Design of a myo-seismic transducer for non-invasive transcutaneous vectorial recording of locally fast muscle-fibre micro-contractions

Abstract

Mechanical recording usually concerns the analysis of movements in bio-mechanical research projects. Mechanical recording of locally fast muscle-fibre micro-contractions, however, is a little-developed and rarely-applied myographic technique. In the last decade, acoustic or myophonic measurements came increasingly into the picture when they were also applied to research on general muscle activity, such as in muscle fatigue studies. In this paper, a new micro-seismic recording technique is introduced. The technique registers extremely local activity in the velocity and force vector of skin movement as a function in time. The recording method is sensitive to micro excursions caused by muscle fibres under the skin. The resolution in time is at least 100 us, which is demonstrated in an experiment where a mechanical contraction is provoked by electrical stimulation of the median nerve. This indicates a seismic variant, referred to as seismic-myography (SMG), of surface EMG's, and offers complementary features. The most important features are: 1. Insensitivity to low frequent, large movement artefacts. 2. Sensitivity to fast mechanical micro-excursions and velocities. 3. Fast and precise discrimination of local mechanical events. 4. Vectorial reconstruction of superficial mechanic activity which can be used for the identification and functional behaviour of subcutaneous muscle fibres and, in addition, for the localisation of motor endplate zones. 5. The method is easy to use. The following, some clinical and physiological applications are discussed: a mechanical variant for the diagnosis of carpal tunnel syndrome (CTS) and measurements of nerve conduction velocities, precise timing of the process chain between electrical fibre membrane potential, and the first contraction of actine and myosine filaments. The function of the Ca++ depots is embedded in this chain. This might open new possibilities for research on muscle disease and muscle fatigue.