• blood content;
  • human physiology;
  • immobilization;
  • physical medicine;
  • peripheral quantitative computed tomography

Introduction: Muscle X-ray attenuation (MXA) in Hounsfield units (HU) is decreased in dystrophy, aging, and immobilization. We hypothesized a decrease also during bed rest (BR). Methods: Twenty-five healthy men performed BR for 90 days, 8 of whom performed flywheel exercise 2–3 times per week. Muscle cross-sectional area (CSA) and MXA were assessed by computed tomography. Results: In the control group, calf muscle CSA decreased during 89-day BR by 26.6 (SD 3.8) %, while MXA increased by 1.2 HU on day 28 and almost reached baseline values on day 89. Changes were mitigated in the flywheel group for the calf (P < 0.001). All changes were reversed within 90 days after BR. Conclusions: Results contradict the initial hypothesis. Increased intramuscular blood content, interstitial fluid space shrinkage, and reduction of intramuscular lipid stores are possible explanations for the moderate increase in MXA during the initial phase of bed rest. Muscle Nerve 47: 722–730, 2013