Motor units within the normal rat medial gastrocnemius

Abstract

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. © 1993 The Physiological Society