Introduction to EMG Part 2

Added: 2026-05-12

[00:00:01]here in part two of our uh EMG module uh we're going to focus on the different uh uh methods in which we can record those Action potentials generated along the muscle fiber membrane so it's very important before we get started here with surface electrodes to revisit the fact that when we are talking about the EMG we are specifically referring to the events taking place and confined to the membrane of the muscle fiber those events take place before the sarom are activated and we get Force production and in subsequent parts of this module we'll take a look at some examples uh where we can produce electrical activities electrical activity and force is not generated uh and so this all begins uh by First Recording the the electr diagram and so what this slide illustrates are the different types of uh what are called electrodes uh these are surface electrodes as they're placed over the essentially the skin surface uh so here we see two electrodes placed over the biceps brachi and so because the biceps brachi is a superficial muscle it is that muscle through which these electrodes will be most sensitive to core to the um to the surface electrode are what are called this the recording surfaces and so what we see here on the bottom this is a disposable and we'll talk a little bit more about the Disposable electrodes in a moment uh the circles illustrate what are called the recording sites uh these are bar electrodes as we'll discuss in a in a little bit more detail on the next slides um but these bar electrodes are the part of the entire or the the bar recording sites are the portions of the entire electrode that are sensitive to those Action potentials and the way this works uh here we see uh a typical surface electrode placed over the skin uh and our interest here is to record the electrical activity of the biceps brachi and in order to produce uh a force we know that the biceps brachi has to be activated uh and that activation eventually results in a force of that muscle uh being applied to its uh biceps tuberosity on the radius and that elbow flexor Force because the uh the the the radius and the ulna uh collectively are attached to the humorus forming the elbow joint it sets up an elbow flexor torque and that will will occur after the uh muscle has experienced action potentials and so an EMG measurement is often done uh most commonly done with two separate electrical recordings and so there are actually two recording sites placed over a muscle to capture the electrical events that take place to set up an elbow flexor Force which will produce an elbow flexor torque this is uh a tracing as you can see here of of of of uh emgs that are recorded during a particular movement we'll talk about the movement here in a second but the first thing that we identify here are what just simply what we refer to as EMG bursts we see these large uh deflections from a baseline activity of three separate muscles where although the pattern of the burst uh are different from one another they're occurring at relatively the same uh uh over the same interval of time and although the font is pretty small along the side here you'll read rectus femoris vasus lateralis vasus medialis and we'll talk about these muscles quite a bit in this module these are the superficial components of the quadriceps for moris muscle that we know functions to produce knee extension and so on this tracing here the elgon refers to an an electrogoniometer it's a device that's taped to a person's leg that will move when that person's leg moves and it generates an electrical signal that we can use to track the movement of the knee and so this tracing shows that when a person is sitting in a chair their knee is at 90° and when they move into full extension we this downward deflection indicates that extension movement and that's going to occur coincident with the burst of The Superficial quadriceps for moris muscles so in order to produce this Movement we have to have action potentials generated to activate the contractile Machinery of the muscle uh now the leg in this example will move back to a knee flexed position and what you'll notice is are those large bursts that will eventually uh Decay back to the resting state but they they don't quite go back to zero that tells us that during the Ecentric phase where the knee extensors are still activated while the muscle is lengthening there's less of a need to actively recruit that muscle the lengthening aspect of the knee flexion movement which is actually controlled by the knee extensor muscle group uh the active component uh we don't need to rely on as much because the lengthening of the muscle presents a resistance to that movement there are different types of EMG electrodes I just introduced uh on the previous couple of slides the surface electrode but for deep muscles very small muscles sometimes the surface electrode does not suffice so the needle electrode is one in which a needle is inserted into a small muscle as you'll see as you see in this picture here and the needle remains in the muscle on the distal end of the needle are recording sites so as a person is performing a contraction the needle stays in and the very very small recording sites will record a limited or will record the action potentials from a limited number of muscle fibers there's also what's called the fine wire in dwelling electrod type and so it's very hard to see but you see on the very end here here we see a very small wire sticking out from the end of this hypodermic needle so the hypodermic needle is inserted into the muscle and we'll go down to the bottom here here we see at the end of the needle um we see that there's uh the end of the of the wires and the end of the wires are uh inserted into the muscle the needle is remove moved and the wires stay in place uh just to talk a little bit more about some of the details of the surface electrode there are what are called disposable electrodes these are onetime usage electrodes and these are just some examples of of surface electrodes uh what we see here there's one type of electrode which only has a single recording site the opposite end this is just a simple clothing button uh because it's metal it can transmit uh an electrical signal it has good conduction qualities that is contained within the that is connected with the recording surface uh and typically because with EMG recordings surface EMG recordings we typically need to put two of these per muscle uh a 1 cm uh diameter for a recording distance is a standard uh uh recording site uh diameter um not the only one so this is just one example of a recording site diameter uh some companies produce what are called double recording electrodes for convenience purposes where a single electrode has the desired two recording sites and this is just a measure of convenience uh instead of using two separate electrodes um the double recording electrode only requires one entire to be placed on a muscle and will always