Medium-term Bed Rest Disrupts Motor Unit Properties and Neuromuscular Transmission in Young Adults
Bibliographic record
Abstract
Background: Short-term disuse minimally affects higher-threshold motor units (MUs) and neuromuscular junction (NMJ) function, but the consequences of prolonged disuse on neuromuscular function remain unclear. This study examined how 21 days of bed rest impact MU properties, NMJ transmission, and NMJ morphology. We hypothesise that this disuse period impairs MU properties across different contraction intensities, and induces alterations in NMJ structure and function. Methods: Nine healthy young males (18–29 years) underwent 21 days of strict horizontal bed rest. Pre- (BR0) and post-bed rest (BR21), we assessed quadriceps maximal voluntary contraction (MVC) and muscle size. Intramuscular (iEMG) and high-density surface electromyography (HDsEMG) were used to evaluate MU properties at 25% and 50% MVC. NMJ transmission was assessed electrophysiologically via near-fibre jiggle and segment jitter, while NMJ morphology was analyzed from muscle biopsies. Serum C-terminal agrin fragment (CAF), a biomarker of NMJ remodeling, was also measured. Results: Quadriceps MVC and size significantly decreased at BR21. MU discharge rate declined, MU potential area changed, and complexity increased at both contraction intensities, overall indicating alterations in MU properties. NMJ transmission impairment was detected at 25% MVC, with increased near-fibre jiggle and segment jitter. Serum CAF levels were elevated at BR21, corroborating NMJ remodeling. Notably, NMJ morphological analysis revealed denervated NMJs exclusively at BR21. Conclusion: Medium-term disuse alters MU properties, impairs NMJ transmission, and triggers early NMJ remodeling, including signs of denervation. These findings highlight disuse duration as a key factor and reinforce the need for countermeasures in clinical, rehabilitation, and spaceflight contexts. Space It Up project funded by the Italian Space Agency, ASI, and the Ministry of University and Research, MUR, under contract n. 2024-5-E.0 - CUP n. I53D24000060005 This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".