**1. Introduction**

[25] Stålberg E & Trontelj JV(1982). Abnormal discharges generated within the motor unit as observed with single-fiber electromyography. In: *Abnormal nerves and muscles as impulse generators,* Culp, W. J. & Ochoa, J. Oxford University Press, New York,

[26] Thesleff, S. (1982a). Fibrillation in denervated mammalian muscle. In: *Abnormal nerves and muscles as impulse generators,* Culp, W. J. & Ochoa, J. Oxford University

[27] Thesleff, S. (1982b). Spontaneous transmitter release in experimental neuromuscular

[28] Trillenberg, P, & Spencer, A. (2010). How precisely can the regularity of spontaneous

[29] Usui, S, & Admiror, I. (1982). Digital low-pass differentiation for biological signal

[30] Wall, P. D, Waxman, S, & Basbaum, A. I. (1974). Ongoing activity in peripheral

[31] Wiederholt, W. C. (1970). End-plate noise" in electromyography. *Neurology,* , 20,

[32] Willison, R. G. (1982). Spontaneous discharges in motor nerve fibers. In: *Abnormal nerves and muscles as impulse generators,* Culp, W. J. & Ochoa, J. Oxford University

activity be recognized acoustically? *Clin Neurophysiol*, , 121, 1969-1971.

443-474.

214-224.

Press, New York, 678-694.

56 Electrodiagnosis in New Frontiers of Clinical Research

Press, New York, 383-392.

disorders of the rat. *Muscle & Nerve,* , 5, S12-S16.

processing. *IEEE Trans Biomed Eng,* , 29, 686-693.

nerve: injury discharge. *Exp Neurol,* , 45, 576-589.

Advances in the study of human spinal cord neurophysiology have been strongly based on the analysis of the electrical activity of muscles (electromyogram - EMG). The EMG measured over the skin reflects the general behavior of motor units (MUs) and hence of spinal moto‐ neurons (MNs). It can be used, for instance, to infer changes in the behavior of neuronal circuits within the spinal cord during the performance of a motor task or in response to peripheral and/or descending inputs.

In the beginning of the 20th century, Paul Hoffmann introduced a non-invasive technique – the H-reflex – that helped to pave the way for subsequent investigations into the mechanisms of stretch reflex regulation [1]. The neuronal organization of the spinal cord is now better understood thanks to studies of reflex modulation in response to different conditionings and motor contexts, e.g., electrical or mechanical stimulation of sensory afferent pathways, magnetic or electric activation of descending tracts (DTs), passive movement of limbs and joints, voluntary isometric contractions and performance of motor tasks.

Reflexes play a fundamental functional role in motor control, as they are involved in the coordination of voluntary movements and maintenance of postural stability. This justifies the high contingent of fibers from peripheral (cutaneous, muscle and joint afferents), segmental (propriospinal interneurons), and supra-segmental (descending tracts) origins that synapse on different spinal cord elements (synaptic terminals, interneurons and MNs). This also highlights the important integrative function of the spinal cord, contrasting with the naive

© 2013 Mezzarane et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Mezzarane et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

notion that it is only a relay station, or a pathway that simply transfers information from the brain to the muscle fibers.

traction, surface electrodes can also record activity of superficial MUs [9] as can be seen in the upper trace in Figure 1. For increased levels of voluntary contraction additional MUs are recruited (see middle trace in Figure 1). For high-intensity contraction the EMG recording tends to show a filled random pattern due to the superposition of a greater number of MU action potentials (MUAPs) known as interference pattern (bottom trace in Figure 1). Thus, the interference pattern of the EMG is associated with the asynchronous firing of different MUs. When the conditioning of an interference pattern EMG is used to infer spinal cord processes, the experimental control of the level of activity is crucial. When the strength of descending commands changes, different populations of MNs and interneurons (INs) are recruited,

Experimental and Simulated EMG Responses in the Study of the Human Spinal Cord

http://dx.doi.org/10.5772/54870

59

**Figure 1.** Surface EMG recordings from soleus (SO) muscle during a weak contraction (unpublished data). Upper pan‐ el: EMG recording during a very weak contraction in which only one MU is recruited. Middle panel: when the subject was told to slightly increase the voluntary contraction, the MU previously recruited increased its firing rate (see the green arrows) and other MUs with different firing rates were recruited (red and blue arrows). The letter "S" indicates a

Some technical aspects need to be considered for an accurate recording of the EMG signal. Here we are going to briefly discuss filtering, sampling rate and electrode positioning.

The spectral composition of a signal has implications on the choice of the band-pass filter cutoff frequencies used before the analog-to-digital conversion as well as for the selection of a suitable

sum of at least two distinct MUAPs. Lower panel: Interference pattern of EMG.

**2.2. Acquisition**

leading to different conditioned EMG responses.

Despite the relative limitations of non-invasive techniques employed in humans, it is currently possible to establish a parallel between the findings from animal preparations (such as cat) and experiments in humans (e.g., [2, 3]). In addition to the use of animal models as aids for understanding human data, another source of information comes from new multi-scale computer simulators of neuronal circuitry and muscle control [4, 5]. Moreover, with the development of these simulators, supported by anatomical and biophysical data from animal experiments, it is also possible to reinforce hypotheses formulated to explain experimental results obtained from humans (e.g., [5]).

The aim of the present chapter is to provide some conceptual and methodological background for researchers and clinicians who intend to use EMG to study human spinal cord neuro‐ physiology. Here we will discuss different methods frequently used in the study of human neurophysiology based on surface EMG. These will be illustrated by results from both experimental studies and simulations performed in a multi-scale model of the spinal cord and leg muscles. Additionally, a brief account will be given of some processing techniques of surface EMG that are used to quantify spinal cord excitability and effects of inhibitory pathways. The methods explored in the chapter have been used in both healthy subjects and patients with a variety of neuromuscular disorders.
