The experimental simulation method was based upon the separate activation of up to 10 small groups of motor units (MU) in an acute nerve-muscle preparation. The investigator was able to precisely control and systematically alter the features of MU pool activation strategies. No implicit assumptions were made regarding MU properties. The purpose of this study was to evaluate the validity of this method. Three criteria were formulated and found to be satisfied: First, in the time domain, visual and audio displays of simulated EMG were indistinguishable from physiological EMG. Secondly, in the frequency domain, power spectra of simulated EMG revealed the typical features of EMG recorded during voluntary activation in the cat. Thirdly, the well-known mono-tonic relationship between EMG magnitude and force was readily reproduced, alüiough strictly linear relations were not found. In addition. the relationship between the pool's ensemble activation rate and EMG magnitude showed distinct gain compression, mostly attributable to signal cancellation.
M. Hulliger is with the Department of Clinical Neurosciences in the Faculty of Medicine at the University of Calgary, Health Sciences Centre, Calgary. Alberta, Canada T2N 4NI. S.J. Day is with the Center for Musculoskeletal Research at the National Institute for Working Life, Umeä. Sweden. A. Guimaraes, W. Herzog, and Y-T. Zhang are with the Human Performance Laboratory in the Faculty of Kinesiology at the University of Calgary.