Central Fatigue Indicators during Repeated Exertional Heat Stress in Untrained Males
Bibliographic record
Abstract
Serotonergic and dopaminergic activity in the brain is associated with increases in core temperature (Tre). Although the development of fatigue and increases in blood-brain barrier (BBB) permeability have been shown to be more pronounced in the heat than in thermoneutral environments, the influence of Tre on the indicators of central fatigue are not well understood. Central fatigue is believed to be dependent upon transport of Tryptophan (TRP), the precursor of Serotonin, into the brain, which is blocked at the BBB by Branch Chain Amino Acids (BCAA). Given that repeated exertional heat stress (EHS) induces a lower resting and exercise Tre, reduced physiological strain, and increased Tre tolerated, it would be of interest to know whether repeated EHS induces a reduced level of central fatigue. PURPOSE: To examine the amino acids and neuroendocrine indicators related to central fatigue during 9 days of exposure to EHS in untrained (UT) males. METHODS: Nine UT males (Mean ± SE: VO2peak = 51.7 ± 1.9 mL·kgLBM-1·min-1, 17 ± 2% fat) walked to exhaustion (EXH) on 9 days (over 2 consecutive weeks) on a treadmill (4.5 km·h-1, 2% incline) in 40°C and 30% R.H., wearing combat clothing and a protective hooded overgarment. Venous blood samples were obtained at rest (PRE; prior to heat) and at a Tre of 38, 38.5, 39°C, and EXH. Amino acids were determined by gas chromatography/mass spectrometry and cortisol (COR) by chemiluminescence. RESULTS: Tre EXH was not different (p < 0.06) from Days 1 (38.8 ± 0.1°C) to 9 (39.1 ± 0.1°C), yet time to EXH increased from Days 1 (95 ± 8 min) to 9 (145 ± 13 min). Free-TRP was attenuated on Day 9 and BCAA values were elevated on Day 9 and at PRE following Day 1. The f-TRP/BCAA ratio was reduced on Day 9 compared to Day 1. No changes were observed for Tyrosine and Phenylalanine from Days 1 to 9 or PRE to EXH. COR responded to absolute thermal strain with greater values at EXH on Day 9 (600.5 ± 71.4 nmol·L-1) than Day 1 (472.5 ± 83.7 nmol·L-1). CONCLUSION: Attenuation of f-TRP and elevated BCAAs on Day 9 of EHS are consistent with reduced central fatigue via increased f-TRP vs. BCAA competition at the BBB, and thus less f-TRP transport into the brain, increased protection of BBB integrity by COR and/or an increase in binding/re-binding of f-TRP to circulating albumin. Funded by DRDC Technology Investment Fund and US Office of Naval Research.
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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".