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WUBU2 (what you been up to)?
I had assumed that during a muscle activation, each muscle produced an electrical signal at a slightly different frequency. To investigate this I used NI SignalExpress to view a real time plot (time domain and frequency domain) of the frequencies present at within an EMG signal. By viewing the signal in the frequency domain I was able to see that there were no specific frequency peaks relating to specific muscle contractions, which directly contradicts my earlier assumption.
Instead of pursuing FFT analysis further I determined that I didn't have a good enough understanding of the fundamentals of EMG, therefore I decided to investigate the science behind muscle contractions and surface EMG sensing.
WUU2 (what you up to)?
I am currently researching the science behind surface EMG sensing, so as to have a better foundational understanding of how to correctly implement EMG sensing and analysis. My findings are detailed below.
How the heck does a muscle work?
Muscles are composed of many Motor Units (MU), which are comprised of muscle fibers. An electrical impulse from your brain or other source (spinal cord) passes down the nerves and stimulates the muscle at the innervation zones (motoneuron synapses). The synapses release charged chemical signals (Na+) to the insides of the muscle fibers, resulting in an electrical potential across the fibers, causing them to contract. This electrical potential, or Motor Unit Action Potential (MUAP) causes the muscle fibers to contract.
And what about all that EMG stuff?
The EMG sensors detect this MUAP, where the electrical signal is then differentiated, amplified and filtered to reduce background noise, reduce muscle crosstalk and bring the signal up to a usable level. The differentiation is produced by using 2 different 'signal' electrodes with respect a 'ground' electrode.
So have you got any thoughts on the shape/size of the electrodes?
The recommended EMG detection surface is 3 parallel metal (material not confirmed) bars. Each 1cm long, 1-2mm wide and spaced 0.5cm apart. The center electrode is the 'ground' and is used with each of the outer 2 electrodes to create a double differential input to reduce the effects of muscle crosstalk.
Got any idea where to put it?
The EMG sensor (as detailed above) should be placed at the midpoint between the innervation zones (where the electrical signal is released as a chemical) and the myotendinous junction (where the muscle joins the tendon). The innervation zones differ between people, but the most reliable location is on the belly of the muscle).
Hey muscles, got any names? It would be swell if they were in latin.
Our current forearm muscle sensing method picks up the activation levels of the following muscles (I have also included the respective nerves which trigger the motor unit action potentials).
Open - Extensor
- Muscle - Extensor carpi radialis longus
- Nerve - Radial nerve
Close - Flexor
- Muscle - Palmaris longus (PL)
- Muscle - Flexor digitorum superficials
- Nerve - Median nerve
Row row row your QUOTE gently down the stream
"These properties of the conduction volume cause multiple diffractions of the electric field vectors at the discontinuities and generate multiple paths of differing impedances between the source and the detection sites causing the signal to be scrambled in the frequency domain and hence uncorrelated"
[C. J. De Luca, 1997, “The use of surface electromyography in biomechanics”, Journal of Applied Biomechanics, 13 (2): 135-163]
The above basically means that due to the various biological inconsistencies within the forearm, any frequencies produced by muscles vary greatly in magnitude and consistency. This means that using 1 or 2 EMG sites in order to try to extrapolate the MUAP's of multiple muscles may not be possible. One solution may be to use some sort of EMG array, similar to that of the Myo armband.