Borrowed Blood

Five minutes into a hard interval, your legs go heavy. But something else changes too — your breathing turns ragged, your chest feels tight, and it's not obvious which one is actually slowing you down.

Here's the thing: your diaphragm is a muscle, just like your quads. And like any muscle worked hard enough, for long enough, it fatigues.

At rest, breathing costs your body almost nothing. But push into a hard effort and the muscles used in breathing can eat up something like 10 to 15 percent of your total oxygen consumption — and they're drawing that oxygen from the same shared blood supply as your legs and arms. 

When those breathing muscles start to fatigue, metabolic byproducts (waste products) start to accumulate and trigger something called the respiratory metaboreflex.  That reflex “steals” blood flow from those hardworking leg muscles and shunts it towards your respiratory muscles.

There's real research behind all of it — nose versus mouth, timed breathing routines, resistance devices that promise more power without another hard pedal stroke. Some of it holds up. Some of it's noise. Worth knowing the difference.

Your lungs have been doing this unsupervised for years. Time to give them a coach.

Explore more: 

Read: Roadman Cycling’s deep-dive on breathing and respiration

Watch: Nasal Breathing for Performance with Steve Magness

Listen: Matching Breathing to Cycling Cadence

See you next week,

Steve