Indices of muscle fatigue.

Myoelectric signal variables and mechanical variables are known to change during sustained voluntary or electrically elicited contractions. These phenomena reflect changes in the properties of the muscle fiber and its membrane. Such changes are generally called "fatigue." Muscle fiber conduction velocity and myoelectric signal spectral parameters (mean and median frequency) show a linear or curvilinear decrease in time, depending on the level of voluntary or electrically elicited contraction. Amplitude paramaters [average rectified value (ARV) and root mean-square (RMS) value] and force often show a dome-shaped pattern with respect to time. In previous research, these patterns have been fitted with least-square regression curves or lines whose parameters (decrement, time constant, initial slope) have been considered indicators of the amount and rate of muscle property changes and therefore of ongoing fatigue. A new index of fatigue is proposed in this work. The product of a reference value (e.g., the first value of the time series) and the time of observation defines a reference rectangle. The area between the upper side of such rectangle and the experimental data points is divived by the area of the reference rectangle to provide this index. This area ratio index may be computed either as an attribute of a contraction or as a function of time; it is regression-free, it is dimensionless, it varies between 0 and 1 for decreasing patterns, it is negative for increasing patterns, and it is little affected by experimental point fluctuations except for the value that defines the reference rectangle. With respect to other indices, the area ratio index provides a quantitative approach to fatigue that is consistent with the intuitive definition of fatigue. If applied to different myoelectric signal variables, the resulting area ratios may be interpreted as the components of a fatigue vector.