AUTHOR=Fernandez-Gonzalo Rodrigo , McDonnell Adam C. , Simpson Elizabeth J. , Macdonald Ian A. , Rullman Eric , Mekjavic Igor B. 

TITLE=Substantial and Reproducible Individual Variability in Skeletal Muscle Outcomes in the Cross-Over Designed Planica Bed Rest Program

JOURNAL=Frontiers in Physiology

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2021.676501

DOI=10.3389/fphys.2021.676501

ISSN=1664-042X

ABSTRACT=<p>To evaluate the individual responses in skeletal muscle outcomes following bed rest, data from three studies (21-day PlanHab; 10-day FemHab and LunHab) were combined. Subjects (<italic>n</italic> = 35) participated in three cross-over campaigns within each study: normoxic (NBR) and hypoxic bed rest (HBR), and hypoxic ambulation (HAMB; used as control). Individual variability (SD<sub>IR</sub>) was investigated as √(SD<inline-formula><mml:math id="INEQ1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi /><mml:mrow><mml:mtext>Exp</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msubsup></mml:math></inline-formula>–SD<inline-formula><mml:math id="INEQ2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msubsup><mml:mi /><mml:mrow><mml:mtext>Con</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msubsup></mml:math></inline-formula>), where SD<sub>Exp</sub> and SD<sub>Con</sub> are the standard deviations of the change score (i.e., post – pre) in the experimental (NBR and HBR) and the control (HAMB) groups, respectively. Repeatability and moderators of the individual variability were explored. Significant SD<sub>IR</sub> was detected for knee extension torque, and thigh and calf muscle area, which translated into an individual response ranging from 3 to −17% for knee extension torque, −2 to −12% for calf muscle area, and −1 to −8% for thigh muscle area. Strong correlations were found for changes in NBR vs. HBR (i.e., repeatability) in thigh and calf muscle area (<italic>r</italic> = 0.65–0.75, <italic>P</italic> &lt; 0.0001). Change-scores in knee extension torque, and thigh and calf muscle area strongly correlated with baseline values (<italic>P</italic> &lt; 0.001; <italic>r</italic> between −0.5 and −0.9). Orthogonal partial least squares regression analysis explored if changes in the investigated variables could predict calf muscle area alterations. This analysis indicated that 43% of the variance in calf muscle area could be attributed to changes in all of the other variables. This is the first study using a validated methodology to report clinically relevant individual variability after bed rest in knee extension torque, calf muscle area, and (to a lower extent) thigh muscle area. Baseline values emerged as a moderator of the individual response, and a global bed rest signature served as a moderately strong predictor of the individual variation in calf muscle area alterations.</p>