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

• Published in: The Journal of physiology 2008-12, Vol.586 (24), p.6037-6048
• Main Authors: Krustrup, Peter, Secher, Niels H., Relu, Mihai U., Hellsten, Ylva, Söderlund, Karin, Bangsbo, Jens
• Format: Article
• Language: English
• Subjects: Adenosine Triphosphate - metabolism; Adult; Atracurium - administration & dosage; Atracurium - analogs & derivatives; Atracurium - pharmacology; Energy Metabolism - drug effects; Exercise - physiology; Fysiologi; Fysiologi och farmakologi; Glycogen - metabolism; Humans; Injections, Intra-Arterial; Lactic Acid - metabolism; Male; MEDICIN; MEDICINE; Muscle Fibers, Fast-Twitch - drug effects; Muscle Fibers, Fast-Twitch - metabolism; Muscle Fibers, Fast-Twitch - physiology; Muscle Fibers, Slow-Twitch - drug effects; Muscle Fibers, Slow-Twitch - metabolism; Muscle Fibers, Slow-Twitch - physiology; Neuromuscular Nondepolarizing Agents - administration & dosage; Neuromuscular Nondepolarizing Agents - pharmacology; Oxygen - metabolism; Phosphocreatine - metabolism; Physiology; Physiology and pharmacology; Quadriceps Muscle - blood supply; Quadriceps Muscle - drug effects; Quadriceps Muscle - physiology; Regional Blood Flow - drug effects; Regional Blood Flow - physiology; Skeletal Muscle and Exercise; Young Adult
• ISSN: 0022-3751; 1469-7793; 1469-7793
• DOI: 10.1113/jphysiol.2008.158162

We tested the hypothesis that a greater activation of fast-twitch (FT) fibres during dynamic exercise leads to a higher muscle oxygen uptake ( ) and energy turnover as well as a slower muscle 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 ( P < 0.05) by 28% in FT fibres, whereas in CON, CP decreased ( P < 0.05) by 33% and 23% in ST and FT fibres, respectively. From 127 s of exercise, muscle was higher ( P < 0.05) in CUR compared to CON (425 ± 25 (± s.e.m. ) versus 332 ± 30 ml min −1 ) and remained higher ( P < 0.05) throughout exercise. Using monoexponential fitting, the time constant of the exercise-induced muscle response was slower ( P < 0.05) in CUR than in CON (55 ± 6 versus 33 ± 5 s). During CUR and CON, muscle homogenate CP was lowered ( P < 0.05) by 32 and 35%, respectively, and also muscle lactate production was similar in CUR and CON (37.8 ± 4.1 versus 35.2 ± 6.2 mmol). Estimated total muscle ATP turnover was 19% higher ( P < 0.05) in CUR than in CON (1196 ± 90 versus 1011 ± 59 mmol) and true mechanical efficiency was lower ( P < 0.05) in CUR than in CON (26.2 ± 2.0 versus 30.9 ± 1.5%). In conclusion, the present findings provide evidence that FT fibres are less efficient than ST fibres in vivo at a contraction frequency of 1 Hz, and that the muscle kinetics is slowed by FT fibre activation.