Electromyography

1. EMG background

What is the Link between Electricity and Muscle Activation?

Electromyography is the study of the electrical signal generated by contracting skeletal muscles. The early work on the relationship between muscle contraction and electricity was conducted in the 1600-1700s.

Before the 17th century, it was believed that movements were produced by moving spirits that travelled from the brain down hollow nerves to the muscles and that caused the muscles to increase in volume during a contraction. In contrast to this belief, in 1664 Jan Swammerdam (1637-1680) demonstrated that stroking the innervating nerve of the frog s gastrocnemius muscle generated a contraction even on an isolated muscle-nerve preparation when the connection to the brain was cut. The first intuition of the relationship between muscles and electricity is documented in the work of Francesco Redi (1626-1697) in 1666. Redi showed that the shock of the electric ray fish was muscular in origin, highlighting the rela-tionship between voluntary contractions and the pro-duction of an electrical signal. A decisive breakthrough came in 1791, when Luigi Galvani (1737-1798), who is credited as the father of neurophysiology, showed that electrical stimulation with metal rods of muscular tissue in frog's legs produced a contraction which generated a force. He reasoned that the factor respon-sible for movement was probably of electrical origin. Carlo Matteucci (1811-1868) definitively proved that electrical currents originated in muscles by showing, that injured excitable biological tissues generated direct electrical currents, and that they could be summed up by connecting elements in series, as in an electric pile.

In 1849, DuBois-Reymond (1818-1896) was the first to report the detection of voluntarily elicited electrical signals from human muscles. He constructed an apparatus that consisted of a surface electrode (a wire attached to a blotting paper immersed in a jar of saline solution) that caused a small deflection on the galvanometer when the fingers immersed in the saline solution were contracted. He understood that the impedance of the skin reduced the current that drove the galvanometer, and that a greater deflection could be produced by induced blisters so that the open wounds were in direct contact with the saline solution. However, methods to detect the electromyographic signal from a human improved slowly in the 19th century, when electricity started to be used for clinical applications. For instance, Guillaume Duchenne (1806-1875) applied electric stimulation to intact skeletal muscles to investigate muscular function in humans. Duchenne was one of the first to investigate electricity for therapeutic purposes, and is considered to be the father of electrotherapy. In 1922, Herbert Gasser (1888-1963) and Joseph Erlanger (1874-1965) used a cathode ray oscilloscope in place of the galvanometer to investigate conduction along the nerve axons that allowed them to observe action potentials. Great impact in the field was made by Edgard Adrian (1889-1977) and Detlev Bronk (1897-1975) in 1929, when they introduced the first needle electrode to record the electrical signal generated during a muscle contraction. It was a groundbreaking innovation with immediate impact on the clinical community. Since then, a series of technological advances have fostered the investigation of muscle function through the analysis of the electrical activity associated with muscle contraction. So how are muscle contraction and electricity related?

2. Muscles, Motor Units and EMG

Skeletal muscles are composed of thousands of muscle fibres that contract when they are activated by nerve pulses. These pulses are generated in motoneurons, whose cell bodies are located in the anterior horn of the spinal cord. Nerve pulses travel down the nerve axons of the motoneuron and come in contact with the muscle at the neuromuscular junction. Each motoneuron innervates a number of muscle fibres, which contract almost simultaneously when they are excited by a nerve pulse. Thus, a single motoneuron and all the fibres innervated by this motoneuron constitute the functional unit of muscles, called motor unit (MU). The muscle fibre membrane has a resting potential of approximately —70 mV. A nerve pulse at the innervation point (i.e. at the neuromuscular junction), temporarily alters this resting potential to +40 mV. The change in membrane potential, referred to as depolarization, propagates in both directions along the muscle fibres from the innervation point. The depolarization voltage travelling down the muscle fibres of a motor unit can be detected with an electrode placed near the muscle fibres or on the skin above the contracting fibres. The characteristic shape of the depolarization voltage is called motor unit action potential (MUAP). In order to sustain a muscle contraction, motor units must be repeatedly activated. The resulting sequences of MUAPs are called motor unit action potential trains (MUAPTs). The EMG signal is the electrical signal associated with a muscle contraction that can be detected with a sensor positioned inside a muscle or on the skin on top of a muscle. It represents the summation of the electrical activity from a number of MUAPTs that are simultaneously activated during a muscle contraction. The EMG signal ranges between a few mikroV to several mV depending on the level of muscle activity. The activation of a motor unit transforms the nerve signals into force and movement so that the electrical manifestation of a MUAP is accompanied by a contraction of the innervated muscle fibres. The force generated by the contraction of a single motor unit is called force twitch. For sustained contractions, force twitches superimpose if they occur in close enough succession, producing a tetanic, or fused, force. The force produced by a contracting muscle is composed of the individual forces generated by all active motor imitc within the muscle.

