The effect of temperature and stimulation scheme on fatigue and recovery in Xenopus muscle fibres.

The influence of temperature and alternations of the stimulation scheme on fatigue development and recovery has been studied in single toe muscle fibres of Xenopus. Fatigue was in all cases produced by intermittent tetanic stimulation. In the temperature experiments easily fatigued (type 1) and fatigue-resistant (type 2) fibres were fatigued in successive series at 10.0, 15.0 and 22.5 degrees C. Lowering the temperature did not markedly influence the time-course of fatigue development in either of the fibre types. At 22.5 degrees C these fibres usually display post-contractile depression (PCD), a delayed force suppression, during the recovery period. At the lower temperatures PCD was not observed in type 1 fibres and it was delayed in type 2 fibres. Only type 1 fibres were studied in the altered stimulation scheme experiments. Neither the time-course of fatigue development nor the recovery process was markedly influenced by an alteration of tetanic stimulus frequency in the range of 40-80 Hz. Increasing the time-tension area produced before the standard fatigue level (40% of the original force) was reached, by increasing the initial interval between tetani, caused a more pronounced PCD. From these results it can be concluded that fatigue development and recovery are complex processes which cannot be readily explained by a single mechanism.