Your diaphragm competes with your legs for blood

Late in a hard effort the breathing muscles fatigue and trigger a reflex that redirects flow away from the limbs doing the work.

It feels like the legs give out. Quite often the argument was settled somewhere else, by a muscle nobody trains and few people think of as a muscle at all.

Breathing at competition intensity is expensive. The diaphragm and the accessory muscles can claim a substantial share of total oxygen consumption during heavy work, and like any other muscle they fatigue when they have been contracting hard for a long time. When they do, they signal it. Metabolic byproducts accumulating in the respiratory muscles stimulate sensory fibres that trigger a sympathetic reflex, and the effect of that reflex is vasoconstriction in the working limbs. Flow gets reprioritised toward the muscles keeping you breathing, because breathing is the older and more urgent problem.

The consequence at the leg is a reduced oxygen supply arriving exactly when demand is highest. Fatigue accelerates. The athlete perceives heaviness and burning in the quadriceps and reasonably concludes that their legs are the limitation, and they are, at that instant, but the cause is upstream in the chest.

This reframes a few things I hear constantly. Being out of breath is not a sign that you should train your lungs, since lung capacity is rarely the limit in a healthy athlete. It is a sign that the pump moving air has a workload of its own. It also explains why efforts that involve trunk stiffening, such as heavy carries or rowing or a contested tackle sequence, feel disproportionately exhausting. The diaphragm is being asked to stabilise the trunk and ventilate at the same time, and it cannot fully do both.

Altitude, illness and asthma all sharpen the same mechanism by making each breath cost more.

I am cautious about the training claim that follows, because dedicated inspiratory muscle work has produced mixed results and is easy to oversell. What I do not doubt is the physiology, which has been shown by loading and unloading the breathing muscles and watching limb blood flow respond within seconds.

The useful takeaway is diagnostic rather than commercial. When an athlete's legs die in the last quarter of every race and their aerobic testing looks fine, look at what their breathing was doing in the third quarter.

There is a competition for blood happening in the middle of a body that only has so much of it.