The properties of electromyogram and force in experimental and computer simulations of isometric muscle contraction: Data from an acute cat preparation

The electromyogram (EMG) is an aggregate representation of the individual action potential (AP) contributions from motor units (MUs) close to the recording electrode, which reflects, in some form, the level of peripheral motor activation and force produced during muscle contraction. Although EMG has been studied extensively during voluntary isometric contraction, the effect of various strategies of recruitment and rate modulation of a MU pool on the relationship between EMG and force has not been resolved. In addition, it is not well understood how the properties (i.e. shape) of individual motor unit action potentials (MUAPs) change with increasing activation rates or how individual MUAPs interact to generate the EMG signal.;As an alternative to conventional experimental approaches (i.e. involving voluntary contraction), an acute animal design was used. This design was based on multi-channel (maximum of 10) independent activation of either single motor units (MUs) comprising a subset, or groups of MUs (multi-MUs) comprising essentially the entire pool of homogenous slow twitch MUs to the cat soleus muscle.;Investigations into EMG signal characteristics revealed that APs (both single- and multi-MUAPs) remained constant with increasing single-channel stimulation rates, as well as, with increasing levels of multi-channel background activation. In addition, no quantitative differences were found between recorded multi-channel EMGs, and EMGs synthesized from the constituent AP trains recorded in single channel stimulation episodes. These findings suggest that individual AP contributions summate algebraically to produce the composite EMG. Further, algebraic summation of APs resulted in signal cancellation due to the overlap of positive and negative phase components. The incidence of AP overlap and subsequent signal cancellation increased as MUs were recruited and as the mean stimulation rate was increased.;Experimental simulations featured the independent activation of either 10 multi-MUs or 40 single MUs. Since a maximum of 10 channels could be activated experimentally, 40 MU simulations were obtained by algebraically combining EMG and force recordings from 4 separate 10 MU constellations. Activation strategies were employed encompassing the spectrum from pure recruitment to pure rate modulation, with emphasis on physiologically plausible combined recruitment and rate modulation designs. Significant differences were observed in relations between strategies featuring large differences in the range of ensemble activation rates (EARs) where recruitment occurred. Differences between strategies were almost exclusively due to alterations in force with increasing EARs. In contrast, the magnitude of EMG was remarkably similar with different forms of combined activation. Only comparisons involving the pure rate modulation strategy were significant differences obtained.;750 experimentally recorded MUAPs were used in a computer simulation of EMG to assess the impact of increasing motor pool size and altering AP properties on signal magnitude. Increasing motor pool size from 1 to 170 resulted in increasing cancellation of AP contributions. Moreover, 170 MUs simulations produced around 81% of the EMG was lost to cancellation at the highest EAR. (Abstract shortened by UMI.).