Synthesising of a motor unit potential based on the sequential firing of muscle fibres

Volume conduction models for surface EMG; confrontation with measurements

Volume conduction models are used to describe and explain recorded motor unit potentials (MUPs). So far it has remained unclear which factors have to be taken into account in a volume conduction model. In the present study, five different models are confronted with measured MUP distributions over the skin surface above the m. biceps brachii generated by MUs at different depths and recorded by small surface electrodes. All model simulations include fibres of finite length. The models differ in the size of the volume conductor (finite/infinite), the number of different layers (1, 2 or 3) and the conductivities of these layers (representing muscle, subcutaneous fat and skin). All measured and simulated MUPs contain a mainly negative propagating wave followed by a positive wave simultaneously present at all electrode positions. The magnitude of the different MUP components relative to each other and as a function of motor unit (MU) and electrode position differ between the models studied and the measurements. All simulated MUPs changed faster with observation distance than the measured MUPs. The three-layer model, in which muscle tissue was surrounded by a subcutaneous fat layer and by a layer of skin resulted in MUPs closest to the measured MUPs.

Finite limb dimensions and finite muscle length in a model for the generation of electromyographic signals

A volume conductor model is presented in which the aspects of finite volume conductor dimensions and finite muscle fiber length are combined. The effects of these aspects on single muscle fiber action potentials (SFAPs) and on motor unit action potentials (MUAPs) are shown and verified with surface recorded motor unit action potentials. It is demonstrated that the influence of the fiber length being finite is enhanced significantly by the finite limb dimensions of the volume conductor model, for single fiber action potentials as well as for motor unit action potentials. The model described is found to be capable of generating surface MUAPs which show a very good resemblance with measured surface MUAPs. Recorded MUAPs illustrate clearly the effects caused by finite muscle fiber length. The effect of finite limb dimensions in simulated intramuscular MUAPs was evidently less dominant than in simulated surface MUAPs.

A multiple-contact EMG recording array for mapping single muscle unit territories

The glycogen-depletion technique has become a well-established method for determining histologically the cross-sectional distribution of a single muscle unit. A major drawback of this method is its low yield of one depleted unit per experiment. Furthermore, this technique is particularly unsuited for determining the longitudinal distribution of single muscle units in long, broad muscles because of the formidable serial sectioning job that would be required. Our alternative, electrophysiological method utilizes a multiple-contact, two-dimensional EMG recording array to map efficiently the cross-sectional and longitudinal distributions of numerous single muscle units in anatomically diverse muscles. Additionally, architectural information on muscle fiber lengths, end-plate locations, motor subunit (MSU) arrangements and muscle conduction velocities can be determined.

Macro EMG

A new EMG recording technique called macro EMG is described. The recording electrode is the cannula of the modified single fiber electromyography (SFEMG) electrode. By means of spike triggered averaging, the contribution from all muscle fibers in a motor unit is extracted. The resulting signal reflects the number and size of muscle fibers in one motor unit. This paper presents the details of the technique, the neurophysiological basis for generation of the macro EMG signal, and the typical findings in normal and diseased muscles. Finally, examples are given of the additional information that can be obtained by combining investigations with SFEMG, conventional EMG, and macro EMG in the same muscles.

Simulation of macro EMG motor unit potentials

Macro EMG motor unit potentials (MUPs) have been simulated. The effects on MUP parameters of number of fibres, fibre diameter, end-plate scatter, nerve branch delay, motor unit territory and electrode position have been studied. The MUP shape is sensitive to electrode position in the vicinity of the end-plates. It is mainly reflecting the end-plate scatter and the nerve delays. Away from the end-plates the wave shape is principally determined by the fibre diameter distribution. Total number of muscle fibres and their mean diameter act as gain factors at all recording positions. In the simulations the area of the signal appears to be a better measure of the strength of the motor unit than the amplitude. Macro EMG offers a much less selective recording than conventional EMG and single fibre EMG and therefore new aspects of the motor unit can be quantified. Preliminary results from simulations of neuromuscular disorders are consistent with actual findings.

Calculation and registration of the same motor unit action potential

In order to increase insight into the electrical phenomena of active motor units, a computer simulation model has been developed. With this model motor unit action potentials (MUAPs) have been calculated. The model has been based on the superposition of the muscle fibre potentials of the fibres of one motor unit. For verification, calculated MUAPs have been compared with the matching recorded MUAPs. During experiments one motor unit was stimulated and the MUAP of this unit was measured with intramuscular wire electrodes. After the experiments the positions of the activated fibres of this unit and of the electrodes were determined by means of histochemical techniques. Other parameters were derived from other experiments or the literature. Using the obtained set of parameters in the model MUAPs were calculated. These MUAPs were compared with the measured MUAPs. From this comparison it has been concluded that the model predicts the MUAP to an appreciable degree. The results clearly show the dominating effect of muscle fibres in close vicinity of the electrode and the important effect of the activation moment of those fibres on the shape of the MUAP.