A morphological analysis of the macro motor unit potential

Abstract

The technique of macro EMG is used to investigate the motor unit architecture in a number of pathological conditions. Amplitude and area are the most commonly used criteria, but these parameters alone are not sufficient to assess the complexity of the macro MUP morphology. In an attempt to examine the morphology of the macro MUP in more detail, additional measures were investigated including, (i) average power, (ii) duration, and (iii) number of phases. Macro MUP duration was defined as the time parameter that contains a particular fraction (90%) of the total power of the potential. The above mentioned parameters were evaluated for normal subjects and for patients suffering with motor neuron disease (MND), spinal muscular atrophy (SMA), and Becker's muscular dystrophy (BMD). It is shown that high amplitude and average power macro MUPs give shorter macro MUP duration than macro MUPs with normal amplitude. In contrast, in low amplitude macro MUPs there is a tendency towards a higher duration measure, as compared with the duration of the normal amplitude macro MUPs. Also, t–test results for the duration measure gave a significant difference between the NOR–MND, and no significant difference between the NOR–BMD and NOR–SMA groups at P<0.05. Significant difference between the NOR and the three disease groups investigated was obtained for the parameters log amplitude, log area, and log average power. The number of phases was not significantly different between the NOR and the rest of the groups. In conclusion, the average power and duration parameters can possibly be used as additional discriminators to detect abnormalities of the macro motor unit potential in both needle and surface EMG but further investigation is necessary.

Introduction

The introduction of the macro EMG technique in neurophysiology [1], [2] has provided additional information to the assessment of patients with neuromuscular diseases [3], [4], [5], [6], [7]. The information gained with macro EMG findings, related to motor unit size, helps to evaluate with a greater certainty the observations made by conventional concentric needle EMG (CNEMG) and single fibre EMG (SFEMG). Equally, knowledge of SFEMG findings such as the fibre density (FD) may help the interpretation of the macro EMG results [8]. Furthermore, the significance of surface detected macro MUP's has recently been demonstrated as a useful source of motor unit size information [9], where motor unit number estimation [10] has been related to the level of muscle activity [11]. According to Stashuk [12], the combination of macro EMG with CNEMG findings and/or firing pattern information may provide new clinical insights.

Amplitude and area are the most commonly used criteria to assess the macro MUP waveform. However, these parameters alone are not sufficient to assess the complexity of the macro MUP morphology. The use of signal representation basis vectors to reduce the dimensionality of the macro MUP was applied successfully by Nandedkar et al. [13]. However, this method is computationally demanding and the parameters estimated are rather difficult to interprete by the physician. In the present study, in an attempt to examine the morphology of the macro MUP in more detail, additional parameters that can easily be computed were introduced including, (i) average power, (ii) duration, and (iii) number of phases. Macro MUP duration was defined as the time measure that contains a particular fraction (90%) of the total power of the potential.