Doublet Discharges Improve Force During Fatigue in Single Fibers

Abstract

Doublet discharge is the brief high discharge of motor units recorded at the very onset of a voluntary muscle contraction, which increases both force and maximal rate of force development in submaximal contractions. However, whether doublet discharges ameliorate or worsen fatigue‐induced reductions in force have not been investigated.MethodsTwenty‐four mechanically‐dissected intact single fibers from flexor digitorum brevis muscles of C57 mice were injected with indo‐1 to assess free myoplasmic tetanic Ca2+ ([Ca2+]i). Fatigue was induced with 150 brief 60 ms tetani every 300 ms, using either a constant frequency 70 Hz stimulation protocol or a doublet stimulation protocol that added a 200 Hz interpulse interval at the start of 70 Hz constant frequency trains.ResultsDuring the first 25 contractions of the fatigue protocol, doublets produced greater force‐time integral and peak force compared with the constant frequency tetani. There was no significant difference between protocols in the plateau tetanic force loss from contractions 50 up to 150. No significant differences in mean tetanic [Ca2+]i were observed between the two stimulation fatigue protocols.ConclusionWithout requiring greater mean tetanic [Ca2+]i than constant frequency stimulation, our findings show that initial doublets increase tetanic force in early but not late stages of fatigue. Supported by The Swedish Research Council