Muscle X-ray attenuation is not decreased during experimental bed rest.

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.