Effect of Hot Spring Therapy on Central Fatigue Elimination During Altitude Training

Altitude training induces central fatigue characterized by decreased descending neural drive from the brain to skeletal muscle, reduced voluntary activation, and impaired cognitive function. This study employs a stratified randomized block design to evaluate whether post-exercise hot spring immersion can accelerate central fatigue recovery compared to pure warm water immersion and natural recovery. All participants will undergo standardized rowing training at approximately 1600 meters altitude. Group C will receive immersion in natural hot spring water at 38°C for 20 minutes. Group B will receive immersion in pure water at the same temperature and duration as a thermal control. Group A will undergo standard training recovery without immersion. The study incorporates partial blinding: Groups B and C are single-blinded to participants (identical appearance and protocol); Group A serves as an open-label control. Assessments will be conducted at baseline (pre-altitude), mid-intervention (Week 2), end-intervention (Week 4), and follow-up (1-2 weeks post-altitude).

• Natural recovery (Behavioral), also known as NR: Standard post-exercise recovery without water immersion. Participants rest passively in a thermoneutral environment (24°C, 50-60% humidity) for 20 minutes with ad libitum water and optional self-directed stretching. No heat, cold, or hydrotherapy interventions are applied.
• Hot water immersion (Behavioral), also known as HWI: After exercise, full-body immersion in daily tap water. Temperature: 38°C ± 0.5°C. Duration: 20 minutes per session, 3 times per week (Tuesday, Thursday, Saturday at 16:00), for 4 consecutive weeks (total of 12 sessions). Protocol: Full-body immersion below the neck for 20 minutes, followed by a 10-minute rest before leaving the facility.
• Hot spring balneotherapy (Behavioral), also known as HSB: After exercise, full-body immersion in natural hot spring water (pH approx. 7.4, Na+ 1210 mg/L, HCO3- approx. 3300 mg/L, total dissolved solids >1000 mg/L). Temperature: 38°C ± 0.5°C. Duration: 20 minutes per session, 3 times per week (Tuesday, Thursday, Saturday at 16:00), for 4 consecutive weeks (12 sessions total). Protocol: Full-body immersion below the neck for 20 minutes, followed by a 10-minute rest before leaving the facility.

Primary outcomes

• Voluntary Activation [Baseline (Week 0), Week 2, Week 3, Week 4 (post-intervention), Follow-up (Week 1-2 post-altitude)]: Voluntary activation assessed by the interpolated twitch technique (ITT) during maximal voluntary contraction (MVC) of the quadriceps femoris using percutaneous electrical stimulation. A single stimulus is superimposed on MVC; VA is calculated as: VA = (1 - superimposed twitch force / potentiated resting doublet force) × 100%. Higher VA indicates greater central neural drive. Unit: percentage (%).
• Central Activation Ratio [Baseline (Week 0), Week 2, Week 3, Week 4, Follow-up (Week 5-6 post-altitude)]: Central activation ratio measured as the ratio of maximal voluntary contraction force to the force evoked by supramaximal electrical stimulation of the femoral nerve during quadriceps contraction. CAR = MVC force / (MVC force + superimposed twitch force). Values closer to 1.0 indicate less central fatigue. Unit: ratio (0-1, no units).

Secondary outcomes

• Stroop Test Performance [Baseline, Week 2,Week 4, Follow-up (Week 1-2 post-altitude)]: Computerized Stroop Color-Word Test measuring executive function and cognitive inhibition. Outcome variables: reaction time (ms) for congruent and incongruent trials, with interference effect calculated as incongruent minus congruent reaction time. Administered in a quiet, temperature-controlled room (22-24°C). Unit: milliseconds (ms).
• Cerebral Oxygenation [Baseline, Week 2, fellow up]: Changes in oxygenated hemoglobin (HbO2) and deoxygenated hemoglobin (Hb) in the prefrontal cortex during rest and cognitive tasks, measured by functional near-infrared spectroscopy (fNIRS) with a multichannel system. Outcome: relative concentration changes (ΔμM) in HbO2 and Hb from baseline. Unit: micromolar (μM).
• Psychomotor Vigilance Test [Baseline, Week 2, Week 3, Week 4, Follow-up (Week 1-2 post-altitude)]: 5-minute Psychomotor Vigilance Test measuring sustained attention and psychomotor vigilance. Outcome variables: mean response time, and number of lapses (reaction time >500 ms). Administered via standardized software on a tablet/computer in a quiet environment. Unit: milliseconds (ms).
• Maximal Oxygen Uptake (VO2max) [Baseline (Week 0), Follow-up (Week 5-6 post-altitude)]: Maximal oxygen uptake measured during a graded exercise test on a rowing ergometer using a portable metabolic cart. Protocol: incremental ramp test (starting at 100W, increasing 20W every 2 minutes) until volitional exhaustion. Unit: mL/kg/min.
• White Blood Cell Count (WBC) [Baseline, Week 2, Follow up (Week 5-6 post-altitude)]: Total white blood cell count measured from fasting venous blood sample using automated hematology analyzer. Unit: 10\^9 cells per liter (10\^9/L).
• Neutrophil Count (NEUT) [Baseline, Week 2, Follow-up (Week 5-6 post-altitude)]: Absolute neutrophil count measured from fasting venous blood sample using automated hematology analyzer. Unit: 10\^9/L. Baseline, Week 4 11 Platelet Count (PLT) Platelet count measured from fasting venous blood sample using automated hematology analyzer. Unit: 10\^9/L.

Sex: All, ages: 12 Years to 16 Years, accepts healthy volunteers

Inclusion Criteria:

* Male or female aged 12-16 years- Healthy with no serious chronic diseases or skin conditions
* At least 1 year of systematic rowing training experience
* Currently undergoing centralized altitude training at approximately 1600 meters- No history of systematic hot spring or spa immersion within the past month
* No allergy history to sodium bicarbonate or hot spring minerals- Ability to provide written informed assent (participant) and written informed consent (parent/guardian)- No acute infection, fracture, or surgery within the past month
* Not currently taking non-steroidal anti-inflammatory drugs (NSAIDs), immunosuppressants, or medications affecting autonomic or cognitive function

Exclusion Criteria:

* History of cardiovascular, respiratory, renal, endocrine, or neurological disorders- Uncontrolled hypertension or hypotension
* History of syncope, heat intolerance, or severe dizziness during hot water immersion- Open wounds, severe eczema, or other skin conditions contraindicating water immersion
* Current use of beta-blockers, anticholinergics, stimulants, sedatives, or other medications affecting central nervous system function
* Color blindness or uncorrected visual impairment affecting Stroop test performance- Pregnancy (for female participants)- Alcohol consumption or smoking
* Shift work or trans-meridian travel within 2 weeks prior to baseline assessment
* Any condition deemed by the investigator as unsafe for participation

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