3. Muscles and Fibre Types

The number of fibres comprising the individual motor units varies widely within a muscle and between muscles. As a general rule, smaller muscles that control fine movements, such as the muscles around the eyes, are composed of smaller motor units containing a smaller number of muscle fibres. In contrast, muscles that are used to generate strong forces and act on large body masses, such as limb muscles, are composed of larger motor units containing a greater number of fibres per motor unit. Muscle fibre types can be characterized as slow twitch (type I) or 'slow twitch oxidative' and fast twitch (type II). Type I fibres are characterized by a slower speed of force production than type II fibres and they produce smaller amplitude and longer duration force twitches. Fast twitch fibres can be further categorized into type IIA and type IIB fibres, which are also known as 'fast twitch oxidative' and 'fast twitch glycolytie, respectively. Type IIB fibres are characterized by a higher force and speed production but are fast to fatigue, whereas type HA fibres are more resistant to fatigue. Type I fibres have a reddish appearance due to the rich vascularization and blood supply providing oxygen, allowing them to be more resistant to fatigue. Type IIA have larger-diameter muscle fibres and have a paler appearance due to lower vascularization and blood supply. The limited amount of energy stored in their mitochondria renders them fast fatiguing. Although muscle fibres are classified into distinct groups (type I, IIA and JIB), each human muscle generally contains motor units of varying size and contractile properties that create a continuum among these groups. The proportion of slow to fast fibres in skeletal muscles varies depending on the type of muscle usage.

4. Motor Unit Recruitment, Firing rate and Force Modulation

Within each muscle, motor units are hierarchically activated in order of increasing size, a property known as the Size Principle (Henneman, 1957). Smaller motor units, characterized by smaller-amplitude and longer-duration force twitches, are recruited at lower contraction forces  i.e. they have a lower recruitment threshold. Larger motor units, characterized by larger-amplitude and shorter-duration force twitches, are recruited at higher contraction forces, i.e. they have a higher recruitment threshold. When a motor unit is activated during a sustained contraction, it fires (contracts) repeatedly. The series of firing times can be represented with a train of bars over time, called the firing train, where each bar represents  one firing or MUAP of the motor unit  The number of pulses that occur over one second is a measure of the motor unit mean firing rate. The mean firing rate is a time-varying function calcu-lated over a unit-area window that slides through the motor unit firing train. The time interval between consecutive firings of a motor unit is called a interpulse interval (IPI) (see picture MU activation)

The force produced by a muscle is modulated by two main processes: 1) by varying the number of active MU, and 2) )varying the their firing rate. When greater amount contraction force is needed, a greater number of motor units are activated, which are progressively larger (higher threshold) and generate larger forces. The firing rates of the active motor units also increase with increasing force and, consequently, their average motor unit IPIs decrease.

These two phenomena translate in an increasing amplitude of the recorded EMG signal during increasing force contractions, due to the increasing number of active motor units and to the increase in their firing rate. In addition, note that the higher-threshold larger motor units are generally characterized by greater amplitude MUAPs in virtue of the greater number of muscle fibres per motor unit.

