Cross-spectral analysis of physiological tremor and muscle activity. II.   Application to synchronized EMG
  We investigate the relationship between synchronized muscle activity and tremor time series in (enhanced) physiological tremor by cross-spectral analysis. Special attention is directed to the phase spectrum and its possibilities to clarify the contribution of reflex mechanisms to physiological tremor. The phase spectra are investigated under the assumptions that the EMG synchronization was caused by a reflex, respectively a central oscillator. In comparison of these results to phase spectra of measured data we found a significant contribution of reflexes. But reflexes only modify existing peaks in the power spectrum. The main agents of physiological tremor are an efferent pace and the resonant behavior of the biomechanical system. 
J. Timmer, M. Lauk, W. Pfleger, G. Deuschl (Center for Data Analysis   and Modeling, University of Freiburg, Germany)