Abstract
Electromyography is the study of the waveforms of the electrical activity which always accompanies muscular contraction. In this thesis are considered the effects of certain factors on the electromyogram, such as the physical arrangement of the fibres of which muscle is constituted, their functional grouping and activity, the form and extent of the action potentials arising in the fibres, the laws governing the distribution of current in a conducting medium, and the form and disposition of the detecting electrodes. Following a short historical introduction, a review is given of the literature on anatomical and functional features, of muscle, electrophysiological properties of muscle and nerve fibres, theories of the form in which muscle' and nerve potentials in a conducting medium can be represented, and the commonly observed types of electromyographic waveform. Some general properties of the isopotential, distribution about a dipole in a conducting medium are deduced, and ah analogy between the dipole and the profile of the muscle, fibre action potential, is developed. The dipole model is used to show, the effect of summation of action potentials from a large number of fibres, as would be found in an idealised muscle. A simple experiment with a tank of electrolyte is described to illustrate the main features of potential distributions about small electrodes. Observations are made on the modifying effect of electrodes on recorded action potentials, in accordance with volume conduction phenomena. Some explanations are offered to account for some-well known characteristics of the electromyogram, for example, the preponderance of low frequencies not present when recording potentials, from single fibres or small groups of fibres.