Mechanomyography Signals in Spastic Muscle and the Correlation with the Modified Ashworth Scale

The modified Ashworth scale (MAS) is the most widely used measurement technique to assess levels of spasticity. In MAS, the evaluator graduates spasticity considering his/her subjective analysis of the muscular endurance during passive stretching. Therefore, it is a subjective scale. Mechanomyography (MMG) allows registering the vibrations generated by muscle contraction and stretching events that propagate through the tissue until the surface of the skin. With this in mind, this study aimed to investigate possible correlations between MMG signal and muscle spasticity levels determined by MAS. We evaluated 34 limbs considered spastic by MAS, including upper and lower limbs of 22 individuals of both sexes. Simultaneously, the MMG signals of the spastic muscle group (agonists) were acquired. The features investigated involved, in the time domain, the median energy (MMG(ME)) of the MMG Z-axis (perpendicular to the muscle fibers) and, in the frequency domain, the median frequency (MMGMF). The Kruskal-Wallis test (p<0.001) determined that there were significant differences between intergroup MAS spasticity levels for MMGME. There was a high linear correlation between the MMGME and MAS (R-2=0.9557) and also a high correlation as indicated by Spearman test (rho=0.9856; p<0.001). In spectral analysis, the Kruskal-Wallis test (p=0.0059) showed that MMG(MF) did not present significant differences between MAS spasticity levels. There was moderate linear correlation between MAS and MMGMF (R-2 = 0.4883 and Spearman test [rho = 0.4590; p < 0.001]). Between the two investigated features, we conclude that the median energy is the most viable feature to evaluate spasticity due to strong correlations with the MAS.