Intramuscular potential changes caused by the presence of the recording EMG needle electrode.

A comprehensive volume conductor study of single fiber needle EMG (SF EMG) has been made. The electrode shaft is described as a passive inhomogeneity in the volume conductor. The most important observation is a substantial increase in single fiber action potential (SFAP) amplitude (up to 70%) for muscle fibers observed from a short distance. For SFAPs from muscle fibers located on the back of the SF electrode a shadow effect occurs which can result in a maximal amplitude decrease of 50%. SFAP wave form changes were observed only in situations without practical consequences or beyond physical reality. The presence of the needle shaft causes an anisotropy-like behavior of the relation between leading-off point to muscle fiber distance. The observed amplification due to the presence of the needle cannula decreases faster in the direction parallel to the cannula than in the direction normal to it: due to the amplification of SFAPs from muscle fibers observed from a short distance, the maximal distance from which SFAPs are included in fiber density measurements (amplitude greater than 0.2 mV) is raised from 380 microns to 460 microns. Also, the consequences of the formation of an edematous layer around the needle cannula have been studied. It was shown that the effect of high conductive fluid around the needle electrode can counteract the effects caused by the presence of the electrical double layer.

Muscle electric activity. II: On the feasibility of model-based estimation of experimental conditions in electromyography.

From regular one-channel registrations of single muscle fiber action potential no measures on the recording configuration can be derived. When multichannel recordings are made, experimental parameters such as the distance between muscle fiber and needle electrode can be estimated. With the help of a volume conductor model, the single fiber activity at each of the electrodes can be predicted as a function of the recording conditions. Within known physical and physiological constraints such a model approach can be inverted (the inverse model) and used to estimate basic experimental conditions. From eight simultaneous single fiber action potential recordings we estimated (a) the distance between fiber and needle, (b) the axial position of the needle with respect to the muscle fiber, (c) a factor related to the muscle tissue anisotropy, and (d) a factor combining the muscle fiber diameter and the effective muscle tissue conductivity. With the help of a model describing the influence of the needle shaft it is made plausible that the needle inhomogeneity influences the results of the proposed procedure.

Muscle electric activity. I: A model study on the effect of needle electrodes on single fiber action potentials.

Needle recorded electromyographic signals can be expected to be influenced by the presence of the needle, the electrical double layer at the metal-electrolyte interface, and by an edematous layer around the needle electrode. The magnitude of each of these effects is derived from a cylinder symmetrical volume conductor model. Analytical solutions of Laplace's equation have been derived. These are used for simulating single muscle fiber action potentials (SFAPs) recorded by a typical single fiber electrode. The results indicate that there is no short-circuiting effect, in spite of the presence of a highly conducting needle shaft, which is due to the high impedance of the electrical double layer. The insulating properties of the double layer cause the SFAP amplitudes to increase, when the muscle fiber passes the electrode at the side of the leading-off point. The edematous layer counteracts this increase depending on the thickness and the conductivity of this layer. Only slight SFAP wave-form changes are found.

A study of motor unit structure by means of scanning EMG.

We used the scanning EMG technique to investigate the structure of human quadriceps muscle motor units. A group of healthy volunteers and 2 groups of patients with proven neurogenic or myogenic neuromuscular pathology have been studied. In total, 86 scans were obtained. An estimate of the motor unit territory (S) separates the 2 patient groups: the majority of territories of myogenic patients are smaller than 4 mm, whereas almost all motor unit territories of neurogenic patients are larger. However, the sizes of the pathologic units only occasionally exceed the upper and lower limits of normal units (2 to 8 mm). The myogenic scans show, on average, a much larger temporal dispersion (T) between the MUAPs within a motor unit than the normal scans. These findings are in accordance with observations with the same technique by others. For the neuropathic scans, there is a significant positive correlation between S and T, which is completely absent in myopathic scans. These observations are confronted with current morphometrical knowledge on motor unit physiology and anatomy.

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.

The equivalent source description representing the extinction of an action potential at a muscle fiber ending.

Extracellular potential waveforms resulting from the termination of the propagation of the transmembrane potential at the end of an active muscle fiber are studied. The transmembrane potential along the muscle fiber is calculated from Hodgkin-Huxley-like equations with axially varying parameters under the condition of increasing axial resistance, decreasing ionic conductances, or decreasing radius at the muscle fiber end. The extracellular potential distribution is calculated using the derived transmembrane current as a line source in an unbounded volume conductor. Potential waveforms with constant latency at extracellular points far from the fiber end are presented and are compared with measurements and simulations from literature. Monophasic waveforms with constant latency are found. The polarity and the amplitude as a function of axial and radial position relative to the fiber end indicate that an equivalent dipole source adequately represents the propagation stop at the fiber end. An expression for the dipole moment in terms of fiber radius, intracellular conductivity, and transmembrane potential is derived.