title: Heat production and heat loss responses to cold water immersion after 35 days horizontal bed rest.
abstract: BACKGROUND
  Bed rest is a terrestrial experimental analogue of unloading experienced during exposure to microgravity. Such unloading causes atrophy predominantly of the postural muscles, especially those of the lower limbs.
  
  
  METHODS
  We tested the hypothesis that 35 d horizontal bed rest alters thermoregulatory responses of subjects (N = 10) immersed in 15 degrees C water, particularly the heat produced by the shivering tremor of the skeletal muscles. Before and after bed rest we measured the thickness of the gastrocnemius medialis (GM), vastus lateralis (VL), tibialis anterior (TA), and biceps brachii (BB) muscles by ultrasonography. During the immersions, we monitored rectal and skin temperatures, heat flux, heart rate, and oxygen uptake.
  
  
  RESULTS
  After bed rest, muscle thickness decreased significantly by 12.2 +/- 8.8% and 8.0 +/- 9.1% in the GM and VL, respectively. No changes were observed in the TA and BB muscles. The 35-d bed rest caused a significant reduction in aerobic power, as reflected in maximal oxygen uptake. There were no significant differences in any of the observed thermoregulatory responses between the pre- and post-bed rest immersions.
  
  
  CONCLUSIONS
  Cardiovascular and muscular deconditioning had no effect on the heat production and heat loss responses. Due to the significant reduction in the mass of the muscles in the lower limbs, concomitant with no change in heat production, we conclude that leg muscles do not play a significant role in shivering thermogenesis.
authors.affiliation: Department of Automation, Biocybernetics and Robotics, Jozef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia. igor.mekjavic@ijs.si
authors.forename: Igor B
authors.initials: IB
authors.lastname: Mekjavic
authors.forename: Stylianos N
authors.initials: SN
authors.lastname: Kounalakis
authors.forename: Michail E
authors.initials: ME
authors.lastname: Keramidas
authors.forename: Gianni
authors.initials: G
authors.lastname: Biolo
authors.forename: Marco
authors.initials: M
authors.lastname: Narici
authors.forename: Ola
authors.initials: O
authors.lastname: Eiken
mesh_terms: D000704:Analysis of Variance; D001833:Body Temperature Regulation / Q000502:physiology*; D003080:Cold Temperature; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D007101:Immersion; D035002:Lower Extremity / Q000502:physiology; D008297:Male; D018482:Muscle, Skeletal / Q000000981:diagnostic imaging; D009133:Muscular Atrophy / Q000503:physiopathology; D010101:Oxygen Consumption / Q000502:physiology; D016684:Prone Position / Q000502:physiology; D012146:Rest / Q000502:physiology*; D012881:Skin Temperature / Q000502:physiology; D016683:Supine Position / Q000502:physiology; D014463:Ultrasonography; D055815:Young Adult