Modifica di Electromyography (sezione)

====Movement Artifacts====
Further disturbances in the faithful recording of an sEMG signal can arise from movement artifacts. This is of particular interest in the recording of surface sEMG because it is evident that movement is inherently generated by the muscle beneath the skin where the electrodes are applied. At least two different types of movement artifacts are described. The first and most obvious is the one that results from a variation in the electrode surface facing the skin. This occurs more easily with large and rigid electrodes rather than small and flexible ones, which can better and more quickly adapt to the changing curvature of the skin over the muscle during contraction. The variation in the electrode surface in contact with the skin produces a sudden change in the electrode's electrical capacitance and, consequently, a variation in the electrode's direct current voltage. The second type is caused by the movement of the cables that connect the electrodes to the amplifier. In this case, the artifact is essentially still due to capacitive variations at the amplifier input, which can be minimized fairly easily with proper design of the amplifier input stages or by shielding the cables. These artifacts typically have a spectrum ranging from 0 to 20 Hz, i.e., outside the useful band for sEMG recording, and can therefore be eliminated by appropriate filtering circuits without significantly altering the useful signal.