VO2 Max Jumped From 58 to 61 in Just 30 Days of Running (That’s 99th Percentile). Here’s the Training Method He Used

Running every day sounds like a dream for some, a nightmare for others.

But what if someone tested exactly what happens to muscle mass, cardio fitness, and body composition when combining daily running with weight training for 30 straight days?

Popular fitness YouTuber Jeremy Ethier did exactly that, using MRI scans, VO2 max testing, and multiple performance assessments to track every change.

His mission: prepare for an international fitness race called Hyrox while discovering whether cardio truly “kills gains.”

Starting Point: Better Than Expected

Ethier began his experiment with a VO2 max test, which measures cardiovascular fitness. Higher scores indicate better endurance capacity.

Average gym-goers typically score in the mid-30s to 40s, while elite athletes like Cristiano Ronaldo allegedly reach 75. Despite rarely running before this challenge, Ethier scored an impressive 58.3 on day one, placing him in the top 2% for his age.

So your VO2 peak was 58.3. It’s really good results.

But deeper analysis revealed critical weaknesses. His body burned through carbohydrate stores too quickly instead of utilizing fat for sustained energy. Even worse, his lactate clearance was dangerously poor—20 minutes post-test, his levels measured 10.5, which is alarmingly high.

Immediate Setback: Injury Strikes

After just his first run, Ethier woke up with sharp pain shooting through his foot. Every step felt like bone fractures.

Emergency physiotherapy revealed he’d likely irritated joints or tendons—common for lifters unaccustomed to running’s repetitive impact. His physiotherapist Rajan delivered sobering news about recovery time.

Best case scenario, it’ll probably get better in a couple weeks with a little bit of rest.

With only 30 days total preparation time, losing two weeks was devastating. Ethier seriously considered withdrawing from the competition entirely.

Adapting Training While Injured

Rather than give up completely, Ethier pivoted. He substituted running with cycling and assault bike sessions during recovery—both easier on injured joints while maintaining cardiovascular stimulus.

This adaptive approach kept conditioning progress alive without risking further damage.

Testing Muscle Loss: MRI Scans Reveal Truth

Day one included comprehensive MRI scans measuring muscle thickness and cross-sectional area throughout Ethier’s upper and lower body. These would be compared against day 30 scans to definitively answer: Does cardio kill gains?

Research offered mixed signals. A 2021 meta-analysis of 27 studies found beginners experienced nearly identical muscle growth whether combining cardio with weights or lifting alone. However, trained individuals showed slightly negative effects from adding cardio.

Three Critical Rules To Prevent Muscle Loss

Scientists identified specific strategies lifters must follow when adding significant cardio:

• Never do intense cardio before lifting – Long or hard sessions deplete energy reserves and tax the nervous system, compromising strength performance
• Fuel properly with carbohydrates – Extra cardio burns through glycogen stores rapidly. Quick carbs like bananas pre-workout and complex carbs like rice post-workout become essential
• Protect leg day recovery – Running heavily taxes lower body muscles. Schedule carefully to ensure legs recover adequately for weight training

Ethier implemented all three religiously throughout his 30 days.

Strategic Training Split: Zone 2 Plus Norwegian Method

To address his fat-burning and lactate clearance deficiencies, Ethier designed a weekly cardio protocol combining two distinct training styles.

Zone 2: Building Aerobic Base

Two weekly sessions focused on zone 2 training—maintaining heart rate at 60-70% maximum (120-140 bpm for Ethier). At this intensity, conversation remains possible without heavy breathing.

Zone 2 trains the body to efficiently burn fat for fuel and improves lactate clearance. The catch? Sessions require 45-60 minutes minimum for meaningful adaptation—painfully boring but scientifically necessary.

Norwegian Method: Elite Performance Protocol

Three times weekly, Ethier employed high-intensity intervals used by Norwegian endurance athletes. The protocol is brutally simple:

• Four minutes maximum effort
• Three minutes easy recovery
• Repeat four total rounds

This 30-minute session produces remarkable VO2 max improvements. Norway, with just 5.5 million citizens, consistently produces world-class endurance athletes partly through this method.

Understanding Hyrox: The Brutal Competition Format

Ethier wasn’t just running—he was preparing for Hyrox, considered one of fitness’s toughest racing formats. Competitors alternate between 1km runs and brutal stations eight times consecutively.

Stations include ski erg, sled push, sled pull, burpee broad jumps, rowing, farmer’s carries, sandbag lunges, and finally 100 wall balls—where many competitors’ legs completely stop functioning. Elite athletes finish under 60 minutes.

Once his foot healed, Ethier enlisted physiotherapist and Hyrox veteran Rajan to teach station techniques and pacing strategies.

You’re a bodybuilder. You haven’t been an endurance athlete before. So, you haven’t done hybrid training. So, I think that transition for you is going to be tough.

Unexpected Benefit: The Runner’s High

Around week two, Ethier experienced something that changed his entire perspective on cardio.

