The Science of Concurrent Training: Why Mixing Cardio and Strength Isn’t as Simple as You Think

Here’s a stat that honestly blew my mind when I first stumbled across it — researchers have found that combining endurance and resistance training in the same program can reduce strength gains by up to 31% compared to strength training alone. Thirty-one percent! That was a gut punch for me because I’d been doing exactly that for years, thinking I was being super efficient with my gym time.

The science of concurrent training is one of those topics that every gym-goer should understand, whether you’re a weekend warrior or someone who takes their programming seriously. It changed how I structure my entire week, and honestly, I wish someone had explained it to me a decade ago.

What Exactly Is Concurrent Training?

Concurrent training simply means performing both aerobic exercise and resistance training within the same training program. Think running three days a week while also hitting the weight room three days a week. Sounds reasonable, right?

The problem was first identified by researcher Robert Hickson way back in 1980, and it’s been called the “interference effect” ever since. Basically, the molecular signaling pathways for endurance adaptations and strength adaptations kind of compete with each other. Your body gets confused about what it’s supposed to be adapting to.

The Interference Effect — My Painful Lesson

I learned about this the hard way. For about two years, I was running 25 miles a week while trying to build muscle with a solid hypertrophy program. My squat numbers were absolutely stuck, and I couldn’t figure out why.

Turns out, the AMPK pathway activated by endurance training directly inhibits the mTOR pathway, which is what drives muscle protein synthesis. In plain English — my running was literally telling my muscles not to grow. That was frustrating to learn, but also kind of liberating because at least I had an answer.

Does This Mean You Should Skip Cardio Entirely?

No way. And this is where people get it wrong all the time. The interference effect is real, but it’s not a death sentence for your gains.

Research from a 2012 meta-analysis showed that the interference effect primarily impacts lower body strength and power development. Upper body gains were been largely unaffected. Also, the type of cardio matters enormously — cycling causes less interference than running because it’s lower impact and more mechanically similar to movements like squats.

Practical Tips That Actually Work

• Separate your cardio and strength sessions by at least 6-8 hours when possible. I started doing strength in the morning and easy cardio in the evening, and it made a noticeable difference.

• Prioritize whatever goal matters more to you. If strength is the priority, do your lifting first or on separate days entirely.

• Keep cardio sessions moderate. High-intensity interval training causes more interference than steady-state work, which was surprising to me at first.

• Focus on nutrition and recovery. Concurrent training demands more calories and more sleep — I bumped my protein to about 1 gram per pound of bodyweight and things improved dramatically.

• Consider cycling over running for your cardio if preserving leg strength matters to you.

Who Should Actually Worry About This?

Here’s the thing — if you’re a recreational lifter who just wants to look good and be healthy, the interference effect probably isn’t gonna ruin your life. It matters most for competitive athletes, powerlifters, and people chasing very specific performance goals.

For the average person, the benefits of doing both cardio and strength training far outweigh the downsides. Cardiovascular health, mental health benefits, bone density — all of that stuff is too important to sacrifice because you’re worried about losing a few percentage points on your deadlift. Context matters so much here.

Train Smarter, Not Just Harder

The science of concurrent training taught me that more isn’t always better — smarter is better. Understanding how your body responds to different training stimuli can save you months or even years of spinning your wheels.

Play around with session timing, cardio modality, and training splits until you find what clicks for your body and your goals. And please, don’t ignore recovery — that’s where the magic actually happens. If you found this helpful and want more evidence-based fitness content, check out other posts on Fitness Nuvra where we break down the research so you don’t have to!