Abstract
ObjectivesThe present study evaluated the effectiveness of a short and versatile daily exercise regime, named locomotion replacement training (LRT), to maintain muscle size, isometric strength, power, and endurance capacity of the leg muscles following 5 days of head-down tilt (HDT) bed rest.Methods10 male subjects (age 29.4 ± 5.9 years; height 178.8 ± 3.7 cm; body mass 77.7 ± 4.1 kg) performed, in random order, 5 days of 6° head-down tilt bed rest (BR) with no exercise (CON), or BR with daily 25 min of upright standing (STA) or LRT.ResultsKnee extensor and plantar flexor cross-sectional area (CSA) were reduced by 2–3 % following bed rest (P < 0.01) for CON and STA, yet maintained for LRT. Knee extensor isometric strength (MVC) decreased by 8 % for CON (P < 0.05), was maintained for STA, and increased with 12 % for LRT (P < 0.05). Plantar flexor MVC remained unaltered during the study. Maximum jump height declined (~1.5 cm) for all conditions (P < 0.001). Neural activation and knee extensor fatigability did not change with bed rest. Bone resorption increased during BR and neither LRT nor STA was able to prevent or attenuate this increase.ConclusionLRT was adequate to maintain muscle size and to even increase knee extensor MVC, but not muscle power and bone integrity, which likely requires more intense and/or longer exercise regimes. However, with only some variables showing significant changes, we conclude that 5 days of BR is an inadequate approach for countermeasure assessments.
Highlights
Human space exploration over the last decades has identified major structural and physiological changes in a variety of organ systems due to weightlessness
The novel finding of the present study was that a versatile, yet relatively mildly intense, upright training regime prevented a loss in knee extensor and plantar flexor muscle size, and even increased knee extensor strength following 5 days of head-down tilt (HDT) bed rest
The present findings indicate that 5 days of bed rest resulted in a loss of both the calf and the thigh cross-sectional area (CSA) of approximately 2–3 %, which is in good agreement with the 3–6 % loss reported after 7–8 days of bed rest, spaceflight and cast immobilization (Crawley 2007; Ferrando et al 1995; LeBlanc et al 1995; Richter et al 1985)
Summary
Human space exploration over the last decades has identified major structural and physiological changes in a variety of organ systems due to weightlessness. Unlike many other previously tested countermeasure schemes, the LRT encompassed various loading schemes (static and dynamic contractions both short and sustained, both unilateral and bilateral)] to mimic the variety of stimuli that are needed to maintain musculoskeletal and cardiovascular systems in daily life With respect to the latter, it seems reasonable to suggest that alterations in the cardiovascular system and fluid shifts may affect the musculoskeletal system during actual spaceflight. The LRT regime was designed to be implementable during centrifugation, and the study was performed with the expectation that the present results would be applicable to using the LRT scheme during centrifugation in real spaceflight Typical everyday activities such as running and jumping on Earth, for instance, generate forces (and stresses and strains) that are high enough for bone strength to be maintained (Frost 2003; Nikander et al 2010). We evaluated whether LRT would prevent increased bone resorption markers, which can be already observed in the first days of bed rest (Baecker et al 1985)
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