The amplitude of a surface electromyogram is dependent on the number of active motor units, their size and the relative position of the recording electrode. It is not possible to interpret the surface electromyogram quantitatively without disentangling these different aspects. In this study the decline of different components of the motor unit potential with increasing radial distance from the motor unit is quantified. Fifty-two motor units in the biceps brachii muscle were studied using 36-channel surface electromyography combined with intramuscular scanning electromyography. Scanning electromyography was used to locate precisely the motor unit. The dependence of the surface motor unit potential magnitude on the radial distance between the motor unit and the recording electrodes can be described fairly well by an inverse power function. The steepness of this function depends on the chosen motor unit potential parameter and the interelectrode distance, but also varies between motor units. The change of the negative peak amplitude of the motor unit potential over the skin surface can be used to give a fairly accurate estimate of the location of the motor unit under the skin surface. We found that for all practical purposes the depth of a motor unit in the biceps brachii muscle can be estimated as 20% of the distance over the skin surface where motor unit potentials can be recorded with higher amplitudes than 50% of the maximal amplitude recorded at the skin surface caused by activity of the same motor unit.
The background of the bioelectric activity of muscle recorded from the surface of the skin (surface electromyography) in terms of the representation of single motor units of the underlying muscle(s) is not very weil documented or understood. An insight into the composition of an electromyogram is essential for the proper interpretation of one of the most widely applied eJectrophysiological techniques. In the present paper, a study of the contribution of single motor unit potentials to the surface electromyogram is presented. To this end, the decline of different components of the motor unit potential with depth of the motor unit is quantified. Experimentally, the action potentials from motor units at several positions in the muscle were recorded by 30 skin surface electrodes. Simultaneous use of scanning electromyography provided information about the actual position and size of the motor unit. Observed linear log-log relationships between motor unit potential magnitudes and distance indicated the usefulness of a power function to describe the motor unit potential's dependence on recording distance. It is shown that different specific surface motor unit potential characteristics fall off differently with depth. The magnitude-distance relationship is shown to be dependent on the recording configuration (unipolar vs. bipolar recording, including the inter-electrode distance) and the chosen motor unit potential parameter (negative peak amplitude, positive peak amplitude and area).
Historically relevant hypotheses on the pathophysiology of muscle cramp are reviewed. Psychosomatic, static, vascular, myogenic and neural theories are highlighted from a clinician's point of view. Modern neurophysiological research leaves little doubt that true muscle cramp is caused by explosive hyperactivity of motor nerves. Several mechanisms may be involved including spinal disinhibition, abnormal excitability of motor nerve terminals and spreading of muscle contraction by ephaptic transmission or axon reflexes.
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