Abstract
PurposeThe objective of this study was to assess whether artificial gravity attenuates any long-duration head-down 60 bed rest (HDBR)-induced alterations in motor unit (MU) properties.MethodsTwenty-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.ResultsThe 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.ConclusionSixty 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.
Highlights
Exposure to microgravity induces a myriad of physiological alterations (Mulavara et al 2018)
There were no significant differences in the age, sex distribution, height, and body mass of the participants between groups at BDC-5 (Table 1)
In both the abductor digiti minimi (ADM) (Fig. 2a) and tibialis anterior muscle (TA) (Fig. 2b), there were no significant changes in Compound muscle action potential (CMAP) at HDT4 and HDT59 compared to BDC-5 in any of the groups
Summary
Exposure to microgravity induces a myriad of physiological alterations (Mulavara et al 2018). NMJ degeneration has been reported in ageing mice, with a concomitant decline in motor unit (MU) number, where the loss of MUs could be prevented by exercise, suggesting a functional rather than anatomical loss of MUs (Giorgetti et al 2019). Such a situation may occur in humans exposed to microgravity, seeing as fundamental MU organization, i.e., presence of a motor neuron and muscle fibres innervated by it, is comparable to that of most mammals (Purves et al 2001)
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