Statistical analysis. After the average activity of the linear
the quiet-standing EMG values for the 4 normalization envelope during quiet stance was calculated, a symmetry
conditions: (1) raw, (2) percent MVC, (3) percent submaxi- ratio could be calculated. The method for calculating the
mal, and (4) percent average of the EMG average during symmetry appears in the “methods section” for study 1.
submaximal tasks. In addition, the repeatability of the sym- After all transformed scores were calculated, the absolute
metry between bilateral muscle groups was also assessed value of each score was used to find the average ratio for the
between days. The symmetry between bilateral muscle group. The absolute values of the symmetry ratios for the
groups was quantified by first using a symmetry transform. painful segment and for the nonpainful segment were then
The transform allows the comparison of ratios between left compared using a 1-way analysis of variance.
and right muscle groups across persons when ratio values of
⬎1 or ⬍1 are produced. A transformed score of 0 indicates Study 3: Asymmetry of Paraspinal EMG During Dynamic Tasks
perfect symmetry. A transformed score of ⫹1 indicates the Patient characteristics. The data analysed in this study had been
right muscular activity was twice as great as the left, previously collected in 2 separate previous studies. The first
Journal of Manipulative and Physiological Therapeutics Lehman 297
Volume 25, Number 5 Clinical Use of PMG

study7 investigated lumbar kinematics and associated trunk ence task, and 17.2% when normalized to the average value
muscle EMG in a healthy population. Data from 14 random found during repeated submaximal reference tasks.
subjects from the study’s 24 subjects (17 men [age ⫽ 22 ⫾ Asymmetry in muscle activity was present regardless of
3.4 years, height ⫽ 178.1 ⫾ 5.1 cm, and weight ⫽ 77 ⫾ 6.7 normalization procedure. On average, differences between
kg] and 7 women [age ⫽ 20.4 ⫾ 0.5 years, height ⫽ 166.1 the left and right side ranged from 65% to 74% (SD ⫽ 54%
⫾ 4.4 cm, and weight ⫽ 71 ⫾ 9.7 kg]) was assessed in the to 60.9%; Tables 2 and 3). The ICCs for symmetry between
present study. The second study population was drawn from the 2 sides showed less repeatability than the other variables
a previous study12 that examined trunk kinematics and measured (ICC ⫽ 0.43-0.773). Normalization to a maxi-
associated EMG in a convenience sample of persons with mum voluntary contraction did not improve the repeatabil-
chronic LBP drawn from the UW-CMCC Research Clinic. ity of symmetry scores relative to using raw EMG. How-
Data from 12 subjects (7 women and 5 men [age ⫽ 29.4 ⫾ ever, the normalization procedures using submaximal ref-
3.05 years, height ⫽ 167.7 ⫾ 11.1 cm, and weight ⫽ 70.6 erence tasks improved repeatability relative to raw EMG
⫾ 13.6 kg]) was assessed. All read and signed information values.
and consent forms approved by the University of Waterloo
Office of Research Ethics. Study 2
Task procedure. With feet shoulder width apart and knees The mean absolute-transformed ratio score was .51 (SD
slightly bent, the subject flexed forward in the sagittal plane ⫽ .36) for the segments exhibiting pain and .69 (SD ⫽ .64)
until full flexion was reached with the arms and head for the segment with no pain. There were no statistically
hanging freely. The subjects were instructed to pause for a significant differences between the 2 segments of interest.
moment at full flexion, then rise to an upright posture. The
subjects performed this task 3 times. Study 3
EMG collection and processing. Raw EMG signals were collected For the upper erector spinae, the mean cross-correlation
during the flexion extension tasks using disposable bipolar coefficient healthy population was .86 (SD ⫽ .06), whereas
Ag-AgCl disc surface electrodes with a diameter of 1 cm the coefficient for the group with LBP was .85 (SD ⫽ .07).
that were placed parallel to the muscle fibers with a center- No significant difference was found between these two
to-center spacing of 3 cm over the following muscles bilat- groups for this muscle study. However for the lower erector
erally: thoracic erector spinae (approximately 3-cm lateral spinae, a statistically significant difference in cross-correla-
to T9 spinous process); lumbar erector spinae (approximate- tion coefficients was found between groups. The population
ly 3-cm lateral to L3 spinous process). EMG signals were without LBP had an average score of 0.93 (SD ⫽ 0.02),
sampled at 1024 Hz (Watscope A/D Convertor 12 bit, whereas the population with LBP averaged a score of 0.86
Northern Digital Inc, Waterloo, Ontario, Canada) and band- (SD ⫽ 0.08).
pass filtered (10 and 500 Hz) for a total of 4 channels. The
amplifier had a common mode rejection ratio of 10,000:1. DISCUSSION
The signals were then full-wave rectified and low-pass Normalization techniques did improve the repeatability
filtered (Butterworth filter cut-off frequency of 2.5 Hz). of the EMG signal; however, all trials, normalized and
Statistical analysis. A cross-correlation comparing the left and non-normalized, demonstrated excellent repeatability of the
right erector spinae dynamic EMG profile (upper and lower) EMG signal, with ICCs above 0.75. This suggests that
for the 3 flexion trials was used. The correlation numbers normalization is not necessary to compare the results of 1
produced for the populations with and without with LBP person with himself over time for quiet-stance trials. How-
were assessed for differences using a 1-way analysis of ever, without normalization it is still difficult to compare
variance. across persons and to allow for a biologically relevant
interpretation of the EMG signal as a result of the various
factors, other than muscle activation, that modulate EMG
RESULTS signal amplitude. A large between-subject variability in
Study 1 EMG amplitude has been demonstrated in previous studies,8
Repeatability was acceptable regardless of the normaliza- as has the risk of non-normalization in proper biologic
tion procedure. Each normalization improved the repeat- interpretation of EMG signals.12 An interesting observation
ability of the measured EMG signal relative to unnormal- was the variability of the EMG signal about the mean during
ized data. It should also be noted that although repeatability within-day trials. This suggests the importance of conduct-
was deemed statistically acceptable, within- and between- ing multiple trials to average the signal across trials. This
variability still existed. The average standard deviation of variablility is most likely a result of the relatively low
EMG activity across all days expressed as a percentage of activity during stance and the way in which this activity can
the mean was 17.3% for raw EMG, 20.9% when normalized be easily influenced by small changes in posture and sub-
to MVC, 17.8% when normalized to a submaximal refer- sequent stability demands on the erector spinae. Subjects
298 Lehman Journal of Manipulative and Physiological Therapeutics
Clinical Use of PMG June 2002

