title: Acute exposure to microgravity does not influence the H-reflex with or without whole body vibration and does not cause vibration-specific changes in muscular activity.
abstract: OBJECTIVE
  Many potential countermeasures for muscle and bone loss caused by exposure to microgravity require an uncompromised stretch reflex system. This is especially true for whole body vibration (WBV), as the main source of the neuromuscular activity during WBV has been attributed to stretch reflexes. A priori, it cannot be assumed that reflexes and Ia afferent transmission in particular have the same characteristics in microgravity as in normal gravity (NG). Therefore, the purpose of the study was to compare Ia afferent transmission in microgravity and NG and to assess how microgravity affects muscle activity during WBV.
  
  
  METHODS
  In 14 participants, electromyographic activity of four leg muscles as well as Hoffmann-reflexes were recorded during NG and microgravity induced by parabolic flights.
  
  
  RESULTS
  The size of the Hoffmann-reflex was reduced during WBV, but did not differ during acute exposure to microgravity compared to NG. The influence of the gravity conditions on the electromyographic activity did not change depending on the vibration condition.
  
  
  CONCLUSIONS
  As far as the electromyographic activity of the recorded leg muscles is concerned, the effect of WBV is the same in microgravity as in NG. Moreover, Ia afferent transmission does not seem to be affected by acute exposure to microgravity when subjects are loaded with body weight and postural sway is minimized.
authors.affiliation: IfSS der Albert-Ludwigs-Universität Freiburg, Freiburg, Germany. andreas.kramer@uni-konstanz.de
authors.forename: Andreas
authors.initials: A
authors.lastname: Kramer
authors.forename: Albert
authors.initials: A
authors.lastname: Gollhofer
authors.forename: Ramona
authors.initials: R
authors.lastname: Ritzmann
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology; D000328:Adult; D005260:Female; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D008297:Male; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology*; D010812:Physical Stimulation / Q000379:methods*; D011930:Reaction Time / Q000502:physiology*; D014732:Vibration; D014893:Weightlessness