July 2018

Bed Rest, Exercise Countermeasure and Reconditioning Effects on the Human Resting Muscle Tone System

Authors: B. Schoenrock, V. Zander, S. Dern, U. Limper, E. Mulder, A. Veraksitš, R. Viir, A. Kramer, M.J. Stokes, M. Salanova, A. Peipsi, D. Blottner

Affiliations: Center of Space Medicine Berlin, Berlin, Germany

Journal: Frontiers in Physiology - July 2018 (DOI: 10.3389/fphys.2018.00810)

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.

The present findings provide further insights to the structural and functional responses of the HRMT system to chronic disuse in bed rest. With the help of the non-invasive myotonometric protocol we were able to define robust bed rest effects in the CTR (without exercise prescriptions) as well as significant training effects in the exercise group (reactive jumps) that we found in global as well as postural muscle groups including tendon and fascia in otherwise healthy male bed rest participants. Soft tissue region and tissue specific biomechanical characteristics found in global and local muscle and myofascial tissue at resting conditions appear to precede functional changes (i.e., property changes prior to functional changes). This allows for real time monitoring of the deconditioned HRMT system in bed rest and provides an early assessment tool if routine (and often highly demanding) neuromuscular tests may not be available, for example, in open field study settings. Myotonometric testing in disuse and exercise proved to be a less costly, easier and more rapid and resilient method than conventional methods and it may be used as a practical evidence-based outcome evaluation tool for an efficient physical countermeasure outcome on the human muscle status/condition in prolonged periods of disuse, perhaps even replacing strenuous functional testing in future human analog studies. The present findings may be even more helpful for a better treatment efficacy in various healthy and clinical populations but also for the monitoring of muscle health status of crew during pre and inflight mission training, and reconditioning thereafter in personalized space medicine.