It was tough, but like I feel incredible. Something about like pushing yourself with cardio, it just like gives you a high that you can’t replace with weights.

This phenomenon has scientific basis. During moderate-to-high intensity cardio sustained for 20-40 minutes, your body releases anandamide—nicknamed “the bliss molecule.”

Anandamide binds to the same brain receptors as THC in cannabis, creating euphoric sensations chemically similar to getting high. Weight training rarely triggers this because short intense sets followed by rest don’t allow sustained effort long enough for anandamide accumulation.

This “healthy high” became one of Ethier’s biggest training motivators moving forward.

Weight Loss Concerns: Adjusting Nutrition

Midway through, Ethier noticed he’d dropped 3 pounds despite eating 2,600 daily calories—normally his maintenance level. Rapid weight loss risked muscle catabolism.

His Built With Science app, which tracks weight trends and automatically adjusts nutrition targets, bumped his calories to 2,800 per day based on detected weight loss rate. This recalibration helped stabilize body composition throughout remaining training.

Practice Run: Testing Race Readiness

With one week remaining, Ethier attempted 80% of full Hyrox distance alongside friend and four-time Hyrox competitor Jeff. His goal: finish in under 60 minutes to gauge competitiveness.

Despite hitting breaking points where his body screamed to quit, Ethier pushed through mentally. Final time: 55 minutes 21 seconds—a promising result indicating genuine race readiness.

Then disaster struck again. Original partner Will Tennyson withdrew due to scheduling conflicts, leaving Ethier partnerless just days before competition.

Final Results: Cardio’s Impact On Muscle And Performance

Before flying to London for the international competition, Ethier completed his final assessments.

VO2 Max Improvement

Retesting revealed his VO2 max increased from 58.3 to 61.3 ml/kg/min—reaching the 99th percentile and representing significant cardiovascular adaptation in just 30 days.

61 is in the 99th percentile. You did excellent.

Additionally, his fat oxidation capacity dramatically improved, addressing the original metabolic inefficiency identified day one.

Muscle Mass: The Verdict

MRI scans provided definitive answers. Upper body muscle showed zero change. Lower body muscle also remained completely stable—no muscle loss occurred despite daily cardio combined with weight training.

DEXA scan revealed body fat dropped 1%, achieved while consuming more calories than typical cutting phases—demonstrating cardio’s metabolic benefits when properly fueled.

Unexpected Strength Gains

Perhaps most surprisingly, Ethier set a volume PR (personal record) during week three despite accumulated training fatigue. How?

Cardiovascular training improves lactate clearance and cellular energy production. Since lactate and accompanying hydrogen ions create muscular burn during higher-rep strength training, better clearance allows pushing closer to failure with less discomfort.

Simultaneously, enhanced mitochondrial function helps muscles work harder and recover faster between sets—potentially explaining why recent research shows cardio may actually enhance rather than hinder muscle gains when implemented strategically.

Race Day: Putting 30 Days To The Test

Competing against 16 countries including winners from Physical 100 (Korea’s elite fitness show), French Hyrox champions, and intimidatingly strong American athletes, Ethier felt outmatched initially.

Fortunately, organizers allowed Jeff to replace Will as Ethier’s partner, forming Team Canada. Professional competitor Cole Learn provided last-minute race strategy, emphasizing controlled pacing over early aggression.

The biggest thing for HYROX is maintaining your heart rate off of the start. If your heart rate is too high, your muscles are not going to work the way they should.

When the race started and competitors surged ahead, Ethier’s instinct was panic—sprint to catch up or face embarrassment. Instead, he trusted his training and maintained planned pace.

The strategy worked. Team Canada steadily climbed standings, eventually crossing the finish line in third place—securing podium position against world-class competition.

Key Takeaways For Lifters Considering Cardio

Ethier’s 30-day experiment provides valuable insights for anyone worried about cardio interfering with strength and muscle goals:

• Muscle loss isn’t inevitable – With proper nutrition and training sequencing, daily cardio caused zero measurable muscle loss
• Performance can improve – Enhanced lactate clearance and energy production may actually boost strength training quality
• Timing matters critically – Never do intense cardio before weights; schedule it after lifting or later in the day
• Fuel accordingly – Significantly increase carbohydrate intake to match increased energy expenditure
• Build gradually – Untrained joints, tendons, and bones need time adapting to running’s impact forces
• Combine training styles – Mix zone 2 aerobic work with high-intensity intervals for comprehensive adaptation
• Mental benefits are real – Runner’s high from anandamide release provides unique psychological benefits weights can’t replicate

For lifters hesitant about cardio destroying gains, Ethier’s scientifically-tracked journey demonstrates that strategic implementation not only preserves muscle but potentially enhances overall fitness and performance.

The key lies in respecting recovery needs, fueling properly, and viewing cardio as complementary rather than contradictory to strength training goals.