Recording from a Single Motor Unit During Strong Effort

Abstract

Abstract-During strong voluntary effort it is rarely possible to identify the action potentials from single motor units. In large muscles the most selective recordings are obtained with bipolar wire electrodes. To elucidate this experimental finding we have calculated the extracellular field around a single muscle fiber from an intracellular muscle action potential. This model showed that the selectivity of a bipolar electrode is high provided: i) the diameter of the recording surfaces is less than half the diameter of the muscle fibers;ii) the center distance between the recording surfaces is of the same order or smaller than the diameter of the muscle fibers, and when iii) the center-line between the recording surfaces is oriented perpendicular to the direction of the muscle fibers.A bipolar electrode with these properties will give a maximal attenuation of the field from distant muscle fibers and only two to nine fibers close to the electrode contribute to the recording.Action potentials were recorded from the anterior tibial muscle of normal subjects with a trifilar cut end electrode and a bifilar side hole electrode. The recording surfaces of the side hole electrode can be positioned close to active muscle fibers by gently pulling the electrode at either side of the muscle.The recording surfaces should not be smaller than 25 Jim. Otherwise the impedances of the recording surfaces are so large that the capacitance between wire and muscle tissue degrades the selectivity of the electrode. Improving selectivity by high-pass filtering was avoided. Narrowing the frequency band made action potentials from different motor units appear similar in shape, thereby making identification more difficult. The amplitudes and the power spectra of action potentials recorded with these electrodes were in accordance with the model and no more than one to three motor units were present up to 60% of maximal effort.

Incomplete Functional Subdivision of the Human Multitendoned Finger Muscle Flexor Digitorum Profundus: An Electromyographic Study

“…Nevertheless, here we used bipolar electrodes with small, closely spaced tips to minimize the effective recording radius. Modeling studies indicate that such electrodes typically record activity within only a few hundred micrometers (Andreassen and Rosenfalck 1978). Electrodes identical to those used here recorded reproducible and categorically distinct patterns of EMG activity in 2 compartments of the smaller macaque FDP (Schieber 1993).…”

“…Although our bipolar electrodes were designed to record relatively selectively from muscle fibers within a radius of only a few hundred micrometers of the electrode tips (Andreassen and Rosenfalck 1978), we frequently recorded EMG activity through a given electrode during flexion of 2 or more contiguous digits (see RESULTS). Such recordings from FDP commonly have been assumed to indicate placement of the electrode near the border between 2 compartments, so that MUs from one compartment are recorded during flexion of one finger and MUs from an adjacent compartment during flexion of the other finger.…”

Characterization of motor units in behaving adult mice shows a wide primary range

“…The area of implantation on the hindlimb was shaved, and both the fine-wire gross and single-motor unit recording electrodes were percutaneously implanted in LG. The single-motor unit electrode was inserted such that the line connecting the two recording surfaces would be oriented perpendicular to the direction of the muscle fibers (Andreassen and Rosenfalck 1978). The gross electrode was inserted prior to the single-motor unit electrode in order to minimize possible damage to the finer, more fragile single-motor unit electrode.…”

Electrical conductivity of skeletal muscle tissue: Experimental results from different musclesin vivo

“…For this reason the interesting frequency range is associated with the frequency components of EMG signals. This range runs from 10 Hz to about 20 kHz (GATH and STALBERG, 1975;ANDREASSEN and ROSENFALCK, 1978;GRIEP et al, 1982). Our experiments with respect to the determination of electrical conductivities were carried out in a frequency range between 10 Hz and 100 kHz.…”