title: Slow-twitch fiber glycogen depletion elevates moderate-exercise fast-twitch fiber activity and O2 uptake.
abstract: OBJECTIVE
  We tested the hypotheses that previous glycogen depletion of slow-twitch (ST) fibers enhances recruitment of fast-twitch (FT) fibers, elevates energy requirement, and results in a slow component of VO2 during moderate-intensity dynamic exercise in humans.
  
  
  METHODS
  Twelve healthy, male subjects cycled for 20 min at approximately 50% VO2max with normal glycogen stores (CON) and with exercise-induced glycogen depleted ST fibers (CHO-DEP). Pulmonary VO2 was measured continuously and single fiber, muscle homogenate, and blood metabolites were determined repeatedly during each trial.
  
  
  RESULTS
  ST fiber glycogen content decreased (P < 0.05) during CON (293 +/- 24 to 204 +/- 17 mmol x kg d.w.), but not during CHO-DEP (92 +/- 22 and 84 +/- 13 mmol x kg d.w.). FT fiber CP and glycogen levels were unaltered during CON, whereas FT fiber CP levels decreased (29 +/- 7%, P < 0.05) during CHO-DEP and glycogen content tended to decrease (32 +/- 14%, P = 0.07). During CHO-DEP, VO2 was higher (P < 0.05) from 2 to 20 min than in CON (0-20 min:7 +/- 1%). Muscle lactate, pH and temperature, ventilation, and plasma epinephrine were not different between trials. From 3 to 20 min of CHO-DEP, VO2 increased (P <0.05) by 5 +/- 1% from 1.95 +/- 0.05 to 2.06 +/- 0.08 L x min but was unchanged during CON. In this exercise period, muscle pH and blood lactate were unaltered in both trials. Exponential modeling revealed a slow component of VO2 equivalent to 0.12 +/- 0.04 L x min during CHO-DEP.
  
  
  CONCLUSIONS
  This study demonstrates that previous glycogen depletion of ST fibers enhances FT fiber recruitment, elevates O2 cost, and causes a slow component of VO2 during dynamic exercise with no blood lactate accumulation or muscular acidosis. These findings suggest that FT fiber recruitment elevates energy requirement of dynamic exercise in humans and support an important role of active FT fibers in producing the slow component of VO2
authors.affiliation: Copenhagen Muscle Research Centre, Institute of Exercise and Sport Sciences, Department of Human Physiology, University of Copenhagen, Copenhagen, Denmark.
authors.forename: Peter
authors.initials: P
authors.lastname: Krustrup
authors.forename: Karin
authors.initials: K
authors.lastname: Söderlund
authors.forename: Magni
authors.initials: M
authors.lastname: Mohr
authors.forename: Jens
authors.initials: J
authors.lastname: Bangsbo
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism; D000328:Adult; D001831:Body Temperature; D015444:Exercise / Q000502:physiology; D006003:Glycogen / Q000378:metabolism*; D006339:Heart Rate; D006801:Humans; D008297:Male; D018656:Muscle Fibers, Fast-Twitch / Q000378:metabolism / Q000502:physiology*; D018657:Muscle Fibers, Slow-Twitch / Q000378:metabolism*; D010100:Oxygen / Q000378:metabolism*; D010725:Phosphocreatine / Q000378:metabolism; D012016:Reference Values; D012119:Respiration; D013548:Sweden
chemical_list: D010725:Phosphocreatine; D000255:Adenosine Triphosphate; D006003:Glycogen; D010100:Oxygen