NORMAL EMG

Fundamental concepts

What is a motor unit?

One motor neuron and all the muscle fibers it innervates.

What is a motor unit action potential (MUAP)?

The electrical activity of a single motor unit. This is what is measured during EMG.

Why do we do EMG (and not just rely on NCS?)

In short, we use needle EMG to evaluate for neurogenic and myopathic patterns of injury. EMG is much more informative about pathology that is either very distal or proximal, whereas NCS gives us information about processes affecting axons between their spinal root and the neuromuscular junction. Another way of thinking about it is that EMG allows us to test nerve-muscle pairs that are not easily accessed on NCS, for example spinal nerve roots and their myotomes. Needle EMG is a very important tool in diagnosing myopathy, since motor NCS is often normal in myopathies but a myopathic pattern of EMG abnormality can help confirm the diagnosis. 

• For example, you wouldn't typically see abnormalities on sensory NCS from a radiculopathy because the lesion is proximal to the dorsal root ganglion (DRG), and NCS only tests the nerve axon after it leaves the DRG. However, you can see changes on EMG (which tests motor nerves) because the end-muscle innervated by the damaged nerve root is affected.

What do we see on EMG?

We insert the EMG needle into a patient's muscle to record and observe if there is any abnormality in electrical activity of muscle fibers while the muscle is at rest and while it is active (patient is contracting it). EMG studies require attention to how things look on the screen as well as how they sound. When the patient's muscle is at rest, we are looking for abnormal "spontaneous activity". During muscle contraction, we are looking at the "morphology" of the MUAPs and the pattern of their overlap ("recruitment") appearing on the EMG needle recording.‍ The recruitment pattern generated when a patient is maximally contracting their muscle voluntarily is called the "interference pattern" and reflects the increasing number of superimposed action potentials from different units.

Process of conducting needle EMG

1. Insert the needle into the target muscle while patient is at rest and the target muscle is relaxed (e.g. abductor pollicis brevis (APB) for median nerve distribution).

--> Record the electrical activity picked up by the needle. We want to see the muscle electrical activity when there is no activation.

2. Ask the patient to gently activate the target muscle (e.g. if testing median nerve, abduction of thumb away from palm).

--> Record electrical activity picked up by the needle. During gentle muscle contraction, only a few motor units are active. This is the optimal time to assess MUAPs generated by muscle fibers from a single motor unit.

3. Ask the patient to contract their muscle more forcefully. You should see the firing rate (frequency) of the first recorded motor unit increase. Eventually, it reaches a certain frequency after which it cannot fire faster, thus an additional motor unit is recruited (which fires slowly when first recruited). This process continues as muscle contraction becomes increasingly forceful, until the patient is contracting at full strength.

Activation, firing frequency, and muscle contraction

The force of muscle contraction is increased both by 1) increasing the firing frequency of active motor units, and 2) by orderly recruitment of successively larger and stronger motor units.

A key concept in needle EMG is "activation", defined as the ability to voluntarily increase the force of muscle contraction. Submaximal activation can reflect reduced effort due to pain, volitional effort, or an underlying upper motor neuron disorder. This in turn can result in an "incomplete interference pattern" where we see fewer MUAPs and also low firing frequency since the force of muscle contraction is small.

Figure 2.1: Normally, when patients increase the strength of muscle contraction, the pattern of recruitment becomes more noisy since individual MUAPs overlap more and more. In this example, the abductor pollicis brevis or APB (innervated by the median nerve) is being tested.

Normal EMG findings

Normal spontaneous activity

Muscle fibers are usually electrically silent at rest, i.e. there should be no "spontaneous activity" when muscles are not being activated. However, there can be BRIEF potentials upon insertion of the EMG needle that are considered normal:

• Insertional activity: seen when the EMG needle is inserted into the muscle.

• End plate activity‍:  seen if the needle happens to be inserted close to the neuromuscular junction. It is often painful for the patient.

‍Voluntary activity

When a muscle is gently contracted, only a few motor units are active, and so it is easier to make out individual MUAPs of individual muscle fibers from a single motor unit. Let's take a closer look at the morphology of these MUAPs.

MUAP components

Figure 2.2: Motor unit action potential (MUAP) labelled.

Duration

• Definition: time from initial deflection to return to baseline.

• Reflects multiple things, including: the number of muscle fibers in a motor unit (size of motor unit), diameter of muscle fibers (hypertrophy), and the synchrony of muscle fiber firing. 

• Sound correlate: longer duration = lower pitch (dull) vs. shorter duration = higher pitch (crisp). 

• Clinical application: if increased, can be due to increased motor fibers in the motor unit OR de-synchronization of existing muscle fiber action potentials (fibers don’t fire together/at the same time, i.e. some are on time and some are delayed). If decreased, can be due to lower proportion of motor fibers in the motor unit.

Amplitude

• Definition: peak to peak height. 

• Reflects: the size, density, and synchrony of the few muscle fibers closest to the needle. It therefore varies a lot with needle position and is not an accurate measure of motor unit size. 

• Sound correlate: higher amplitude = higher volume.

• Clinical application: increased in axonal injury after re-innervation, and decreased in myopathic injury due to loss of muscle fibers.

‍‍Polyphasia

• Definition: the number of phases or turns (number of time it crosses the baseline + 1). Normal # of phases is 4, except deltoid where it is normal to have more.

• Reflects: the synchrony of muscle fiber firing within a motor unit. 

• Sound correlate: higher frequency = clicking.

• Clinical application: increased in axonal injury and myopathy due to injured nerves / muscle fibers leading to asynchronous firing.

Examples of normal EMG findings

I recommend the YouTube channel "neuromuscular teaching" to see examples of normal findings. I have included a few of their videos here.

Normal EMG: Self-test