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Or get your hybrid score firstThe Interference Effect: Does Running Kill Your Gains?
The interference effect is the reason every powerlifter's training partner warns them not to start running. "Cardio kills gains." You've heard it. Maybe you believe it. Here's what actually happens, with my own numbers as evidence.
The science, briefly
In 1980, Robert Hickson published a study showing that combining strength and endurance training produced smaller strength gains than strength training alone. This became the foundation of the "interference effect" — endurance training blunts strength adaptations at the molecular level (AMPK vs mTOR signaling, if you care about the mechanism).
The research is real. But the practical significance is wildly overstated in gym culture.
What the research actually shows
Meta-analyses over the past decade have clarified the picture:
- Strength gains are reduced by ~5-10% when combined with endurance training, compared to strength training alone. Not eliminated. Reduced.
- Hypertrophy is barely affected. You can still build muscle while running. The interference is primarily neurological (peak force production), not structural (muscle size).
- Type of cardio matters. Running creates more interference than cycling because of the eccentric muscle damage from impact. Cycling is mechanically more compatible with lifting.
- Volume is the key variable. Low-to-moderate running volume (3-4 sessions, 30-50km/week) causes minimal interference. High volume (60km+) is where it gets real.
- Timing matters. Separating strength and endurance sessions by 6+ hours reduces interference significantly. Same-session concurrent training is the worst case.
The gym bro interpretation — "if you run, you lose all your gains" — is a caricature of what the research says.
My actual numbers
I went from competitive powerlifting to hybrid training with serious running volume. Here's what happened to my lifts across one marathon prep cycle:
| Lift | Start of prep | 9 weeks out | 3 weeks out | 1 week out |
|---|---|---|---|---|
| Deadlift | 200kg x5 | 180kg x5 | 170kg x5 | 170kg x3 |
| Squat | ~130kg x5 | 110kg x5 | 100kg x4 | 100kg x3 |
| Bench | ~110kg x5 | 100kg x5 | 100kg x5 | DB 30kg x10 |
Context: i was running 55-62km/week during the build phase, tapering to 46km in the final week. Training 6 days a week total (4 runs + 2 strength).
The squat took the biggest hit — it always does. Your legs are never fully fresh when you're running 50+ km per week. Deadlift held better, especially at submaximal loads. Bench barely moved because upper body doesn't compete for the same recovery resources.
For the full powerlifting-to-hybrid transition numbers (including the bigger drops from my peak), i wrote about that here.
Why squat suffers more than deadlift
This is consistent across hybrid athletes and it makes sense mechanically:
- Squat requires quad-dominant strength. Running fatigues your quads chronically. You're squatting on pre-fatigued legs every session.
- Deadlift is more hip-dominant (especially conventional). The hip extensors get some running fatigue but recover faster because the eccentric component is lower.
- Bench uses completely different muscle groups. Unless running is causing weight loss (it might), bench is mostly unaffected.
If you're a lifter adding running and your squat drops 20-30% while your deadlift drops 10-15%, that's normal. It's not a training error.
How to minimize interference (practically)
Forget the molecular biology. Here's what actually works in a real training week:
1. Separate sessions by time. If you can run in the morning and lift in the evening (or vice versa), do it. Same-session is worst case. If you have to do both in one session, lift first. The pairings that work and the two that never do are in running and lifting on the same day.
2. Keep easy runs easy. This is the single biggest mistake lifters make when they start running. An easy run at zone 2 causes minimal interference. An easy run that's actually zone 3-4 because you're too proud to run slowly creates recovery debt that eats into your lifting.
3. Don't do hard running the day after heavy lower body. Your legs need 48 hours between heavy squats/deadlifts and hard running (tempo, intervals, long runs). Easy runs are fine anytime. The full sequencing and deload rules — including when to back both off at once — go deeper.
4. Two strength sessions per week is enough. You won't grow. But you'll maintain more than you'd expect. I've held a 170kg deadlift for months on two sessions per week.
5. Cut accessories, keep compounds. When recovery is limited, spend it on the lifts that matter. I dropped leg press, leg curl, and most isolation work months ago. Squat, deadlift, press, pull-ups. That's it.
6. Eat enough. This is the silent killer. Running burns a lot of calories. If you're in a deficit — intentional or accidental — you'll lose strength faster than the interference effect alone would cause. Eat at maintenance or above.
If the thing on the calendar is a Hyrox start line, these six rules become an eight-week block: how to build a Hyrox training plan without giving up your lifting.
The case for running anyway
The interference effect is real. You will be weaker than if you only lifted. But consider what you gain:
- Cardiovascular health. Lifters have notoriously poor cardiovascular fitness. A resting heart rate of 75+ and getting winded going up stairs isn't something to be proud of.
- Body composition. I look better now than i did as a pure powerlifter. Lower body fat, still solid muscle mass, more athletic proportions.
- Longevity. The combination of muscle mass and cardiovascular fitness is the strongest predictor of all-cause mortality. This isn't bro science — it's epidemiology.
- Mental health. Easy runs clear your head in a way that lifting doesn't. It's meditative.
- A different challenge. If you've been lifting for 10+ years, PRs come slowly. Running offers a new progression curve with rapid improvement in the first year.
The honest summary
Running will cost you 10-30% on your squat, 10-15% on your deadlift, and barely anything on your bench. In exchange, you get cardiovascular fitness, better body composition, and the ability to run a half marathon or marathon — something 95% of powerlifters can't do.
Whether that trade-off is worth it depends on your goals. If you're competing in powerlifting, probably not. If you're training for health, performance, and longevity? It's hard to argue against it.
The interference effect is real. It's just not the boogeyman that gym culture makes it out to be.
Want to see where you stand across both strength and endurance? Check your Hybrid Athlete Score — it's free. And if you're ready to train both without guessing, HybridX generates weekly plans that manage the interference effect for you.