Lab A16: Muscle activity recorded with skin electrodes:

Background:  When current moves across the interface of a metal electrode and biological tissue (in the case here, your skin...) there is a chemical reaction that occurs. In the metal, electrons account for current flow, and in the tissue charged ions (Na+, K+, Ca++, Cl-...).

Find a pair of Grass gold-plated silver electrodes. Make sure the connecting wires are in "braided ground strap" shielding. (After you've set up all the electronics and VI, you will start by holding the electrodes to your thumb muscle on the palm.)

Form a circuit of
1. Electrodes attached to (say) thumb muscle, leading to
2. Two capacitors as parts of two low-freq first order HP filters, leading into
3. Two UGVFs, whose outputs project to

4. The + and - inputs (pins 1 and 2) of a 524 instrumentation amplifier, set for gain 100
5. that is fed to a first-order LP anti-aliasing filter (optional) whose output goes
6. to a LabVIEW analog-in channel which is
7. Displayed as a waveform graph and sent to
8. A calculation of its mean, subtracted from the Ain itself, fed to
9. Power Spectrum icon whose output is on another waveform graph.

Run the electrode leads out to high pass filters with about 1Hz cutoff leading into the inputs of unity-gain voltage followers. Let the UGVF outputs be the inputs to your instrumentation amp. You should see reasonable activity at gain = 100.

(optional) After the instrumentation amp place a first-order LP anti-alias filter good for a sample rate of 1000Hz. Send this filtered signal into LabVIEW on an analog input channel.

Monitor the 524 output on your scope. Without proper grounding, you should see 60Hz noise, at least. Attach a 100K resistor across the electrode contacts and see if you can generate 60Hz noise by touching the resistor. If so, then you're ready to send your signal into LabVIEW for frequency analysis.

Make your measurements in AC, not DC, mode on the scope.

Have two cases for your VI:
1. have the VI act as a continuous strip chart and show the EMG activity until a STOP button is pressed. Let about 1 second of activity be on the chart at a time.

2. In the second (more important) mode, have the VI capture 2 seconds of data per run. Show the freq. spectrum of the activity (tense vs resting muscle) on a waveform graph, with frequency resolution 0.5Hz and maximum frequency of 500 Hz.

In the main experiment, connect two electrodes to a thumb muscle of one of the lab partners. Have that partner wear metal jewelry (watch band, bracelet) and/or touch ground. Relax the thumb as it rests horizontally at the edge of the labtop. The other lab partner will record a 2 second interval of relaxation, and save (ALT-Print Screen) the front panel view of time response and frequency spectrum. Next record 2 seconds of isometric contraction in the same thumb position. Note (and save) the difference in the time and frequency domains...

Other signals: If you take the electrodes off your thumb and grip one in each hand between the the thumb and finger, you should be able to see your electrocardiogram on the scope or in LabVIEW.

If you attach the electrodes one to each temple, across the eyes,and move your eyes back and forth, you (or your lab partner looking at the scope or DMM) should be able to see the EOG, electro-oculogram, a means for monitoring horizontal eye movements. You may need to bypass the DC-blocking capacitors on the UGVFs to see good EOGs.

Subject isolation: the capacitor in series with the electrode and the UGVF isolates you the subject from the rest of the circuit. The HP filter will also reduce the DC fluctuations at the electrode-skin interface.

Free Advice: (1) Refer to the archive website Lab 4 writeup for other details.
(2) Try cleaning your electrodes with an old toothbrush, in saline, if electrode paste has hardened on them.
(3) Learn more from the University of Oklahoma Health Sciences Center Biomechanics of the thumb website.
What muscle are we recording from in this experiment?
(4) You the subject should ground yourself--run a connection from ground to metal jewelry--a metal watch band or bangle, for example--to reduce 60Hz noise on the signal. The more surface area the better.
(5) Your "tense" waveform and spectrum should look something like below (Kyle+Kao 09). No auto-scale Y on spectrum; The x-axis on the right graph should be labelled "Freq."

Likely the spectrum you see on your thumb experiment is the summary of about 100 oscillators in the form of motoneuron axons projecting to thousands of muscle fibers in the thumb. The thumb is different from the fingers in that its muscles are in the hand, not the forearm.

Suggestion: There are two kinds of people in the world: those who text and play video games with their thumbs, and those who don't. Find someone in the first category and compare the thumb muscle spectrum to that of a "normal person."

Possible FTQ: Read the lecture notes on metal-aqueous interface and be prepared to answer a question about Nernst potentials, or chemical reactions at electrodes.