Over the past five decades, there has been a common acceptance of the notion that higher-threshold moto-neurons are characterized by greater firing rates than lower-threshold ones (Eccles et al., 1958; Kernell, 1965, 2003). This notion stems from the observation that, when the nerves of anaesthetized cats are electrically stimulated, the larger-diameter (higher-threshold) motoneurons exhibit a shorter after-hyperpolarization and greater firing rates than the smaller-diameter (lower-threshold) ones. This firing rate organization would match the motor unit contractile properties: lower-threshold motor units have wider and smaller amplitude force twitches than higher-threshold ones and require lower firing rates to produce tetanic force. This arrangement would conceivably 'optimize' the force-generating capacity of the muscle because motor units would fire at a rate that produces the greatest amount of force. However, we now know that, in humans performing voluntary contractions, motor unit firing rates are organized in an opposite manner: lower-threshold motor units have greater firing rates than higher-threshold ones. This firing rate arrangement is known as the Onion-Skin scheme (De Luca & Erim, 1994) and it demonstrates that the physiological firing behaviour of motor units when they are voluntarily activated in humans is dramatically different than that observed when isolated motoneurons are electrically stimulated in animals. One ostensive shortcoming of the Onion-Skin scheme is that the motor unit firing rates do not match the motor unit contractile properties, so that motor units do not always produce fused force and they do not always take full advantage of their force-generating capacity. Why is that? One reason is that the high-threshold motor units are fast-fatiguing and would not be able to sustain force production for a is long period if they were to fire fast. Thus, the Onion-Skin scheme is not designed to 'optimize' muscle force. Instead it provides a lower maximal force with the capacity to sustain it over longer time (De Luca & Erim, 1994, Contessa & De Luca, 2013).

5. Detecting the EMG signal

Thera are two main possibilites of detecting EMG signal: surface EMG, which involves placing the electrodes on the skin overlying the muscle; intramuscular EMG, which involves inserting an electrode into the muscle itself.

We will discuss only the surface EMG (sEMG). If you are more interested in intramuscular EMG Im gonna reffer you on The Comprehensive Textbook of Clinical Biomechanics  by J. Richards (2018) or Muscles Alive, Their Functions Revealed by Electromyograph by Basmajian)

5.1. Surface EMG

Surface EMG signals can be detected with electrodes of various shapes and sizes positioned on the skin.

Surface EMG signals can be detected with electrodes of various shapes and sizes positioned on the skin above the muscle of interest. The monopolar arrangement is the most basic configuration. It requires one electrode to be placed over the muscle that detects the surface EMG signal with respect to a second reference electrode. This arrangement is sometimes used in clinical EMG biofeedback devices and is subject to a variety of contaminations from ambient noise and other sources. The bipolar or single differential arrangement uses two electrodes, generally placed 1 to 2 cm apart along the orientation of the muscle fibres, to measure the EMG signal with respect to a third reference electrode. Surface electrodes can be either passive or active. In the passive configuration, the electrode consists of a detection surface that senses the current on the skin through the skin—electrode interface. The signal is then transmitted to the amplifier for conditioning. In the active configuration, the amplifier stage is at the sensor site, decreasing the path length between detection area and signal conditioning. This characteristic greatly increases the input impedance of the electrodes, reduc-ing noise and artefacts and ensuring high-quality signal recording. Electrodes are either of solid reusable construction with pure silver bar electrodes. or disposable silver/silver chloride (Ag/AgC1) electrodes. The shape and size of the electrodes and their interelectrode distance determine the area of muscle tissue being examined and, therefore, the number of motor units that may be detected within this area.
. The greater the area and the inter-electrode distance, the greater the electrode pick-up area and the number of detected motor units. However, if the electrodes are too large or if they are placed too far apart, they are more susceptible to detecting signals from surrounding muscles in addition to the target muscle. Interference from electrical signals originated by nearby muscles are referred to as crosstalk signals and can mislead the identification of the activity of the target muscle.

Surface EMG electrodes are exclusively used to pick electrical signals from superficial muscles located close to the skin. Signals from deeper muscles can not be recorded reliably. Even for superficial muscles, a thick layer of fatty tissue between the electrode and the muscle acts as an insulator and can affect both the magnitude and frequency response of the EMG signal recorded. In these cases, there are no options other than using intramuscular EMG recording techniques. Grids of surface electrodes are used when the spatial properties, rather than temporal properties, of a muscle are under investigation.

