Concurrent training and the interference effect: what the research actually says

Concurrent training is the boring scientific name for doing endurance and strength work in the same program. Running and lifting. Rowing and squatting. Whatever your version is, the question it raises is always the same: does one cancel out the other?

The interference effect says it can. The gym version of that idea says running "kills your gains" and you should pick a side. The research says something much less dramatic and much more useful, and it's the reason exrsz schedules your runs and lifts as one plan instead of two. Here's what the studies actually found, where they disagree, and what to do about it.

What is concurrent training?

Concurrent training means training two qualities that drive different adaptations, aerobic endurance and muscular strength, within the same program. Same week, sometimes the same day, sometimes the same session.

It's what every hybrid athlete, Hyrox racer and runner-who-lifts is doing, whether they call it that or not. The only question is whether they're doing it deliberately. A deliberate version looks like a week-by-week hybrid plan: one quality leads, the other supports, and the sessions that compete are kept apart. The accidental version looks like a running app and a lifting app fighting over your Tuesday.

If you're new to the idea, start with the complete guide to hybrid training. This post is the research layer underneath it.

Where the interference effect came from

The story starts with Hickson in 1980. He compared three groups over ten weeks: strength training only, endurance training only, and both. The combined group did the same strength sessions (five days a week) and the same endurance sessions (six days a week) as the other two groups. That's a lot of training.

What he found:

• ›The combined group gained strength at the same rate as the strength-only group for about seven weeks.
• ›Then their strength leveled off and declined in weeks nine and ten.
• ›Their aerobic gains (VO2max) were the same as the endurance-only group.

That's the interference effect in its original form: endurance training blunting strength gains, but not the other way round. It's a real finding. It's also one small study with a training load most people with a job will never come near, and it's been carrying a lot of gym folklore on its back for over forty years.

What the meta-analyses say

Single studies are noisy. Meta-analyses pool lots of them. Three are worth knowing.

Wilson et al., 2012: it depends on the cardio

Wilson and colleagues pooled 21 studies to work out which parts of endurance training cause interference. Their findings:

• ›Concurrent training still produced strength and size gains, just smaller ones on average than strength training alone.
• ›Running alongside lifting, but not cycling, was linked to significant decrements in both muscle growth and strength.
• ›The more frequent and the longer the endurance sessions, the bigger the interference with muscle growth, strength and power.

Their conclusion: interference is a function of the modality, frequency and duration of the endurance work. Not "cardio bad". Specific cardio, in specific doses, bad.

Schumann et al., 2022: muscle and max strength are fine

The biggest update is Schumann et al. (2022) in Sports Medicine, which pooled 43 studies comparing concurrent training with an identical strength program without the aerobic work. Their conclusion:

• ›Concurrent training did not compromise muscle hypertrophy or maximal strength development.
• ›Explosive strength gains may be attenuated, especially when aerobic and strength training are done in the same session. When sessions were separated by at least three hours, the effect wasn't significant.
• ›The results appeared independent of cycling vs running, training frequency, training status and age.

Note the disagreement with Wilson on running vs cycling. Newer data, more studies, different answer. That's how science is meant to work, and it's a reason to be suspicious of anyone quoting one paper as gospel.

Petré et al., 2021: training status matters

Petré and colleagues looked specifically at lower-body maximal strength (leg press and squat 1RM) across 27 studies, split by training status. They found:

• ›Concurrent training negatively affected lower-body 1RM gains in trained individuals.
• ›It did not in moderately trained or untrained people.
• ›The impairment seemed more pronounced when training was done in the same session.

Their advice: trained individuals should separate endurance from resistance training during periods when maximal strength is the priority.

So is the interference effect real?

Yes, in a narrower form than the folklore. Putting the three together:

And the dose matters. More endurance sessions, longer endurance sessions, more interference (Wilson 2012). Which is exactly the variable you control when you write the plan.

Why does interference happen?

Two explanations, one fancy and one obvious.

