#9 2023 F-EMG Fast Twitch Muscle Fibers

F-EMG directly measures motor function, so there is no need to wait for development the advanced muscle atrophy that is required for 1944 EMG to detect structural damage. It has long been recognized that damage to motor innervation initially involves fast twitch muscle fibers. In the mid 2000s Hedgecock demonstrated that within hours of nerve damage the fast twitch muscle fibers, which are the main fibers involved in Reflex Withdrawal, require stronger voltage of a specifically modulated frequency to cause the voltage-gated calcium channels to initiate muscle twitch. He also demonstrated that modulations and frequencies out of this specific range cannot initiate twitch response with as mild a voltage as does his myo-selective signal.

A-delta fiber signals are afferent, i.e., traveling from the periphery to the CNS. Afferent signals pass along the dorsal (posterior) spinal pathways. However, many A-delta fibers synapse with interneurons carrying signals to the ventral side of the spinal cord where they synapse with motor neurons. If a sufficient number motor neurons are activated a Reflex Withdrawal is initiated. It is reasonable to assume that this reflex, out of necessity, must be a rapid interaction so as to limit damage. Based on this I reasoned that the neuro-chemical processes between A-delta, interneurons and motor neurons must be quite similar and the same frequency and modulations selectively stimulating A-delta fibers probably selectively initiate motor unit action potentials and the muscle twitch response. With some experimentation this was proved to be another discovery.

The first demonstration of functional electromyography was carried out at the 2007 AASEM Annual Conference in Newport Beach, California. A doctor attendee with a right C-6 radiculopathy volunteered and it was demonstrated that the corresponding right C6 motor unit required over 100% more voltage to initiate muscle twitch than the uninvolved left C6 motor unit. I also demonstrated that only the A-delta neuroselective frequency initiates muscle twitch. The selective frequencies for the A-beta and C-Type fibers will not at the maximum device output produce muscle twitch.

F-EMG Purpose

Once A-delta F-NCS locates nerve root injury/pathology, it is justified to assess the corresponding motor unit with the F-EMG. F-EMG does not require catastrophic damage because it directly triggers the Calcium-gated channels that cause muscle contraction. Also keep in mind that the F-EMG can be used where independent motor unit dysfunction is suspected, such as demonstrated in my wife’s case.

F-EMG Electrodes

The F-EMG detection electrodes could literally be placed at any cutaneous location and detect motor activity. Electrocardiography (ECG), for example, places electrodes at distances as far away as the ankles. F-NCS pickup electrodes are placed, as described in the AMA EDx Guidelines, “near the nerve being tested.” The F-EMG signal acquisition is enhanced by an extremely fine needle attached to the detection electrode that is positioned in/near the target muscle’s epimysium. Painful needle insertion into the deeper muscle tissue is only required in 1944 EMG, because it is assessing gross loss of myofibril structural integrity. The F-EMG functional assessment is based on measuring the minimum voltage required to initiate myoneural action potentials. These signals are easily accessed near the motor unit. The ultrathin (basically painless) needle insures detection of the myoneural action potential.

F-EMG Protocol

F-EMG seemed to me to be of academic interests. However, this opinion changed when my wife had a standard EMG. I realized that some patients have separate motor unit dysfunction without sensory involvement. Plus there are cases where motor weakness and/or muscle atrophy requires testing to rule in/out motor unit involvement. Additionally, the added data helps in assessing the overall efficacy or interventions.

Setup: See the charts in the manual for electrode placement

1. Record the skin temperature at the pickup electrode sites.

2. Clean the sites with alcohol.

Testing Steps:

1. Place the potentiometer electrode with the needle at the site.

2. Place the saline soaked electrode proximal to the pickup electrode.

3. Increase the stimulus until the target muscle twitches.

4. Turn the stimulus down below the intensity causing twitch.

5. Watch the potentiometer to record the amplitude microvoltage jump accompanying twitch.

6. Repeat on contralateral side.