Effect of Internal and External Cooling on High-Intensity Intermittent Cycling Performance and Cognitive Function in the Heat

Study Aim
The study examined whether combining internal cooling (ice slurry ingestion) and external cooling (ice collar) can improve performance and cognitive function during high-intensity intermittent exercise in hot conditions (33 °C, 50% relative humidity).

Participants & Protocol

• 29 trained male athletes participated.
• Each performed two trials:
  1. Control (CON): thermoneutral water (~37°C).
  2. Cooling (COOL): ice slurry (7.5 g/kg before exercise + 1 g/kg during) and an ice collar.
• Exercise: 40 min intermittent cycling (repeated 5 s maximal sprints with active recovery).
• Cognitive tests: battery of tasks (Stroop, Sternberg, Visual Search, Rapid Visual Information Processing) before and after exercise.
• Core (rectal), neck, and forehead temperature, heart rate, and subjective measures of comfort and effort were recorded.

Main Findings

1. Performance Outcomes

• No significant difference in peak or mean power output between cooling and control trials.
• Cooling did not enhance sprint performance in this protocol.

Interpretation:
The exercise intensity and duration did not elevate core temperature (> 38.5 °C) enough to induce the level of heat strain that typically limits performance. Therefore, cooling offered no mechanical advantage under these specific conditions.

2. Physiological Responses

• Rectal temperature: 0.2 °C lower in COOL (37.39 °C vs. 37.59 °C).
• Neck temperature: 4 °C lower in COOL.
• Heart rate: slightly reduced in COOL (123 bpm vs. 127 bpm).
• Sweat rate, glucose, and lactate: no significant differences between trials.

Interpretation:
The combined cooling approach effectively reduced body and skin temperatures and lowered cardiovascular strain, confirming its thermoregulatory benefits even if performance didn’t improve.

3. Perceptual Responses

• Participants felt cooler, more comfortable, and less exerted during COOL.
• Thermal sensation and comfort improvements coincided with lower neck temperature.

Interpretation:
Cooling significantly improved perceived comfort, which can help athletes tolerate heat better and potentially sustain motivation and pacing during prolonged activity.

4. Cognitive Function

• Cooling improved reaction time in some cognitive tests:
  • Faster responses in complex Stroop tasks (executive control).
  • Slightly faster responses in Sternberg working memory and simple number tasks.
• However, in other tasks (visual search, sustained attention), performance improved more in the control condition.

Interpretation:
Cooling may preserve or enhance complex decision-making under heat stress but doesn’t necessarily improve all cognitive domains. Benefits seem task-specific, likely linked to better comfort and localized brain cooling (via the neck).

Conclusions

• Combined internal and external cooling did not enhance sprint performance, but it:
  • Lowered core and skin temperatures
  • Reduced heart rate and perceived exertion
  • Improved comfort and some cognitive functions
• Practical combined cooling (ice slurry + neck ice collar) can be used safely and effectively during high-intensity intermittent exercise in heat to reduce thermal and perceptual strain.

Practical Applications and Tips for Athletes

For Training and Competition in Hot Conditions:

1. Pre-cooling strategy:
   • Consume ice slurry (≈7.5 g/kg body mass) about 30 minutes before exercise, in 2–3 servings.
   • Wear an ice collar or cooling wrap around the neck to pre-cool the carotid area.
2. During breaks (e.g., halftime or between intervals):
   • Ingest smaller ice slurry doses (≈1 g/kg) and reapply a cooled collar.
   • Focus on the neck region—it provides strong thermal relief due to its proximity to the brain’s thermoregulatory center.
3. Perceptual benefits:
   • Expect to feel cooler and less fatigued, which can sustain mental focus and pacing, even if direct power output gains aren’t measurable.
4. Cognitive protection:
   • Cooling can help maintain executive function, supporting faster decision-making in tactical sports (e.g., football, basketball, rugby).
5. When it matters most:
   • Cooling is particularly valuable in matches or sessions where heat stress is high and athletes’ core temperature exceeds 38.5 °C.
   • For events with lower heat load, benefits may be mostly perceptual rather than performance-enhancing.

Key Takeaway

While cooling may not directly boost sprint power, it reduces heat strain and perceptual discomfort, protects cognitive sharpness, and enhances heat tolerance — making it a valuable practical tool for athletes training or competing in hot and humid conditions.