Characterization of muscle activity by electromygraphy versus tocography.
1. Introduction
The understanding of risk factors and mechanisms related to preterm birth has been advancing
through several public health and medical interventions to reduce the incidence (DSOG, 2010).
The preterm birth rate is rising in most industrialized countries. In Denmark the latest accessible
statement shows that 17% of all births are preterm (SST, 2011). Compared to international
statistics this is a high number; in the US preterm births account for 11-13% and in other
industrialized countries it is 5-9%. These numbers are predicted to rise further as consequence to
increased numbers of infertility treatments and thereby associated multiple pregnancies. For now
preterm births account for 75% of perinatal mortality and more than 50% of long term morbidity
among newborn in the US (Golberg RL, Culhane JF, Jams JD, Romero R, 2008). The socioeconomical interest in this clinical challenge may be exemplified by an health technology
assessment (HTA) comparing hospitalization costs four years after the birth showing a difference
of 150% between mature and premature groups of children, ranging from 8.807 dkr (mature) to
143.506 dkr (premature)(Korvenranta E. et al. 2010).
Besides these socio-economical perspectives there is of cause the interest of the parents, the
premature children them-selves, relatives, and the clinical staff managing their pregnancies and
births. A peer-reviewed meta analysis proves that mental and psycho-social liabilities which
occurs during pregnancy have significant effect on the pregnant, her partner, the fetus and family
(Austin MP. Priest SR. Sullivan EA, 2008). Alone the risk of giving birth premature with all of the
enclosed worries would apply a liability, let alone the actual liability in actually going in to labor
prematurely. A kohorte study involving all neonatal care units in Denmark and all extreme
premature children born in a period of 2 years (1994-95) showed that 56% survived until leaving
the neonatal units, and they were in average admitted 81 days. The incidence of cerebral paresis
was 12% of the surviving children, and further more did 21% show intellectual development
disorders and/or motor skill disabilities. This study applies the most vulnerable of preterm
children, those born from gestation week 24+0 until 28+0 (ETFOL, 2000), and thereby is this study
not comparable to the premature group of children as whole, but serves to highlight which
morbidity and mortality issues the parents and children are facing.
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Characterization of muscle activity by electromygraphy versus tocography.
Besides the psychological effects on parents and serious consequences in terms of morbidity and
mortality for children, there is also a risk concerning clinical staff managing these risk situations,
due to possible ineffective diagnostic tools. According to motivation theoreticals Hackman and
Oldham effective management in job situations could influence on motivation and improve the
work environment for the clinicians (Jacobsen. Thorsvik. 2007).
Summarized there is potential in reducing the incidence of premature labor in regards of a socioeconomical, socio-psychological, child mortality and morbidity incidence, and additionally by
improving motivation and work environment for the clinical staff by improving clinical diagnosis of
preterm labor.
An evaluation on managing premature labor in clinical practice is thereby justified in order to
investigate possible innovative improvements of diagnostic possibilities.
2. Uterine contraction and clinical practice
 2.1 Introduction
It is well established that the pregnancy monitoring techniques are essential to assess the risk
factors and permit timely medical intervention; however accurate prediction of the delivery time,
which can be the key parameter for timely treatment of preterm labor, still remains a major
challenge (Golberg R.L, Culhane J.F, Jams J.D, Romero R. 2008). Clinical practice in Denmark
(DSOG, 2010) and most other industrialized countries involves a technique to detection and
evaluation of uterine contractions based on the use of an external tocodynamometer (TOCO),
which provides a noninvasive indication of contraction onset timing based on external strain
gauges (Garfield RE. Maul H. Shi L. Maner W. Fittkow C. Olsen G. 2001). This method of measuring
muscle activity provides an indirect mechanical detection as consequence of electrical activity in
myometrial cells.
Direct measurements of electrical activity in the myometrial cells, surface-electromyography
(sEMG) with uterine contractility as consequence could provide the clinical practice of diagnosing
preterm labor with a more reliable assessment technique than the one used to day (Lucovnik M.
et al. 2011).
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Characterization of muscle activity by electromygraphy versus tocography.
Focus of this paper is to validate the clinical value of the methods of electromyography versus
tocography as monitoring techniques to interpret the contraction time-frequency, duration and
force by amplitude in muscle activity. Thereby how often, how long, and how strong the muscle is
working.
Since the clinical use of these methods involve measuring uterine activity in pregnant women, this
paper serves as part 1 in a two-part study, conducted respectively on 2. and 3. semester in the
Candidate in Clinical Science and Technology, Aalborg University. This part 1 serves the purpose of
establishing a substansial monitoring platform to perform more in-dept monitoring parameters on
pregnant women in labor at Aalborg Hospital, Aarhus University hospital. In this early stage of the
study, it is unnecessary to put pregnant women and their fetuses at possible risk and
inconvenience. Therefore this trial is conducted on non-pregnant individuals measuring muscle
activity in musculus abdominus recti. The methodology and conclusions will afterwards be used in
part two of the study. Hypothesis and research question for this part 1 of the study are as follows:
2.1 Hypothesis.
Poor sensitivity as consequence of indirect mechanical measurement of pressure increase during
muscle activity can affect the clinical estimation accuracy of muscle contraction time-frequency,
duration and amplitude.
2.2 Research question
How is the clinical estimation accuracy of musculus abdominus recti contraction
time-frequency, duration and amplitude affected using the methods of
electromyography versus tocography?
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Characterization of muscle activity by electromygraphy versus tocography.
Through the following chapters it is elaborated which physiological parameters could be of
importance in clinical diagnosis of preterm labor using both electromyographic and tocographic
methods of measurements and why these are of importance.
The study and its method is described.
It is described in which way each of the two methods could proof to have clinical feasibility
features in measuring contraction time-frequency, duration and amplitude.
Empiric results of both methods of measuring are analyzed and validated against each other and
against the literature.
Finally conclusive considerations are proposed and evaluated in regards of constituting a platform
for further studies of uterine activity measurements.
