Using Electromyography to test Muscle Function
When a muscle contracts, an electrical current is generated. One way to test whether a muscle is working properly is to measure this current. In this lab, you will practice measuring the current produced by a muscle and use the measurements to determine when a muscle is being used. This is called ELECTROMYOGRAPHY.
Measuring the current in a muscle involves attaching an electrode to each end of the muscle tissue and detecting the charge difference between the two electrodes. The charge difference is displayed on a monitor as a trace similar to an EKG trace. Any charge difference between the two ends of the muscle appears as a peak on the screen, and the higher the peak the greater the amount of muscle activity.
If you look at your MacLab, you’ll see that an electrode assembly has been attached to it. There are three electrode heads; red, black, and white. These will attach to the positive, negative, and ground electrodes attached to your subject.
Because your muscles don’t produce much electricity, the electrode assembly has been plugged into an amplifier, the Eth-250. Have a look at this little box and make sure the electrode assembly is plugged into a port on the back of it. There should be a cable from the same side of the Eth-250 to channel 1 of the MacLab.
Open the EMG setup file in the MacLab folder in the BI 231-Net course folder. Make sure the screen is showing channel 1.
Now, hook up your experimental subject.
Begin by choosing a muscle which is easily accessible on one of your lab group. Fill in the table below to review some muscles you might test:
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Muscle |
Movement |
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Biceps brachii |
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Extends forearm |
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Extends ankle (points toe) |
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Vastus medialis |
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Rectus abdominis |
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Sternocleidomastoid |
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Pick one of these muscles and find it on your subject. Have your subject perform the movement against a resistance, and see if you can feel or see changes in the muscle.
Find the belly of the muscle and pick two spots, one at each end of the belly. If you pick a spot too far toward the end, on a tendon or ligament, you will get poor results because tendons do not generate the electrical impulses you are trying to measure.
Snap each of the electrode heads onto an adhesive electrode. Now fix the red and black electrodes on the belly of the muscle. Put the white electrode on one of the subject's ankles or the other wrist.
Click "Start" and adjust the computer until you see a trace. You will see a trace of the muscle's electrical potential on the screen. The trace will not be clear, since many muscles are firing at once and the skin brings in some electric impulses from other parts of the body. As the subject performs different motions, however, you should see both the AMPLITUDE (height) and the FREQUENCY of the peaks change. When the muscle you are studying is being used, the peaks will be larger and closer together.
If you can’t see the peaks:
Start by adjusting the sensitivity on the MacLab.
If that doesn’t work, look at the Eth-250. You can increase the amount it amplifies the trace from 0 to 10x to 100x. You can also adjust the filter, or move the trace up and down on the screen.
Have your subject perform a series of movements. Before each movement, type the proposed motion into the computer; then, just before she does the movement, hit "return". Remember, a motion uses much more muscle firing if it's performed against a resistance!
Do EMGs on three of the muscles from the table. Identify which other movements use those muscles.
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muscle |
Main movement |
Other movements using it |
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Why would a muscle be active when the person is not doing the movement it causes? There are several reasons.
Muscle tone – muscles remain partly contracted even when they are not being used. They never relax entirely.
Stabilizing a joint – a muscle that flexes a joint is pulling the two bones closer together. Which bone will move? When you flex your elbow, will it move your wrist up or your shoulder down? This is controlled by the other muscles, which must stabilize the joint you don’t want to move.
Controlling a movement – if all you used to flex a joint was the flexor, how would you make the joint move smoothly instead of just banging? The opposing muscles work like the spring on a door, to keep the joints from swinging too fast or too loosely.