A simulation study on the depth information of motor units

Abstract

Purpose The depth information of muscle motor units is important in clinical applications. The purpose of this paper is to investigate relationship between motor unit action potentials from surface electromyography and motor unit (MU) depth. Methods Firstly, the skin structure was depicted, and the action potential was considered as a constant current source, which was generated by a tripole model. Secondly, the potential generated by the tripole on the surface was given. Finally, the relationships between MU depth and MU action potential (MUAP) parameters on the skin were studied. Results The regression analysis showed a low relative error for the simulated signals. When the position of the detected electrodes was 18 mm, the linear regression analysis yielded 2.47% in the case of arc, and 2.48% in the case of side. The relationships between the peak to peak amplitude of MUAP and the lengths (arc and side) were tested by the nonlinear regression analysis. The sum of the squared errors was less than 0.0202, and the coefficient of determination was more than 0.9939. Conclusions These results show that it is possible to estimate MU depth by action potentials on surface electrodes.

Alterations of Motor Unit Characteristics Associated With Muscle Fatigue

“…FWHM is essentially a continuous variation of amplitude value of MUAP at the electrode. Although MU depth and MU size are related to MUAP, MU size has much less effect on FWHM compared to MU depth, which has a high correlation with FWHM [23][24][38][39]. As such, FWHM has been used in MU depth estimation in previous studies [40][41].…”

“…Since the MUAP initiates from the neuromuscular junction (indicated as IZ) and propagates in two opposite directions toward the fiber endings, the surface location of MU IZ can be visually identified from the bipolar MUAP mapping by observing either the reversal of signal polarity or minimum amplitude [21,22]. In addition, according to previous studies [23,24], the depth of MU recruited at a low contraction level can be estimated according to the full width half maximum (FWHM), defined as the muscle-fiber-transverse-direction distance over the skin surface where the absolute value of the MUAP negative peak is higher than 50% of the maximal absolute value. Therefore, by combining the surface MU IZ location with the identified depth information, the same MU recruited at different contraction levels can be identified.…”

“…(3) MU depth was estimated by FWHM [23,24]. As shown in Figure 1E, take the waveform on the electrodes in the column (perpendicular to the direction of the muscle fibers) where the position of the MU IZ is located, and estimate the MU depth; (4) Confirm the same MUs extracted before/after fatigue at the same IZ location and depth;…”