An EMG measures the electrical activity produced by your muscles. When muscle cells contract, they generate tiny electrical signals, and an EMG records those signals to determine whether your muscles and the nerves controlling them are working normally. A healthy muscle at rest produces no electrical activity at all. When a muscle is damaged or the nerve supplying it is injured, abnormal electrical signals appear, either at rest when there should be silence or in unusual patterns during movement.
How Muscles Produce Electrical Signals
Every voluntary movement starts with a signal from your brain traveling down through your spinal cord, out through a nerve, and into a muscle. The point where a nerve connects to a group of muscle fibers is called a motor unit. When a motor unit fires, it creates a brief burst of electrical activity. The more force you need, the more motor units your brain recruits, and the stronger the electrical signal becomes.
An EMG picks up these electrical bursts using electrodes, either placed on the skin surface or inserted directly into the muscle with a thin needle. The signals are amplified and displayed as waveforms on a screen. The shape, size, and firing rate of those waveforms tell a specialist whether your nerves and muscles are communicating properly. Muscle activation typically produces signals in the range of 20 to 60 Hz, and the pattern of those signals changes depending on how hard you’re contracting.
Needle EMG vs. Surface EMG
There are two main ways to record muscle electrical activity, and they serve different purposes.
A needle EMG uses a thin electrode inserted through the skin directly into the muscle. This gives a precise, detailed reading of individual motor units. It can distinguish between nerve damage and muscle disease, identify which specific muscles are affected, and detect subtle abnormalities that wouldn’t show up any other way. Needle EMG is the clinical standard for diagnosing neuromuscular disorders.
Surface EMG uses adhesive sensors placed on the skin over a muscle. It captures a broader signal across a wider area, which makes it useful for studying muscle fatigue, movement patterns, and overall muscle activation during physical tasks. However, skin-surface recordings are distorted by the tissue between the electrode and the muscle, and signals from neighboring muscles can bleed in. The American Academy of Neurology has concluded that surface EMG is substantially inferior to needle EMG for evaluating neuromuscular disorders. It can detect that something is abnormal, but it generally cannot pinpoint what type of disease is present.
What the Test Actually Looks For
During a needle EMG, the specialist evaluates your muscles in two phases: at rest and during contraction.
At rest, a healthy muscle is electrically silent. If the needle picks up spontaneous activity in a resting muscle, that’s a red flag. The two most common abnormal findings at rest are fibrillation potentials and positive sharp waves. Both indicate that individual muscle fibers have lost their nerve supply and are firing on their own. This is a sign of ongoing or recent nerve damage. In some cases, it can also point to inflammatory muscle diseases where muscle fibers are being actively destroyed.
Fasciculation potentials are another type of spontaneous activity: irregular, involuntary twitches of an entire motor unit rather than a single fiber. You might even see or feel these as small muscle twitches under the skin. Occasional fasciculations are common and harmless, but when they appear widely across many muscles, they raise concern for motor neuron diseases like ALS.
During contraction, the specialist watches how motor units recruit as you gradually increase your effort. In a healthy muscle, more and more motor units join in smoothly as you push harder. If nerve cells have been lost, the remaining motor units have to fire faster to compensate, creating a distinctive pattern of fewer units working overtime. The shape of each motor unit’s waveform also matters. In nerve damage, surviving nerve branches often sprout to take over orphaned muscle fibers, producing motor unit signals that are larger than normal and more complex in shape. In muscle diseases, the opposite tends to happen: motor units become smaller and shorter because muscle fibers themselves are damaged.
How EMG Differs From a Nerve Conduction Study
An EMG is almost always performed alongside a nerve conduction study, and the two are often lumped together under the term “EMG.” But they measure different things. A nerve conduction study delivers a small electrical impulse to a nerve and records how fast and how strongly the signal travels. It measures the nerve itself. An EMG measures what happens at the muscle end. Together, they paint a complete picture: is the problem in the nerve, the muscle, or the connection between the two?
For a condition like carpal tunnel syndrome, for example, the nerve conduction study is the primary diagnostic tool. It measures how quickly signals travel through the median nerve as it passes through the wrist. Compression damages the nerve’s insulating sheath, which shows up as delayed signal speed and reduced signal strength. The EMG portion then checks whether the muscles supplied by that nerve have started to show signs of damage, which helps determine severity.
Conditions an EMG Can Help Diagnose
EMG is a key diagnostic tool for a wide range of conditions affecting the nerves and muscles. The patterns it reveals help distinguish between problems originating in the nerve versus the muscle itself.
In ALS, EMG findings are distinctive: sensory nerves test completely normal, but the muscles show widespread signs of nerve loss. Fibrillation potentials, fasciculations, and large, complex motor unit waveforms appear across multiple body regions. The motor nerve signals are reduced in strength. This combination of normal sensation with diffuse motor nerve damage is a hallmark of the disease. In fact, EMG evidence of nerve damage in multiple body regions is part of the diagnostic criteria for ALS.
For peripheral neuropathies, such as those caused by diabetes or autoimmune conditions, EMG and nerve conduction studies together can identify where along the nerve the damage is occurring and whether the insulating sheath or the nerve fiber itself is affected. Finding specific patterns like conduction block, where a nerve signal drops off at a particular point, can point toward treatable conditions like multifocal motor neuropathy.
In myasthenia gravis, the problem is at the junction between nerve and muscle. Repetitive nerve stimulation during the test reveals a characteristic pattern where the muscle response drops off significantly, typically by more than 20%, as the nerve is stimulated repeatedly. This distinguishes it from other conditions where a smaller drop might occur.
Inflammatory muscle diseases like polymyositis and muscular dystrophies produce their own patterns: small, brief motor unit potentials along with spontaneous activity at rest, reflecting direct muscle fiber damage rather than nerve loss.
What to Expect During the Test
A typical EMG session takes 60 to 90 minutes, though this varies depending on how many muscles and nerves need to be tested. If a nerve conduction study is included, that portion usually comes first. Small electrodes are placed on your skin, and you’ll feel brief electrical pulses that feel like a mild shock. It’s startling more than painful.
For the needle EMG portion, a thin needle electrode is inserted into specific muscles. You’ll feel some discomfort during insertion, similar to a quick pinch or a deep ache. The specialist will ask you to relax completely so they can check for abnormal resting activity, then ask you to contract the muscle at varying levels of effort. The needle may be repositioned several times within a muscle and inserted into multiple muscles during the session.
Before your appointment, let the testing physician know if you take blood thinners, aspirin, or have hemophilia, since the needle insertion carries a small risk of bleeding or bruising. If you have a pacemaker, mention that as well. If you’ve been diagnosed with myasthenia gravis, ask whether you should adjust any medications beforehand. Avoid applying lotions or creams to your skin on the day of the test, as they can interfere with electrode contact.
Most people experience only mild soreness afterward, similar to the feeling after a blood draw, and can return to normal activities the same day.