**2. EMG signal fundamentals**

EMG is the recording of the electrical activity produced within the muscle fibers. The relation of surface EMG to torque makes EMG an attractive alternative to direct muscle tension measurements, necessary in many physical assessments. However, the complexity of the EMG signal origin has been a barrier for developing a quantitative description of this relation. The EMG signal origin and character is necessary background to understand the difficulty of establishing a relationship between surface EMG and torque.

The nervous system controls the voluntary movement of various body parts in humans by contracting and relaxing various skeletal muscles. To instantiate a contraction, a neuron generates a small electrical potential on the surface of the muscle fiber. This electrical potential causes depolarization of the muscle fiber tissue and a following depolarization waveform. This waveform travels the length of the muscle fiber and is known as the Action Potential (AP). Fig. 1 depicts the generation of electric fields in muscle fibers.

Muscle fibers are excited by nerve branches by one motoneuron in groups known as motor units. These motor units are defined as the fundamental unit of contraction and can range from a few muscle fibers for small muscles such as those in the hand and fingers, to thousands of muscle fibers in large muscles such as those in skeletal muscle. Because each motor unit contains a number of muscle fibers that are attached to the motor neuron at various points, the electrical signal of a motor unit is the summation of the action potential of each muscle fiber, which may be phase shifted from the other muscle fibers in that unit (Perry & Bekey,1981).

**Figure 1.** Muscle Fibers Composition

This notion is reinforced in Fig. 2. The electrical potential due to contraction of all fibers in a motor unit during a single activation is referred to as the Motor Unit Action Potential (MUAP). This MUAP can be recorded by using electrodes placed on the surface of the skin above the muscle. Also, a muscle is not typically excited by only one action potential. In order to hold a contraction for any length of time, the motor units must be repeatedly activated. This repeated activation gives rise to a series of MUAPs that can be modeled as a pulse train in classical signal processing terms. This series of MUAPs that is produced is referred to as a Motor Unit Action Potential Train (MUAPT). When the electromyography measured using a surface electrode, the electromyography can be defined as the superposition of numerous MUAPTs firing asynchronously. Fig. 3 reinforces the notion that the superposition of motor unit action potentials gives rise to surface EMG. The surface electromyography signal typically does not exceed 5-10 mV in amplitude with the majority of signal information being contained between the frequencies of 15 and 400 Hz. As a result, the amplitude of the EMG contains a great deal of the signal information which can be modeled as a Gaussian random process. The EMG amplitude can thus be defined as the time-varying standard deviation of the EMG signal and is a measure of the activity level of the muscle under observation.

**Figure 2.** MUAP with Phase shifted from the other muscle fibers

366 MATLAB – A Fundamental Tool for Scientific Computing and Engineering Applications – Volume 1

difficulty of establishing a relationship between surface EMG and torque.

Potential (AP). Fig. 1 depicts the generation of electric fields in muscle fibers.

EMG is the recording of the electrical activity produced within the muscle fibers. The relation of surface EMG to torque makes EMG an attractive alternative to direct muscle tension measurements, necessary in many physical assessments. However, the complexity of the EMG signal origin has been a barrier for developing a quantitative description of this relation. The EMG signal origin and character is necessary background to understand the

The nervous system controls the voluntary movement of various body parts in humans by contracting and relaxing various skeletal muscles. To instantiate a contraction, a neuron generates a small electrical potential on the surface of the muscle fiber. This electrical potential causes depolarization of the muscle fiber tissue and a following depolarization waveform. This waveform travels the length of the muscle fiber and is known as the Action

Muscle fibers are excited by nerve branches by one motoneuron in groups known as motor units. These motor units are defined as the fundamental unit of contraction and can range from a few muscle fibers for small muscles such as those in the hand and fingers, to thousands of muscle fibers in large muscles such as those in skeletal muscle. Because each motor unit contains a number of muscle fibers that are attached to the motor neuron at various points, the electrical signal of a motor unit is the summation of the action potential of each muscle fiber, which may be phase shifted from the other muscle fibers in that unit

This notion is reinforced in Fig. 2. The electrical potential due to contraction of all fibers in a motor unit during a single activation is referred to as the Motor Unit Action Potential (MUAP). This MUAP can be recorded by using electrodes placed on the surface of the skin above the muscle. Also, a muscle is not typically excited by only one action potential. In order to hold a contraction for any length of time, the motor units must be repeatedly activated. This repeated activation gives rise to a series of MUAPs that can be modeled as a pulse train in classical signal processing terms. This series of MUAPs that is produced is referred to as a Motor Unit Action Potential Train (MUAPT). When the electromyography

**2. EMG signal fundamentals** 

(Perry & Bekey,1981).

**Figure 1.** Muscle Fibers Composition

**Figure 3.** Superposition of Motor Unit Action Potential Gives Rise to Surface EM
