The Role of Long-Term Head-Down Bed Rest in Understanding Inter-Individual Variation in Response to the Spaceflight Environment: A Perspective Review

Abstract

Exposure to the spaceflight environment results in profound multi-system physiological adaptations in which there appears to be substantial inter-individual variability (IV) between crewmembers. However, performance of countermeasure exercise renders it impossible to separate the effects of the spaceflight environment alone from those associated with exercise, whilst differences in exercise programs, spaceflight operations constraints, and environmental factors further complicate the interpretation of IV. In contrast, long-term head-down bed rest (HDBR) studies isolate (by means of a control group) the effects of mechanical unloading from those associated with countermeasures and control many of the factors that may contribute to IV. In this perspective, we review the available evidence of IV in response to the spaceflight environment and discuss factors that complicate its interpretation. We present individual data from two 60-d HDBR studies that demonstrate that, despite the highly standardized experimental conditions, marked quantitative differences still exist in the response of the cardiorespiratory and musculoskeletal systems between individuals. We also discuss the statistical concept of “true” and “false” individual differences and its potential application to HDBR data. We contend that it is currently not possible to evaluate IV in response to the spaceflight environment and countermeasure exercise. However, with highly standardized experimental conditions and the presence of a control group, HDBR is suitable for the investigation of IV in the physiological responses to gravitational unloading and countermeasures. Such investigations may provide valuable insights into the potential role of IV in adaptations to the spaceflight environment and the effectiveness of current and future countermeasures.

Contractile behavior of the gastrocnemius medialis muscle during running in simulated hypogravity

“…To prevent these detrimental effects, the crewmembers of the International Space Station (ISS) perform daily exercise countermeasures, including treadmill running, cycling, and resistance training 3 . Due to the implementation of new exercise hardware and improvements of the in-flight exercise hardware and exercise prescriptions, μg-induced physiological deconditioning has been reduced, although variable inter-individual physiological responses to the exercise induced stimuli persist 4,5 .…”

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

“…Anecdotal evidence has suggested that there is substantial individual variability in physiological variables following bed rest, including losses in muscle mass and function (Akima et al, 1997;Downs et al, 2016;Scott et al, 2021). However, the limited number of participants included in previous studies and the lack of an ambulatory control group prevented any firm conclusion.…”

“…Only large national or international space agencies can afford to run long-duration bed rest campaigns, which include a rather limited number of subjects per intervention group. The small sample size is indeed a significant constraint when exploring individual variability in bed rest studies (Scott et al, 2021). Furthermore, the lack of a genuine control group in bed rest studies, i.e., an ambulatory group, has limited the validity of any past attempt to detect individual responses to bed rest interventions (Atkinson and Batterham, 2015;Scott et al, 2021).…”

“…The small sample size is indeed a significant constraint when exploring individual variability in bed rest studies (Scott et al, 2021). Furthermore, the lack of a genuine control group in bed rest studies, i.e., an ambulatory group, has limited the validity of any past attempt to detect individual responses to bed rest interventions (Atkinson and Batterham, 2015;Scott et al, 2021). Overcoming these limitations could translate into improved health management of astronauts and optimized individual programs to counteract the negative effects of unloading both during space missions and on Earth (Scott et al, 2021).…”

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

“…Therefore, studying humans in space can be viewed as an “experiment” to both identify in more detail how humans are regulated, and to identify and find solutions to enhance the health and safety of astronauts. Thus, from this “experiment,” it appears that human's exhibit heterogeneity in response to space conditions (reviewed in Scott et al, 2021 ), and that some of the same physiological systems affected by age/aging in many humans on Earth are also affected by space conditions (Ray, 1991 ; Vernikos and Schneider, 2010 ). As such, space flight may be viewed as “accelerated aging” (Ray, 1991 ; Vernikos and Schneider, 2010 ), or for the context of this discussion, acute (short term) and chronic (longer term) “deconditioning” based on removal from the boundary conditions of Earth and the mechanical activity required to maintain system integrity.…”