Effect of number of stimuli and timing of twitch application on variability in interpolated twitch torque

Application of a supramaximal electrical twitch to the voluntarily contract ed muscle is used to assess the level of muscle activation. Large variabili ty in the interpolated twitch torque (ITT) has been observed when repeated stimulations are performed. It is hypothesized that this variability in ITT is caused by the stochastic nature of the timing of twitch application rel ative to pulses of voluntary excitation trains: Two experiments were perfor med on 12 subjects each to test this hypothesis. For the first experiment, a single twitch was superimposed on a train stimulation at different time i ntervals relative to the train pulses. For the second experiment, single, d ouble, triple, or quadruple twitches were applied on a voluntarily contract ed muscle. The ITT critically depended on the time point of twitch applicat ion: a single pulse applied 5 ms before a train pulse consistently evoked h igher ITTs than all other stimulation conditions. Furthermore, variability of the ITT decreased as the number of applied,twitches increased. The resul ts support the hypothesis that a large part of the variability in the ITT m ay be caused by the timing of the superimposed twitch relative to the motor unit trains. The variability may be reduced by increasing the number of su perimposed twitches.