Breathing is the most fundamental function in endurance sport, yet it receives a fraction of the attention devoted to training load, nutrition, and gear. Most athletes breathe in patterns developed through habit rather than training — and in many cases, those patterns are suboptimal for endurance performance. The cost of breathing — the oxygen consumed by respiratory muscles just to power inhalation and exhalation — accounts for 3–5% of total oxygen consumption at easy effort and rises to 10–15% at near-maximal intensities. Reducing this respiratory oxygen cost through technique optimisation directly improves how much oxygen is available for locomotor muscles.
The diaphragm is the primary muscle of breathing — a dome-shaped muscle sitting beneath the lungs that contracts to create negative pressure and draw air in. At rest and low exercise intensity, diaphragmatic breathing is effortless and efficient. As exercise intensity increases, most untrained athletes progressively shift to thoracic breathing (chest lifting, shoulders rising) — a pattern that activates less efficient accessory muscles and reduces tidal volume compared to full diaphragmatic excursion. Elite endurance athletes maintain more efficient diaphragmatic breathing patterns at higher intensities, contributing to lower perceived effort at the same workload.
Nasal vs Mouth Breathing
Nasal breathing during low-intensity exercise (Zone 1–2) offers several physiological advantages over mouth breathing: (1) Air is humidified and warmed, reducing respiratory water loss by approximately 33% during long efforts; (2) Nasal airflow stimulates production of nitric oxide in the paranasal sinuses — a powerful vasodilator that improves pulmonary circulation; (3) Nasal breathing increases airflow resistance, creating a modest elevation in CO2 retention that improves carbon dioxide tolerance over time. Research has shown that habitual nasal breathing training over 6 weeks improves CO2 tolerance by 15–25%, reducing the respiratory drive sensation that limits pacing confidence at threshold intensities. At high intensities above lactate threshold, mouth breathing is necessary and appropriate — the airflow rate required exceeds nasal capacity.
Rhythmic Breathing for Runners
Running's cyclical impact creates an opportunity for synchronised breathing patterns that reduce respiratory muscle strain and improve efficiency. Research by Bramble and Carrier demonstrates that most experienced runners entrain breathing to footstrike in consistent ratios — typically 3:2 (3 footstrikes per inhale, 2 per exhale) at easy pace, 2:1 at moderate pace, and 2:2 or 1:1 at high intensity. The practical benefit of deliberate rhythmic breathing is load distribution: if you always exhale on the same footstrike, that leg's muscles consistently absorb ground impact while your diaphragm is contracting — asymmetric loading that can contribute to side stitches and lateral core fatigue over distance. Experiment with both-side exhalation patterns to distribute load more evenly across the 30,000+ footstrikes in a marathon.
Diaphragmatic Strengthening: Inspiratory Muscle Training
The diaphragm is a muscle that responds to training. Inspiratory muscle training (IMT) — using a device that adds resistance to inhalation — has been studied extensively in endurance athletes. A 2021 meta-analysis of 21 randomised trials found that IMT over 4–8 weeks improved endurance performance by an average of 4.7% and reduced perceived exertion at submaximal workloads. Practical IMT protocol: 30 inspiratory efforts at 50–75% of maximal inspiratory pressure, twice daily, 5 days per week. Commercial IMT devices are widely available and the protocol requires only 2–3 minutes per session. Outside of device training, swimming (which creates forced exhalation against water resistance) develops respiratory muscle strength with secondary endurance benefits.
CO2 Tolerance and Breathing at Race Intensity
At race intensities near or above lactate threshold, many athletes overbreathe relative to their metabolic CO2 production — a pattern called hyperventilation that paradoxically reduces performance by lowering blood CO2 levels, constricting cerebral blood flow, and increasing perceived breathlessness. Controlled exhalation — deliberately extending the exhale to 1.5–2x the length of the inhale — is the most effective real-time technique for reducing hyperventilation and restoring CO2/O2 balance during high-intensity efforts. Practise this in hard training intervals before using it in racing. Respiratory efficiency and metabolic fueling interact directly: adequate carbohydrate availability reduces the ventilatory response to exercise (less lactic acidosis means less CO2 production demanding ventilation). Athletes who under-fuel training sessions experience increased perceived breathing difficulty at a given pace — another reason to prioritise carbohydrate availability in quality sessions. Use the NorthLine Race Day Nutrition Planner to ensure your fueling keeps respiratory demand aligned with your target effort during key sessions and races.