CAFFEINE ALTERS VOLUNTARY BUT NOT REFLEX ACTIVATION OF SINGLE HUMAN MOTOR UNITS
Bibliographic record
Abstract
Vibrating a muscle tendon generates sufficient Ia afferent input to the spinal cord to recruit individual single motor units (MU) and cause a reflex contraction. When the force profile is reproduced voluntarily, the same MU is recruited at a higher recruitment threshold (RT) and at a higher firing rate (FR) compared to reflex activation. This suggests that MU may be activated via two different pathways. To determine if this change is due to alterations in descending drive we increased central excitability by administering caffeine, which is known to act centrally and is excitatory to the MU pool during voluntary activation. PURPOSE The purpose of the present experiment was to compare the effects of caffeine on the RT and FR of MU in the vastus lateralis (VL) during reflex and voluntary contractions. We hypothesized that there would be a decrease in the RT and an increase in the average FR in the caffeine condition during voluntary activation and that caffeine would have no effect during reflex activation. METHODS MU recordings were obtained using tungsten microelectrodes from the VL on 2 separate days (caffeine and placebo). Vibration (175 Hz) was applied to the patellar tendon for 30 to 35s. After a 90s rest period, and without moving the electrode, the force profile was reproduced voluntarily. This allowed us to observe the behaviour of the same MU activated under both conditions. RESULTS The results showed that during reflex activation the RT and FR were significantly lower for all conditions. There was a significant increase in the FR of MU during voluntary activation in the caffeine condition but no changes were observed in FR during reflex activation. There were no alterations in the RT in either condition. CONCLUSION We demonstrated that caffeine only exerts its effects during voluntary activation of muscle, and does not alter peripheral feedback from muscle spindles. Caffeine's potentiating effects on MU FR may be due to an increase in the central excitatory neurotransmitters serotonin and norepinephrine, which are known to increase with caffeine ingestion. Supported by NSERC
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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) | 0.001 | 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".