The Accuracy of Determining Onset of Muscle Activity from the Surface Electromyogram

Abstract

Amplitude cancellation is a characteristic of the surface EMG that reduces the estimated amplitude due to the overlap of opposing phases of motor unit potentials. Presumably, this attenuation occurs at the onset of a muscle contraction as well, and likely leads to errors in detecting the onset of motor unit activity. PURPOSE: To quantify the influence of amplitude cancellation on the accuracy of detecting the onset of motor unit activity in the rectified interference EMG. METHODS: A generic lower-limb muscle with 350 motor units in 20 random locations was simulated using the Fuglevand model (Fuglevand et al J Neurophysiol 70: 2470–2488, 1993). The force profile for the Achilles tendon during the stance phase of walking was used to validate the simulated net muscle force. The surface EMG signal was simulated by summing the motor unit potentials before (cancellation EMG) and after (no-cancellation EMG) they had been rectified. The two EMGs were compared across 20 trials at four levels of excitation: 10, 20, 40, and 100% of maximum. The influence of amplitude cancellation was examined by comparing the mean amplitude of the cancellation and no-cancellation EMGs. The experiment was repeated with median filtering (window sizes of 10, 20, 50, 80, and 100 ms, incremented by 1 ms) and low-pass filtering (cutoff frequency of 13, 20, 40, 60, and 100 Hz) applied to the cancellation EMG signal. Onset time was determined by visual discrimination as well as statistically as the last sample of the current window when the window average exceeded 3 SDs above the baseline mean. RESULTS: The no-cancellation EMG exhibited a greater rate of increase at the onset of motor unit activity for all four levels of excitation. Onset time differed by up to 12 ms between the cancellation and no-cancellation EMGs and was greatest for window sizes above 50 ms, and for low-pass filter cutoff frequencies below 60 Hz. For example, the comparisons between the cancellation and no-cancellation EMG at 40% excitation indicated that the error in detecting the onset of the contraction from the cancellation EMG was 8–26 ms, and that from the no-cancellation EMG was 7–17 ms. CONCLUSION: Amplitude cancellation did not influence the accuracy of visually detecting the onset time of a contraction in the surface EMG. However, errors were introduced when the EMG signal was processed with median and low-pass filtering techniques; the errors were most pronounced for larger window sizes (median filtering) and lower cutoff frequencies (low-pass filtering). Supported by NINDS NS043275.