 2.4 Abbreviations and delimitations
Table 1. Abbreviations.
Amplitude
Muscle fiber action potential from the
contracting muscle measured in volt (Merletti
R. Parker PA. 2003).
Clinical feasibility
The utility of use in clinical practice
Duration
Time from contraction onset to contraction
onset (Bennett RV. Brown LK. 2001).
Neural network
Artificial computional model inspired by
structure and/or functional aspects of
biological neural network (Merletti R. Parker
PA. 2003).
Preterm labor
Induced labor before gestational age 37 weeks
and 0 days(DSOG, 2010).
sEMG
Surface Electromyography, a direct, noninvasive method to record electromygraphic
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Characterization of muscle activity by electromygraphy versus tocography.
signals from muscle cells (Merletti R. Parker PA.
2003).
Time-frequency
Assessment of repeatability of EMG vaiables of
cnsiderable relevance for the clinical use of
surface EMG
(Merletti R. Parker PA. 2003).
TOCO
Tocography is a non-invasive method to record
indirect mechanical pressure as result of
muscle contraction (Bennett RV. Brown LK.
2001).
Uterine activity
Contractions of the myometrial tissue in uterus
(Bennett RV. Brown LK. 2001).
Cross-talk
Signals from activity in neighbouring muscle
(Merletti R. Parker PA. 2003).
Delimitations
Beyond the scope of this project is the various mathematical calculations on data, as mean,
amplitude and standard deviations due to the comparability of the methods, while the TOCO data
is visually assembled on a monitoring device in the clinic, the EMG data feasibility should be
compared in a similar manor.
Another method used in the clinic to assess labor induction and/or progression as IUPC, intra
uterine pressure catether, is not commonly used in Denmark (DSOG, 2010) and not comparable as
method due to its invasive nature (Bennett RV. Brown LK. 2001).
Tocography assessments are currently combined with other clinical approaches, such as palpating
the cervical condition and thereby performing a subjective assessment on risk of labor induction
called Bishops Score (DSOG, 2010). This approach is non- comparative to the method of sEMG due
to the clinical setting (not including pregnant women in this part of the trial), and due to the high
degree of subjectiveness of the assessment.
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Characterization of muscle activity by electromygraphy versus tocography.
 2.5 Physiology of contractions
2.5.1 Anatomy of uterus and m. abd. recti
The uterus is a hollow, muscular pear-shaped organ situated in the pelvis. In non pregnant women
it is 7.5 cm long, 5 cm wide, 2.5 cm in depth, each wall being 1.25cm thick. In pregnancy it enlarges
by a factor 4-5. The uterine wall consists of three distinct layers, of which the middle muscle layer
is by far the thickest. The inner layer, the endometrium forms a lining of mucous membrane which
lines the lumen of the organ, the middle layer, the myometrium which is the contractile element
of the uterus and consists of an inner layer with fibers running diffusely and interlace to surround
the blood vessels, and an outer layer which is formed by longitudinal bundles of smooth muscle
cells (Bennett RV. Brown LK. 2001). Smooth muscle cells of the myometrium are generally long
spindle shaped cells (fig. 1).
Fig. 1. Striated multi-nuclear muscle cells and single-nuclear smooth muscle cells.
These cells progressively increase in size during the last stage of pregnancy. Contraction of smooth
muscle cells occurs, as in the skeletal muscle, through the interaction of myosin and actin filament.
On cellular level there are mainly two differences in which smooth muscle cells differ from skeletal
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Characterization of muscle activity by electromygraphy versus tocography.
muscle cells: The myosin and actin filaments are not arranged in the same striated composition as
in skeletal muscle cells, and in smooth muscle cells a large number of filaments are attached to
dense portions of smooth muscle, referred to as dense bodies (Rhoades RA. Tanner GA, 1995).
In skeletal muscles as the one measured in this trial, musculus abdominus recti the muscle fiber is
arranged lengthwise into parallel myofibrils consisting of sarcomeres where parallel structures are
combosed by actin and myosin filaments (Rhoades RA, Tanner GA, 1995).
Musculus abdominus recti originates from pubis and inserts on the costal cartilage on ribs 5-7 and
xiphoid process of sternum. It is a paired muscle running vertically on each side on the frontal
abdominal wall, and is separated by a mid-line band of connective tissue, linea alba. It has an
important postal function and is responsible for flexing the lumbar spine and assist in breathing. It
furthermore protects inner organs and helps as example in parturition (Bojsen-Moller. 1996).
2.5.2 Cell activation
Similar for both smooth muscle and skeletal muscle is basic of activation. The contraction results
from the progatation of electrical activity through the muscle cells in the form of action potentials
(AP). The intracellular AP results from changes in the membrane ionic permeability, that are
caused by hormonal changes or by cell- to –cell excitation (Rhoades RA, Tanner GA, 1995).
The cell membrane potential depends on the distribution of ions across the plasma menbranes. At
rest, the ionic distribution is such that the concentration of sodium (Na+) and calcium (Ca2+) ions
are higher outside the cell than inside, whereas the concentration of potassium (K+) ions is higher
inside the cell. This distribution of ions corresponds to the resting membrane potential
(repolarization) and the myometrium is in a quiescent status.
The muscle cells respond a rise in the level of positive ions Na+ and Ca2+ inside the cell
(depolarization); as a consequence a transmembrane ionic current is established and the
contractile event of the muscle occurs. Conversely, a reduction of intracellular free Ca2+, either as
result of efflux into extracellular space or re-uptake into cellurlar storage sites, terminates the
contraction (Rhoades RA. Tanner, 1995).
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Characterization of muscle activity by electromygraphy versus tocography.
