Differences in the profile of unfused tetani of fast motor units with respect to their resistance to fatigue in the rat medial gastrocnemius muscle.

In most studies performed on motor units in mammalian muscles the division of these units into fast and slow types has been based on the 'sag' visible in the profile of unfused tetanus. The time course of the sag in unfused tetani of fast motor units was analysed in the present study. Fast units of rat medial gastrocnemius muscle were classified as fast fatigable (FF) or fast resistant to fatigue (FR) on the basis of a fatigue index calculated during the standard fatigue test. In middle-fused tetani (fusion index 0.25-0.75), it was observed that for FF motor units the sag was shorter and occurred earlier than for FR units. Moreover, in FF units, the sag was followed by potentiating tension, whereas for FR units this potentiation was weaker or even absent. A tetanus shape index, which expressed the ratio of the area of the first part of the tetanus record (between the tension record and the baseline, from the beginning of tetanus up to the lowest point during the sag in the tension record) to the area under the second part of tetanus (from this lowest point up to the end of the record) was introduced. For FF units, this index ranged from 0.13 to 0.47, whereas for FR units it ranged from 0.54 to 17.8 (with one exception). These results showed that the difference in unfused tetanus expressed in this tetanus shape index could be used as an accurate alternative method of dividing fast units into FF and FR groups. Moreover, the difference in sag time course in FF and FR groups. Moreover, the difference in sag time course in FF and FR units suggests that the metabolism responsible for this contractile phenomenon is significantly different time courses in IIa and IIb muscle fibres, constituting FF and FR units, respectively.