Table 2. ICC scores for erectors spinae activity during quiet Table 3. Average Symmetry score across the population on all
stance three different experiment days

Normalization Technique Right ES Left ES Normalization Technique Asymmetry Score and ICC

None raw amplitude 0.78 0.87 None raw amplitude .65 (.609) ICC ⫽ .59
% MVC 0.88 0.926 % MVC .66 (.54) ICC ⫽ .43
% submaximal MVC 0.82 0.925 % submaximal VC .74 (.56) ICC ⫽ .67
% average submax MVC 0.84 .93 % average submaximal VC .67 (.54) ICC ⫽ .773

ES, Erector spinae; MVC, maximum voluntary contraction. ICC, Intraclass correlation coefficient; MVC, maximum voluntary con-
traction; VC, voluntary contraction.

also showed a high asymmetry score, indicating that, on

average, one muscle was at least 60% more active than the of dysfunction present at the upper erector spinae, consid-
other in an a population without LBP. ering the pain population was presenting with LBP. With no
This study is limited to clinical applicability because the fewer than 7 segments between the muscle sites, different
relevance of these measures to dysfunction is still unknown. fascicles would be recruited to perform the same bending
However, the results imply that abiding to a stringent test task. Study 2 only had 5 segments between possible muscle
protocol can result in reproducible measurements across sites, resulting in the recruitment of the same muscle fasci-
days; therefore it is possible to track a patient through cles during stance. Nonetheless, the differences between
rehabilitation program using these measures. populations are noteworthy, but are they clinically signifi-
Study 2 demonstrates that in terms of amplitude asym- cant? What does a cross-correlation of 0.86 mean in the
metry, problematic spinal segments are no more asymmet- symptomatic population compared with a cross-correlation
rical than pain-free segments in a population with LBP. An coefficient of 0.93 in the control group? How dysfunctional
argument may be made that the 5 segments between muscle is this difference? Once again, the clinical relevance for
sites is too close to allow for a problematic/non-problematic rehabilitation is unknown.
comparison. This is a just argument, considering the muscle
evaluated by the EMG electrodes, longissimus thoracis, CONCLUSION
crosses more than 5 segments. It is important to note that
EMG signals during quiet stance show excellent repeat-
smaller segmental muscles are not measured by the elec-
ability; segmental differences in asymmetry between dys-
trodes since the electrode pick-up depth is 1 cm.13 With this
functional and nonproblematic segments are not evident,
anatomical consideration, it may be impossible to detect
which suggests that the diagnostic validity of EMG evalu-
segmental dysfunction (ie, discriminating L4 from L5) with
ations during simple quiet-stance tasks is highly suspect.
the use of surface or scanning EMG. This study questions
Bilateral amplitude asymmetry appears not to be indicative
the validity of using a simple static posture (stance) to
of pathology but rather a natural phenomenon in both symp-
identify muscle dysfunction. However, clinicians should be
tomatic subjects and controls. This null finding was con-
cautious in concluding that surface EMG provides no in-
trasted with the observation that differences in phasic asym-
sight into abnormal muscle function. The simplicity of the
metry during dynamic flexion were found between symp-
test protocol rather than the EMG tool may be at fault. To
tomatic and control populations in the lower erector spinae
properly exploit surface EMG measurements and EMG
EMG linear envelope but not in the upper erector spinae.
amplitude ratios between muscle groups, additional exper-
The implications of these findings are unclear but suggest
imental rigor and more challenging static exertions may be
that patterns of muscle activation rather than mere ampli-
necessary. Edgerton and colleagues14 have acquired a data-
tude comparisons may better identify pathology.
base of muscle amplitude ratios during 7 “static end range
positions” and proposed a theoretic model that may be
useful in detecting hyperactive, hypoactive, and compensa- ACKNOWLEDGMENTS
tory muscle function by assessing 14 separate muscle The author gratefully acknowledges the financial support
groups. This protocol is exceedingly more complex than the of the Natural Sciences and Engineering Research Council
protocols evaluated in this study and is typically used with (NSERC) under SM McGill’s research grant.
scanning EMG equipment. However, it may be this com-
plexity that is essential to adequately assess muscle dys-
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