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2 1996 Elsevier Science B.V. All rights reserved. Recent Advances in Clinical Neurophysiology. J. Kimura and H. Shibasaki, editors. 271 Far-field potentials in surface EMG Dick F. Stegeman1*, Karin Roeleveld1*, Daniel Dumitru2 and Dick M. Vingerhoets1 institute of Neurology, Department of Clinical Neurophysiology, University Hospital Nijmegen, The a Netherlands; and Department of Rehabilitation Medicine, The University of Texas, Health Sciences Center at San Antonio, San Antonio, Texas, USA Abstract. A short summary of the mechanism behind Far-field potential generation of propagating electrophysiological activity is given. The nonmoving waveforms caused by the blocking of motor unit action potentials at the tendon are presented as an example. Key words: far-field, nonmoving potentials, propagation block, surface electromyography. Introduction There is a general familiarity of neurophysiologists with potential waveforms which can be interpreted on the basis of a local current distribution caused by nearby neurophysiological sources: the near-field. A near-field potential waveform has clear changes in amplitude, polarity, wave shape and/or in latency when the position of the active electrode is changed over a small distance. Conversely, in a far-field the signal characteristics are not influenced by a changing electrode position. Since the lack of changing latencies with electrode position is a striking characteristic of far-field components, the term nonmoving potentials may often be more appropriate. The obvious background of such lack of waveform changes is the location of the electrode at a large distance from the bioelectrical source. Far-fields gained much interest with the observations of short latency far-field components in SEP experiments by Cracco and Cracco [1]. Bioelectric activity from the nervous system, measurable over distances of the order of the dimensions of the body, did not and still does not fit into the intuitive perception of such signals. Many papers on the physiological and physical origin of far-field potentials have been published (e.g., [2 7]). A dipole in a finite volume conductor The potential field of a dipole has a well known spatial distribution. A potential change, as presented in Fig. 1A, is predicted by calculation. The horizontal axis of Address for correspondence: Dick F. Stegeman, Institute of Neurology, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands. *Stegeman and Roeleveld are members of the Institute of Fundamental and Clinical Human Movement Sciences of the Vrije Universiteit Amsterdam and the University of Nijmegen.

3 272 Fig. 1. Potential profiles in an infinite (A) and in a finite (B) (cylindrical) volume conductor caused by a dipole source in the direction of the horizontal axes of the curves, which is also the axis of the cylinder for part B. The axes denote distance (horizontal) and voltage. this figure denotes distance, the vertical axis voltage. For this calculation, the volume conductor is assumed to be infinitely extended in all directions. It is clear that the potential decreases monotonously with distance. Therefore, this combination of a dipolar source and an infinite volume conductor cannot produce a far-field potential distribution. In Fig. IB, a potential profile is calculated for a finite volume conductor; in this case a long cylinder with a dipole source at its axis. From comparing Fig. 1A with Fig. IB one can conclude that the finiteness of the human body as a volume conductor is essential. The curves are much alike, except for the tails between which a constant potential difference appears. This difference does not further decrease with distance to the left or to the right. When measuring with two electrodes across the source these tails are the cause of a far-field component. Far-fields and propagating bioelectric activity The source of a nerve or muscle action potential is travelling along the fibre. Theoretical studies have shown that propagating action potentials effectively have no dipole component. Such sources can be described with two equal dipoles with opposite polarity (schematically: + +, often denoted as a tripole). A calculated extracellular potential profile of such a source in conducting tissue is presented in Fig. 2. The volume conductor in this result is a finite cylinder again, as for Fig. IB. No far-field component is observed for Fig. 2. The far-field contributions of both dipoles in a tripole cancel each other. This is in accordance with the daily practice in clinical neurophysiology: a propagating impulse of a group of nerve or muscle fibres cannot be electrically observed beyond a distance of a few millimeters to a few centimeters at most. Now the problem has been turned into the question: if this is so, why then are far-fields present in the peripheral neuromuscular system where all bioelectric activity is of a travelling nature? The answer to this question was given in the last decade. When the constant propagation is disturbed or when the extracellular environment of an action potential is inhomogeneous in some way, the