5.2. Wireless EMG systems

The benefit of a fully wireless device is that the subject is free to move with little or no restriction. The wireless EMG system transmission needs to be reliable to avoid loss of packets of data and to minimize interference from radio frequencies, Bluetooth, microwaves, etc. The wired systems require a physical connection, usually in the form of a cable that con-nects each EMG sensor to a separate amplifier unit.

The drawbacks of the wired systems are that they restrict the subjects movements and that the cables i need to be well shielded to avoid introduction of electrical noise in the recording. EMG signals are low voltage and need to be amplified, with values for the amplification typically between 1000 and 10 000 times. Amplifiers are often positioned close to or as part of the electrodes, in the case of active electrodes, reducing the length of the connecting wire that can pick up and introduce electrical interference. Most EMG systems are capable of measuring EMG signals at sampling frequencies of 1000 Hz or higher. This is the minimum sampling frequency required to correctly sample the EMG signal and analyse its fre-quency content, which is negligible above 450 Hz. For intramuscular EMG recordings, which have higher frequency content, a sampling rate of 2000 Hz is recommended. When individual motor unit action potentials are being investigated, up to 10 000 Hz or 20 000 Hz (10 kHz or 20 kHz) is generally needed.

For biomechanical applications, or any application involving dynamic contractions, the bandwidth provided by a surface EMG system should ideally be between 20 and 450 Hz. Surface EMG signals are known to have no or little energy at frequencies greater than 450 Hz. At frequencies lower than 20 Hz, the disturbances from noise and motion artefacts are prevalent and complicate the interpretation of the EMG data. This low-frequency cut-off is particularly relevant for biomechanists who work with dynamic con- be tractions where movement artefacts contribute energy components below the 20 Hz range. For more detailed te] information on this point refer to De Luca and col. (2011). The bandwidth should be greater for intramuscular EMG signals, which present considerable energy up to 1000Hz.

5.3. Factors affecting the quality of the EMG signal

There are somw factors, which need to be metioned when talking about EMG signal. Following factors may affects the signal and may do the signal interpretaion more difficult.

1) Signal-to-noise ratio (SNR) is the best generally  accepted measure of the quality of the EMG signal. It indicates the ratio of the wanted EMG signal verus unwanted signal (baseline noise, line noise, movement artefacts).

The amplitude of the wanted EMG signal depends on the contraction strength, but also on the location of the sensor on the muslce of interest, its orientaion with respect to muslce fibres, and the sensor characteristics, such as the size of the electrodes and the inter-electorde distance.

It means that one of the factors is the

2) Position of electrodes

The location of the sensor on the muscle is the single most important factor for obtaining good SNR. This advice was first provided by Basmajian and De Luca in 1985 in their book Muscles Alive and has been confirmed by numerous studies since. The muscle, fat and skin tissues present an internal impedance to the propagation of electric currents and act as a low-pass filter on the detected EMG signal. The filtering characteristics of these tissues are a function of the distance between the active muscle fibres and the detection surfaces of the electrodes. Thus, the location of the sensor on the muscle renders dramatically different surface EMG signal characteristics. For instance, locating the sensor in the proximity of the tendon origin or the innervation zones yields lower amplitude signals. The fibres in the middle of the muscle belly generally have a greater diameter than those at the edges of the muscle or near the origin of the tendons. Because the amplitude of action potential from the muscle fibres is proportional to the diameter of the fibre, the amplitude of the EMG signal will be greater in the middle of the muscle.

For example of differneces of electrode position see picture Electrode spacing

There are more factors which affects EMG siganls: As interelectrode distance, Skin-electrode interface, Motion artefact, Physiological Noise, Power Line and Electrical Noise, Crosstalk...

For more infromation about these factors I will reffer you to Comprehensive Textbook of Clinical Biomechanics by J. Richards (pp218-222)

5.4. How to obtain good-quailty sEMG signal?

Sensor technology: use active differential sensors of highquality technology. Ensure that there is no instrumentation or electrical connection introducing line interference in the recording.

EMG sensor: use sensors with small (1 cm or less), and possibly fixed, inter-electrode spacing. 

Reference electrode: ensure the reference elec-trode, if separate from the EMG sensor, is con-nected to the instrumentation and to the skin of the subject

Sampling rate: ensure that the surface EMG signal is sampled at least at 1000 Hz.

