The Interference Effect: Why Strength and Endurance Training Compete
If you've ever tried to build serious strength while also putting in serious endurance mileage, you've probably run into the interference effect, the physiological reason concurrent strength and endurance training can blunt your gains in one or both qualities. It shows up in predictable, frustrating ways:
Your strength stalls.
Your power drops.
Your legs feel constantly fatigued.
Or the opposite happens:
You focus on lifting heavy.
Your endurance performance declines.
Conditioning tests start to suffer.
This is the interference effect at work, one of the most important concepts in hybrid and tactical training, and one of the most poorly understood. Most athletes assume their progress has stalled because they're not training hard enough. The opposite is usually true: they're training hard in two directions at once, with no structure to manage the collision. Athletes who want a program specifically built to manage that collision can explore our CF ONE hybrid strength and conditioning programs.
One clarification first, since the phrase is shared. In psychology, "the interference effect" means something entirely different, interference theory, where newly learned information disrupts recall of older material (retroactive interference) or older material disrupts recall of new (proactive interference). That is a memory phenomenon and it is unrelated to what follows. This article is about the training-science sense: the competition between strength and endurance adaptations in concurrent training.
What the Interference Effect Is
The interference effect refers to the phenomenon where endurance and strength training compete with each other at a physiological level, reducing the adaptation to one or both qualities when training is poorly structured. It isn't a marketing concept or a gym-floor opinion, it's a documented training-science finding that has been replicated since the early 1980s.
In simple terms:
Strength training tells the body to build force and muscle.
Endurance training tells the body to become more efficient and fatigue-resistant.
Both are beneficial. But they rely on different physiological pathways.
When training stress is excessive or poorly organized, the body may struggle to fully adapt to either one. For common questions about how to navigate this in a structured program, the hybrid training program FAQ addresses the most important variables to understand before building a concurrent training plan.
Where the Interference Effect Concept Comes From
The interference effect was first formally documented in 1980 by Robert Hickson, an exercise physiologist who designed a now-classic study comparing strength-only, endurance-only, and concurrent training groups over a 10-week period. The concurrent group's strength gains plateaued and then declined at the back half of the study, while the strength-only group continued to progress. The endurance group's aerobic gains were unaffected. The pattern Hickson documented has been replicated repeatedly since, and athletes performing both strength and endurance training often see:
Smaller strength gains
Reduced power development
Greater fatigue
Compared to athletes who only trained strength. That paper, Interference of strength development by simultaneously training for strength and endurance, published in the European Journal of Applied Physiology (Hickson, 1980), has been cited hundreds of times and effectively founded the field. It is also, importantly, forty-five years old, and what the research has done with it since is the subject of the next section.
Since Hickson, the picture has been refined rather than simply confirmed:
Concurrent training can reduce gains in some qualities, but not all of them equally.
The effect is heavily influenced by volume, intensity, modality, and scheduling.
Proper programming can minimize or, for most qualities, effectively eliminate it.
So the interference effect is real, but it is narrower and more manageable than the 1980 finding alone suggests, and that distinction is where most hybrid and tactical training programs succeed or fail. Understanding the broader context of what concurrent training is and how it differs from a sequential or block approach is the parent concept that underpins everything in the rest of this post.
What the Research Says Now
Hickson's finding held up. What changed is our understanding of how far it extends, and if you learned about the interference effect more than a few years ago, the current picture is meaningfully different from the one you were taught.
The short version: it is smaller and more specific than the folklore suggests.
Recent systematic reviews and meta-analyses pooling dozens of concurrent-training studies have converged on a consistent pattern:
Maximal strength appears largely unaffected. Across the pooled literature, concurrent training groups do not show meaningfully worse maximal strength development than strength-only groups. This is the finding that most contradicts how the interference effect is usually described.
Hypertrophy appears largely unaffected. Same conclusion. Adding endurance work does not, on the available evidence, meaningfully compromise muscle growth.
Explosive strength and power are affected. This is where interference reliably shows up in the pooled data. Rate of force development, jump performance, and sprint qualities are the ones that suffer when endurance volume is high.
That last point matters more for tactical populations than for the general lifting audience most of this research is written about. Power is not a nice-to-have in this job. Getting over a wall, driving a casualty drag, breaking contact, moving fast under load, these are power expressions, and power is precisely the quality the evidence says is genuinely vulnerable.
Two qualifications worth holding onto.
First, the reassuring meta-analytic findings come mostly from studies of recreational and moderately trained subjects on controlled programs. They describe what happens at reasonable training loads. They do not describe what happens to someone running high mileage while also training strength hard, which is closer to the tactical reality, and the researchers making the "less scary" argument say so explicitly, framing interference as something that shows up when someone is already pushing their limits in multiple directions at once.
