Apnea has long fascinated exercise physiologists. Free divers routinely show higher circulating hemoglobin than untrained people, and researchers have spent two decades trying to work out whether deliberately holding the breath can produce the same kind of adaptation seen in altitude training — without the altitude. A study published this month in Physiological Reports adds some of the clearest evidence yet that it can, at least under the right protocol.
How researchers tested a structured breath-hold protocol on cyclists and military recruits
The research team, led by Matthew Burley and colleagues at the University of Exeter, ran two separate studies using the same breath-hold technique: fifteen breath-holds performed lying down, starting at 30 seconds and increasing by 10 seconds each round up to 2 minutes 50 seconds, with a minute of normal breathing between each hold. The whole session takes about 40 minutes.
In Study A, eleven healthy men did a single session of this protocol before a 15-minute steady-state cycling bout and a 5-minute time trial, compared with a resting control condition. In Study B, 48 US Marine Corps officer candidates were split into a training group and a control group and completed the protocol five days a week for three weeks, folded into their normal officer candidate training schedule, with performance tracked through the standard 3-mile run test.
A single 40-minute breath-hold session boosted cycling performance by over 8%
The acute results were striking for a one-off intervention. After the 40-minute breath-hold session, participants produced 8.2% more total work during the cycling time trial than after simply resting (84.4 kJ vs 78.0 kJ). Hemoglobin concentration rose by 3% during the breath-hold session itself, and blood lactate during steady-state cycling trended lower in the breath-hold condition, suggesting participants were relying less on anaerobic metabolism at the same workload. Heart rate patterns and end-tidal gas measurements pointed to a genuine shift in gas exchange, not just a placebo response — though a psychological contribution, like reduced perceived effort, can’t be ruled out either.
Three weeks of daily training improved 3-mile run times in Marine Corps candidates
The chronic results are arguably more relevant to anyone thinking about breath-hold work as a training tool rather than a one-off trick. Both groups improved their 3-mile run time over the three-week training block, as expected from standard officer candidate training. But the breath-hold group improved more — 7.2% versus 4.9% in the control group. Hemoglobin concentration rose by 4.3% in the breath-hold group and actually fell in the control group, a pattern the researchers link to training-induced plasma volume expansion diluting red cell concentration in the untrained comparison. Maximal breath-hold duration itself increased by well over 100% in the trained group, more than in controls, showing the expected training effect on tolerance to breath-hold itself.
What’s driving the improvement: hemoglobin, oxygen delivery, and lactate tolerance
The leading explanation is that structured breath-holding creates a combined hypoxic and hypercapnic stimulus — low oxygen and high carbon dioxide — strong enough to trigger the same kind of physiological response associated with altitude exposure: more circulating hemoglobin, better buffering of the acid load produced during hard exercise, and possibly reduced reliance on anaerobic pathways at a given intensity. It’s the same mechanism proposed to explain why free divers carry higher baseline hemoglobin than the general population. The authors are careful to note that spleen contraction and erythropoietin weren’t directly measured in this study, so the exact biological pathway remains an inference from prior literature rather than something demonstrated here directly.
What seems to matter most is dose. Earlier studies using shorter, non-progressive breath-holds — three to five holds near maximal capacity, with short rests — have mostly failed to move the needle on VO2max or hemoglobin. The protocol here is different: it progressively pushes breath-hold duration well past what participants would choose unprompted, sustaining the hypoxic-hypercapnic load for closer to 40 minutes rather than a few minutes. That distinction lines up with earlier work from the same group and with a study by Zoretić and colleagues, who found similar hemoglobin and VO2max gains using extended breath-holds during walking.
Where this research fits in the broader science of apnea and breath-hold training
This sits squarely in the same research territory as much of the work on structured breathing and apnea: free diver adaptation, altitude and hypoxic-hypercapnic training, and voluntary breath-hold practice more broadly. The specific protocol tested here is a progressive breath-hold sequence, building from 30 seconds up to nearly three minutes across a session — a different shape than the hyperventilation-then-hold rounds used elsewhere in the field, though built on the same underlying physiology of oxygen and carbon dioxide tolerance. It’s a reminder that this is an active and still-developing area of research, with different protocols being tested for the same broad question: how much of the diver’s advantage can be trained deliberately, and what’s the most effective way to do it.
For anyone following the science of breathwork and apnea, that makes it a genuinely useful data point: gains that show up after a single 40-minute session, and that compound into measurable endurance improvements over three weeks of repeated practice, in a population — military candidates already in hard physical training — where a small edge matters. The authors frame it as an early but promising result, calling for larger trials that directly measure spleen response and erythropoietin to pin down the mechanism.