The fancy one is molecular. Endurance and strength exercise trigger different signaling pathways in muscle, and the theory is that they can get in each other's way. A 2025 review of training order (Feng et al.) notes that these acute molecular responses do vary with sequence, but their translation into long-term adaptations is complex and non-linear. In other words: interesting, real, and not something you can program around from a lab paper.

The obvious one is fatigue. Hard running tires the same legs you squat with. Lift on tired legs and you lift less, so you get less out of the session. Run hard on legs still sore from deadlifts and the run suffers too. Most practical interference is just residual fatigue on the wrong day. That one you absolutely can program around.

What actually causes the problem in real life

When someone says running ruined their lifting, it's rarely the running itself. It's usually one of these:

• ›Too much, too fast. Going from zero to five runs a week on top of a full lifting program. Wilson's frequency and duration findings, in the wild.
• ›Hard on hard on hard. Intervals the day after heavy squats, then a heavy pull the day after that. Nothing ever recovers.
• ›Eating like a lifter, training like a triathlete. More training, same food. That's an energy problem dressed up as an interference problem, and it's the real story behind why cardio doesn't have to cost you muscle.
• ›No priority. Trying to set a squat PR and a 10K PR in the same month.

How to minimize interference

The research translates into a short list of rules:

1. ›Separate the sessions. At least a few hours apart on the same day, ideally different days for heavy legs and hard running. Schumann's same-session finding is the clearest practical signal in the literature.
2. ›Order by priority. On a shared day, the priority goes first. The details of should you run or lift first have their own post.
3. ›Keep most endurance work easy. Easy aerobic work, the kind usually called zone 2 training, builds the engine without leaving much fatigue behind. Save the hard running for two sessions a week, not five.
4. ›Control the dose. Add running volume gradually. Wilson's data says the frequency and duration of endurance work drive the interference, so don't jump both at once.
5. ›Rotate the priority by block. A strength block where running holds, then a running block where lifting holds. That's how periodization works for hybrid athletes, and it's the cleanest way to dodge interference over months instead of fighting it every week.

For lifters, runners and everyone in between

If you're a lifter adding cardio, the evidence says you can relax. Keep it mostly easy, keep it away from your heavy leg days, and you're very unlikely to lose muscle or strength. The detail of how lifters should do cardio is mostly about picking the right type and dose.

If you're a runner adding strength, the classic finding runs in your favor: Hickson's combined group improved VO2max as much as the endurance-only group. The interference he saw was in strength, not endurance.

If you want to be properly good at both, which is what being the hybrid athlete actually means, interference is a scheduling constraint, not a reason to quit one side. The same logic applies to marathoners: marathon training with strength work is just concurrent training with a long run in it.

How exrsz uses this

exrsz builds one periodized plan from your goal, schedule, level and equipment, with running and lifting balanced in it from the start. It keeps hard runs and heavy leg days apart, picks which quality leads each block, adapts each week to how you actually trained and reshuffles missed sessions so the hard days don't end up stacked.

FAQ

What is concurrent training?

Concurrent training is combining endurance training and strength training in the same program, whether that's the same week, the same day or the same session. Running and lifting in one plan is the most common version.

Is the interference effect real?

Yes, but it's smaller than gym folklore suggests. A 2022 meta-analysis of 43 studies found concurrent training didn't compromise muscle growth or maximal strength, though explosive strength gains may be blunted, especially when both are done in the same session.

Does concurrent training affect beginners?

Less than it affects trained lifters. A 2021 meta-analysis found lower-body max strength gains were impaired in trained individuals but not in untrained or moderately trained people.

How do I avoid the interference effect?

Separate hard runs from heavy lower-body lifts, keep most endurance work easy, add running volume gradually, and pick one quality to prioritize per training block. Splitting same-day sessions by at least a few hours also helps.

Does lifting hurt endurance gains?

Not in the classic research. In Hickson's 1980 study, adding strength training didn't reduce VO2max improvements compared with endurance training alone.