ELECTROMYOGRAPHY
I.
Objective:
Electromyography (EMG), the recording of electrical activity in muscles, is a
clinically useful technique in patients with neuromuscular disorders.
After completing this assignment, students will be able to:
• Explain how EMG measurements can be made using surface electrodes
• Describe how the myotatic reflex can be studied using EMG
• Describe how EMG recordings indicate motor unit recruitment in bicep curls
II.
Background:
Relevant lecture material was presented in 111A (Part II Neural Systems)
by Professor Esdin.
When somatic motor nerves simulate skeletal muscles to contract, the action
potentials produced by the muscles induce potential differences in the surrounding tissue
that can be detected by surface electrodes placed on the overlying skin. The recording
obtained is called an electromyogram (EMG).
Although it had been known since the middle of the nineteenth century that
contraction of muscle tissue is accompanied by an electrical change that can be recorded
and measured, it was not until the introduction of the concentric needle electrode by Adrian
and Brink in 1929 that important technical progress in EMG was made. This type of
electrode, basically a stainless steel cannula, similar to a hypodermic needle, but with a
wire in the center of the shaft of the needle, both of which are bare at the tip, allows the
potential difference between the needle tip and the shaft to be recorded when the needle is
near a source of electrical activity. Using this type of recording system with its restricted
pick-up range, Adrian and Bronk were able to show that in human triceps during voluntary
contraction, the gradation in force was associated with increases in the measured discharge
frequency.
These observations simulated interest in the application of electromyography as a
clinically diagnostic tool. In the early 1950’s several investigations revealed a relationship
between the amplitude of the EMG signal and the force and velocity of a muscle
contraction. This finding led to increased use of electromyography in studies concerned
with muscle function, motor control, and kinesiology.
Figure 4.1 shows an electromyogram recorded from the biceps brachii at rest using
a concentric needle electrode (courtesy of Dr. M. Graves, EMG laboratory, Dept. of
Neurology at UCLA; subject: R.H.C).
Fig. 4.1: Motor unit action potential recorded within the brachial biceps.
A typical triphasic waveform is observed, in contrast to the monophasic shape of the
intracellular action potential. This is a reflection of the large radial extent of the
extracellular medium in which the current spreads in space. Mathematically, it can be
shown that the extracellular action potential measured at the fiber surface is proportional to
the second derivative of the intracellular action potential, a situation different from
extracellular recording using the frog sciatic nerve, where the dissected nerve is in the air.
This relationship is illustrated further in Fig. 4.2:
Fig. 4.2: Triphasic waveform of the extracellular action potential at the surface of an
active fiber in situ (lower panel) compared with the intracellular action potential
(upper panel).
The EMG trace shown in Fig. 4.1 is characteristic of a single motor unit action
potential (MUAP), although many factors can influence the shape of the MUAP, in
particular the relative geometrical relationship of the electrode surfaces and the muscle
fibers of the motor unit in the vicinity. The motor unit consists of the anterior horn cell, its
axon, and all of the muscle fibers innervated by that axon (Fig. 4.3):
Fig. 4.3: Schematic representation of the single motor unit, comprising of the
anterior horn cell, axon, and all of the muscle fibers innervated by the axon.
The amplitude of the MUAP is influenced by a number of factors; in normal muscle the
amplitude of the action potential typically measured, varies from 300 µV to 5 mV. While
needle electrodes can provide information about individual MUAPs, use of surface electrodes
applied on the skin overlying the muscle gives an EMG signal which reflects the activity of
many muscle fibers. Using surface electrodes placed on the anterior surface of the upper arm
you will make EMG recordings from the biceps brachii, and examine how the pattern of
electrical activity changes during forearm extension and flexion, when performing different
types of contraction, and during fatigue.
III.
Experimental Procedure
Equipment:
Powerlab with built-in Bio Amp (Channel
Surface electrodes (resting EKG type)
Weights
The Bio Amp is a differential input amplifier which allows the Powerlab system to
record biological signals such as EMG’s or EKG’s from humans, with full electrical
isolation. Electrode leads (ADInstruments shielded lead wires MLA2504) are connected to
the Bio Amp input using the 6-pin socket on the front panel of the Powerlab.
Note the color designation of the subject’s leads:
GREEN = GROUND
BLACK = POSITIVE (+)
WHITE = NEGATIVE (–)
Open SCOPE program. Bio amplifier settings (from channel 3) should be as follows:
Range
= 2mV
High pass filter = 0.5Hz
Low pass filter = 50 Hz
Experiment 1: Recording of myotatic reflexes from the biceps brachii.
You will use a reflex hammer to induce a tendon tap reflex for the biceps brachii and
record the resulting EMG signal.
Place two surface electrodes over the mid-point of the biceps brachii; the upper electrode
should be connected to the black (+) lead, and the lower electrode to the white (–) lead.
Place the ground electrode (green lead) on the inner elbow (on the medial epicondyle).
Now setup the SCOPE program – you will only be recording data from channel 3. Go to
Set Up Menu, select Sampling, and when the dialog box appears, set source to External.
The reflex hammer that you will be using is now set up so that at the point of impact it tells
the “oscilloscope” to start recording events from that time, on the millisecond time scale.
Other settings should be as follows:
Samples: 256
Time base: 50 msec
Before collecting each trace (data page) you will need to hit start, and then when you are
ready, use the reflex hammer to initiate the recording. You can test the triggering
mechanism by depressing the hammer (not on the table, use finger), and seeing a recording
in channel 3.
on
(+
¥1501.10m)
Trial
latency
I
22.75ms
2
22.75ms
You are now ready to start recording the EMG trace. It is important to obtain as stable a
baseline as possible – this is best accomplished by holding your arm still in the air with the
elbow flexed at slightly > 90°. Initiate recording (after clicking start) by tapping the
subject’s tendon at or just below the elbow crease. Obtain a clear recording of the resulting
EMG signal (flat baseline before and after biphasic curve).
