We all know that awful feeling — trying to run right after heavy lunges or deadlifts and your legs just won’t move.
In the latest video from Wod Science, researchers at Adidas HQ tested elite CrossFit and HYROX athletes to find out exactly why this happens.
They tracked oxygen use, muscle fatigue, and energy systems before and after tough functional workouts. The results were eye-opening — and could change how you train.
This article breaks down what the data showed, why your running gets worse, and how to fix it. If you want to run faster after lifting heavy, keep reading.
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What Is Running Economy and Why It Matters
Running economy refers to the amount of oxygen your body uses at a given running speed.
If two athletes are running at 14 km/h and one consumes 40 ml/kg/min of oxygen while the other uses 55 ml/kg/min, the latter has worse running economy — they’re using more fuel to move at the same pace.
Running economy is crucial in hybrid sports like CrossFit and HYROX, where athletes are required to perform both strength and endurance tasks, often back to back. Any inefficiency means more fatigue and slower times.
The Adidas Study: Measuring the Effects of Functional Movements on Running
To explore how running economy is impacted by functional workouts, scientists at Adidas HQ ran a test on elite athletes.
Here’s what they did:
- Athletes completed a 6-minute submaximal run, during which oxygen uptake and biomechanics were measured.
- Immediately after, they performed a high-intensity functional workout:
- For CrossFit: A 21-15-9 set of deadlifts and walking lunges.
- For HYROX: 100 meters of front rack walking lunges.
- Then, athletes completed another 6-minute submaximal run, and the same metrics were measured again.
The goal? To see how high-intensity strength work would affect their efficiency in running.
What the Data Showed: Running Economy Drops Dramatically Post-Workout
In one CrossFit athlete, oxygen consumption increased by 12.5% during the second run — even though the running speed was identical to the first. That’s a clear indication of reduced efficiency.
Put simply: the athlete needed significantly more oxygen to maintain the same pace.
Key Findings from the Data
- Same VO₂, Different Source: VO₂ stayed the same between running and functional movements (~40 ml/kg/min), but the respiratory exchange ratio (RER) shot up during functional tasks. RER values hit 1.2+, meaning nearly all energy came from anaerobic carbohydrate metabolism — a stark contrast to the aerobic dominance during the run.
- Crossover to Fatigue: This switch toward anaerobic energy pathways is typical of movements like wall balls, deadlifts, or sled pushes — they create localized muscle fatigue and systemic lactate buildup, making post-workout running feel brutally inefficient.
- HYROX Athletes Handle It Better: An Elite 15 HYROX athlete showed only a 4.4% increase in oxygen consumption post-lunges, suggesting better fatigue resistance. The nature of HYROX — where athletes train for compromised running specifically — plays a major role.
Why Does Running Economy Drop After High-Intensity Work?
The scientists outlined three physiological reasons behind the efficiency drop:
1. Blood Flow Redistribution
After high-intensity leg movements, blood is redirected toward fatigued, working muscles like the quads and glutes. This can deprive stabilizing muscles (such as those supporting ankle or knee movement) of oxygen-rich blood, disrupting running mechanics and economy.
2. Acidosis and pH Drop
Functional strength work causes a buildup of hydrogen ions and lowers pH in the muscle environment (a state known as acidosis). This acidic shift inhibits mitochondrial enzymes, making it harder for muscles to use oxygen efficiently.
“Fatigue can make your mitochondria less efficient. You need more oxygen to produce the same amount of energy,” said the lead researcher.
3. Reduced Oxygen Extraction
Fatigued muscles struggle to extract oxygen from the blood. Using near-infrared spectroscopy (NIRS), previous research has shown that fatigued tissue shows altered oxygen saturation patterns, requiring more blood flow to do the same work.
Practical Takeaways: How to Train Smarter
The Adidas team didn’t just study elite athletes — they drew clear, actionable insights from the data.
Here’s how to use it in your own training:
1. Don’t Introduce Compromised Running Too Early
Many gyms introduce compromised running workouts (e.g., run after wall balls or sleds) too soon — before athletes have developed a solid aerobic base.
Instead, build a phase of pure running volume first.
“Implementing compromised running too early will just fully decrease running economy and shift the focus away from the right things,” the scientist warned.
2. Test Your Own Running Economy
Use a wearable or mobile app that tracks running biomechanics.
- Run a baseline test (pre-fatigue).
- Do a short, heavy functional workout (4–5 minutes).
- Repeat the run and compare metrics.
This gives you insight into how fatigue affects your form, not just the lab averages.
3. Add Running Drills and Plyometrics
Running economy improves with better neuromuscular efficiency and tendon stiffness. Drills like A-skips, bounding, and plyometric hops help improve both.
Include:
- Jump rope or box jumps
- Stride mechanics drills
- Strength training (especially posterior chain)
All of these help improve energy transfer and reduce ground contact time — key factors in running efficiency.
Final Thoughts
Running economy isn’t just a buzzword — it’s a direct measure of how efficiently your body turns oxygen into movement. In hybrid sports like CrossFit and HYROX, where fatigue is a constant companion, optimizing this metric could mean shaving minutes off your time.
As the Adidas data shows, how you train — and when you introduce compromised running — makes a massive difference. Focus on building a solid aerobic engine first, strengthen your biomechanics, and use targeted strength to support rather than sabotage your stride.
Your lungs, your legs, and your leaderboard rank will thank you.
