EVALUATION OF NEUROMUSCULAR FATIGUE THROUGH INNOVATIVE SURFACE EMG PARAMETERS IN HUMANS

The surface electromyogram (EMG) undergoes several changes during an isometric fatiguing muscle contraction. Amplitude and spectral parameters, as well as muscle fiber conduction velocity and fractal dimension of the surface EMG signal, may be used as indirect fatigability indices to monitor these changes. The aims of this thesis were to determine whether surface EMG is a reliable tool for estimating conduction velocity, through a systematic review of the literature; to determine the relationship between muscle force and the selected fatigability indices; and to study the behavior of the fatigability indices in patients with facioscapulohumeral muscular dystrophy (FSHD) during a fatiguing task, with respect to healthy controls. It was hypothesized that the fractal dimension was not related to the intensity of muscle contraction, and that FSHD patients would have shown significant differences in the considered fatigability indices. In the first study (p. 32) high reliability was reported in eight studies and was, in general, associated with using the initial or mean conduction velocity value, using several electrodes (3 to 8), ensuring appropriate electrode positioning, and evaluating muscles with fibers that run parallel to the skin. It was next demonstrated in the second study (p. 52), that the values of fractal dimension and mean frequency of the power spectrum increased with force unless a plateau was reached at 30% maximal voluntary contraction. Finally, the third study (p. 61) showed that FSHD patients presented significantly less steeper slopes of mean frequency of the power spectrum, conduction velocity and fractal dimension, compared to the controls. The results of this thesis demonstrated firstly, that sEMG is suitable for use when investigating conduction velocity, which is sensible to peripheral aspects affecting performance fatigability; secondly, the use of fractal dimension, as index of central factors affecting performance fatigability, may be considered above a certain level of force, regardless of muscle contraction intensity; and lastly, that impaired neuromuscular function caused patients with FSHD to exert a smaller force, yield a longer endurance time and experience lower levels of performance fatigability compared to healthy participants. In conclusion, the use of the fractal dimension of the sEMG signal to infer central aspects of performance fatigability should be promoted; in particular, in those muscles were motor unit decomposition techniques are limited by anatomical constraints.