Muscle surface mechanical and electrical activities in myotonic dystrophy

The dimensional changes of the muscle fibres of the active motor units generate a signal, labelled as mechanomyogram (MMG), related to the number and firing rate of the active fibres. Aim of the study was to evaluate if the root mean square (RMS) of the surface EMG, of the MMG and their relationship (electromechanical coupling efficiency, EMCE) are related to the clinical stage of patients affected by myotonic dystrophy. The EMG and the MMG were recorded during isometric contractions at 20%, 40% and 60% of the maximal voluntary contraction (MVC), in less (elbow flexors, EF) and more (finger flexors, FF) affected muscles in 10 myotonic dystrophy (DM) patients (duration of disease: 15-22 yr.; MDRS scale: 3-4) and in 10 age matched controls (C). It resulted that I) the MVC was only 30% lower in DM than in C (P > 0.05) for the EF but 74% lower (P < 0.001) for the FF; II) at the same relative force the MMG-RMS and the EMG-RMS in DM were significantly lower than in C (P < 0.05); III) the EMCE was almost the same in DM and C for EF but dramatically lower for FF. In conclusion these findings provide evidence that the EMCE changes due to the dystrophic process can be monitored by this new and non invasive technique. A good concurrence of clinical findings and experimental results was observed.