Jeremy Ethier set out to answer one of fitness’s most debated questions: does cardio kill muscle gains?
In a recent YouTube video, the Built With Science founder spent 30 days running every day while continuing his regular strength training in preparation for an international HYROX race in London. Along the way, he tracked every major change using VO2 max testing, MRI muscle scans, and DEXA body composition assessments.
A Strong Start, But Clear Weaknesses
Ethier’s initial VO2 max score of 58.3 placed him among the top 2% for his age. However, deeper testing revealed poor fat utilization and unusually high lactate accumulation. To address these issues, he implemented a combination of Zone 2 training, HYROX-specific workouts, and high-intensity intervals, including the Norwegian 4×4 protocol.
The challenge nearly ended after his first run when severe foot pain limited him to less than a minute of running.
“I’m realizing it’s not enough to just be in shape,” Ethier said. “My joints, tendons, and even bones had never been trained for running.”
Did Cardio Cost Him Muscle?
One of Ethier’s biggest concerns was whether the added endurance work would negatively affect his physique. To minimize interference with muscle growth, he followed three key rules: perform cardio after lifting, increase carbohydrate intake, and avoid compromising leg-day performance.
As the weeks progressed, his perspective on endurance training also began to change.
“Something about pushing yourself with cardio, it gives you a high that you can’t replace with weights,” he explained.
The Final Results
After 30 days, Ethier improved his VO2 max from 58.3 to 61.3, increased his ability to use fat as fuel, and reduced body fat by 1%. Most importantly, MRI scans showed no loss of upper-body muscle mass, while his lower body remained stable despite the increased training load.
The experiment culminated in a third-place finish at the HYROX event, where Ethier and teammate Jeff fought through a stacked international field. Reflecting on the experience, Ethier summed up the biggest lesson from the challenge:
“When you’re scared of failing, when everything in your mind is telling you to quit, keep going.”
For lifters worried that cardio automatically destroys gains, his results suggest the opposite may be true when training, nutrition, and recovery are managed correctly.
About the Author
Jeremiah Oliva
Jeremiah Oliva is a writer passionate about fitness, sports, and active living. He has experience in songwriting and managing content and social media for online radio and magazine platforms.
He covers HYROX, CrossFit®, and competitive fitness, with a focus on performance, mindset, and athlete development.
Outside of writing, Jeremiah trains in boxing, cycles, explores the outdoors with his kids, and plays the guitar.
