So hello everyone, today we will talk about the electro-vegan or called the EMG.

We've seen in the previous lectures the circuitries at the brain level and the spinal level that

project to the muscle, they project to the spinal motor neurons, which is the final path

to a movement, and then the motor neuron projects to the muscle.

And this generates an electrical activity that we can record with some electrodes that

we place either on the surface of the muscle or inside the muscle.

The first person to demonstrate the existence of electrical activity within the muscle was

a German physiologist, Bois-Remoind, in Berlin.

And he demonstrated that this with a galvanometer and by inserting both hands in a saline solution.

Then in the subsequent year, this was demonstrated in several experiments and it was around the

year 1910-1920 that the first action potential from individual motor units were identified.

So if we look at the basic concept, the EMG signals arises from current fields that are

associated with the propagation of action potential along the muscle fibers.

So if we look here, this is the skin, here we have a differential electrode that you

see it records a voltage, and then you have the volume conductor which consists of the

subcutaneous fat tissue, the subcutaneous layer which is mostly the fat layer, and then you

have the muscle fibers.

Then you know what there is behind because we look at the first lectures, but this is

a simplification of what's inside the muscle.

But you can see here in this example that you have a motor axon from one motor neuron.

And this motor axon, usually motor axon are very distinct, in the distinct portion of

the muscles.

And when they discharge an action potential, this action potential travels in two directions

that you can see here.

And this generates a current field among the muscle fibers with a source and a sink.

And this generates a voltage with negative and positive fields.

The negative and positive fields generate a field, a current field that we can detect.

We can detect this field both from the surface and inside the muscle with intramuscular

electrode.

You can already imagine from this slide that if you record the EMG from the top of the

muscle, the action potential will be filtered by the volume conductor.

And the subcutaneous tissue, it is indeed a low-pass filter.

And it attenuates the action potential shape.

And indeed, just to make you an example, if you want to record an EMG from the surface,

of the muscle, the NICLIS theorem limit frequency is satisfied for frequencies above 2000 Hz.

On the other hand, if you want to record inside the muscle, you need to sample a much higher

frequency because the action potential, it is not filtered by the volume conductor, in

this case by the subcutaneous fat layer that attenuates significantly the action potential

shape.

And you need to sample from 10 to 20,000 Hz.

So when we look at EMG-C to a person that is not expert, it will look extremely messy,

which is okay, it's normal to everyone that is not an expert, EMG looks extremely messy,

but there is a very strong order in EMG, especially if we sample inside muscle and if we sample

from a large number of electrodes.

And this you should not really know by knowing at the previous lectures that each of these

action potential corresponds to the activity of one motor unit action potential.

But now we will see this again.

So just to remind you, you have one motor neuron in the spinal cord, in the ventral

horn of the spinal cord that within its axon innervates a distinct group of muscle fiber.

And if this distinct group of muscle fiber is unique in the position of the muscle and