Electromyography (EMG) is a technique that measures the electrical activity of muscles when they contract. In this article, I present the fundamental principles of this method, starting with an introduction to key concepts and the underlying biophysics.

I then discuss two commonly used EMG methods: surface EMG and intramuscular EMG. Finally, I briefly address the different types of analyses that can be applied to the signals and the information that can be extracted from them. 

Whether you are a student, professional, or simply curious to learn more about how muscle activity can be measured and explore the links between muscles and the nervous system, this article aims to provide you with a first approach to understanding the basics of EMG.

Electromyography in brief

Electromyography measures the resultant electrical activity from the interaction between the nervous system and muscle fibers. When the brain sends signals to the muscles to contract, these signals propagate along the nerves in the form of an electrical charge. These impulses reach the muscle fibers and cause muscle contraction.

EMG allows the capturing of electrical signals associated with these muscle contractions. These signals, called action potentials, are detected using electrodes placed on the skin or directly inside the muscles. The captured signals are then amplified, filtered, and analyzed to provide information on the state and functioning of the studied muscles.

This technique is widely used in various fields, including biomedical research, rehabilitation, sports performance evaluation, the diagnosis of neuromuscular diseases, and ergonomics. EMG allows researchers, clinicians, and health professionals to better understand muscle function, identify dysfunctions, and design strategies to optimize performance and rehabilitation.

The biophysics of EMG​

The process of muscle contraction, action potentials, and neuromuscular junctions

To fully grasp the foundations of electromyography, it is essential to understand the mechanism of muscle contraction. Muscle contraction is a complex process that involves the transmission of electrical signals along motor neurons and muscle fibers, as well as biochemical interactions at the neuromuscular junctions.

Let's look in more detail at what happens at each of these steps:

What exactly do we measure?

We understand that EMG signals reflect the electrical activity of muscles, but what is actually measured and how is it related to muscle function?

Motor units: A key element in understanding EMG

A motor unit is composed of a single motor neuron and all the muscle fibers it innervates. Motor units are the smallest functional components of the neuromuscular system capable of producing a muscle contraction. They play a crucial role in the generation of muscle force and the coordination of movements.

Recruitment of motor units

The recruitment and firing frequency of motor units are the main mechanisms by which the nervous system controls muscle force and coordination. Recruitment refers to the number of motor units activated to produce a given muscle force.

The higher the number of motor units recruited, the greater the muscle force. Firing frequency, however, refers to the rate at which motor neurons generate action potentials. A higher firing frequency leads to faster and stronger muscle contractions.

Motor units and EMG

EMG measures the action potentials of the individual muscle fibers that make up motor units. By analyzing EMG signals, we can obtain valuable information on motor unit recruitment and firing frequency, as well as the synchronization and coordination of muscle contractions. 

However, what we measure will depend on the method used and the level of analysis chosen. Depending on whether we measure the overall activity of a muscle or focus on specific muscle fibers, the information gathered will not be the same. It will not represent the same underlying phenomena. 

Two common electromyography techniques

1) Surface electromyography (sEMG)

Surface electromyography, or sEMG, is a non-invasive technique that measures the electrical activity of muscles through the skin. This method uses surface electrodes placed on the skin directly over the muscle of interest. The electrode captures the electrical signals that trigger the contraction of the underlying muscle fibers. 

Non-invasive: One of the main advantages of sEMG is that it is non-invasive, which means that no needles or other instruments are inserted into the muscle to measure electrical activity. This makes it more comfortable for the subject and reduces the risk of infection or complications.

Ease of use: sEMG is relatively simple to set up and use. The electrodes can be easily applied to the skin, without the need for specialized training. However, it is important to properly prepare the area of the skin where the electrodes will be placed (by cleaning it and removing hair, for example) to ensure good adhesion and optimal signal quality.

Less precise for deep muscles: sEMG is particularly suitable for measuring the activity of superficial muscles, that is, those close to the skin surface. For deeper muscles, sEMG may be less precise, as the electrical signals must pass through additional layers of tissue to reach the electrodes.

