17-07-2012, 05:01 PM
Introduction to EMG
[attachment=27867]
In the muscle physiology lecture I showed you how an externally measured action potential
from a single muscle fibre produced a characteristic biphasic waveform. Imagine what
happens when a recording is made externally to the muscle in a living subject. Instead of
recording from a single muscle fibre you are recording from thousands of muscle fibres and
instead of recording just next to the fibre you are recording some distance away through
various layers of connective tissue and skin. This means that instead of a nice clean signal of
about 100 mV you get a much more complex signal of about 5 mV. The actual size of the
signal is highly variable and depends on the thickness of the connective tissue, the quality of
the contact between the electrode and the skin, the size of the electrodes and the sizes and
time courses of the individual motor unit action potentials. These will vary from experiment
to experiment so the size of an EMG is a largely qualitative measure although it does increase
in size with the activation level of a muscle.
5 mV is quite a small signal. In a laboratory surrounded by lots of electrical equipment the
human body makes a good aerial and picks up some quite strong electrical signals – perhaps
as much as a volt or two. If we tried to measure the electrical signal using two electrodes as
shown at the top of figure 2 most of what we would measure would be this electrical noise
and we would be lucky if we could measure the much smaller signal from the muscles. The
solution to this is shown in the bottom half of the figure. By using three terminals, two placed
over the muscle and the other placed anywhere on the body, we can take two sets of
measurements which can be subtracted. The noise signal is exactly the same in each
measurement so that it disappears when the two measurements are subtracted. This is a very
common approach when measuring small electrical signals (for example high quality
microphones work this way). It is done using a differential amplifier and how well it
performs this subtraction is known as the Common Mode Rejection Ratio (CMRR). It is
important that the correct electrodes are used: the earth electrode can go anywhere on the
body but the two active electrodes must be placed over the body of the muscle a few
centimetres apart.
[attachment=27867]
In the muscle physiology lecture I showed you how an externally measured action potential
from a single muscle fibre produced a characteristic biphasic waveform. Imagine what
happens when a recording is made externally to the muscle in a living subject. Instead of
recording from a single muscle fibre you are recording from thousands of muscle fibres and
instead of recording just next to the fibre you are recording some distance away through
various layers of connective tissue and skin. This means that instead of a nice clean signal of
about 100 mV you get a much more complex signal of about 5 mV. The actual size of the
signal is highly variable and depends on the thickness of the connective tissue, the quality of
the contact between the electrode and the skin, the size of the electrodes and the sizes and
time courses of the individual motor unit action potentials. These will vary from experiment
to experiment so the size of an EMG is a largely qualitative measure although it does increase
in size with the activation level of a muscle.
5 mV is quite a small signal. In a laboratory surrounded by lots of electrical equipment the
human body makes a good aerial and picks up some quite strong electrical signals – perhaps
as much as a volt or two. If we tried to measure the electrical signal using two electrodes as
shown at the top of figure 2 most of what we would measure would be this electrical noise
and we would be lucky if we could measure the much smaller signal from the muscles. The
solution to this is shown in the bottom half of the figure. By using three terminals, two placed
over the muscle and the other placed anywhere on the body, we can take two sets of
measurements which can be subtracted. The noise signal is exactly the same in each
measurement so that it disappears when the two measurements are subtracted. This is a very
common approach when measuring small electrical signals (for example high quality
microphones work this way). It is done using a differential amplifier and how well it
performs this subtraction is known as the Common Mode Rejection Ratio (CMRR). It is
important that the correct electrodes are used: the earth electrode can go anywhere on the
body but the two active electrodes must be placed over the body of the muscle a few
centimetres apart.