Amplifier gain: a gain of 1000 is usually appro-priate for surface EMG signals. Ensure that no clipping is present in the signal, i.e. that the signal amplitude does not exceed the maximum value allowed by the recording system (satura-tion). If this occurs, check that the EMG sensor and reference electrode are adequately attached and connected. If possible, reduce the gain or reposition the EMG sensor to reduce the signal level.

Skin preparation: remove excessive hair; clean the skin with alcohol to improve electrical contact with the sensor; if necessary peel off the dead skin (i.e. with a hypo-allergic adhesive tape).

Sensor placement: place the EMG sensor in the in the middle of the muscle belly, away from known innervation zones and tendon origins, with the electrodes aligned parallel to the muscles fibres. Position the sensor as far as possible from sources of physiological noise, ensuring that both electrodes are equidistant from the source in single differential sensors.

6. EMG amplitude interpetation

The relation between the amplitude of the EMG signal and the force exerted by the muscle has caused many debates in the literature. It is known that the amplitude of the EMG signal increases as the contraction strength increases, but how? See chaptres below.

6.1. EMG amplitude, Isometric Muscle Force and Joint Moment

EMG signal amplitude and muslce force production  is best studied during isometric contractions, that is, during contractions where the joint angle is maintained fixed so that the length of the muscle does not change. Isometric contractions are better suited to relate EMG amplitude and muscle force because some confounding factors are minimized, such as the changes in the EMG amplitude as the relative position between the sensors and the muscle fibres changes during anisometric contractions. A study by Lawrence and De Luca (1983) investigated the relation between EMG and force for different muscles (biceps, deltoid and first dorsal interosseous), rate of force production, and training level of the subjects (pianists, long-distance swimmers, power lifters and normal subjects). Only the muscle type influenced the relation between EMG signal and muscle force, with the smaller muscle (first dorsal interosseous) showing an almost linear relation with the force, that was instead non-linear for bigger muscles.

The difference among muscles is due to several factors that affect the relation between EMG and force: the motor unit recruitment and firing rate properties; the relative amount and location of slow-twitch and fast-twitch fibres with respect to the recording electrodes, the crosstalk contamination from other muscles; the muscle viscoelastic properties; and the force contributions from agonist and antagonist muscles. This last factor is particularly important because a single muscle may not be solely responsible for producing a moment around a joint. Additionally, there is often a high degree of variation between subjects, not only between different muscles in the same subject but also in how the surface EMG signal translates to muscle force.

For anisometric (dynamic) contractions, the relation between EMG amplitude and muscle force is not linear. When the muscle contracts, the length of the muscle shortens, the joint angle decreases, and the moment arm of the muscle (the distance from the muscle to the centre of rotation of the joint) increases. If we apply a constant force, as the moment arm increases, the force produced by the lengthening muscle must decrease, resulting in a non-linear EMG—force relation. In addition, two other factors contribute to the non-linearity between the force output and the EMG signal amplitude. The first factor is the change in the relative position of the source of the EMG signal and the sensor which remains attached to the skin as the muscle moves below the skin. The second factor is the non-linear relationship between the force produced by the muscle and the varying length of the contracting muscle fibres.

6.2. EMG Amplitude During Concentric and Eccentric (Anisometric) Muscle Contractions

Muscle length remains more or less constant during isometric contractions, whereas it shortens during concentric contractions, and it lengthens during eccentric contractions. Concentric contractions are generally weaker and require greater muscle activation than isometric and eccentric contractions for a particular load. ( If a surface EMG signal is recorded from the biceps i brachii muscle when flexing (concentric) and extending (eccentric) the arm at the elbow while holding a fixed load, the raw and EMG patterns in picture Con vs ECC EMG bellow are generated. The highest muscle activity is bserved during the concentric phase and the lowest are during the eccentric movement.

 It should be noted that the difference in EMG amplitude during dynamic eccentric and. concentric contractions may be affected by several factors, including changes in the length of the muslce fibres. THe relative movment between the surface sensors on the skin and the shifting muscles fibres underneath laso cause changes in the amplitude of the EMG. Thus, the use if MEG amplitude as in indication of muscle activity during anisometric concractions souhld be conisedered with caution.