Abstract
There are contradictory reports regarding changes in motor unit firing rates during steady force contractions. Inconsistencies are likely the result of previous studies disregarding motor unit recruitment thresholds and not examining firing rates on a subject-by-subject basis. It is hypothesized that firing rates are manipulated by twitch force potentiation during contractions. Therefore, this study examined time-related changes in firing rates at steady force in relation to motor unit recruitment threshold in the first dorsal interosseous and the influence of twitch force potentiation on such changes in young versus aged individuals. Subjects performed a 12 second steady force contraction at 50% maximal voluntary contraction with evoked twitches prior to and following the contraction to quantify potentiation. Firing rates, in relation to recruitment thresholds, were determined at the beginning, middle, and end of the steady force. There were no firing rate changes for aged individuals. For the young, firing rates decreased slightly for lower-threshold motor units but increased for higher-threshold motor units. Twitch force potentiation was greater for young than aged subjects, and changes in firing rates were correlated with twitch force potentiation. Thus, individuals with greater increases in firing rates of higher-threshold MUs and decreases in lower-threshold MUs possessed greater twitch force potentiation. Overall, changes in firing rates during brief steady force contractions are dependent on recruitment threshold, and partially explained by twitch force potentiation. Because firing rate changes were measured in relation to recruitment threshold, this study illustrates a more complete view of firing rate changes during steady force contractions.
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