Four healthy subjects (26±4 years 3 males, 1 female) were tested in both a Back Analysis System, for the production of a sustained static isometric contraction, and a LIDO-Lift® Controller (Loredan), for repetitive lifting and lowering of a weighted box. Our purpose was to use this new procedure to identify (a) whether changes in the instantaneous median frequency among concurrently active paraspinal muscles during repetitive trunk extension produces a `fatigue pattern' that is indicative of normal functioning, and (b) whether this pattern is different when the subject produces a sustained isometric trunk extension. The technique is then used to process real EMG signals from paraspinal muscles during repetitive lifting. Spectral compression is measured by defining the instantaneous median frequency from time-frequency representations of the signal derived from a transformation of the Cohen class.
The basic properties of the method are first developed using simulated EMG signals. A new approach to estimating the frequency compression of the surface EMG signal during cyclical dynamic exercise is described.
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