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

Abstract

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 (n = 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 (SDIR) was investigated as √(SDExp2–SDCon2), where SDExp and SDCon 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 SDIR 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 (r = 0.65–0.75, P < 0.0001). Change-scores in knee extension torque, and thigh and calf muscle area strongly correlated with baseline values (P < 0.001; r 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.

Learning From Human Responses to Deconditioning Environments: Improved Understanding of the “Use It or Lose It” Principle

“…Some of the changes occurring after prolonged bedrest can persist for long after a return to normal activity (Trudel et al, 2009 ) and thus the system compromise from such deconditioning can be protracted even when reinstitution of physical activity is implemented. However, similar to space flight, the responses to bedrest can show considerable individual variation (Fernandez-Gonzalo et al, 2021 ; reviewed in Scott et al, 2021 ).…”

Optimal fibre length and maximum isometric force are the most influential parameters when modelling muscular adaptations to unloading using Hill-type muscle models

“…Indeed, shorter and less pennate muscle fibres have been reported alongside reduced muscular strength following spaceflight ( Koryak, 2019 ) and bed rest ( Reeves et al, 2002 ; De Boer et al, 2008 ). Further evidence of the complexity inherent to muscle adaptations, are i) a preferential shift from type I to type II fibres observed following disuse ( Trappe et al, 2004 ), ii) the inter-individual variation reported in the literature ( Fernandez-Gonzalo et al, 2021 ; Scott et al, 2021 ), which also complicates the adjustment of muscle model parameters. For example, muscle fibre CSA and peak isometric fibre force ranged between 49%–106% and 30%–90%, respectively, across eight astronauts from a single study ( Fitts et al, 2010 ).…”

A novel computational framework for the estimation of internal musculoskeletal loading and muscle adaptation in hypogravity

“…Astronauts can present with substantial muscular adaptations, such as atrophy and reduced strength, power, and endurance, in as little as 7–14 days of spaceflight ( Winnard et al, 2019 ). To complicate matters, substantial inter-individual variability has been reported in the literature ( Fernandez-Gonzalo et al, 2021 ; Scott et al, 2021 ), both in terms of the magnitude and the nature of the muscular adaptations experienced by astronauts. For example, Widrick et al (1999) reported a 2-fold variation in responses of muscle contractile properties following 17 days spaceflight.…”