Propagation of smooth muscle cells differ from propagation of skeletal muscle cells as follows: The
time required for the diffusion of ions (latent period) is on average 200-300 ms, which is
approximately 50 times longer than the latent period measured in skeletal muscle. Furthermore
Ca2+ is removed from intracellular space by a calcium pump in smooth muscle cells, whereas Ca2+
in skeletal muscle cells is removed by a much faster sarcoplasmic reticulum pump. Compared to
skeletal muscle cells, smooth muscle cells have far more volatage-gated Ca2+ channels, but fewer
voltage-gated Na+ channels. Therefore the propagation of smooth muscle cells is mainly regulated
by Ca2+, while the contraction of skeletal muscles are mainly regulated by Na+ channels. Calcium
channels open significantly slower than Na+ channels and also remain open longer. This accounts
for the slow onset of contraction and relaxation in smooth muscles compared to skeletal muscles
(Guyton AC, Hall JE. 2006).
In skeletal muscles, the contraction is initiated by the nervous system. The motor neuron initiates
an action potential that propagates through the neuromuscular junction to the muscle end plate
(Motor unit, MU), causing the muscle fiber to contract. A motor unit thereby consists of an αmotor neuron and the muscle fibers it innervate, fig. 2.
Fig. 2. Motor Unit
Motor neuron
Muscle fiber containing
myofibrils.
Neuromuscular junctions which are highly structured in placement do not occur in smooth muscle.
In smooth muscles the initiated action potential starts in pacemaker cells and propagates to
surrounding pacemaker cells and open ion channels and thereby allows entry of Ca2+ into cells to
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Characterization of muscle activity by electromygraphy versus tocography.
induce contraction (Guyton AC, Hall JE. 2006). Up till now there is no recognized conduction
pathway, but more so evidence of a strongly non-linear propagation mechanism consisting of
pace-maker cells (Hassan M. Terrien J. Karlsson B. Marque C. 2009).
2.5.3 Cell contraction
Contraction and relaxation of muscle cells is regulated by phosphorylation, i.e., removing of
phosphate groups by hydrolysis of myosin which leads to the conformational changes in the
myosin heads to interact with actin, forming crossbridges which shortens the muscle fibers by the
myosin and actin filaments sliding over each other. The myosin heads then release the actin
filament and adopt their original conformation. They can then re-bind to another part of the actin
molecule and drag it along further. This process is called cross-bridge cycling and is the same for
both muscle types (Rhoades RA. Tanner GA, 1995).
Differently from the skeletal muscle, the cross-bridges in smooth muscles are arranged in a way
that the bridges on one side hinge in one direction and those on the other side hinge in the
opposite direction. This arrangement allows the myosin to pull the actin filament in one direction
in one side and in the other direction on the other side, allowing smooth muscle cells to contract
as much as 80% of their length instead of 30% as in skeletal muscle cells (Guyton AC. Hall JE.
2006).
2.5.4 Uterine contraction
The electrical activity in smooth muscle cells of the myometrium initiate the mechanical
contraction of the uterus. The main purpose of uterine contractions is shortening the cervix and
helping the fetus to descend into the birth canal (Bennett RV. Brown LK, 2001). The tension
developed in the uterus is dependent on both strength and duration of the hormone stimulus. The
myometrium requires relatively long time to respond to stimulus compared to skeletal muscle as
described above. This is partly due to differences in ion channeling and cross- bridge cycling. Cross-
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Characterization of muscle activity by electromygraphy versus tocography.
bridge cycling which in smooth muscle allow cells to contract 80% of their length are also 10-300
times slower than in skeletal muscles.
However the cross-bridge cycling in smooth muscle is believed to have a longer attachment
between myosin and actin filaments which results in a maximum specific force of concentration
greater than the one in skeletal muscle (Guyton AC. Hall JE. 2006).
This description of muscular functionality leads to the question as to how these contractions are
monitored:
2.6 Clinical practice
2.6.1 Contraction monitoring
External tocography is besides manual palpation the most common used technique to monitor the
uterine contractions during pregnancy and labor (Bennett RV. Brown LK, 2001). A
tocodynamometer consists of a strain gauge transducer placed on the external surface of the
abdomen with an elastic band. The primary advantage of the tocodynamometer is its
noninvasiveness and many years of introduction as part of the clinical monitoring abilities
(SonicAid. 2011). However, since it monitors the indirect mechanical measure of the pressure
increase, the signal by external tocography can be affected by many variables, such as the sensor
position above the uterus, how tight it is put on the skin surface using the elastic band, thickness
of subcutaneous fat. More so gastric activity, such as vomiting, body movements as laughter, or
other non labor-induced activity can affect the tocodynamometer and be mistaken for uterine
contractions (Garfield et al. 2001).
2.6.2 Clinical value of monitoring
As a result the use of external tocodynamometer is dependent on the clinical examiners skills and
is characterized by a low sensitity. The use of extern tocodynamometer possible only provides
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Characterization of muscle activity by electromygraphy versus tocography.
information related to the time-frequency and duration of uterine activity, whereas
measurements concerning amplitude or strength of contraction seem inaccessible.
Premature birth as a consequence of spontaneous preterm labor is still a major cause of fetal
morbidity and mortality as described in the introduction. There are two main areas in which early
detection of preterm labor could be helpful in clinical practice today:
The first area concerns asymptomatic pregnancies where the patients anamneses is the reason for
risk treatment throughout the pregnancy, either through timely visits to the obstetric unit where
tocodynametri, Doppler-ultrasound of cervix, bacteria-smears, palpation of softening, dilatation
of cervix performed by attending midwife etc is conducted (DSOG. 2010).
The second area concerns the symptomatic pregnant women, who themselves contact the
obstetric unit with symptoms of preterm labor. This could be preterm. Pre-rupture- of membranes
(PPROM), contractions, vaginal –heaviness etc. The latter group is possibly the most prone to
preterm birth because of symptoms already in a state of irreversible progression, but are
examined through same procedure as above.
For both groups in risk of preterm birth monitoring contractions are conducted by tocography,
cervical Doppler and/or cervical palpation (DSOG. 2010)
Current method used
Accuracy
Invasiveness
External tocography
Erratic
No
Self-monitoring
Poor
No
Cervical Doppler
Moderate/high
Yes
Cervical palpation
Moderate/high
Yes
It seems that clinical diagnosis of preterm birth are either invasive or have drawbacks in terms of
inaccuracy issues.
