Freediving, a discipline that involves prolonged breath holding (apnoea) whilst submerged, represents a unique intersection of human physiology and extreme sport. Unlike most athletic pursuits, freediving intentionally disrupts homeostasis by suppressing respiration, thereby inducing states of hypoxia and hypercapnia. This blog examines the physiological mechanisms underpinning apnoea, with a focus on the mammalian dive reflex, breath-hold phases, and training ratios. Drawing from recent research on trained freedivers, we explore how the body adapts during repeated breath holds, the implications for safety, practical training strategies, and broader applications in breathwork therapies.

Understanding Apnoea and Freediving Physiology

Apnoea, or voluntary breath holding, triggers a cascade of physiological responses designed to conserve oxygen and maintain vital functions. Central to this is the mammalian dive reflex, an evolutionary adaptation observed in marine mammals such as seals and whales, and humans. This reflex is primarily initiated by immersion of the face in water, stimulating the trigeminal nerve, which signals the brainstem to activate protective mechanisms. Upon breath cessation, particularly in aquatic environments, the reflex induces bradycardia (a reduction in heart rate), peripheral vasoconstriction, and selective blood flow redistribution. Oxygen is prioritised for the brain and heart, whilst blood vessels in non-essential areas, such as skeletal muscles, constrict to minimise oxygen consumption. This vascular shunting enhances survival by delaying hypoxia-induced blackout, enabling marine mammals like Weddell seals to dive for over an hour, compared to human breath-hold times typically ranging from 2 to 7 minutes in trained individuals.

In humans, the dive reflex is amplified in water due to contextual factors such as immersion, hydrostatic pressure, and temperature changes, which can extend breath-hold duration compared to dry-land training. For example, facial immersion in cold water potentiates bradycardia, with heart rates dropping by 20–50% or more, as observed in training scenarios. Physiologically, breath holding begins with an inhalation to near-total lung capacity (vital capacity, typically 4–6 litres in adults, higher in males), followed by glottal closure to prevent air escape. Oxygen levels (measured as SpO2) decline gradually, whilst carbon dioxide (CO2) accumulates, stimulating chemoreceptors in the carotid arteries and aorta, which increase respiratory drive.

Key physiological markers during apnoea include:

  • Oxygen saturation (SpO2): Baseline values around 97% may drop to 94% or lower in prolonged holds, with desaturation defined as a ≥3% decline.

  • Heart rate: Initial tachycardia from intrathoracic pressure shifts gives way to bradycardia via parasympathetic activation.

  • Cerebral and muscle oxygenation: Near-infrared spectroscopy reveals increased cerebral blood flow to combat hypoxia, contrasted with muscle deoxygenation.

  • CO2 levels: End-tidal CO2 (EtCO2) rises from a baseline of approximately 35 mmHg during holds, influencing air hunger.

These responses are modulated by training, with experienced freedivers exhibiting enhanced tolerance to hypoxia and hypercapnia, often supported by spleen contraction, which releases additional red blood cells to increase oxygen-carrying capacity.

Breath-Hold Phases: Easy vs. Struggle

A breath hold comprises two distinct phases:

  • Easy phase: Characterised by relaxation and the absence of involuntary diaphragmatic contractions. This phase relies on initial oxygen stores and minimal air hunger, lasting longer with relaxation techniques and controlled breathe-up (preparatory breathing).

  • Struggle phase: Marked by diaphragmatic contractions as CO2 accumulates and chemoreceptors signal the brain to resume breathing. The breakpoint occurs when voluntary control yields to respiration.

Prolonging the easy phase is a primary training goal, achieved through relaxation, emotional regulation, and avoiding hyperventilation, which can reduce CO2 excessively and heighten blackout risk by delaying air hunger cues relative to oxygen depletion. Techniques such as diaphragmatic breathing and mindfulness can extend this phase, particularly in novices.

Training Ratios in Apnoea: Insights from a 1:1 Recovery Study

A recent study by Mulder et al. (2025) investigated whether trained freedivers could sustain a 1:1 apnoea-to-recovery ratio during repeated breath holds, assessing impacts on oxygenation and safety. Twenty-one participants (15 males, 6 females; average age 40 years; ≥3 years apnoea experience) performed seven 2-minute breath holds on dry land in a supine position, each followed by 2-minute normal breathing recovery periods. Hyperventilation and lung packing were prohibited to standardise conditions, and vital capacity averaged 6.4 litres in males and 4.2 litres in females.

Measurements included:

  • SpO2 and heart rate via pulse oximetry.

  • Cerebral and muscle oxygenation via near-infrared spectroscopy.

  • Diaphragmatic movement via a chest force sensor.

  • EtCO2 via nasal cannula.

Key findings highlighted progressive adaptations:

  • Oxygenation: Initial SpO2 desaturation (≈3%, to 94%) stabilised in subsequent holds (≈2% drop). Cerebral oxygenation increased during the first apnoea (1.0 ± 2.3%) due to vasodilation, then remained slightly below baseline. Muscle oxygenation declined consistently (-6.7 ± 3.1% in the first apnoea) but recovered between holds.

  • Heart rate: Initial tachycardia diminished by 10 bpm across the series, with the lowest heart rate rising from 61 ± 15 to 65 ± 13 bpm, indicating reduced sympathetic activation.

  • CO2 dynamics: Baseline EtCO2 dropped subtly from 4.6 kPa (≈34.5 mmHg) to 4.3 kPa (≈32.2 mmHg), potentially delaying contractions by 10–15 seconds. Post-apnoea rises were modest (≈1 kPa or 7.5 mmHg), with no progressive increase.

