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Download APA 4314 Laboratory No. 8 Electromyography: Isometric Contractions
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Transcript
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. 1 2 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. 3 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.