Age-related muscle morphological alterations are associated to changes in the motor units' (MU) control properties, to a decrease in maximal isometric torque and to an higher resistance to fatigue. This study is aimed at investigating how these age-related physiological modifications are reflected by peculiar signs of myoelectric fatigue and/or changes in the deterministic content of the surface electromyographic signal (sEMG). For this purpose the sEMG was analysed in time and frequency domains using as global indexes of myoelectric fatigue the sEMG Root Mean Square (RMS), average conduction velocity (CV) and the Median Frequency (MDF). Concerning the non-linear approach, the recurrence quantification analysis (RQA) was applied, and one of its parameters, the percent of determinism (%DET), was computed. Besides, we also sought to test if the above indicators correlate with the ultimate end-point (endurance time, ET) for the task. Five healthy young (Y) subjects (27+-4) and five healthy elderly (E) subjects (72+-1) participated in the study. After the assessment of right biceps brachii maximal voluntary contraction (MVC), they were asked to perform three endurance test at 80, 50 and 30% MVC, spaced each other by one week. The task consisted in matching a target value displayed on a PC screen for as long as they can and eventually to continue also after the ET point was reached. Biceps brachii sEMG signals were detected using a 16 electrodes linear array. The ET was defined as the interval between the beginning of the steady state phase in force recording and the point at which subjects were unable to maintain the force within a 2.5% of the target for more than 0.25s. Normalized MVC (N/BMI) was greater in Y than in E subjects. Elderly seemed to show greater endurance at low force level (30%MVC) than Y; MDF and CV decays in E group were lower at higher (80%MVC) force level, while %DET increase behaved similarly in the two groups as function of force. Finally, MDF, CV and %DET were all well correlated with ET. These preliminary results lead us to conclude that sEMG reflects neuromuscular changes associated with ageing process, although the overall control scheme is not changed. Interestingly, it seems that the studied parameters are suitable predictors of ET in young and elderly subjects, being the CV the more robust.
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