  • Diaphragmatic contractions: The easy phase extended by up to 31 seconds by the seventh apnoea, with 63% of participants avoiding the struggle phase entirely in later holds.

These findings suggest a robust initial dive reflex response (bradycardia, cerebral vasodilation) that attenuates in subsequent holds, possibly due to spleen contraction, increased parasympathetic tone, or psychological familiarisation. The 1:1 ratio appeared sufficient for recovery at submaximal efforts, with no progressive hypoxia accumulation.

Practical Training Strategies

For practitioners, these findings offer actionable insights:

  • Beginner training: A 1:1 ratio is a safe starting point for static apnoea, ensuring adequate recovery without excessive CO2 offloading. Novices should focus on relaxation techniques, such as diaphragmatic breathing or meditation, to extend the easy phase.

  • Intermediate to advanced divers: For longer holds (e.g., 3–4 minutes), a 1:2 ratio may be necessary, particularly in dynamic apnoea, where energy expenditure and acidosis amplify physiological stress.

  • Surf apnoea: Short, repeated holds under stress (e.g., 15–30 seconds for surfers) benefit from understanding the dive reflex to foster calmness. Training should emphasise psychological resilience to perceived danger.

  • Avoiding blackout risks: Coaches should monitor for excessive CO2 reduction (e.g., from over-breathing), as low CO2 can delay air hunger, increasing hypoxia risks. Pulse oximetry and buddy supervision are critical.

Critiques and Limitations of the Research

The study by Mulder et al. (2025) has notable limitations:

  • Breath-hold duration: Two-minute holds are submaximal for trained freedivers (maximum holds ranged 4–7.5 minutes), potentially underestimating risks in competitive or dynamic scenarios.

  • Context: Dry-land, supine positioning lacks water immersion, hydrostatic pressure, and movement, which amplify the dive reflex and energy demands.

  • Participant variability: Experience differed by sex (7 years for males vs. 3 years for females), and menstrual cycle effects on female physiology were unaddressed, potentially influencing CO2 sensitivity.

  • CO2 oversight: The subtle baseline EtCO2 drop was not discussed, yet it could explain extended easy phases by reducing initial respiratory drive.

  • Methodological gaps: The absence of comparison groups (e.g., 1:2 ratios or longer holds), blood gas analyses, or dynamic apnoea testing limits generalisability. Measurement artifacts were also noted.

The hypothesis focused on 1:1 ratio safety, but the discussion shifted to adaptive responses, suggesting unexpected results influenced the paper’s framing, potentially reducing its alignment with the original intent.

Advanced Concepts: Cortical Modulation and Air Hunger

Traditional views attribute air hunger to CO2-sensitive chemoreceptors, but emerging paradigms highlight cortical gain modulation. Chemoreceptors relay CO2/pH changes to the brain, where regions like the insula, anterior cingulate cortex, and prefrontal cortex amplify (high gain) or attenuate (low gain) signals based on emotional state and habituation. High stress (e.g., poor sleep, caffeine) increases gain, heightening air hunger despite stable CO2 levels; relaxation reduces it, extending the easy phase. For example, a stressed diver may experience diaphragmatic contractions within seconds, while a calm diver sustains the easy phase longer, even with identical CO2 levels.

This distinguishes “air hunger tolerance” (psychological adaptation) from “CO2 tolerance” (chemoreceptor sensitivity). Training via calm exposure—such as progressive breath-hold tables or mindfulness—desensitises this pathway, aligning with study observations of prolonged easy phases independent of significant CO2 shifts.

Broader Applications: Breathwork Therapies

Beyond freediving, apnoea training principles inform therapeutic breathwork, particularly in managing stress, anxiety, and respiratory conditions. Controlled breath holds enhance CO2 tolerance, reducing hyperventilation tendencies in anxiety disorders. The dive reflex’s calming effects (bradycardia, parasympathetic dominance) are leveraged in practices like cold-water immersion therapy, which can lower heart rate and promote relaxation. For example, protocols inspired by freediving, such as 1:1 breath-hold cycles, are being explored in clinical settings to improve autonomic regulation.

Safety Implications and Future Directions

A 1:1 ratio appears viable for submaximal, static training, stabilising physiology and potentially reducing blackout risk by maintaining oxygen levels. However, low CO2 may delay air hunger cues, increasing hypoxia risks in prolonged holds. For beginners or surf apnoea (short, repeated holds under stress), understanding the dive reflex fosters confidence, as holds rarely exceed the easy phase. Coaches must educate clients on physiological cues and avoid over-breathing to ensure safety.

Future research should:

  • Test ratios at 50% of individual maximum holds to reflect real-world demands.

  • Compare static vs. dynamic or water-based apnoea to account for immersion and movement.

  • Investigate cortical modulation using neuroimaging to quantify neural gain effects.

  • Assess longer holds (>2 minutes) and the impact of metabolic acidosis in dynamic apnoea.

  • Explore female-specific physiological responses, including menstrual cycle influences.

In conclusion, freediving physiology underscores the body’s remarkable adaptability, from the mammalian dive reflex to neural modulation. Whilst a 1:1 training ratio offers a safe starting point for many, personalised approaches—considering experience, context, and physiological markers—are essential to balance performance and safety. These insights not only enhance freediving training but also inform therapeutic breathwork, highlighting the versatility of apnoea principles.

References

Mulder, E. R., Bouten, J., Holmström, P. K., & Schagatay, E. K. (2025). Progressive changes of oxygenation, diving response, and involuntary breathing movements during repeated apneas. Respiratory Physiology & Neurobiology, 336, 104455. https://doi.org/10.1016/j.resp.2025.104455