have a fixed uh distance between the C the center to Center distance of the two recording sites is 2 cm and once again very similar we have a 1 cm recording diameter for each of the two recording sites as we had with this single recording electrode um there are also what are called reusable surface electrodes the first type is what's called a recessed electrode and so that's what you see on the picture here on the right here we see uh the electrod casing that is essentially taped to the muscle of interest and recessed up to 5 mm it does vary depending upon the depending upon the type of electrode but there the recording site is a recessed uh within the entire housing um and they come in different um uh as different models so some have diameters of 1 mm it can be up to 5 5 mm diameter as well and the advantage of the reusable uh is that well it doesn't produce garbage right because the dispo the Disposable electrode is thrown in the garbage um but the recess uh the the depth of the recording site uh will actually reduce what's called movement artifact one of the problems with EMG is if the wires move the wires themselves can generate its own electrical signal which may suggest that the muscle is active when it is actually not active this depth which has to be filled with electrolytic gel uh the gel will actually absorb some of the movement and produce actually less artifact so it will produce a relatively higher quality signal uh uh that is recorded now some of the problems with the recessed electrode of course it has to be cleaned and it's often difficult uh to to get uh the gel out of this recessed electrode uh but it it's just a a minor inconvenience for people that use it uh the second type of reusable is called the dry electrode and the dry electrode really has no need for conductive gel as the recessed electrode does um I put need in quotations uh usually uh a very thin film of electrolytic gel should be uh basically just wiped along the two our recording surfaces and so here we see uh what's called uh a differential electrode uh as I indicated on an earlier slide when we record surface CG signals we want to have two recording sites per muscle when the recordings are made from each of the two recording sites as they are in this uh bar electrode the the voltage that's recorded is subtracted from each other and that produ produces a difference voltage the voltage subtraction does not eliminate the electrical signal that's recorded um because they're not recorded at the same point in time uh and that will produce what's called a differential recording which is a standard type of recording for surface electrodes this particular uh dry electrode on the bottom has three recording sites a triar uh record Tri uh uh uh recording a triar site uh recording electrode um and it's referred to as a double differential electrode uh the Double differential electrode is thought to produce a cleaner EMG signal that is less contaminated by electrical noise um there are also what are referred to as active electrodes and these active electrodes contain what are called onsite preamplifiers because the electrical signal generated from the muscle is very very weak um we have to amplify that signal on the recording which is exactly the same as turning up the volume on on your television or your radio or your music player turning up the volume increases the magnitude of that sound and these uh preamplif fires do just that it increases the recording strength of those action potentials so uh but by increasing the uh volume of those electrical signals coming off the muscle uh the pre-amplifiers which are contained as you can see here very close to the recording sites uh these bar electrodes the pre-amplifiers are built into these electrodes that you see here built into the housing it will produce a relatively cleaner signal so we're not amplifying noise that may be picked up uh on the way to the computer and that's what's referred to as the Fidelity of the signal a signal that has relatively less noise contamination uh so the Fidelity refers to the extent to which a recorder in this case an EMG recorder receives and transmits input signals without uh Distortion um the other type of of uh em recording that is also quite popular is referred to as Telemetry EMG on the previous uh slides those EMG recordings required a hardwire connection to a what's called a data acquisition unit that is physically plugged into a computer uh but Telemetry EMG is the recording of action potentials from a muscle and transmitting those data to a remote location so there's no hardwire connection necessary between the recording sites and a computer that's used to record those uh those signals um your polar if you ever used a Polar heart rate monitor um that you strap across your chest and you have and you either transmit the signal to your phone or to a watch a smartwatch uh that is a Telemetry type of device so for EMG uh this is used as a uh it's transmitted through a frequency modulated signal uh so there has to be a transmitter and a frequenc frequency modulated receiver uh some commercially device commercially available devices can transmit from the person up to the receiver uh up to 100 ft um the farther you go it can play a role in diminishing uh the quality of the signal uh but this type approach is actually good for athletic tasks where uh having a person hardwired to a computer would actually interfere with the ability to perform a biomechanical analysis and so in this ill in this photo on the right hand side here we see a person performing a squatting maybe even a jumping activity and here we see surface electrodes taped to various muscles and these surface electrodes have what are called preamplifiers so it's recording the very small uh magnitude Action Potential from the muscle and at the point of recording it's turning up the volume uh so that we can actually perform a reasonable analysis on it uh these signals are trans transmitted to a or sent to a transmitter that is uh in essentially a belt that the person is wearing and that transmitter uh is sending a radio signal it's broadcast it's frequency modulated broadcast to this and captured by this antenna along a fixed frequency uh and the receiver uh is either the data acquisition unit or can be connected to other data AC position units and that goes off to what's called analog to digital conversion uh because these signals are voltages that are recorded computers cannot interpret voltages computers can only interpret zeros and ones which is the digital language so there is a level of what's convert what's called ad conversion where the analog is converted to a digital signal uh often times there's a a second level of amplification uh and then it goes off to the computer where we can uh graph the signal and perform various analyses on that signal and so this will we're going to stop here on part two of this module and we'll start part three of this module by examining uh the individual components of the EMG signal