Running Daily for 30 Days: Jeremy Ethier Explores the Effects on Muscle Growth and Endurance
Jeremy Ethier, the founder of Built With Science, aimed to tackle a common question in fitness: does cardio impact muscle growth?
In a recent YouTube video, Ethier documented his experience of running every day for 30 days while maintaining his strength training. This was part of his preparation for an international HYROX competition in London. Throughout this month, he monitored his body’s changes using VO2 max tests, MRI muscle scans, and DEXA body composition assessments.
**A Strong Start, But Clear Weaknesses**
Ethier began with a VO2 max score of 58.3, placing him in the top 2% for his age group. However, further tests showed that he had trouble effectively using fat for energy and faced high levels of lactate accumulation. To tackle these problems, he incorporated Zone 2 training, HYROX-specific workouts, and high-intensity intervals, particularly the Norwegian 4×4 method.
His running journey faced a challenge right from the start when severe foot pain restricted him to less than a minute of running. "I’m realizing it’s not enough to just be in shape," Ethier mentioned. "My joints, tendons, and even bones had never been trained for running."
**Did Cardio Cost Him Muscle?**
Ethier worried that increased endurance training might harm his physique. To prevent interference with his muscle growth, he adhered to three essential rules: perform cardio after weightlifting, increase his carbohydrate intake, and protect his leg-day workouts.
As he progressed, his attitude towards endurance training began to shift. "Something about pushing yourself with cardio, it gives you a high that you can’t replace with weights," he said.
**The Final Results**
After completing 30 days of daily running, Ethier improved his VO2 max from 58.3 to 61.3, enhanced his fat utilization for fuel, and reduced his body fat by 1%. Most notably, MRI scans indicated no upper-body muscle loss, while his lower body remained stable despite the increased training intensity.
The experiment concluded with Ethier finishing third at the HYROX event, where he and his teammate Jeff competed against a highly competitive field. Reflecting on the experience, Ethier shared an important lesson: "When you’re scared of failing, when everything in your mind is telling you to quit, keep going."
For those worried that cardio might harm their gains, Ethier's results suggest that proper management of training, nutrition, and recovery could yield the opposite effect.
**About the Author**
Jeremiah Oliva is a writer dedicated to fitness, sports, and healthy living. He has experience in songwriting and managing content for radio and magazine platforms. Oliva covers HYROX, CrossFit®, and competitive fitness, focusing on performance, mindset, and athlete development. Outside of writing, he enjoys boxing, cycling, and exploring the outdoors with his children, along with playing guitar.
