Doublets Initiating Low Frequency Stimulation Improve Dynamic Contractility Without Exacerbating Fatigue

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PURPOSE: Doublets, two action potentials (AP) with a very short inter-impulse interval, frequently occur initially in impulse trains from motor nerves during muscle activation. Such doublets improve dynamic contractile parameters such as rate of force development (RFD), velocity and power. In addition, previous studies have shown delayed fatigue development from repeated isometric contractions when a doublet initiates a sub-tetanic train compared with constant frequency stimulation. However, these comparisons were not matched for impulse number, train duration and work, which may have confounded the interpretations. Furthermore, it is unknown how doublets would affect endurance during dynamic contractions. We, therefore, compared the development of fatigue from repeated shortening contractions activated with doublet-initiated sub-tetanic train (DT) or constant-frequency sub-tetanic train (CFT) using protocols matched for work, AP and train duration. METHODS: EDL muscles were isolated from 4 week old rats and incubated at 30°C in Krebs Ringer buffer. To induce fatigue, 100 shortening contractions were elicited either by a doublet-initiated sub-tetanic train (DT; 60 Hz) or a constant-frequency sub-tetanic train (CFT; 71 Hz). Contractions in the two groups were matched for work, train duration and pulse number. Isometric contractility was tested before and immediately after fatigue. RESULTS: Work of each contraction was similar in DT- and CFT-stimulated muscles both initially (368 ± 10 vs 369 ± 13 μJ, respectively) and after 100 fatiguing contractions (202 ± 9 vs 193 ± 8 μJ, respectively). However the contraction force and shortening developed faster in the DT-fatigued than the CFT-fatigued muscles: Initial values (RFD 20 ± 0.7 vs 13 ± 0.3 N/s and shortening velocity 48 ± 2 vs 41 ± 1); fatigued values (RFD 16 ± 0.7 vs 10 ± 0.4 N/s, velocity 29 ± 2 vs 23 ±1 mm/s ). Isometric tetanic force was equally affected in both groups; DT-fatigued muscles decreased to 73 ± 2 % and CFT-fatigued muscles to 75 ± 1 % of initial force. CONCLUSION: The results indicate that during repeated contractions matched for total work, stimulation duration and pulse number, an initiation by a doublet does not exacerbate fatigue development, but provides an activation strategy for muscles to produce a higher RFD and shortening velocity.