We tested the hypothesis that a greater activation of fast-twitch (FT) fibres during dynamic exercise leads to a higher muscle oxygen uptake (JOURNAL/jphy/04.02/00005245-200812150-00020/ENTITY_OV0312/v/2017-10-03T050651Z/r/image-pngO2) and energy turnover as well as a slower muscle JOURNAL/jphy/04.02/00005245-200812150-00020/ENTITY_OV0312/v/2017-10-03T050651Z/r/image-pngO2 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 JOURNAL/jphy/04.02/00005245-200812150-00020/ENTITY_OV0312/v/2017-10-03T050651Z/r/image-pngO2 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 JOURNAL/jphy/04.02/00005245-200812150-00020/ENTITY_OV0312/v/2017-10-03T050651Z/r/image-pngO2 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 JOURNAL/jphy/04.02/00005245-200812150-00020/ENTITY_OV0312/v/2017-10-03T050651Z/r/image-pngO2 kinetics is slowed by FT fibre activation.