Time-frequency parameters of the surface myoelectric signal for assessing muscle fatigue during cyclic dynamic contractions

The time-dependent shift in the spectral content of the surface myoelectric signal to lower frequencies has proven to be a useful tool for assessing l ocalized muscle fatigue. Unfortunately, the technique has been restricted t o constant-force, isometric contractions because of limitations in the proc essing methods used to obtain spectral estimates. A novel approach is propo sed for calculating spectral parameters from the surface myoelectric signal during cyclic dynamic contractions. The procedure was developed using Cohe n class time-frequency transforms to define the instantaneous median and me an frequency during cyclic dynamic contractions. Changes in muscle length, force, and electrode position contribute to the nonstationarity of the surf ace myoelectric signal. These factors, unrelated to localized fatigue, can be constrained and isolated for cyclic dynamic contractions, where they are assumed to be constant for identical phases of each cycle, Estimation erro rs for the instantaneous median and mean frequency are calculated from synt hesized signals. It is shown that the instantaneous median frequency is aff ected by an error slightly lower than that related to the instantaneous mea n frequency. In addition, we present a sample application to surface myoele ctric signals recorded from the first dorsal interosseous muscle during rep etitive abduction/adduction of the index finger against resistance. Results indicate that the variability of the instantaneous median frequency is rel ated to the repeatability of the biomechanics of the exercise.