During recent years research the use of electromyography has been conducted as diagnostic tool
in monitoring uterine activity (Lucovnik et al. 2011).
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Characterization of muscle activity by electromygraphy versus tocography.
Because of its ability to measure uterine contractility as a direct consequence of the changing
electrical activity in the myometrical cells it is possible to monitor the time-frequency, amplitude
and duration of contractions determined mainly by the frequency of occurrence of the uterine
electrical bursts, the total number of muscle cells that are active during the bursts, and the
duration of the uterine electrical bursts (Garfield et al. 2001). Several mathematical approaches
for the purpose of assessing and quantifying elelctromyographic measurements of contractions
have been applied, and a review of the literature shows that the so far best parameter for
predicting birth is related to the electrohysterographic spectral content shift, calculated by Fourier
transform, time-frequency, or wavelet-analysis (Vinken MP. Rabotti C. Mischi M Oei SG. 2009).
These types of electromyographic analysis approaches will be discussed in the following chapter.
3. Electromyography analysis
 3.1 Introduction
Tocographic measurements of uterine contractions seems to be affected by several drawbacks in
terms inaccuracy as described above. The literature shows furthermore that this method not been
useful in reducing the rate of preterm delivery in randomized trials (Iams JD. et al. 2002).
Consequently there is a need for better diagnostic tools in prediction of preterm births.
Electromyography is currently the best choice of diagnostic improvement in predicting preterm
births using dedicated signal processing techniques to permit the required clinical benefits (Vinken
MP. Rabotti C. Mischi M Oei SG. 2009).
In this chapter we introduce the trial-set up in order to compare electromyographic
measurements to tocographic measurements on musculus abdominus recti. Furthermore we
describe in the chapter the EMG data collected by the means of its feature extractions and clinical
feasibility towards measuring uterine activity.
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Characterization of muscle activity by electromygraphy versus tocography.
 3.2 Data acquisition
For the assessment of muscle contraction in musculus abdominus recti caution was taken to meet
the following criteria:
 Body movements to activate m. abd. recti should be performed in order to simulate
uterine contractions, hereby facilitate frequency and duration similar to uterine
contractions.
 Both electromyographic sensor electrodes and tocographic strain gauge device are
applied simultaneously to secure similar acquisition paragraphs.
 Trial-protocol inclusion- and exclusion criteria are met in order to minimize potential risks
to the trial subject (appendix 1).
 Both methods are similar analyzed through discussion of data feature extractions and their
clinical feasibility.
Method:
The experimental data were collected at Aalborg University, Frederik Bajers Vej 7, Aalborg.
Denmark. 6 recordings of simultaneously recorded electromyoraphic and tocographic
meassuremets of muscle activity in musculus abdominus recti on both men and women were
included in the trial.
After accurate skin preparation using shaving and alcohol cloths, three disposable Ambu 720
electrodes were placed on the abdomen as shown in fig. 3. The electrode position was chosen
according to SENIAM proposed surface electrode sites (Konrad P. 2005) and the common
reference electrode (REF) for the recording electrodes was placed on the left costa C12. The trial
subjects were instructed in being in resting position flat on her/his back in start of acquisition to
define baseline activity. Then they were instructed in positioning the upper body in angle of 45%
for 30 seconds, and on instruction returning in resting position. This exercise was repeated 3 times
in total for all 6 subjects. The set-up of acquisition is shown I fig. 3:
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Characterization of muscle activity by electromygraphy versus tocography.
The main settings of the used equipment as follows:
Sampling frequency is set at 1000 Hz i.e., a sufficiently high sampling frequency to avoid aliasing of
the components deriving from the subjects electrocardiogram (ECG) and skeletal electromyogram
(EMG). Sweep length of recording is set at 2000 seconds, acquisitions though stopped at
approximately 250 seconds. Signals are processed using gain at 10K and low pass/ high pass filter
at 500 – 5 Hz.
The recording program used is an early version of Mr. Kick, which was originally intended for use
in robot development technique by laboratory engineer at SMI, AAU, Knud Larsen.
Fig 3. Acquisition set-up.
3
2
1
4
1. EMG electrodes applied on skin surface and connected to amplifier
2. Amplifier connected to AC/DC converter
3. AC/DC converter connected to PC recording signals by Mr. Kick-program
4. Tocography strain gauge applied on skin surface connected to registration unit.
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Characterization of muscle activity by electromygraphy versus tocography.
3.2.1 Data processing
The electromyography signal can be affected by several noise sources, e.g. the subjects ECG, the
EMG generated by contraction of other muscles than m. abdominus recti (cross-talk),
subcutaneous fat layer, respiration and different motion and electrical artifacts (Merletti R. Parker
PA. 2004).
As seen below in fig. 4 an example of the EMG indicates that we achieved a somewhat acceptable
visual presentation:
Fig 4. Matlab plot.
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Characterization of muscle activity by electromygraphy versus tocography.
All 6 files were recorded continuously in a 2000 seconds window as suggested by the recording
programs inventor Mr. Knud Larsen. Additional changes in the actual time-frame are done by
recording cessation after 250 seconds because of duration of exercises.
After trial completion data were given trial numbers 1-6 in order to achieve anonymity of the trial
subject personas. Then data were loaded in to Matlab 7.8.0 (R2009a) which is a high- level
technical computing language and interactive program for algorithm development, data
visualization, data analysis and more (Matlab, 2011). After data processing in order to find suitable
recordings matching measurements done by TOCO, the data were plotted in to visual graphs (Yaxis showing EMG potential in mV, and X-axis showing time in seconds). There is a considerable
amount of noise showing on the plots, possible reasons for this is discussed later in this chapter.
