Analysis of the surface myoelectric signal by means of a cross-time-frequency based technique

This paper presents a cross-time-frequency (XTF) based algorithm for the estimation of the instantaneous frequency (IF) of nonstationary stochastic processes and discusses its application to the analysis of the surface myoelectric signal (SMES) recorded during dynamic muscle contractions. This original algorithm derives from that previously proposed by Boashash and O'Shea [1] and further investigated by Ristic and Boashash [6], which was originally proposed for the analysis of deterministic signals. We assessed the estimation error of the modified algorithm by means of a synthesized process with statistical properties similar to those of SMES. Bias and standard deviation of the IF estimates were obtained for signal to noise ratio (SNR) values ranging from 5 dB to 100 dB and for different frequency contents of the SMES. The algorithm was found satisfactory for research as well as for clinical applications. A sample application is shown where we utilize this method for the estimation of the IF of real SMES recorded from the extensors and flexors during repetitive isokinetic knee flexion-extension movements.