The human resting muscle tone (HRMT) system provides structural and functional support to skeletal muscle and associated myofascial structures (tendons, fascia) in normal life. Little information is available on changes to the HRMT in bed rest. A set of dynamic oscillation mechanosignals ([Hz], [N/m], log decrement, [ms]) collected and computed by a hand-held digital palpation device (MyotonPRO) were used to study changes in tone and in key biomechanical and viscoelastic properties in global and postural skeletal muscle tendons and fascia from a non-exercise control (CTR) and an exercise (JUMP) group performing reactive jumps on a customized sledge system during a 60 days head-down tilt bed rest (RSL Study 2015–2016). A set of baseline and differential natural oscillation signal patterns were identified as key determinants in resting muscle and myofascial structures from back, thigh, calf, patellar and Achilles tendon, and plantar fascia. The greatest changes were found in thigh and calf muscle and tendon, with little change in the shoulder muscles. Functional tests (one leg jumps, electromyography) showed only trends in relevant leg muscle groups. Increased anti-Collagen-I immunoreactivity found in CTR soleus biopsy cryosections was absent from JUMP. Results allow for a muscle health status definition after chronic disuse in bed rest without and with countermeasure, and following reconditioning. Findings improve our understanding of structural and functional responses of the HRMT to disuse and exercise, may help to guide treatment in various clinical settings (e.g., muscle tone disorders, neuro-rehabilitation), and promote monitoring of muscle health and training status in personalized sport and space medicine.

A broad variety of countermeasures on the effects of weightlessness on human physiology have been developed and applied in the course of space exploration. Devices like treadmills, stretch ropes etc. have several disadvantages in common: they require a significant amount of crew time and they may not efficiently counteract the degradation of physiological structures and cellular functions. Some methods even include potentially painful or uncomfortable procedures for the astronauts. Thus, the application of Artificial Gravity (AG) generated by short radius centrifuges (they fit into space vessels) has been discussed and proposed by a number of scientists and space agencies as an alternative countermeasure during long-term space missions. Although there is a profound knowledge concerning, e.g., the cardiovascular system and immune responses acquired on long radius centrifuges, there is a remarkable lack of knowledge concerning the same issues on devices operating with short radius. In strict contrast to long radius centrifuges, there is a significant gravity gradient in the head-to-toe axis which comes along with the short radius and higher relative rotation velocity. Thus it is of utmost importance to continue investigating the effects of AG, especially by use of short radius centrifuges. The Short Arm Human Centrifuge (SAHC) at the German Aerospace Center (DLR) in Cologne, Germany, is the most advanced type of short radius centrifuges presently commercially available. Experience gained so far using the SAHC at DLR revealed that future projects on centrifuge devices with short radius should aim at a clear identification of the threshold level of the g-load, which is necessary to efficiently counteract the degradation of physical structures and an efficient support of cellular functions. A satisfying result would be combined countermeasure methods applied at a threshold concerning g-load and exposition time in the course of long-term sojourn in microgravity. Another future control or monitoring method to exactly dose AG training is heart rate variability, which offers an insight into neurovegetative and cardiovascular regulation. Centrifuges like the SAHC are also useful platforms to accommodate small biological experiments, e.g., experiments addressing the response of cultured cells to hypergravity. Here, we briefly review the issue of short radius centrifuges and also address our experience hitherto gained during a number of scientific projects carried out at the SAHC at DLR.

Abstract. Direct object selection in an Augmented Reality environment that is coded outside of human body frame of reference is deteriorated under short-term altered gravity. As countermeasures we developed a gravity-adapted resizing technique based on the Hooke's law that resulted in two techniques of target and interface deformation (compression, elongation). To prove the concept of this resizing approach we initially conducted two experiments under simulated hypergravity conditions. While during the first study hypergravity was induced by a long-arm human centrifuge, in the second study hypergravity was simulated by additional arm weightings that were balanced and attached to the participants' pointing arm. We investigated the difference of the task performance with respect to the pointing frequency, response time, pointing speed and accuracy, when participants performed a visuomotor task under the resizing conditions compared to the unchanged condition. During the second study we additionally evaluated the speed-accuracy tradeoff of the resizing techniques according to Fitts' law and the physiological workload by cardiac responses analyzing the heart rate variability. Both experiments showed that the online adaption of the present gravity load to targets' size and distance influences the performance of direct AR direct pointing. The results revealed that the pointing performance benefits from elongation target deformation by increased target sizes and distances, while pointing towards compressed targets mostly decreases the physiological workload under increased gravity conditions.

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The aim of this study was to determine the hemodynamic and neuroendocrinological responses to different levels and protocols of artificial gravity, especially in comparison to what is expected during a moderate bout of exercise. Ten male participants were exposed to artificial gravity using two different protocols: the first was a centrifugation protocol that consisted of a constant phase of 2 Gz for 30 minutes, and the second consisted of an intermittent phase of 2 Gz for two minutes, separated by resting periods for three minutes in successive order. Near infrared spectroscopy (oxyhemoglobin and deoxyhemoglobin) at the prefrontal cortex, Musculus biceps brachii, and Musculus gastrocnemius, as well as heart rate and blood pressure were recorded before, during, and after exposure to artificial gravity. In order to determine effects of artificial gravity on neuroendocrinological parameters (brain-derived neurotrophic factor, vascular endothelial growth factor, and insulin-like growth factor 1), blood samples were taken before and after centrifugation. During the application of artificial gravity the concentration of oxyhemoglobin decreased significantly and the concentration of deoxyhemoglobin increased significantly in the prefrontal cortex and the Musculus biceps brachii muscle. Participants exposed to the continuous artificial gravity profile experienced peripheral pooling of blood. No changes were observed for brain-derived neurotrophic factor, vascular endothelial growth factor, or insulin-like growth factor 1. Intermittent application of artificial gravity may represent a better-tolerated presentation for participants as hemodynamic values normalize during resting periods. During both protocols, heart rate and arterial blood pressure remained far below what is experienced during moderate physical activity.

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