What to Expect During an EMG and NCS for CMT

If you are being tested for Charcot-Marie-Tooth disease (CMT), your doctor will likely order an Electromyography (EMG) and a Nerve Conduction Study (NCS). These tests evaluate how well your nerves send electrical signals and how your muscles respond to them. The entire appointment usually takes between 45 to 90 minutes. While the idea of electrical shocks and needles can be intimidating, knowing exactly what the process feels like and what the results mean can help reduce anxiety ^[1][2].

The Nerve Conduction Study (NCS): Checking the “Wiring”

The Nerve Conduction Study (NCS) is typically done first. The technician will tape small patches (electrodes) to your skin over the nerves and muscles being tested across different limbs. They will then apply a small electrical pulse to the nerve to see how fast and how strong the signal travels ^[3].

What it feels like:
You will feel mild, brief electrical shocks or tapping sensations. Most patients describe it as a quick “zap” or a static shock, similar to rubbing your feet on carpet and touching a doorknob. The technician will test several nerves, so you will experience this zap multiple times. It causes your muscle to briefly twitch, which can feel startling but is generally well-tolerated ^[1][2]. If you are feeling highly anxious, let the technician know — they can often pace the test to make it more comfortable for you.

The Electromyography (EMG): Checking the “Engine”

After the NCS, the doctor will perform the Electromyography (EMG). This test looks directly at how your muscles respond to nerve signals. The doctor will insert a very thin, sterile needle electrode into specific muscles ^[4]. Importantly, the needle does not send shocks into your muscle; it only listens to the electrical activity already happening inside you.

What it feels like:
You will feel a quick poke or pinch as the needle goes in, similar to a minor injection or an acupuncture needle. Once the needle is in place, the doctor will ask you to relax the muscle and then gently flex it. You might feel a dull ache or cramping sensation while the muscle is being tested, and the machine will make a crackling sound like radio static as your muscle contracts ^[4][5].

What the Numbers Mean: The 38 m/s Cutoff

For patients with CMT, the most critical number from your NCS is your motor nerve conduction velocity (MNCV), which measures the speed of the signals sent to your muscles in meters per second (m/s) ^[6]. Your nerves have a central core (the axon) that acts like a copper wire, and an outer coating (the myelin sheath) that acts like rubber insulation. CMT affects these parts differently.

Doctors use a speed cutoff of 38 m/s in the median motor nerve (the nerve running down your arm) to help classify which type of CMT you have:

• Under 38 m/s (Demyelinating - CMT Type 1): If your signals travel slower than 38 m/s, it indicates that the myelin “insulation” is damaged. Without good insulation, the electrical signal leaks out and slows down ^[7][8].
• Over 38 m/s (Axonal - CMT Type 2): If your signals travel at relatively normal speeds (often well over 38 m/s), it suggests the myelin insulation is intact, but the actual “wire” (the axon) is damaged. In this case, the signals are fast, but the total amount of signal reaching the muscle is smaller ^[7][8].

While the 38 m/s cutoff is a standard guide, some patients have an “intermediate” speed (usually between 25 and 45 m/s) ^[9]. Because of this overlap, the EMG/NCS results are used alongside genetic testing to give you an exact diagnosis ^[4].

How to Prepare

• Skip the lotion: Do not use any lotions, oils, or creams on your arms and legs on the day of the test, as they can prevent the electrodes from sticking ^[10].
• Stay warm: Cold skin can artificially slow down nerve signals, so wear warm clothes to the appointment ^[11].
• Tell your doctor: Let your doctor know if you have a pacemaker, or if you use a blood thinner or have a bleeding disorder, as this increases the risk of bruising and bleeding from the EMG needle insertions ^[4].

After the Test

Once the testing is complete, you can return to your normal daily activities, including driving yourself home. You might experience minor bruising or temporary muscle soreness for a day or two where the EMG needles were inserted, but this usually resolves quickly. Your doctor will review the data to recommend the next steps, which typically involve targeted genetic testing based on your results.