Matched wavelet approach in stretching analysis of electrically evoked surface EMG signal

During a sustained muscular contraction, variations in the myoelectric signal parameters can be noticed, before a force decrease is observed. This phenomenon is known as localized muscular fatigue, and always preceedes the true physical tiredness. In particular, for the electrically elicited contraction, the signal variations are essentially due to two different phenomena: a time stretching, that can be interpreted as the occurrence of a scaling factor, due to either an enlargement of the depolarized area or a decrease of the conduction velocity; a shape variation, which takes into account all those phenomena that are not due to a time stretching, and could be associated to a change in the recruited muscular fibers activity (for example, a different distribution of the conduction velocity). Both of these effects are of potential clinical interest, as they are related to different physical phenomena. In this paper, we analyze electromyography signals with the objective of quantifying the muscular fatigue by means of proper parameters. We look for indicators which are simple, correct, reliable, easy to interpret, able to separately quantify the time stretching and the morphological asymmetry of the signal during contraction.