Head-down tilt bed rest with or without artificial gravity is not associated with motor unit remodeling
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ORIGINAL ARTICLE
Head‑down tilt bed rest with or without artificial gravity is not associated with motor unit remodeling Julia Attias1 · Andrea Grassi2 · Alessandra Bosutti3 · Bergita Ganse1 · Hans Degens1,4 · Michael Drey5 Received: 6 March 2020 / Accepted: 31 July 2020 © The Author(s) 2020
Abstract Purpose The objective of this study was to assess whether artificial gravity attenuates any long-duration head-down 6 0 bed rest (HDBR)-induced alterations in motor unit (MU) properties. Methods Twenty-four healthy participants (16 men; 8 women; 26–54 years) underwent 60-day HDBR with (n = 16) or without (n = 8) 30 min artificial gravity daily induced by whole-body centrifugation. Compound muscle action potential (CMAP), MU number (MUNIX) and MU size (MUSIX) were estimated using the method of Motor Unit Number Index in the Abductor digiti minimi and tibialis anterior muscles 5 days before (BDC-5), and during day 4 (HDT4) and 59 (HDT59) of HDBR. Results The CMAP, MUNIX, and MUSIX at baseline did not change significantly in either muscle, irrespective of the intervention (p > 0.05). Across groups, there were no significant differences in any variable during HDBR, compared to BDC-5. Conclusion Sixty days of HDBR with or without artificial gravity does not induce alterations in motor unit number and size in the ADM or TA muscles in healthy individuals. Keywords Motor unit · Bed rest · Neuromuscular · MUNIX · Microgravity · Spaceflight · Muscle strength Abbreviations ADM Abductor digiti minimi AG Artificial gravity BDC Baseline data collection CMAP Compound muscle action potential EMG Electromyography ESA European Space Agency HDBR Head-down bed rest HDT Head-down tilt MFCV Muscle fibre conduction velocity MU Motor unit Communicated by Guido Ferretti. * Julia Attias [email protected] 1
Research Centre for Musculoskeletal Science & Sports Medicine, Manchester Metropolitan University, Manchester, UK
2
Schulthess Clinic, Zurich, Switzerland
3
Department of Life Sciences, University of Trieste, Trieste, Italy
4
Lithuanian Sports University, Kaunas, Lithuania
5
Department of Medicine IV, University Hospital, LMU Munich, Munich, Germany
MUNE Motor unit number estimation MUNIX Motor unit number index MUSIX Motor unit size NMJ Neuromuscular junction SIP Surface interference pattern TA Tibialis anterior VIC Voluntary isometric contraction
Introduction Exposure to microgravity induces a myriad of physiological alterations (Mulavara et al. 2018). Head-down bed rest (HDBR) is an accepted earth-based model of the microgravity experienced during spaceflight (Pave-Le Traon et al. 2007). Both spaceflight and HDBR are accompanied with a significant loss in muscle strength, which is a major concern for astronauts during long-term space missions (Rittweger et al. 2018). Maintenance of muscle strength during spaceflight is crucial, not only because crew are required to perform physical work during extravehicular activities, but also to ensure a prompt return to one’s functional capaci
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