Human motor unit characteristics of the superior trapezius muscle with age-related comparisons
Bibliographic record
Abstract
Current understanding of human motor unit (MU) control and aging is mostly derived from hand and limb muscles that have spinal motor neuron innervations. The aim here was to characterize and test whether a muscle with a shared innervation supply from brainstem and spinal MU populations would demonstrate similar age-related adaptations as those reported for other muscles. In humans, the superior trapezius (ST) muscle acts to elevate and stabilize the scapula and has primary efferent supply from the spinal accessory nerve (cranial nerve XI) located in the brainstem. We compared electrophysiological properties obtained from intramuscular and surface recordings between 10 young (22–33 yr) and 10 old (77–88 yr) men at a range of voluntary isometric contraction intensities (from 15 to 100% of maximal efforts). The old group was 41% weaker with 43% lower MU discharge frequencies compared with the young (47.2 ± 9.6 Hz young and 26.7 ± 5.8 Hz old, P < 0.05) during maximal efforts. There was no difference in MU number estimation between age groups (228 ± 105 young and 209 ± 89 old, P = 0.33). Furthermore, there were no differences in needle detected near fiber (NF) stability parameters of jitter or jiggle. The old group had lower amplitude and smaller area of the stimulated compound muscle action potential and smaller NF MU potential area with higher NF counts. Thus, despite age-related ST weakness and lower MU discharge rates, there was minimal evidence of MU loss or compensatory reinnervation. NEW & NOTEWORTHY The human superior trapezius (ST) has shared spinal and brainstem motor neuron innervation providing a unique model to explore the impact of aging on motor unit (MU) properties. Although the ST showed higher MU discharge rates compared with most spinally innervated muscles, voluntary strength and mean MU rates were lower in old compared with young at all contraction intensities. There was no age-related difference in MU number estimates with minimal electrophysiological evidence of collateral reinnervation.
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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 | 0.001 |
| 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) | 0.002 | 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".