Sensitivity to Artifacts: sEMG can be sensitive to artifacts, that is, unwanted signals that can interfere with the EMG signals of interest. Artifacts can come from various sources, such as motion, electrocardiograms, sweating, electromagnetic interference and baseline noise, or even contractions of adjacent muscles. Signal processing techniques, such as filtering and signal decomposition, can mitigate artifacts' impact on EMG measurements.

Surface electromyography is a widely used method for measuring muscle activity due to its ease of use and non-invasive nature. It is particularly useful for studies in neurophysiology, functional rehabilitation, ergonomic assessment, and biomechanical analysis.

2) Intramuscular Electromyography (iEMG)

Intramuscular electromyography, or iEMG, is an invasive technique that measures the electrical activity of muscles by directly inserting fine electrodes (needles or wires) into the muscle of interest. This method allows for more detailed information on the activity of individual muscle fibers and is particularly useful for studying deep muscles or small groups of muscle fibers. 

Invasive: Unlike sEMG, iEMG is an invasive technique that requires the insertion of electrodes into the muscle. This method can be more uncomfortable for the subject and must be performed by a qualified professional.

Requires specialized training: The setup and use of iEMG require specialized training and practical experience, as needles or wires must be inserted precisely to avoid injuries and obtain reliable measurements.

Increased precision for deep muscles: One of the main advantages of iEMG is its ability to measure the activity of deep muscles with increased precision compared to sEMG. By directly inserting electrodes into the muscle, iEMG allows capturing electrical signals directly at the source, without them passing through multiple layers of tissue.

Less Sensitive to Artifacts: iEMG is generally less sensitive to artifacts than sEMG, as the electrical signals are captured directly inside the muscle. This allows for more precise measurements and minimizes the impact of external interferences.

Intramuscular electromyography is a valuable method for studying muscle activity in detail, particularly in cases where sEMG may not provide sufficiently precise information. iEMG is more widely used in research and medical diagnostics. Despite its invasive nature, iEMG offers unique advantages in terms of precision and signal resolution, making it a key tool in analyzing muscle pathologies, assessing neuromuscular disorders, and understanding the underlying mechanisms of muscle contraction.

The Information Contained in EMG Signals

Three types of analysis are possible to extract information contained in EMG signals: 1) amplitude analysis, 2) temporal analysis, and 3) frequency analysis. Each of these analyses provides different information on muscle function and can complement each other to give a comprehensive understanding of muscle activity. Let's look at the data that can be extracted with these analyses.

Amplitude analysis

Amplitude analysis focuses on the magnitude of the EMG signal, which correlates with the number of motor units, activated muscle fibers, and the level of muscle force generated. This approach is commonly used to assess muscle strength, muscle activation patterns, and detect muscle imbalances. Amplitude analysis can be applied to both sEMG and iEMG signals.

Temporal Analysis

Temporal analysis examines the time domain of the EMG signal, focusing in particular on the moments of activation and deactivation of muscle contraction. For sEMG signals, temporal analysis involves studying the variations of the signal over time, thus allowing the identification of muscle activation patterns and the evaluation of coordination between different muscle groups. For iEMG signals, it refers more to the examination of individual muscle action potentials, their duration and intervals, offering a more detailed perspective on the recruitment of muscle fibers and muscle activation mechanisms.

Frequency Analysis

Frequency analysis involves examining the frequency content of the EMG signal, which reflects various aspects of muscle contraction, such as the type of muscle fibers, contraction speed, and fatigue. By analyzing the different frequency components of the EMG signal, it is possible to obtain information on endurance, strength, and muscle power and detect early signs of fatigue or other functional changes. This information will be representative of the overall activity of muscle fibers beneath the skin surface for sEMG signals, while reflecting the contraction and relaxation mechanisms of individual muscle fibers for iEMG signals.

For each of these analysis methods—amplitude, temporal, and frequency—signal processing techniques and specific indicators exist and are widely documented in the scientific literature. These indicators allow for the extraction of detailed and quantitative information on muscle activity, thus facilitating the understanding of underlying mechanisms and the implementation of appropriate intervention strategies.