Neuromuscular blockade of slow twitch muscle fibres elevates muscle oxygen uptake and energy turnover during submaximal exercise in humans

<jats:p>We tested the hypothesis that a greater activation of fast‐twitch (FT) fibres during dynamic exercise leads to a higher muscle oxygen uptake (<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/TJP_3184_mu1.gif" xlink:title="inline image"/>) and energy turnover as well as a slower muscle <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/TJP_3184_mu2.gif" xlink:title="inline image"/> on‐kinetics. Subjects performed one‐legged knee‐extensor exercise for 10 min at an intensity of 30 W without (CON) and with (CUR) arterial injections of the non‐depolarizing neuromuscular blocking agent cisatracurium. In CUR, creatine phosphate (CP) was unaltered in slow twitch (ST) fibres and decreased (<jats:italic>P</jats:italic> < 0.05) by 28% in FT fibres, whereas in CON, CP decreased (<jats:italic>P</jats:italic> < 0.05) by 33% and 23% in ST and FT fibres, respectively. From 127 s of exercise, muscle <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/TJP_3184_mu3.gif" xlink:title="inline image"/> was higher (<jats:italic>P</jats:italic> < 0.05) in CUR compared to CON (425 ± 25 (±<jats:sc>s.e.m.</jats:sc>) <jats:italic>versus</jats:italic> 332 ± 30 ml min<jats:sup>−1</jats:sup>) and remained higher (<jats:italic>P</jats:italic> < 0.05) throughout exercise. Using monoexponential fitting, the time constant of the exercise‐induced muscle <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/TJP_3184_mu4.gif" xlink:title="inline image"/> response was slower (<jats:italic>P</jats:italic> < 0.05) in CUR than in CON (55 ± 6 <jats:italic>versus</jats:italic> 33 ± 5 s). During CUR and CON, muscle homogenate CP was lowered (<jats:italic>P</jats:italic> < 0.05) by 32 and 35%, respectively, and also muscle lactate production was similar in CUR and CON (37.8 ± 4.1 <jats:italic>versus</jats:italic> 35.2 ± 6.2 mmol). Estimated total muscle ATP turnover was 19% higher (<jats:italic>P</jats:italic> < 0.05) in CUR than in CON (1196 ± 90 <jats:italic>versus</jats:italic> 1011 ± 59 mmol) and true mechanical efficiency was lower (<jats:italic>P</jats:italic> < 0.05) in CUR than in CON (26.2 ± 2.0 <jats:italic>versus</jats:italic> 30.9 ± 1.5%). In conclusion, the present findings provide evidence that FT fibres are less efficient than ST fibres <jats:italic>in vivo</jats:italic> at a contraction frequency of 1 Hz, and that the muscle <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/TJP_3184_mu5.gif" xlink:title="inline image"/> kinetics is slowed by FT fibre activation.</jats:p>