This section completes the picture for the related muscle measurement methods, although these are used in the context of muscles more commonly studied by occupational therapists. EMG signals are affected by muscle fatigue. The frequency spectrum becomes narrower with fatigue, losing parts of the high-frequency content (Herzog et al, 1994). Other '-myographys' behave differently. Acoustic myography (AMG) in which muscle sounds are recorded from the vibrations of contracting muscle applied to quadricep muscles. Vibromyography (VMG) (Matheson et al., 1997) uses an accelerometer to measure lateral oscillations during contraction. The mean power frequency does not change, with fatigue, leading to the claim that it is a measure of intrinsic mechanical activity (Zwarts and Keidel, 1991). Interestingly VMG was found to discriminate better the absolute forces between subjects than EMG. In comparison with EMG, where the median frequency decreased continuously, an abrupt drop in the median frequency of the VMG signal occurred after fatigue. This change in signal was readily observed in the time domain for the raw data leading to identification of the onset of fatigue (Herzog et al. 1994).
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