3.2.2 Feature extractions
As described in chapter 2.3 the musculus abdominus recti originates from pubis and inserts on the
costal cartilage on ribs 5-7 and xiphoid process of sternum. It is a paired muscle running vertically
on each side on the frontal abdominal wall. It is chosen primarily because of its relatively large
size, thereby making it possible to record with both electrodes and TOCO strain gauge
simultaneously, and its location above the uterus and under the skin thus prone to possible
confounding factors such as skin movement, artifacts in terms of “filter-function” composed from
the biological tissue between the muscle and the recording site on the skin. Whereas the fat- and
skin layer, cross-talk from nearby situated muscles while the signal recorded at the skin-surface is
the weighted average of the electrical activity of all underlying excited cells (Zwarts MJ. Stegeman
DF. 2003).
A systematic review of literature concerning clinical applications in the usage of multi-channel
surface EMG shows that temporal activity and spatial activity, as muscle fiber velocity (MFCV)
measurements and evaluation of single motor unit (MU) characteristics are prone to the usage as
clinical tools (Drost G. Stegeman DF. vanEngelen BG. Zwarts MJ. 2006). While interest in this trial
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Characterization of muscle activity by electromygraphy versus tocography.
concerns both the temporal and the force perspectives of the EMG features with concern to the
initial research question, we will focus on the following parameters (fig. 5).
Duration
Timefrequency
Amplitude
Clinical feasibilty
Fig. 5 illustrating the parameters concerning clinical feasibility.
3.2.3 Results
In terms of assessing the temporal features of the EMG measurements, they are initially addressed
separately:
Frequency
It is commonly known, that a time series, such as uterine contractions or these performed EMG
measurements can be described in terms of their harmonic components. However this approach
of measuring dynamic contractions in addition to those encountered in measuring isometric
contractions affects the results. One of the problems being the high degree of nonstationarity of
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Characterization of muscle activity by electromygraphy versus tocography.
the signal as result of possible sudden change in their spectral properties (amplitude, especially).
During movement the muscle change size and move in respect to the electrodes on the skin
surface, thus the skin itself moves.
Time-Frequency (TF) defined as the time in seconds from onset of registered EMG potential to
next onset. In the 6 recordings the interval varies from 40 to 95 seconds, with a mean duration of
45 seconds of measured EMG potential. There exists numerous mathematical techniques to
quantatively describe the recorded signal resulting from physiological events. One of the most
widely used method for this is the short-time Fourier transform (STFT), in which the original signal
is decomposed into smaller frequency- dependent fractions (spectrograms) (Merletti R. Parker PA.
2003) .
For the use in this project it is apparent that the usage as clinical method to estimate contractions
is compared to the method commonly used in the clinic. As later described in chapter 4,
tocography, as the currently used method to estimation of uterine contractions, is assessed by
visual quantification of temporal parameters. To compare the two methods in terms of their
clinical use we estimate the temporal parameters from a visual reading on the visualized graphs
rather than conducting spectrogram analysis. The values visualized on each graph for EMG
measurements on m.abdominus recti is seen below in table 2.
Trial no.
Time-frequency (s)
Duration (s)
1
10-95 (85 sec), 95-190 (85 sec)
10-65 (55 sec), 95-145(55 sec), 190-250(60 sec).
2
20-140 (120 sec), 140-220(80 sec)
20-48(28 sec), 140-180(40 sec), 220-250(30 sec).
3
40-100(60 sec), 100-175(75 sec)
40-78(38 sec), 100-150(50 sec), 175-230(55 sec).
4
05-90(85 sec), 90-180(90 sec)
05-60(55 sec), 90-145(55 sec), 180-230(50 sec).
5
20-80(60 sec), 80-160(80 sec)
20-65(45 sec), 80-120(40 sec), 160-225(65 sec).
6
20-85(65 sec), 85-160(75 sec)
20-40(20 sec), 85-140(55 sec), 160-190(30 sec).
Mean
80 sec.
46 sec.
Table 2. Time-frequency and duration of contractions.
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Duration
The clinical evaluation of induction of labor or labor progression is partly performed by assessing
uterine contractions by their time-frequency (interval) and duration Whereas short and regular
intervals between contractions indicate good labor progression, and the opposite; irregular and
longer intervals indicate non – or poor induction or progression of labor (RegionMidt, 2011).
In this trial we conducted body movements to evoke activity in m. abdominus recti to simulate
uterine contractions in their temporal somewhat harmonic appearance. The mean duration of
contractions is 46 seconds ranging from 20- 65 seconds.
In general the quality of the 6 performed EMG recordings is of an acceptable standard in order to
obtain a visual evaluation of both time-frequency and duration. There is however a considerably
amount of noise on the plots made in the matlab computering program. It is described in chapter
3.2.2. which confounding factors could have impact on noise recorded. Furthermore it is also of
relevance which preprocessing data techniques could enhance the quality of the registered signal.
Proper skin preparation, electrode placement and the analog filtering set-up in the amplifier
apparatus is of apparent importance. Once these components are optimal set, there still is
possible methods to enhance the raw “true signal”.
A whitening filter can be applied to a frequency domain in raw EMG and is formed by estimating
the power spectrum density (PSD) (summation of the spectral densities of the motor unit (MU)
action potentials of the active MUs). Hereby the relevant and “true” EMG will show more clearly
and is assumed to be noise free and uncorrelated. From this point of enhancing a form of nonlinearity can be applied to the signal, hereby raising the value of each sample to a demodulation
power, and afterwards soothed forming one amplitude estimate of a time- average of all the
sample. Cutting the entire signal in to short-time Fourier transform by spectrogram may also
enhance the detection of change in the EMG frequency content (Merletti R. Parker PA. 2003).
These enhancement techniques are not applied to the trial recordings, thus in spite of their
relevance in order to obtain EMG of good quality, are not essential to our focus being how the
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Side 19
Characterization of muscle activity by electromygraphy versus tocography.
clinical estimation accuracy of the temporal aspects of contractions in order to perform the later
randomized controlled trial on uterine activity.
