Abstract-The time-dependent shift in the spectral content of the surface myoelectricsignal to lowerfrequencieshas proven to be a useful tool for assessinglocalized muscle fatigue. Unfortunately, the technique has been restricted to constant-force, isometric con tractions because of limitations in the processingmethods used to obtain spectral estimates. A novel approach is proposed for cal culating spectral parameters from the surface myoelectric signal during cyclicdynamic contractions. The procedure was developed using Cohen class time-frequency transfonns to define the instan taneous median and mean frequency during cyclic dynamic con tractions. Changes in muscle length, force, and electrode position contribute to the nonstationarity of the surface myoelectricsignal.These factors, unrelated to localized fatigue, can be constrained and isolated for cyclic dynamic contractions, where they are as sumed to be constant for identical phases of each cycle. Estima tion errors for the instantaneous median and mean frequency are calculated from synthesized signals. It is shown that the instan taneous median frequency is affected by an error slightly lower than that related to the instantaneous mean frequency. In addi tion, we present a sample application to surface myoelectricsignals recorded from the first dorsal interosseous muscle during repeti tive abduction/adduction of the indexfinger against resistance. Re sults indicate that the variability of the instantaneous median fre quency is related to the repeatability of the biomechanics of the exercise.