title: High-intensity interval training alters ATP pathway flux during maximal muscle contractions in humans.
abstract: OBJECTIVE
  High-intensity interval training (HIT) results in potent metabolic adaptations in skeletal muscle; however, little is known about the influence of these adaptations on energetics in vivo. We used magnetic resonance spectroscopy to examine the effects of HIT on ATP synthesis from net PCr breakdown (ATPCK ), oxidative phosphorylation (ATPOX ) and non-oxidative glycolysis (ATPGLY ) in vivo in vastus lateralis during a 24-s maximal voluntary contraction (MVC).
  
  
  METHODS
  Eight young men performed 6 sessions of repeated, 30-s 'all-out' sprints on a cycle ergometer; measures of muscle energetics were obtained at baseline and after the first and sixth sessions.
  
  
  RESULTS
  Training increased peak oxygen consumption (35.8 ± 1.4 to 39.3 ± 1.6 mL min(-1) kg(-1) , P = 0.01) and exercise capacity (217.0 ± 11.0 to 230.5 ± 11.7 W, P = 0.04) on the ergometer, with no effects on total ATP production or force-time integral during the MVC. While ATP production by each pathway was unchanged after the first session, 6 sessions increased the relative contribution of ATPOX (from 31 ± 2 to 39 ± 2% of total ATP turnover, P < 0.001) and lowered the relative contribution from both ATPCK (49 ± 2 to 44 ± 1%, P = 0.004) and ATPGLY (20 ± 2 to 17 ± 1%, P = 0.03).
  
  
  CONCLUSIONS
  These alterations to muscle ATP production in vivo indicate that brief, maximal contractions are performed with increased support of oxidative ATP synthesis and relatively less contribution from anaerobic ATP production following training. These results extend previous reports of molecular and cellular adaptations to HIT and show that 6 training sessions are sufficient to alter in vivo muscle energetics, which likely contributes to increased exercise capacity after short-term HIT.
authors.affiliation: Department of Kinesiology, University of Massachusetts, Amherst, MA, USA; Department of Health Science and Technology, Aalborg University, Aalborg, Denmark.
authors.forename: R G
authors.initials: RG
authors.lastname: Larsen
authors.forename: L
authors.initials: L
authors.lastname: Maynard
authors.forename: J A
authors.initials: JA
authors.lastname: Kent
mesh_terms: D000255:Adenosine Triphosphate / Q000378:metabolism*; D000328:Adult; D001642:Bicycling / Q000502:physiology; D004734:Energy Metabolism / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D006801:Humans; D009682:Magnetic Resonance Spectroscopy; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000378:metabolism*; D010101:Oxygen Consumption / Q000502:physiology; D055815:Young Adult
chemical_list: D000255:Adenosine Triphosphate
keywords: exercise capacity; glycolysis; magnetic resonance spectroscopy; oxidative phosphorylation; phosphocreatine