Amplitude
Estimation of the EMG amplitude is used as indicator of muscular activity and its developing
gradient or fatigue (Merletti R. Parker PA. 2003). The force production in muscle is regulated
mainly by two mechanisms: Recruitment of MUs and the increase of the firing rate of the already
active MUs. These two parameters are present in different numbers in different muscles. In
uterine activity MU action potential occur in bursts. They arise in cells acting as pacemakers and
propagate from cell to cell through gap-junctions, which are low resistance electrical connections
(Rhoades RA. Tanner GA. 1995).
In estimating amplitude in direct relation to force it seems there are several aspects to be taken
into account. As estimating time-frequency and duration in EMG signals is prone to aliasing noise
confounders, it shows in the literature that estimating EMG amplitude is even more affected by
submaximal recording set-up: Electrode displacement is shown by Jensen et al. to affect especially
the amplitude/force relation (Merletti R. Parker PA. 2003). The subcutaneous fat layer, the
distribution of the conduction velocity of the active MUs, where the MUs are placed in the muscle,
the muscle size, bearing in mind that the anatomy of the uterus enlarges by a factor 4-5 (Chapter
2.3, are also factors which affects the EMG amplitude measurements. More so the muscle
condition (especially fatigueness) shows in amplitude measurements. Because of the physiology of
EMG signals with larger amplitude compared to EMG from a muscle in unfatigued condition
(Merletti R. Parker PA. 2003). Thus there seems to be a larger number of limitations in the use of
amplitude measurements compared to time-frequency and duration measurements.
Because of the low-strain, short-time exercises in this trial, it is irrelevant to evaluate muscle
fatigue. Instead we can focus on the possible difference between the amplitude in EMG signals
recorded because of the similarity in the exercises.
The trial set-up was the same for each recording, thus same skin preparation, same positioning of
electrode according to SENIAM literature, same positioning of the TOCO strain gauge, same
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Side 20
Characterization of muscle activity by electromygraphy versus tocography.
exercise and exercise interval, same location and thereby same risk of signal distribution from
other power sources etc. The parameter that assumingly varies the most is the volume of
subcutaneous fat and skin between the electrodes and m. abdominus recti. on the trial subjects.
This in-depth spatial volume or distance is not accessible in this trial, however we have
information regarding the trial subjects waist measurements. This we could compare to the
measured EMG amplitude on each subject to possible access a connection between the two:
Trial number
Amplitude (mV x10-3)
Waist (cm)
1
0.4, 0.6, 0.7 (mean 0.57)
84
2
0.8, 0.4, 0.3 (mean 0.57)
74
3
0.5, 0.4, 0.5 (mean 0.47)
95
4
0.4, 0.5, 0.6 (mean 0.5)
63
5
0.4, 0.3, 0.5 (mean 0.3)
69
6
0.4, 0.3, 0.3 (mean 0.3)
78
Mean
0.46
73.7
Table 3. Amplitude mean and waist relation.
100
90
80
70
60
0.6
0.57
0.57
95
84
0.5
0.5
74
0.47
78
69
0.4
63
50
0.3
0.3
0.3
40
0.2
30
20
10
0.1
Correlation Waist/Amp
0
0
Waist
Amp
Table 4. Correlation between waist and amplitude.
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Characterization of muscle activity by electromygraphy versus tocography.
There is no indication that waist measurements and amplitude are connected (Table 4). For
illustrative purpose compared to the amplitude measurements by TOCO, the amplitude by EMG is
measured in similar way (from 0 to max. positive value, instead of min-max), see also delimitation
section 2.4 on page 4.
The second method to assess the same muscular contractions is described by similar
methodology:
4. Tocography analysis
 4.1 Introduction
Tocography has been the primarily used clinical assessment tool to diagnose possible preterm
labor and labor progression through 30 years (RegionMidt, 2011). The method is non-invasive and
consists of a strain gauge (Doppler-sound) which applies on the abdomen skin above the uterus
with an elastic band tied around the waist line and measures the indirect uterine activity as the
mechanical pressure from the muscle strengthening and enlarging (SonicAid, 2011).
The major benefit from this contraction measuring method is it non-invasiveness and the fact that
it needs no skin preparation to function. The drawbacks are its high sensitivity and low specificity,
thus it is easily affected by the way it is applied to abdomen (tight or loose), where it is placed on
the uterus, whether the pregnant has a high volume of subcutaneous fat or is very slim, whether
the pregnant is standing, sitting, lying on her back or on her side etc. The low sensitivity is to
whether it is measuring the fetus moving, the pregnant breathing, vomiting, coughing, or
measuring nothing at all due to the pregnant position or fat layer (SST. 2007),(Iams et al. 2002).
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Characterization of muscle activity by electromygraphy versus tocography.
4.2 Data acquisition
4.2.1 Data processing
The data acquisition set-up is according to the trial protocol (app. 1). 6 trial subjects are included
in the trial and instructed as described in chapter 3.2 to perform 3 upper-body movements in a 45
degree angle of 30 seconds duration and with 20 seconds rest in between.
The strain gauge (tocodynamometer) is applied on the opposite side of the bilateral musculus
abdominus recti as the EMG electrodes, and with no physical contact to these. The EMG signal is
tested on each subject to make sure there is no visual interference between the two methods.
The gauge is apr. 5x5x2 cm and round, connected to a monitoring device with wire. The
registrations is seen in real-time on the monitor, can be registered in an assigned computer
program or printed in paper. It registers the mechanical pressure by scaling in percent, with
baseline/ no mechanical muscle activity equals <10%. The model used in the trial is SonicAid
FetalCare cardiotocograph. Oxford instrument (SonicAid, 2011), (appendix 2).
The recordings are given succeeding trial numbers 1-6, and are printed on paper which afterwards
is scanned into computer program for visualization and comparison purpose (Table 5).
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Characterization of muscle activity by electromygraphy versus tocography.
Table 3 showing how each square represents 60 seconds in the X-axis, and amplitude is measured
by percentage in the Y- axis
4.2.2 Feature extractions
As described the parameters concerning time-frequency and duration are visually assessable.