(i) Note the shape of the signal and measure the latency (time from initiation of recording
to peak amplitude) of the EMG signal using the cursor. Make at least three measuremtns
so that you can determine mean ± standard deviation for latency.
(ii) What happens to the shape of the EMG signal when you reverse the polarity of the
22.25ms recording electrodes?
3
Trial
I
hardtm
"
25.75N
26
2
3
f-
2550
On top , t
on
(iii) Obtain a printout of an overlay showing the effect of reversing electrode polarity.
soft
bottom )
-
22.25 , 27.75 22.75
,
Experiment 2: Measurement of the EMG signal from the biceps brachii during
isotonic contractions and relation to force development.
The amount of force generated by a muscle is dependent upon the summation of the
contractions of many motor units, and tension can be increased by increasing the number of
motor neurons that are firing action potentials (recruitment). This is evident from the
correlation that is observed between the quantitative electrical activity recorded by EMG,
who weights
1st
2nd
3rd
4th
weight
"
"
"
( bar)
/ bar
:
sibs)
+
4
lbs
:
9
: 14
( bar +1016s)
19
:
bar +151611
(
:
(bar +1916J
24
and the amount of force developed (Bigland, B. and Lippold, O.C.J., 1954 J. Physiol. 123,
214-224).
You will use the Chart program to record and quantitate EMG changes in response to
lifting a series of weights (dumbbell bar (4lbs) and incremental weights added to the bar,
e.g. 5, 10, 15, & 20 pounds to reach 24lbs for final set).
(i)
Measurement of the EMG signal.
Set the recording time (channel 3) to 1 sec/div; also set “absolute value” to record data.
Set the range and filters as you did for Scope (2mV, 0.5Hz, 50Hz).
Coordinate the recording of the EMG between the weight lifter and computer operator.
The computer operator needs to hit “Enter” at (1) the beginning, (2) when the forearm is
vertical, & (3) at the end of the lift. Subject will lift the dumbbell from the table while
resting their elbow on a foam pad on the table. It is important to maintain a stable
baseline by using the elbow as a fulcrum on the table; rest the forearm on the table in a
relaxed manner with the hand prepared to grasp the dumbbell bar from below. Try to
raise and lower each weight at a constant rate. After lifting each weight, add a comment
to the trace to record the weight.
Put all five EMG recordings into Zoom Window and obtain a printout.
(ii)
Integration of the EMG signal.
The two phases of each EMG recording (concentric and eccentric contractions) will be
integrated separately using the Data Pad as follows:
Open Data Pad and click on channel 3 box. A window will pop-up showing a series of
functions to choose from. On the right side change “mean” to “integral” then click OK.
For each EMG profile, highlight the defined time period between the initiation of the lift
and the time at which vertical was reached (concentric phase). Add to Data Pad. Do
this for all the weights before analyzing the eccentric phase. You should then have five
values listed in Data Pad under channel 3. Then highlight the defined time period
between the vertical and the end of the lift (eccentric phase). Add to Data Pad. Do this
for all the weights.
You are now ready to plot your data by copying the values from Data Pad and pasting
them in Microsoft Excel. (include both concentric and eccentric in same graph, plot
weight vs integrated EMG).
How do the graphs for concentric and eccentric contractions compare?
Concentric
Eccentric
¥ : : :::: ::::
9
§§
:
0.0004744
0-0003662
19 :
0.0009967
0.0006793
24 :
0-001216
0.0008473
yg §É ^
•
-
•
conc
.
"
"
.
Load ( tbs)
Experiment 3: Comparison of recording from the triceps.
w/
1416s
(bar +10 lbs)
This is best performed by a different member of the group. Attach electrodes to the biceps
and record a lift with a low/intermediate weight (e.g. 9lbs) as for experiment 2. Now attach
electrodes to the triceps and repeat the same lift (biceps curl).
Obtain printouts of EMG data recorded from the biceps and triceps (put both in the same
zoom window and print).
(
Calculate integrated EMG values for concentric vs eccentric phases for the recording from
each muscle.
Experiment 4: Effect of fatigue on the EMG signal.
Place recording electrodes on the head of the biceps of one arm and record a single lift with
an intermediate weight. Now perform repetitive lifts with the same weight to exhaustion
(recording is not necessary). Record the EMG signal with the same weight at the point of
exhaustion. Integrate the EMG signals (concentric & eccentric) before and after the fatigue
using the same procedure as in experiment 2.
Now repeat the experiment with the other arm, but this time use a considerably higher
weight than before.
Record the EMG signal before and after exhaustion with that particular weight. Integrate
with Data Pad.
Plot the 8 data points in a histogram using Excel.
Obtain printout of the EMG traces for intermediate and heavy weights.
Ecc
conc
.
0.0004139
Biceps
b÷§É§=i
""
"'
.
.ie
0.0003532
Triceps 0<000075780.0001150
Interned
.
Conc
( 9 lbs)
The fatigue
Ecc
.
.
0.0004131
0.0002886
0.0007277
0.0005284
-
Post fatigue
-
Heavy
(24ᵗb
cone
pry fat
.
.
Ecc
.
0.000715s 0.0006122
,
Post fat
-
.
0.001989
0.001258
Int
Heart
j§Ñ°%%%
"
_
Fig
"