Cardiopulmonary Responses to Sub-Maximal Ergometer Exercise in a Hypo-Gravity Analog Using Head-Down Tilt and Head-Up Tilt

Abstract

After more than 50 years of spaceflight, we still do not know what is the appropriate range of gravity levels that are required to maintain normal physiological function in humans. This research effort aimed to investigate musculoskeletal, cardiovascular, and pulmonary responses between 0 and 1 g. A human experiment was conducted to investigate acute physiological outcomes to simulated altered-gravity with and without ergometer exercise using a head-down tilt (HDT)/head-up tilt (HUT) paradigm. A custom tilting platform was built to simulate multiple gravitational loads in the head-to-toe direction (Gz) by tilting the bed to the appropriate angle. Gravity levels included: Microgravity (-6°HDT), Moon (0.17g-Gz at +9.5°HUT), Mars (0.38g-Gz at +22.3°HUT), and Earth (1g-Gz at +90° upright). Fourteen healthy subjects performed an exercise protocol at each simulated gravity level that consisted of three work rates (50W, 75W, 100W) while maintaining a constant cycling rate of 90 rpm. Multiple cardiopulmonary variables were gathered, including volume of oxygen uptake (VO 2 ), volume of carbon dioxide output (VCO 2 ), pulmonary ventilation (V E ), tidal volume (V T ), respiratory rate (R f ), blood pressure, and heart rate (HR) using a portable metabolic system and a brachial blood pressure cuff. Foot forces were also measured continuously during the protocol. Exercise data were analyzed with repeated-measures ANOVA with Bonferroni correction for multiple comparisons, and regression models were fitted to the experimental data to generate dose-response curves as a function of simulated AG-levels and exercise intensity. Posture showed a main effect in all variables except for systolic blood pressure. In particular, VO 2, VCO 2 , V E , V T , R f , and HR showed average changes across exercise conditions between Microgravity and 1 g (i.e., per unit of simulated AG) of -97.88 mL/min/g, -95.10 mL/min/g, -3.95 L/min/g, 0.165 L/g, -5.33 breaths/min/g, and 5.05 bpm/g, respectively. In the case of VO 2 , further pairwise comparisons did not show significant differences between conditions, which was consistent with previous studies using supine and HDT postures. For all variables (except HR), comparisons between Mars and Earth conditions were not statistically different, suggesting that ergometer exercise at a gravitational stress comparable to Mars gravity (∼3/8 g) could provide similar physiological stimuli as cycling under 1 g on Earth.

The Influence of Altered-Gravity on Bimanual Coordination: Retention and Transfer

“…Two gravity levels were simulated by tilting the platform to the designated angle: Earth [90 • Head Up Tilt (HUT)] and microgravity [6 • Head Down Tilt (HDT)]. Specifically, the 6 • HDT position is a widely accepted analog for microgravity conditions (Diaz-Artiles et al, 2019). Force transducers were fixed in an adjustable fashion such that subjects' arms would be at their side and elbows bent 90 • with forearms resting against the force transducers.…”

“…Finally, while HDT/HUT paradigms are very well established analog to investigate altered-gravity environments (Clement et al, 2015;Diaz Artiles et al, 2016;Diaz-Artiles et al, 2019;Petersen et al, 2021;Whittle et al, 2021), 6 • HDT is not a fully accurate representation of microgravity conditions during spaceflight. The presence of a transverse gravitational component (front-to-back, or Gx) and a small longitudinal gravitational component (foot-to-head, or Gz) are limitations inherent to our ground-testing simulation.…”

Computational model of cardiovascular response to centrifugation and lower body cycling exercise

“…Several studies have investigated the short-term hemodynamics response of exercise in the upright and supine posture (1,11), and as a potential countermeasure in ground-based bedrest studies (40). Our focus here is on the acute cardiovascular response to a gravitational stress that can be recreated in space and that is not constant along the major body axis.…”

Artificially altered gravity elicits cell homeostasis imbalance in planarian worms, and cerium oxide nanoparticles counteract this effect

“…Moreover, several studies demonstrate that altered gravity affects signaling pathways 13‐16 and modifies the transcriptional profile of factors not exclusively associated with stress response 10,17‐22 . The understanding of the biological effects produced by altered gravity conditions is a relevant issue for spaceflight, bearing in mind that – after long‐term missions – astronauts experience a number of health problems, like atrophy of muscles, bone demineralization, cardiopulmonary, ocular and vestibular complications, and modifications in the nervous system connections 23‐26 . Special challenges are to be faced when attempting to study altered gravity on a whole organism, and therefore our knowledge about the effects of gravity perturbations in vivo is still scarce and fragmented.…”

“… 10 , 17 , 18 , 19 , 20 , 21 , 22 The understanding of the biological effects produced by altered gravity conditions is a relevant issue for spaceflight, bearing in mind that – after long‐term missions – astronauts experience a number of health problems, like atrophy of muscles, bone demineralization, cardiopulmonary, ocular and vestibular complications, and modifications in the nervous system connections. 23 , 24 , 25 , 26 Special challenges are to be faced when attempting to study altered gravity on a whole organism, and therefore our knowledge about the effects of gravity perturbations in vivo is still scarce and fragmented. Using simple worms (planarians, Platyhelminthes) as a model system, we simulated altered gravity conditions by using a large diameter centrifuge (LDC) and a random positioning machine (RPM) at the European Space Research and Technology Centre of the European Space Agency (ESA‐ESTEC) ground‐based facilities, in Noordwijk, the Netherlands.…”