title: Effects of lengthening contraction on calcium kinetics and skeletal muscle contractility in humans.
abstract: OBJECTIVE
  We have tested the hypothesis that the altered muscle contractility after lengthening contractions (LC) is caused by altered calcium (Ca2+) kinetics.
  
  
  METHODS
  Subjects (n = 8) performed 100 drop jumps and muscle contractility was measured pre- and post-exercise by maximal voluntary contraction (MVC) and transcutaneous electrical stimulation (1, 20 and 50 Hz). Muscle biopsies were analysed for muscle metabolites, rates of SR Ca(2+) uptake (CaU) and release (CaR) and myosin heavy chain (MHC) composition.
  
  
  RESULTS
  The rates of torque relaxation and CaU were positively related to muscle fibre type composition (% MHC II). Muscle creatine (Cr) decreased and the ratio between phosphocreatine (PCr) and Cr increased 3 and 24 h post-exercise (P < 0.05 vs. pre-exercise). LC resulted in reduced MVC (-19%), twitch torque (-41%) and 20/50 Hz torque ratio (-30%) and a faster relaxation rate (P < 0.05). The contractile parameters recovered partially but remained altered 24 h post-exercise (P < 0.05). The average CaR was unchanged after LC (P > 0.05). However, the response varied between subjects and the relative post-exercise CaR was significantly related to the degree of LFF (post/pre 20/50 Hz force ratio) and to the decline in twitch force (post/pre twitch ratio). CaU was lower in seven of eight subjects after LC (P > 0.05).
  
  
  CONCLUSIONS
  The decline in torque after LC could not be explained by metabolic factors since PCr/Cr ratio increased. The relation between CaR and fatigue suggests that the mechanism of fatigue in part may be attributed to intrinsic changes in the SR Ca2+ release channel. The faster torque relaxation after LC could not be explained by an increased rate of CaU.
authors.affiliation: Institute of Sports Science and Clinical Biomechanics, University of Southern Denmark, Odense, Denmark.
authors.forename: J S
authors.initials: JS
authors.lastname: Nielsen
authors.forename: K
authors.initials: K
authors.lastname: Madsen
authors.forename: L V
authors.initials: LV
authors.lastname: Jørgensen
authors.forename: K
authors.initials: K
authors.lastname: Sahlin
mesh_terms: D000328:Adult; D002118:Calcium / Q000493:pharmacokinetics*; D003401:Creatine / Q000032:analysis; D003402:Creatine Kinase / Q000378:metabolism; D015444:Exercise / Q000502:physiology; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018485:Muscle Fibers, Skeletal / Q000378:metabolism; D009126:Muscle Relaxation / Q000502:physiology; D018482:Muscle, Skeletal / Q000378:metabolism / Q000502:physiology*; D018995:Myosin Heavy Chains / Q000378:metabolism; D010725:Phosphocreatine / Q000032:analysis; D012519:Sarcoplasmic Reticulum / Q000378:metabolism; D004561:Transcutaneous Electric Nerve Stimulation / Q000379:methods
chemical_list: D010725:Phosphocreatine; D003402:Creatine Kinase; D018995:Myosin Heavy Chains; D003401:Creatine; D002118:Calcium