How Running Completely Changes The Human Body

Start a free two-week trial of BWS+ here: https://bws.plus/19a I did 30 minutes of cardio a day (running every day for 30 days) while still lifting to settle the debate every lifter has: does cardio kill gains? It comes down to questions like cardio before or after weights, whether cardio and weight training can coexist, the problems with running vs lifting, and the two fears that keep people from running at all — is cardio bad for muscle gain, and does running make you lose muscle? To find out, I tested my VO2 max, got an MRI body scan, used a DXA scan, and trained for Hyrox. Going into this, I hated running. I lifted consistently but barely did cardio. So when Browney invited me to represent Team Canada alongside Will Tennyson in an international Hyrox race, I had 30 days to build cardio without losing muscle. On Day 1, my VO2 max came back at 58 — around the top 2% for my age. But the deeper data showed two problems: my body was relying too heavily on carbs instead of using fat efficiently, and my lactate levels were extremely high. I had a strong engine but couldn't sustain it for a full race. So the plan was simple but brutal. I settled the cardio before or after weights question by lifting first and doing cardio after or later in the day, then built my aerobic base with Zone 2 cardio, added intervals like the Norwegian 4x4, and practiced Hyrox stations like the sled push, SkiErg, rowing, and wall balls. Then after my first run, I hit a setback: brutal foot pain. My physiotherapist Rajan suspected I had inflamed one of the joints and tendons across the bottom of my foot, and for a moment I was 50/50 about continuing. I couldn't run 60 seconds, but I was supposed to run 8km broken up by workout stations. For the first few days, I swapped running for cycling and the assault bike to keep training without hammering my foot. But the bigger fear was still there: is cardio bad for muscle gain, and would all this cardio cost me muscle? To answer does running make you lose muscle, I got MRI scans of my upper and lower body before the experiment. These measured muscle thickness and cross-sectional area, so I'd know if and where I lost it. I also used DXA scans to track body fat. The research was reassuring. A 2021 meta-analysis found that people who combined cardio and weight training saw similar muscle growth to lifting alone. But this was mainly true for beginners, while more trained lifters saw a slightly negative effect. So I followed three rules: no hard cardio before weights, refuel properly, and protect leg days. That meant more carbs than usual. Even though I was eating around 2,600 calories a day, the extra training pushed me into a bigger deficit than expected. After two weeks, I was down almost 4 pounds, so I increased my intake to 2,800 calories. As the weeks went on, running everyday started to feel better. I finally understood the runner's high — a feeling often linked to a compound called anandamide, the so-called "bliss molecule." Despite all the extra cardio, I hit a volume PR on the hack squat. That may be partly because better lactate clearance and energy production can help during higher-rep lifting, but the real test was the final MRI scan. With just over two weeks left, I did my first mock Hyrox with Jeff. We ran 800 meters into each station and repeated the circuit, aiming to finish 80% of a Hyrox in under an hour. We finished in 55:21, and for the first time, doing well actually felt possible. Then Will had to drop out, so Jeff became my new teammate. One week before the race, we tried to complete a full Hyrox at the pace we thought we'd need to match the strongest teams. We made it about 70% of the way. So the focus shifted from conditioning to race strategy. Before London, I spoke with Cole Learn, a pro Canadian Hyrox competitor. His advice: control heart rate from the start and treat the rower like a recovery station so we wouldn't burn out early. After 30 days, I repeated the tests. My VO2 max went from 58 to 61. My body's ability to burn fat instead of carbs improved dramatically. My DXA scan showed my body fat dropped by 1%. And the MRI results showed no upper-body muscle loss. I didn't gain upper-body muscle, but I didn't lose it either. My lower body even showed a small trend toward muscle growth, possibly from indirect leg volume in Hyrox training. Then came race day in London. Team Korea looked strong. Team France had a Hyrox champion. Team USA looked way stronger than expected. Jeff and I started conservatively, stuck to the plan, and moved from around 10th into 4th. By the final wall balls, there was one podium spot left. We crossed the finish line in third place. So, does cardio kill gains? In this 30-day experiment, no. But adding cardio aggressively while lifting demanded smarter timing, more carbs, better recovery, and more humility than I expected. Click below to subscribe for more videos: https://www.youtube.com/jeremyethier/?sub_confirmation=1