Slowed convective blood flow delivery speeds the onset of fatigue in highly‐oxidative skeletal muscle

Abstract

It was the purpose of this study to examine the role of slowed blood flow kinetics at the onset of contractions on fatigue characteristics in highly‐oxidative skeletal muscle. A step change in metabolic rate was elicited by stimulating canine gastrocnemius‐plantaris muscles (n=6) via their sciatic nerves (6‐8 V, 0.2 ms duration, 50 Hz, 200 ms train) at a rate of 1/2 s. With arterial O2 concentration maintained constant, in random order trials, blood flow tau was set at a control transition speed between rest and contractions (determined in pilot work) of 20 s (CT20) or was slowed to tau values of 45 s (EX45) and 70 s (EX70). Resting and steady‐state (4 min of contractions) blood flows were not altered between the transition conditions. Peak force was not significantly different (p>0.05) between conditions (mean±SD; CT20=293±64 N; EX45=265±64 N; EX70=256±56 N). Final normalized force was also not significantly different (p>0.05) between conditions (CT20=87±6; EX45=88±7; EX70=88±11 %). Time from peak force to 50% of final force was significantly faster (p=0.0163) for EX45 (76±25 s) in comparison to CT20 (103±12 s). Time to 50% of final force was also significantly faster (p=0.0195) for EX70 (72±19.391 s) vs. CT20 while EX45 and EX70 were not significantly different. To our knowledge, these are the first experiments to alter only the time course of blood flow at contractions onset without changing the baseline and steady‐state flows, and our data suggest that fatigue occurs more rapidly with slowed blood flow only in the transition period at the onset of contractions in highly‐oxidative skeletal muscle.