There is however a wide range of limitations in usage of the contraction force measured as
amplitude because of poor sensitivity (Iams JD et al. 2002).
The benefits are that the strain gauge is easy to apply on the non-prepared skin-surface together
with its non-invasive nature.
4.2.3 Results
In terms of analyzing the temporal parameters, time-frequency, duration and force by amplitude
we again address these separately.
Time-frequency
As an indirect measure of uterine activity, in terms of measuring the motor units action potential
(MUAP) in cell-level, the recordings show no visual delay on contraction. This is also the case when
the subject takes in resting position between activity.
Duration
Accordingly to the precise registrations of time-frequency intervals, the duration of the m.
abdominus recti activity is visually accessible by their waveform illustrations of muscle activity.
Trial number
Time-frequency (s)
Duration (s)
1
25-110 (85), 110-190 (80)
25-70(45), 110-140(30), 190-245(55)
2
50-115 (65), 115-170 (55)
50-75(25), 115-140(25), 155-185(30)
3
25-95 (70), 95-155 (60)
25-55(30), 95-110(15), 155-185(30)
4
05-75 (70), 75-170 (95)
05-50(45), 75-115(40), 170-200(30)
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Characterization of muscle activity by electromygraphy versus tocography.
5
30-115 (85), 115-190 (75)
30-60(30), 115-140(25), 190-245(55)
6
25-80 (55), 80-170 (90)
25-55(30), 80-125(45), 170-200(30)
Mean
75.8 sec.
34.2 sec
Table 6. TOCO registered time-frequency and duration.
The recordings show a time-frequency mean difference between the two methods 80.0-75.8
seconds = 4.2 seconds. Regarding that these numbers are made by visual interpretation of the
plots, it seems that the method is possible usefull in clinical practice by its easy accessibility.
The duration mean, however differ considerable: 46.0-34.2 seconds = 11.8 seconds. This could be
as result of the difference in the physiology, hereby the direct electrical activity versus the
mechanical pressure as indirect activity. Thus a mean between the two methods of 11.8 seconds
still is within an acceptable range in terms of visual interpretation of contraction duration, and
thereby also seems possible useful in clinical practice.
Amplitude
The tocography recording suffers from limitations in measuring amplitude as result of force in
spite of it mechanical nature. This is prone to the high sensitivity of the strain gauge, the major
issues being affected by how tight or lose the elastic band connecting it to the skin surface is
tightened and how thin/thick the subcutaneous fat layer is on each individual pregnant person
(Iams et al. 2002).
5. Discussion
 5.1 Matching methods of measuring muscle activity
The methods of measuring contractions by muscle activity by electromypgraphy and tocography
are described in the two previous chapters. The temporal parameters in terms of time-frequency,
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Side 25
Characterization of muscle activity by electromygraphy versus tocography.
duration, and also force by amplitude have been evaluated each by each for both EMG recordings
and tocography recordings.
To this point the next apparent step is answering the initial research question by matching the
methods of measuring techniques with focus on how the clinical estimation accurarcy of musculus
abdominus recti contraction time-frequency, duration and amplitude is affected using the methods
of electromyography versus tocography?
Clinical use of measuring uterine activity is currently performed by visualizing the wave-formed
amplitudes not by their force measured in percentage, but more so by the means of their timefrequencies and durations. These wave-formed amplitudes can possibly be imitated in EMG
recordings using spectral temporal mapping to ease the translation by midwives and physicians in
the clinic (Maul H. Maner WL. Olson G. Saade GR. Garfield RE, 2004).
The use of this knowledge could be transferred to part 2 of this trial, which is performed later this
year in terms of defining outcome measures over a period of time. Correlation between the two
methods could be achieved by time-frequency and duration detected both by EMG and standard
TOCO compared to time-frequency and duration detected by EMG only (positive predictive value
(PPV)). In order to enhance the clinical value of the registered EMG it is critical that some sort of
preprocessing other than the ones performed in this trial (high-pass filter at 5 Hz, and low-pass
filter at 500 Hz). Additional filtering or interference reduction techniques may be applied to
remove ECG artifacts when recording signals in the truncus area. Whitening of raw EMG can be
applied to enhance the raw signal, as well as raising the value of each sample by demodulation.
Smoothing distributions by forming one amplitude estimate of a time-average can also enhance
the quality of the signals. Cutting the entire signal in to short-time Fourier transform by
spectrogram may also enhance the detection of change in the EMG frequency content.
Another approach to possible enhance the clinical value of the registered EMG could be
converting the EMG into wave-formed TOCO-like signals o facilitate the ease of translation to
clinicians from the point of view that one could argue that the TOCO registrations are more exact
when analyzing the temporal features in a monitor device.
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Characterization of muscle activity by electromygraphy versus tocography.
Fig. 6.1 of both EMG and TOCO
Fig. 6.2 of both EMG/TOCO
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Characterization of muscle activity by electromygraphy versus tocography.
Fig. 6.3 of both EMG and TOCO
Fig. 6.4 of both EMG and TOCO
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Side 28
Characterization of muscle activity by electromygraphy versus tocography.
Fig. 6.5 of both EMG and TOCO
Fig. 6.6 of both EMG and TOCO
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Characterization of muscle activity by electromygraphy versus tocography.
Concerning the third aspect of the measured temporal parameters, the amplitude, this trial shows
in consent with the existing literature that this is a parameter currently vitiated with limitations.
The limitations consist of mainly of poor sensibility to muscle state in terms of fatigue - nonfatigue, number and placement of MU in the measured area, high or low skin-subcutaneous fat
volume, electrode/tocodynamometer strain gauge placement. Thus given the relatively large
variability of the EMG amplitude depending on these factors, a reliable EMG force is currently not
possible (Merletti R. Parker PA. 2003). This applies also to the clinical use of TOCO, unless
numerous improvements regarding both the technological features of the strain gauge and the
manor of which it is applied on the pregnant abdomen by elastic band are presented. The fact that
this method of evaluating contractions has existed for last 25 years in clinical practice, it could
seem less than plausible that it is prone to severe changes in the near future.