4 273 + Fig. 2. The extracellular potential profile of a travelling action potential (tripole source) in a finite volume conductor. balance between the two dipolar sources in a tripole is disturbed and a net dipolar source is generated at that site. In the case of the early SEP components, various dipoles are generated at sites of anatomical changes and/or changes in the extracellular tissue around the stimulated nerve [4,7]. Motor unit action potentials in the surface EMG A less often mentioned, but also very illustrative, example of a far-field is found in proximal 100uV 50 ms distal Fig. 5. Measurement of the spatio-temporal potential profile along the skin surface of a motor unit of the biceps brachii muscle. The 15 electrodes are located in parallel to the main direction of the fibres of the muscle (electrode spacing 6 mm). The ipsilateral elbow is the site of reference. The profile is the average

5 274 muscle [8 11]. When unipolar measurements are made along the fibre direction of a muscle, single motor unit action potentials (MUAPs) can be isolated from the surface EMG after triggered-averaging of the signal. An example of such a registration with 15 electrodes in a row along the fibre direction of human biceps brachii muscle is given in Fig. 3 [12]. The waveforms are clearly composed of two main components. The propagating negative (= upward) part in two directions reflects the travelling muscle fibre impulses along the sarcolemma [13]. The positive nonpropagating components (see arrow) clearly represent a nonmoving potential field with far-field properties in the upper nine traces and near-field characteristics in the lowest six traces. This underlines the suggestion in the intrqduction that a distinction between moving and nonmoving components often better separates different field distributions than far-field vs. near-field. Compare the profile of Fig. IB of which the left half could represent a snapshot at the moment of the arrow. The dipolar source causing the nonmoving part is evoked at the muscle-tendon transition when the travelling tripolar source loses its leading and its trailing dipole components in subsequent order. So, the nonmoving components in the surface MUAPs are now, theoretically, well embedded, which is an achievement since the early studies of these components by Kosarov and Gydikov [10]. Far-fields sometimes raise associations of magic among clinical neurophysiologists. It is more and more clear, however, that dipole fields are presenting themselves at numerous places in the peripheral neuromuscular system, each of which, depending on the volume conductor environment, can be the source of a far-field. Acknowledgements Part of this work has been supported by the Netherlands Organization of Scientific Research (NWO). References 1. Cracco RQ, Cracco JB. Somatosensory evoked potential in man: far-field potentials. Electroenceph Clin Neurophysiol 1976;41: Cunningham K, Halliday AM, Jones SJ. Simulation of stationary SAP and SEP phenomena by twodimensional potential field modelling. Electroenceph Clin Neurophysiol 1986;65: Dumitru D, Jewett DL. Far-field potentials. Muscle Nerve 1993;16: Dumitru D, King JC. Far-field potential production by quadrupole generators in cylindrical volume conductors. Electroenceph Clin Neurophysiol 1993;88: Kimura J, Yamada T. Physiologic mechanisms underlying the generation of far-field potentials. Electroenceph Clin Neurophysiol 1990;41(Suppl): Sohmer H. Action potentials recorded with evoked potential techniques: modes and sites of generation. J Basic Clin Physiol Pharmacol 1991;2: Stegeman DF, van Oosterom A, Colon EJ. Far-field evoked potential components induced by a propagating generator: computational evidence. Electroenceph Clin Neurophysiol 1987;67: Dumitru D, King JC. Far-field potentials in muscle. Muscle Nerve 1991;14: Gootzen THJM, Stegeman DF, van Oosterom A. Finite limb dimensions and finite muscle length in a model for the generation of electromyographic signals. Electroenceph Clin Neurophysiol

6 1992;81: Kosarov D, Gydikov A. The influence of various factors on the shape of the myopotentials in using monopolar electrodes. Electromyogr Clin Neurophysiol 1973;13: Lateva ZC, Dimitrova NA, Dimitrov GV. Effect of recording electrode position along a muscle fibre on surface potential power spectrum. J Electromyogr Kinesiol 1993;3: Roeleveld K, Stegeman DF, Falck B, Stalberg EV. Estimation of motor unit location and size from surface electromyography. Electroenceph Clin Neurophysiol 1995;97:S Masuda T, Sadoyama T. Distribution of innervation zones in the human biceps brachii. J Electromyogr Kinesiol 1991; 1: