COMPUTATIONAL METHODS FOR IMPROVING ESTIMATES OF MOTOR UNIT TWITCH CONTRACTION PROPERTIES

Estimates of mechanical properties of human motor units have usually b een made indirectly, using the technique of ''spike-triggered averagin g'' (STA), In this method, a single motor unit action potential is use d to synchronize the accumulation of an ensemble average of correlated force transients. However, under most realizable conditions, these tr ansients are recorded during periods of sustained motor unit discharge , in which each motor unit is producing a partially fused tetanus. The refore, the STA technique extracts the characteristics of the unfused force transient, instead of the desired single motor unit twitch, Alth ough the STA method has been widely used, there is as yet no well-esta blished relation between the force transient in the unfused tetanus, a nd the twitch contraction properties of the motor unit, To evaluate th e accuracy of the STA as a measure of the motor unit mechanical proper ties, we applied two types of muscle models to the force transients re corded in an unfused tetanus, using data derived from experiments in w hich the response to a single twitch was also recorded. Our objective was to see whether accurate predictions of single motor unit mechanica l characteristics are possible, working backward from the STA. The mod els chosen for this task were a linear second order model, and the dis tribution-moment (DM) model, These model predictions were then compare d with the STA response, and with the twitch properties of the individ ual motor units, We also evaluated the utility of extrapolating the in itial slope of the STA backward to improve the accuracy of the mechani cal estimates. The results of our simulation suggest that there is no straightforward relation between the characteristics of the unfused te tanus and the mechanical properties of the single twitch. Although our attempts to predict the properties of the single twitch from the STA were only partly successful, the results of the simulations were fat-m ore accurate than those derived from the STA alone, Because the errors in the use of the STA method were so substantial, we would urge that the STA technique be used with great caution as a measure of twitch co ntraction properties, unless accompanied by appropriate simulations of muscle mechanical behavior. (C) 1995 John Wiley and Sons, Inc,