Motor units within the normal rat medial gastrocnemius

The contractile properties, fatiguability and axonal conduction velocity were determined for 118 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 comparison the fast-twitch units were classified on the basis of their fatiguability as fast-fatiguable (FF), fast intermediate (FI) and fast fatigue-resistant (FR). As such, FR units tended to have a longer isometric twitch time course than other fast-twitch units. On the basis of peak 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. Although units were categorized, it is clear that for all characteristics investigated the fast-twitch units exist as a continuum. Some of the largest FF units (peak force 350-500 mN) failed to maintain force with high-frequency stimulation (300 Hz) and this was associated with a failure of the EMG signal.