Berger L;  Institute of Aerospace Medicine, Cologne, GERMANY
Mulder E; Zange J; Frings-Meuthen P; Frett T; Simon P; Hoenemann
JN; Poczatek MJ; Laurie SS; Huang AS; Macias BR; Jordan J; Tank J;
Rittweger J; Schmitz MT; Pesta D

Medicine & Science in Sports & Exercise. 57(12):2887-2901, 2025 Dec 01.
VI 1

PURPOSE: During spaceflight, countermeasures are crucial to counteract
cardiorespiratory and musculoskeletal deconditioning, similar to effects
seen after prolonged immobilization. Within a bed rest study simulating
the effects of microgravity, we investigated the effectiveness of
submaximal aerobic exercise and postexercise venoconstrictive thigh cuffs
(VTC) for maintaining cardiorespiratory fitness and muscle function.

METHODS: Twenty-four males and females were randomly allocated to a
countermeasure ( n = 12, age = 34 +/- 9 yr, body mass index = 22.9 +/- 2.9
kg.m -2 ) or control group ( n = 12, age = 35 +/- 8 yr, body mass index =
23.3 +/- 2.0 kg.m -2 ). Participants underwent 30 d of strict 6degree
head-down tilt (HDT) bed rest. Six days per week, the countermeasure group
completed 60 min of submaximal cycling in HDT position, followed by 6 h of
VTC (50 +/- 5 mm Hg). The control group maintained HDT bed rest without
any countermeasure. Cardiorespiratory fitness and muscle strength were
assessed before and after bed rest via cardiopulmonary exercise testing
and isokinetic dynamometry, respectively.

RESULTS: Peak oxygen uptake was maintained in the countermeasure group
(-106 +/- 148 mL.min -1 ) compared with the control group (-607 +/- 343
mL.min -1 ; P < 0.001). The countermeasure also mitigated the decrease in
plasma volume (countermeasure: -147 +/- 95 mL vs control: -286 +/- 153 mL;
P = 0.014), peak estimated stroke volume (countermeasure: -6 +/- 5 mL vs
control: -21 +/- 13 mL; P < 0.001), and cardiac output (countermeasure:
-0.7 +/- 0.9 L.min -1 vs control: -3.8 +/- 2.1 L.min -1 ; P < 0.001). Both
groups showed reductions in fat-free mass and muscle strength after bed
rest.

CONCLUSIONS: Submaximal exercise with subsequent VTC preserves
cardiorespiratory fitness during bed rest, likely by maintaining peak
stroke volume and cardiac output. However, this countermeasure did not
prevent declines of fat-free mass and muscle strength.