David Jacko Jackson played over 300 games for Nottingham Rugby Football Club. In 2013, a month after getting married, he had a seizure on the pitch and a small bleed on the brain. His rugby career was over.

What followed was months of emotional dysregulation, extreme fatigue, sensitivity to light, and a nervous system so disrupted he once burst into tears in a supermarket because he couldn't choose a yogurt. Doctors told him to sit in a dark room and wait. Nobody talked about breathing.

Years into his recovery, Jacko stumbled across research connecting traumatic brain injury with disrupted respiratory patterns, and everything changed. By retraining his breathing mechanics, he knocked a minute off his 5K time in four weeks. He retrained as a strength and conditioning coach, worked with the British Paralympic swim team ahead of Rio, and eventually built an entirely new career around what he'd discovered. His book, Breathe Smarter, Run Stronger, with a foreword by Patrick McKeown of The Oxygen Advantage, came out two months ago.

This week, Rich sits down with Jacko to talk about the brain injury, the recovery, and why breathing might be the most overlooked performance tool in every runner's kit bag. They get into the CO2 science, why your perception of effort is more closely linked to your breathing rate than your heart rate or blood lactate levels, and the three things that should always be in play regardless of whether you breathe through your nose or your mouth.

They also talk about what Jacko is doing next - running 200 marathons in 100 days for two brain injury charities, Headway UK and Head for Change, starting the day after his 44th birthday and taking a clockwise loop of the UK. You can join him for as little as a 5K at 25 locations along the route.

Find Jacko at thebreathrunningcoach.com or on Instagram @TheBreathRunningCoach.