The Heat Does Not Just Drain Your Legs. It Hijacks Your Breathing.
Every Queensland pre season, the same thing happens. Players who look sharp in May are gasping, glassy eyed and making poor decisions in the back half of a session in November.
Most of the conversation is about fluids, ice towels and acclimation. Those matter. But one part of the heat response gets almost no attention from coaches: what happens to your breathing as your core temperature climbs, and what that does to blood flow to your brain.
Why You Over Breathe When You Get Hot
Breathing rises with effort. That is expected. In the heat, something extra happens on top of it.
Once core temperature passes a threshold, ventilation keeps climbing even when the workload does not change. Physiologists call this hyperthermia induced hyperventilation. A 2015 review by Hayashi in the Journal of Physical Fitness and Sports Medicine describes this response and links it to central fatigue, one of the reasons performance drops in hot conditions.
The problem is not the extra air. It is what the extra air takes with it.
Blowing off CO2, losing blood flow to the brain
Carbon dioxide is the main signal that controls how wide the blood vessels in your brain open. Breathe more than you need and arterial CO2 falls. When CO2 falls, those vessels narrow and cerebral blood flow drops.
In the heat, that is the opposite of what you want. Your brain is the organ making every read, every call and every pass under pressure. The Hayashi review proposes that reduced cerebral perfusion from heat driven hyperventilation lowers brain oxygenation and slows the removal of heat from the brain itself.
This is the same mechanism behind over breathing costing you performance in normal conditions. Heat simply turns the volume up.
What the Research Shows When Athletes Control It
The key question for a coach is simple: can a player actually do anything about this, or is it automatic?
Tsuji and colleagues tested exactly that in a 2015 study published in the American Journal of Physiology. Twelve healthy men cycled at 50% of their peak oxygen uptake for up to 60 minutes in 37°C heat at 50% humidity. In one trial they breathed normally. In the other, they deliberately held their ventilation at the level they reached after five minutes of exercise.
The results at the same rise in core temperature (+1.1°C) were clear:
Ventilation: rose 7.4 litres per minute with normal breathing, but only 1.6 litres per minute with controlled breathing.
Estimated arterial CO2: fell 3.4 mmHg with normal breathing versus 0.8 mmHg with controlled breathing.
Brain blood flow velocity: dropped 10.4 cm per second with normal breathing versus 3.9 cm per second with controlled breathing.
Just as important, controlling the breath did not interfere with cooling. Sweating and skin blood flow were unchanged. The body still dumped heat. It simply stopped dumping CO2 it did not need to lose.
What that study does not prove
Be clear on the limits. These were healthy men, not elite team sport athletes. The exercise was steady cycling at moderate intensity, not repeated high intensity efforts. And the study measured breathing, CO2 and brain blood flow. It did not measure performance, decision making or time to fatigue.
A second study adds important nuance. Périard and Racinais (2015) had 11 cyclists complete a self paced time trial in 35°C and 20°C conditions. Brain blood flow velocity fell more in the heat, yet ventilation and end tidal CO2 changed similarly in both conditions. The authors attributed the larger drop mainly to thermal strain itself: more blood sent to the skin and lower blood pressure.
So breathing is one factor in how the brain copes with heat, not the whole story. Anyone telling you breathing control alone beats the heat is overselling it.
Where Breathing Fits in Your Heat Plan
Heat acclimation, hydration, cooling and smart scheduling are the foundation, and your sports science staff should own them. Breathing is the layer most programs leave untouched, and it costs nothing to add.
Here is how we use it with teams heading into hot conditions.
1. Build the skill when it is cool
You cannot learn breath control at 38°C in the 70th minute. Start with light, slow nasal breathing at rest and during low intensity work, where the goal is to breathe less air without strain. A small study by Dallam and colleagues (2018) found recreational runners who had trained with nasal only breathing for around six months reached a similar peak oxygen uptake and time to exhaustion breathing through the nose as through the mouth. Treat that as early evidence from non elite athletes, not a guarantee.
2. Use it in breaks, not during the sprint
During maximal efforts, breathe as hard as you need. The opportunity is in the gaps: between drills, at stoppages, on the bench and in breaks. That is where players spiral into rapid, shallow mouth breathing that keeps CO2 low long after the effort is over.
Coach a cue: exhale long, close the mouth, slow the next breath down. Thirty seconds of control in a break is a repeatable habit under fatigue.
3. Train CO2 tolerance off the field
Breathing less when your body is screaming for air is uncomfortable. Athletes who are more comfortable with rising CO2 find it easier. Short breath holds after a normal exhale, done seated and calm in a cool environment, are one way to build that comfort. A BOLT score is a useful self check of breathlessness and CO2 sensitivity, but it is not a validated predictor of performance, so use it as a guide, not a selection tool.
Safety: never practise breath holds in or near water or while driving, stop if dizzy, never do it alone, and get medical clearance first if pregnant or with heart, lung or blood pressure conditions. Keep hard breath hold sprint work out of extreme heat.
4. Make breathing part of your heat monitoring
Rapid, uncontrolled mouth breathing in a low intensity part of a session is a visible warning sign. Coaches and medical staff can use it alongside temperature, heart rate and perceived exertion data as a prompt to cool, rest or pull a player.
FOR ELITE ATHLETES, THIS CAN HELP YOU
✔ Understand why you over breathe as your core temperature rises in hot sessions and games
✔ Learn a breathing pattern that controlled research shows can limit the CO2 drop during exercise in the heat
✔ Protect brain blood flow during prolonged efforts in the heat, without blunting sweating or skin cooling
✔ Recover your breathing faster in breaks instead of staying stuck in rapid mouth breathing
✔ Add a no cost layer to the heat plan your sports science staff already run
Performance takeawayHeat makes you over breathe. Over breathing costs your brain blood flow.
You cannot control the weather. You can train how you breathe in it.
Want this in your program? Enquire about Vincere Protocol for your team.
