Statistical validation of wavelet transform coherence method to assess the transfer of calf muscle activation to blood pressure during quiet standing

Background: Continuous and discrete wavelet transforms have been established as valid tools to analyze non-stationary and transient signals over Fourier domain methods. Additionally, Fourier transform based coherence methods provide aggregate results but do not provide insights into the changes in coherent behavior over time, hence limiting their utility. Methods: Statistical validation of the wavelet transform coherence (WTC) was conducted with simulated data sets. Time frequency maps of signal coherence between calf muscle electromyography (EMG) and blood pressure (BP) were obtained by WTC to provide further insight into their interdependent time-varying behavior via the skeletal muscle pump during quiet stance. Data were collected from healthy young males (n = 5, 19-28 years) during a quiet stance on a balance platform. Waveforms for EMG and BP were acquired and processed for further analysis. Results: Low values of bias and standard deviation (<0.1) were observed and the use of both simulated and real data demonstrated that the WTC method was able to identify time points of significant coherence (>Threshold) and objectively detect existence of interdependent activity between the calf muscle EMG and blood pressure. Conclusions: The WTC method effectively identified the presence of linear coupling between the EMG and BP signals during quiet standing. Future studies with more human data are needed to establish the exact characteristics of the identified relationship.