Download APA 4314 Laboratory No. 8 Electromyography: Isometric Contractions

APA 4314
Laboratory No. 8
Electromyography: Isometric Contractions
Resistance. The opposition of a device or material to the flow of direct-current (DC), equal to
the voltage drop across the element divided by the current through the element (R=V/I). In a
alternating-current resistance is equal to the real part of the complex impedance.
Impedance. Impedance is the measure to which an electric circuit resists electrical current when
a voltage is applied. Impedance, expressed in ohms, is the ratio of the voltage impressed across a
pair of terminals to the current flow between those terminals. In direct-current (DC) circuits,
impedance corresponds to resistance. In alternating current (AC) circuits, impedance is a function
of resistance, inductance and capacitance. Inductors and capacitors build up voltages that oppose
the flow of current. This opposition, called reactance, must be combined with resistance to find
the impedance. The reactance produced by inductance is proportional to the frequency of the
alternating current, whereas the reactance produced by capacitance is inversely proportional to
the frequency.
Full-wave rectifier. Electronic equivalent of taking the absolute value of an analog signal.
Linear envelope detector. Circuitry that combines a full-wave rectifier followed by a low-pass
filter.
Integrator. Device that performs a time integral of an electrical signal.
Frequency spectrum, power spectrum, spectral density. Plot of the distribution of the
intensity of a signal versus the frequencies of the signal.
Motor unit. A single motor nerve and all of the muscle fibres that it enervates.
Part 1:
Equipment:
Pellet electrodes and electrode paste
Ohmmeter or multimeter
Electrode leads and electrode cabling
Software: Vicon Workstation and BioProc3
BNC-BNC cables
Bioamplifier (e.g., Bortec)
Analog-to-digital computer
5.
Fatigue is the inability of a muscle to produce force during a sustained contraction or
multiple contractions. After sufficient rest, have a subject hold a maximum isometric
contraction until fatigue occurs. Before beginning the sustained contraction establish the
subject’s maximum isometric contraction force (MVC). Collect the EMG signal at the
rate of 1000 Hz for 30 to 60 seconds. Use Vicon Workstation to collect the data.
6.
Process the EMG signal(s) produced in step 5 with BioProc3, to obtain the frequency
spectra for 2 second durations before and after fatigue. Are the two spectra similar? It
is expected that the fatigued contraction will have fewer high frequency components and
therefore the median frequency will be lower.
7.
Run the data through the Fatigue analysis option under the Analysis menu, EMG analysis
submenu. Plot the fatigue curve and print it. From this curve decide when the person first
showed signs of neuromuscular fatigue (mean frequency < 70-80% MVC). The median
frequency may also used for this purpose.
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Part 2:
Equipment:
Pellet electrodes and electrode paste
Ohmmeter or multimeter
Electrode leads and electrode cabling
Oscilloscope
Bioamplifier (single channel)
BNC-BNC cables
1.
Record the skin resistance across pairs of electrodes applied to (a) unprepared skin (i.e.,
unshaven and with no abrasion or alcohol preparation) and (b) prepared skin over the
flexor digitorum longus muscle. Skin resistance (Rskin) is the average of the resistances
taken in both directions across the electrode pair.
2.
Given that the bioamplifier that you are using has an input resistance or input
impedance (Rinput) of 10 megohms (MW), what is the percentage attenuation of the EMG
signal by the bioamplifier for step 1 above? Use the following equation:
What would be the attenuation if your subject had skin resistances of 2 megohms or
20 kilohms? What would be the attenuation if the input impedance of the amplifier
was 10 gigohms (10 GW, 10 000 MW)?
3.
Record the raw (band-passed filtered), linear envelope and integrated EMG of several
contractions and examine the phasic (temporal) relationships among these different
processed versions of the EMG. E.g., does each signal start and finish simultaneously;
do maxima occur simultaneously?
4.
Try to produce a single motor unit EMG. In other words, try to repeatedly produce a
single triphasic EMG spike with a very light contraction. With practice try to recruit a
second then third motor unit. The shape of the EMG (amplitude and direction) of a motor
unit’s EMG does not change for a specific electrode placement.
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Part 3:
Equipment:
Pellet electrodes and electrode paste
Ohmmeter or multimeter
Electrode leads and electrode cabling
Force platform
Software: BioAD2 and BioProc3
Weights and weight holder
Bioamplifier (e.g., Bortec or Noraxon)
2 or 3 BNC-BNC cables
Analog-to-digital computer
Using an A/D program such as BioAD2 collect the vertical force and the raw EMG level
produced during an isometric contraction. Record the EMG of the biceps brachii during an
isometric contraction that increases from zero force to maximum and back to zero over a 6
second duration. Consult figure 1 for how collect the EMG and force information. Note the
transducer is not needed if you have a force platform. Furthermore the subject should not actually
list the weights from the ground. Import the files into BioProc3 then convert the EMG data to a
linear envelop using the EMG Analysis submenu under the Analysis menu. Use a cutoff
frequency of 5 hertz. Correlate the signal to determine the r-value (linearity) and the slope
(sensitivity) of the line of best fit in N/V.
Plot the relationship between force (X-axis) and LE-EMG (Y-axis). Is the relationship
linear or nonlinear? You may use a spreadsheet program (Quattro Pro, Lotus 1-2-3, Excel,
etc.) to produce the graph.
Figure 1. EMG-isometric force comparison test.