title: Adjustments in the force-frequency relationship during passive and exercise-induced hyperthermia.
abstract: BACKGROUND
  We examined the extent to which fatiguing cycling exercise in the heat influences contractile function in modulating the force-frequency relationship.
  
  
  METHODS
  Before (∽37.0 °C) and after (∽38.5 °C) exercise (ExH) and passive (PaH) hyperthermia, an 8-s train of stimulation at 10, 20, 50, and 100 Hz (2 s per frequency) and a potentiated twitch were evoked on the relaxed knee extensors using percutaneous stimulation.
  
  
  RESULTS
  ExH and PaH produced a decrease in the 20:50 Hz force ratio, indicative of low-frequency fatigue (P < 0.01). This adjustment was more pronounced after ExH than PaH (P < 0.01). A rightward displacement in the force-frequency relationship occurred after ExH and PaH (P < 0.05) and was exacerbated by ExH (P < 0.05). Peak twitch force also decreased after ExH (P < 0.05).
  
  
  CONCLUSIONS
  ExH reduces force summation due to development of skeletal muscle fatigue, exacerbating the shift in force-frequency to the right relative to PaH.
authors.affiliation: Aspetar-Qatar Orthopaedic and Sports Medicine Hospital, Research and Education Centre, P.O. Box 29222, Doha, Qatar.
authors.forename: Julien D
authors.initials: JD
authors.lastname: Périard
authors.forename: Sebastien
authors.initials: S
authors.lastname: Racinais
authors.forename: Martin W
authors.initials: MW
authors.lastname: Thompson
mesh_terms: D000328:Adult; D000704:Analysis of Variance; D001703:Biophysics; D004558:Electric Stimulation; D004576:Electromyography; D015444:Exercise / Q000502:physiology*; D005334:Fever / Q000503:physiopathology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018763:Muscle Fatigue / Q000502:physiology*; D053580:Muscle Strength / Q000502:physiology*; D018482:Muscle, Skeletal / Q000503:physiopathology*; D009469:Neuromuscular Junction / Q000502:physiology; D010101:Oxygen Consumption / Q000502:physiology; D011795:Surveys and Questionnaires
keywords: core temperature; fatigue; heat stress; low-frequency fatigue; muscle temperature