Force response of rat soleus muscle to variable-frequency train stimulation.

1. The purpose of this study was to study the effects of a high-frequency burst of pulses at the onset of a subtetanic train of pulses on the force output of the rat soleus muscle. 2. The soleus muscle was studied in eight rats deeply anesthetized with urethan. The effects of two-, three-, or four-pulse bursts at the onset of subtetanic trains containing a total of 12 pulses were studied in detail. 3. The results showed that two-pulse bursts at the onset of the train produced approximately 20% augmentation in average force and nearly a 50% reduction in the time required to reach a targeted level of force, compared with a comparable 12-pulse subtetanic constant-frequency train; three- or four- pulse bursts produced progressively less additional improvement. In contrast, the two-pulse bursts produced approximately 13% increase in the force-time integral (Area), the three-pulse burst did not significantly further increase the Area gain, and the use of four-pulse bursts markedly decreased the gain in Area. 4. For all three bursts, the observed force augmentation rapidly declined over the 12-pulse trains. Extrapolation beyond the actual data suggested that the force augmentation should last for between approximately 16 and 19 interpulse intervals. 5. To describe the characteristics of the contractile response of the muscle that explains or predicts the amount of force augmentation seen, we made three measurements of the response to the burst of pulses: 1) the peak force produced by the initial burst of pulses (PeakBURST), 2) the force at the time of arrival of the pulse that followed the burst (CatchBURST), and 3) the rise in force produced by the pulse that followed the burst (PotBURST). Of these three measurements, the CatchBURST was the best predictor of the force augmentation seen. 6. The present results showed 1) the importance of the stimulation pattern on the force output of skeletal muscle; 2) that the force-frequency relationship is multivalued, with force depending on both the stimulation history and stimulation frequency; and 3) that a relatively simple discharge strategy, where each train of pulses begins with one or two brief interpulse interval durations, will produce the maximum force from the muscle and result in a predictable force-frequency relationship.