Consequently our initial division of temporal parameters being time-frequency, duration, and force
measured by amplitude as the main aspects of interest to the clinic seems to be inadequate.
The amplitude, thereby the force quantification, is hardly obtainable and hence not reliable. The
question arises, is it possible by the features of sEMG to reveal the force characteristic in another
way?
As previous described in chapter 2.3 the contractile element of the uterus, the myometrium,
consists of two smooth muscle layers. The inner layer where the muscle fiber run difusely, and an
outer layer which is formed by longitudinal bundles. At cell-level the myometrial tissue the smooth
muscle cell are depolarizing and repolarizing, generating electrical activity. This electrical activity is
generated at any area of the myometrium (pacemaker-area) and propagates to a neighbor cell, by
an opening of ion channels (low-resistance electrical paths) which allows entry of Ca2+ into cells
(action potentials) and induce contraction. Up till now there is no recognized conduction pathway,
but more so evidence of a strongly non-linear propagation mechanism. Early in pregnancy the
pacemaker cells are poorly coupled by the ion channels. This status changes towards term (Hassan
M. Terrien J. Karlsson B. Marque C. 2009).
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Characterization of muscle activity by electromygraphy versus tocography.
For this reason the spatial propagation proportion could be of interest in evaluation of force
instead of the initial chosen amplitude parameter. How much of the myometrial tissue is involved
in the contraction – and is this a reliable measure in terms of analyzing onset or progression of
labor?
The questions could be simplified by designing a trial where spatial sampling is conducted by
placing a number of detection system electrodes in different locations on the skin covering the
uterus (in longitudinally and transversal directions). By this the action potentials conduction
velocity can be quantified as result of the inter channel changes. With this, one should bear in
mind the limitations towards recording on the posterior side of the uterus. As spreading of
electrical activity is in the myometrium is the first trigger of a coordinated contraction (Hassan M.
Terrien J. Karlsson B. Marque C. 2009). A multi channel analysis of direction and speed of the
propagating EMG signal could provide a fundamental contribution for predicting labor.
5.1.1 Clinical value.
In the presence of uterine activity in labor progress the contraction rate can be reliably read by an
external tocodynamometer because of its temporal features. However, due to the weak relation
between contraction rate and preterm delivery ( Iams DJ. et al. 2002), and the possible diffuse
contraction pattern as either irregular or non-sensed contractions, an early prediction of labor
cannot be provided by tocography.
Electromyography as method to evaluate the temporal features of time-frequency and duration
proves by this trial to be as accurate as TOCO, thus possible not as easily visualized in its spikeform compared to the wave-form of TOCO.
The clinical value of both methods in terms of evaluating amplitude as measure of force
conducted by contraction is of poor quality due to their low specificity and high sensitivity.
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Characterization of muscle activity by electromygraphy versus tocography.
5.1.2 Shape variation modeling and feasibility
A new aspect of the definition of “force” or the quantitative impact of the electrical activity on the
myometrium is possible revealed by the measure of conduction velocity throughout the uterus.
Thus the myometrium is quiescent due to poor cell to cell ion channeling in early pregnancy, and
closer to term the number of these channels rise with the risk of preterm birth. If inter channeling
electrode recordings results in a method of shape variation modeling, which could be applied in
the clinic in relation to that of neural networks (NN), possible future predictions of preterm labor
may one day make the technology routine for monitoring and management of pregnancies. Neural
network is driven by information (data) methodologies, based on artificial computional model
inspired by structure and/or functional aspects of biological neural network, rather than
mathematical rules( Merletti R. Parker PA. 2003).
6. Conclusions
The first part of the uterine activity trial concludes that EMG as method detects muscle
contractions accurately during non-stationary circumstances compared to the method of TOCO.
However the visual appearance of the contractions is not directly comparable to that of the
currently used clinical evaluation due to the difference in the spike-like EMG recordings and the
wave-like TOCO recordings.
There is a need for improvement of the signal quality both in terms of quantification and
visualization purposes. A number of preprocessing techniques involving short-time Fourier
transformation, filtering, demodulation and more should be applied to obtain a reliable signal.
Different limitations concerning both methods in terms of high sensitivity and low specificity are
evaluated in regards of the methods clinical estimation accuracy. Mainly in terms of the muscle
condition, fatigue - non-fatigue, number and placement of MU in the measured area, high or low
skin-subcutaneous fat volume and electrode/tocodynamometer strain gauge placement.
These limitations appear to impact the amplitude parameter the most. The amplitude in EMG is
according to the literature to this point not feasible in terms of quantifying force conducted.
Project paper, Clinical Science and Technology, HST, AAU. 03.06.11.
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Characterization of muscle activity by electromygraphy versus tocography.
Because of the trial set-up we are unable to evaluate a possible force-development. We conclude
that in this trial there is no direct connection between the skin-subcutaneous volume defined by
waist measurements.
In regards to amplitude measured by TOCO limitations. They seem prone to mainly the high or low
skin-subcutaneous fat volume and electrode/tocodynamometer strain gauge placement, and the
fact that amplitude is measured by a visual scale divided in percent of the mechanical pressure,
which is directly affected by how tight the elastic ban is problematic.
In order to enhance these limitations towards measuring force, other possible methodologies
could appear:
7. Perspectives
The clinical temporal parameters time-frequency and duration are reliable aspects of contraction
quantification and visualization purposes. However steps should be taken to enhance the quality
beyond the quality obtained in this trial when it comes to performing part two of this trial.
The interesting and unexpected finding in this part of the trial is the massive limitations in
assessing the amplitude parameter both measured by EMG and TOCO.
By achieving further knowledge of electromyography applications it seems possible that the
conduction velocity of the action potentials in the myometrium as spectral changes is the answer
to interpret the onset of uterine contractions leading to labor. That is to be considered in the
second part of this trial of uterine activity.
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Characterization of muscle activity by electromygraphy versus tocography.
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Side 34