title: The mechanistic bases of the power-time relationship: muscle metabolic responses and relationships to muscle fibre type.
abstract: The power-asymptote (critical power; CP) of the hyperbolic power-time relationship for high-intensity exercise defines a threshold between steady-state and non-steady-state exercise intensities and the curvature constant (W') indicates a fixed capacity for work >CP that is related to a loss of muscular efficiency. The present study reports novel evidence on the muscle metabolic underpinnings of CP and W' during whole-body exercise and their relationships to muscle fibre type. We show that the W' is not correlated with muscle fibre type distribution and that it represents an elevated energy contribution from both oxidative and glycolytic/glycogenolytic metabolism. We show that there is a positive correlation between CP and highly oxidative type I muscle fibres and that muscle metabolic steady-state is attainable <CP but not >CP. Our findings indicate a mechanistic link between the bioenergetic characteristics of muscle fibre types and the power-time relationship for high-intensity exercise.
  
  
  
  We hypothesized that: (1) the critical power (CP) will represent a boundary separating steady-state from non-steady-state muscle metabolic responses during whole-body exercise and (2) that the CP and the curvature constant (W') of the power-time relationship for high-intensity exercise will be correlated with type I and type IIx muscle fibre distributions, respectively. Four men and four women performed a 3 min all-out cycling test for the estimation of CP and constant work rate (CWR) tests slightly >CP until exhaustion (Tlim ), slightly <CP for 24 min and until the >CP Tlim isotime to test the first hypothesis. Eleven men performed 3 min all-out tests and donated muscle biopsies to test the second hypothesis. Below CP, muscle [PCr] [42.6 ± 7.1 vs. 49.4 ± 6.9 mmol (kg d.w.)(-1) ], [La(-) ] [34.8 ± 12.6 vs. 35.5 ± 13.2 mmol (kg d.w.)(-1) ] and pH (7.11 ± 0.08 vs. 7.10 ± 0.11) remained stable between ∼12 and 24 min (P > 0.05 for all), whereas these variables changed with time >CP such that they were greater [[La(-) ] 95.6 ± 14.1 mmol (kg d.w.)(-1) ] and lower [[PCr] 24.2 ± 3.9 mmol (kg d.w.)(-1) ; pH 6.84 ± 0.06] (P < 0.05) at Tlim (740 ± 186 s) than during the <CP trial. The CP (234 ± 53 W) was correlated with muscle type I (r = 0.67, P = 0.025) and inversely correlated with muscle type IIx fibre proportion (r = -0.76, P = 0.01). There was no relationship between W' (19.4 ± 6.3 kJ) and muscle fibre type. These data indicate a mechanistic link between the bioenergetic characteristics of different muscle fibre types and the power-duration relationship. The CP reflects the bioenergetic characteristics of highly oxidative type I muscle fibres, such that a muscle metabolic steady-state is attainable below and not above CP.
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.
authors.forename: Anni
authors.initials: A
authors.lastname: Vanhatalo
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.
authors.forename: Matthew I
authors.initials: MI
authors.lastname: Black
authors.affiliation: Teachers College, Department of Biobehavioral Sciences, Columbia University, New York, NY, USA.
authors.forename: Fred J
authors.initials: FJ
authors.lastname: DiMenna
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.
authors.forename: Jamie R
authors.initials: JR
authors.lastname: Blackwell
authors.affiliation: Department of Nutrition, Exercise and Sports, The August Krogh Building, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Jakob Friis
authors.initials: JF
authors.lastname: Schmidt
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.
authors.forename: Christopher
authors.initials: C
authors.lastname: Thompson
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.
authors.forename: Lee J
authors.initials: LJ
authors.lastname: Wylie
authors.affiliation: Faculty of Natural and Health Sciences, University of the Faroe Islands, Tórshavn, Faroe Islands.|Center of Health and Human Performance, Department of Food and Nutrition, and Sport Science, University of Gothenburg, Gothenburg, Sweden.
authors.forename: Magni
authors.initials: M
authors.lastname: Mohr
authors.affiliation: Department of Nutrition, Exercise and Sports, The August Krogh Building, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Jens
authors.initials: J
authors.lastname: Bangsbo
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.|Department of Nutrition, Exercise and Sports, The August Krogh Building, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Krustrup
authors.affiliation: College of Life and Environmental Sciences, St Luke's Campus, University of Exeter, Exeter, UK.
authors.forename: Andrew M
authors.initials: AM
authors.lastname: Jones
mesh_terms: D000328:Adult; D001642:Bicycling / Q000502:physiology*; D004734:Energy Metabolism; D015444:Exercise / Q000502:physiology*; D005221:Fatigue / Q000503:physiopathology; D005260:Female; D006801:Humans; D008297:Male; D018485:Muscle Fibers, Skeletal / Q000502:physiology*; D055815:Young Adult