title: Biorhythmic influences on functional capacity of human muscle and physiological responses.
abstract: Previously, this laboratory has demonstrated that exhaustive aerobic exercise performance is not subject to significant chronobiological variation between 0800 and 2000 h, but certain physiological responses to maximal aerobic effort do fluctuate significantly within that time frame.
  
  
  OBJECTIVE
  The purpose of the present investigation was to determine whether muscle performance, and selected physiological responses to resistance exercise, was significantly influenced by time of day effects.
  
  
  METHODS
  Ten healthy, but untrained, men (21.1+/-0.6 yr, mean +/- SE) volunteered to participate in the study. In a balanced and randomized study design, each subject performed resistance exercise protocols on an isokinetic dynamometer with maximal effort at 0800 h, 1200 h, 1600 h, and 2000 h. Selected physiological and hormonal data were recorded before and immediately following the exercise stimulus.
  
  
  RESULTS
  The data demonstrated significant chronobiological oscillation in peak torque, average power, maximal work in a single repetition, and total work per set. Interestingly, this oscillation was manifested only at the fastest velocities of limb movement utilized. Pre- and postexercise rectal temperature exhibited significant time of day fluctuation, as did postexercise blood pressure. Plasma levels of testosterone and cortisol also displayed significant biorhythmicity under both pre- and postexercise conditions. However, exercise-induced responses (pre- to postexercise differences) of those steroid hormones did not exhibit significant biorythmic variation. Although plasma concentrations of both testosterone and cortisol were highest at 0800 h, testosterone to cortisol ratios were greatest at 2000 h.
  
  
  CONCLUSIONS
  In summary, these data suggest that maximal muscle performance does vary within the segment of the day when exercise typically occurs (0800-2000 h) but that this variation is specific to speed of movement.
authors.affiliation: Department of Kinesiology, The College of William & Mary, Williamsburg, VA 23187-8795, USA. mrdesc@facstaff.wm.edu
authors.forename: M R
authors.initials: MR
authors.lastname: Deschenes
authors.forename: W J
authors.initials: WJ
authors.lastname: Kraemer
authors.forename: J A
authors.initials: JA
authors.lastname: Bush
authors.forename: T A
authors.initials: TA
authors.lastname: Doughty
authors.forename: D
authors.initials: D
authors.lastname: Kim
authors.forename: K M
authors.initials: KM
authors.lastname: Mullen
authors.forename: K
authors.initials: K
authors.lastname: Ramsey
mesh_terms: D000328:Adult; D001794:Blood Pressure / Q000502:physiology; D001831:Body Temperature / Q000502:physiology; D002940:Circadian Rhythm / Q000502:physiology*; D004734:Energy Metabolism / Q000502:physiology; D015444:Exercise / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D006854:Hydrocortisone / Q000097:blood; D007773:Lactates / Q000097:blood; D008297:Male; D018763:Muscle Fatigue / Q000502:physiology; D018482:Muscle, Skeletal / Q000502:physiology*; D013739:Testosterone / Q000097:blood; D019415:Torque
chemical_list: D007773:Lactates; D013739:Testosterone; D006854:Hydrocortisone