Effect of 3 Weeks of Strict Head‑Down Tilt Bed Rest on Human Muscle Fuction and Architecture

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The aim of this study was to first, experimentally measure in vivo changes in the length, fiber angle and thickness of the medial gastrocnemius muscle (MG) in young men in response to changes in foot position and joint moment during isometric plantar flexion and, in secondly, to compare the changes in the above characteristics of the muscular architecture that occur during the transition from a state of rest to a given isometric intensity during plantar flexion. The internal architecture of the MG was determined after 21-day of strict head‑down tilt bed rest (HDT). MG scanning was performed using ultrasound at rest at ankle joint angles of –15° (dorsiflexion), 0° (neutral), +15° and +30° (plantar flexion). Additional ultrasounds were performed during maximal voluntary contraction (MVC) and additionally at 80, 60, 40, and 20% of the MVC with the ankle in neutral position. In each position, longitudinal ultrasound images of the MG were obtained in a relaxed (passive) state with the determination of the length (Lf) and angles of fascicles (Θf) relative to the aponeurosis. After HDT, the thickness of the MG during graduated isometric force up to 80% of the MVC in the neutral position of the ankle joint remained constant. Various Lf and Θf and their changes after HDT can be a limiting factor in the generation of muscle contractile functions. The results of the study show that the change in muscle structure during contraction compared to rest, as measured by changes in muscle architecture, can be used to assess muscle mechanical output.