MOTOR UNITS WITHIN THE NORMAL RAT MEDIAL GASTROCNEMIUS

The contractile properties, fatiguability and axonal conduction veloci ty were determined for 11 8 motor units in the medial gastrocnemius of eighteen rats. Fast-twitch and slow-twitch units could be categorized on the basis of the 'sag' test. For the purposes of statistical compa rison the fast-twitch units were classified on the basis of their fati guability as fast-fatiguable (FF), fast intermediate (FI) and fast fat igue-resistant (FR). As such, FR units tended to have a longer isometr ic twitch time course than other fast-twitch units. On the basis of pe ak tetanic force FF units were largest (mean 341 +/- 120 mN) followed by FI (145 +/- 85 mN), FR (87.3 +/- 38) and slow units (40.4 +/- 11.0 mN). There were no differences in motor axonal conduction velocity. Al though units were categorized, it is clear that for all characteristic s investigated the fast-twitch units exist as a continuum. Some of the largest FF units (peak force 350-500 mN) failed to maintain force wit h high-frequency stimulation (300 Hz) and this was associated with a f ailure of the EMG signal.