Doublets and low-frequency fatigue in potentiated human muscle.

AIM

To test the hypothesis that doublets compensate for low-frequency fatigue. Doublets increase force output from muscles stimulated at low frequencies. Low-frequency fatigue is a decline in the force elicited by low-frequency stimulation.

METHODS

Human flexor carpi radialis muscles were stimulated with 20 Hz trains with and without an initial doublet and with and without low-frequency fatigue and the resulting force response measured.

RESULTS

An initial doublet caused an increase in the maximum rate of force rise of 179.6 +/- 27.9% in rested and 242.9 +/- 37.7% in muscles with low-frequency fatigue, and a substantial enhancement in force in the first three inter-pulse intervals after the extra pulse. The magnitude and time course of the early doublet enhancement were very similar regardless of low-frequency fatigue, consistent with current theories regarding the mechanisms of the doublet effect and of low-frequency fatigue. By the end of the 1 s stimulus train, force enhancement was insignificant in rested muscles and was small and subject-dependent in muscles with low-frequency fatigue (17.3 +/- 8.1% of force without a doublet), reducing the force deficit by 25.2 +/- 5.5%.

CONCLUSIONS

The time course of doublet force enhancement implies that an initial doublet may effectively compensate for the deficit in rate of force rise in muscles with low-frequency fatigue, but may not compensate for force deficits beyond the first few inter-pulse intervals.