Second, interference appears to be largely a function of excessive total load and poor structure, not an unavoidable law of physiology. That is the same conclusion the practical sections of this article reach from the coaching side, and it is why the programming advice below matters more than the mechanism does.
So the honest summary: if you train strength and endurance at sensible volumes with sensible structure, you will very probably build both, and the older "cardio kills gains" framing overstates the risk substantially. If you are running high volume while trying to develop maximal power, interference is real and you should plan for it.
Why Interference Happens
Several mechanisms contribute to the effect.
1. Molecular Signaling Conflicts
Strength and endurance training activate different, and partly antagonistic, cellular signaling pathways. Strength training preferentially activates the mTOR pathway, which drives muscle protein synthesis and hypertrophy. Endurance training preferentially activates AMPK, which drives mitochondrial biogenesis and aerobic efficiency. When AMPK is highly active, it can suppress mTOR signaling, which is the cellular root of the interference effect.
Strength training promotes muscle growth and force production.
Endurance training promotes mitochondrial development and efficiency.
The competition is sharpest in the same muscle group on the same day. Running heavy intervals and then squatting two hours later doesn't just fatigue your legs, it tells the same tissue to make two opposing adaptations at the cellular level. Across separate days or with sufficient recovery, the two pathways can coexist far more comfortably. When both signals are constantly competing, adaptation can be blunted.
Worth a note of caution on this mechanism, because it is the one most confidently repeated and the one with the widest gap between theory and outcome. The AMPK–mTOR interaction is well characterized in cell and animal work, and it is a genuinely elegant explanation. But the meta-analytic evidence above shows hypertrophy largely surviving concurrent training in practice, which means the molecular conflict, real as it is at the signaling level, does not translate cleanly into lost muscle in trained humans on sensible programs.
Treat it as the reason same-muscle, same-session pairing is a bad idea, not as proof that endurance work costs you muscle in general. The fatigue and total-load mechanisms below are doing more of the practical work.
2. Neuromuscular Fatigue
High-volume endurance work can leave the nervous system and muscles fatigued.
This is where the interference effect shows up first for most tactical athletes. Strength testing in a fatigued state systematically underrepresents true capacity, and over weeks of high-volume conditioning, that under-recruitment becomes the new training stimulus. The lift stops being a force-production exercise and starts being a fatigue-management exercise, which is why heavy days that follow long runs feel disproportionately hard for the load on the bar.
This reduces:
Force output
Power production
Motor unit recruitment
Trying to push high-intensity strength and high-intensity endurance simultaneously usually exceeds these limits within two to three weeks. The body doesn't break suddenly, it leaks performance slowly, through declining lift numbers, slower interval times, and recovery sessions that don't feel restorative anymore. Most athletes interpret the slow leak as a need to train harder. It's almost always the opposite.
3. Energy and Recovery Limits
The body has limited resources for recovery:
Glycogen stores
Hormonal balance
Tissue repair capacity
Sleep and recovery time
Trying to push high-intensity strength and high-intensity endurance simultaneously often exceeds these limits.
Does the Type of Conditioning Matter?
It does, and it is one of the more useful practical findings in this literature.
Running appears to interfere more than cycling. The likely reason is mechanical rather than metabolic: running involves substantial eccentric loading through the same muscles you are trying to develop in the squat and deadlift, producing more muscle damage and a longer recovery cost. Cycling is largely concentric and lands more softly on the tissue.
Rucking sits in an awkward place for tactical athletes. It is load-bearing and heavily eccentric on descents, with a long recovery cost, and it is usually non-negotiable because the job requires it. If your programme includes serious ruck volume, treat it as competing directly with lower-body strength work rather than as ordinary aerobic conditioning.
Non-impact modalities are the lowest-cost aerobic option when strength or power is your current priority, cycling, rowing, the ski erg, swimming. They build the same aerobic engine with less mechanical interference.
Duration and frequency matter too. Longer and more frequent endurance sessions carry more interference cost than shorter, less frequent ones at the same weekly volume.
The practical version for a tactical athlete: you cannot stop rucking and running, because the job tests them. What you can do is choose the modality for your discretionary aerobic volume based on what you are currently prioritising. If you are in a strength block and already rucking twice a week, make your extra conditioning cycling or rowing rather than more running.
When the Interference Effect Is Strongest
Interference tends to increase when:
Endurance volume is very high.
Strength sessions are performed in a fatigued state.
High-intensity intervals are frequent.
There is little recovery between sessions.
Training lacks a clear priority.
For example:
Heavy squats immediately after a long interval run
High-volume lifting combined with daily intense conditioning
Programs with no periodization or focus
These setups almost guarantee stalled progress.
When Interference Is Minimal
Hybrid training can be highly effective, even for athletes chasing both strength and endurance simultaneously, when the structure is right. The interference effect shrinks dramatically under a small set of conditions:
Endurance work is mostly low-intensity (zone 2).
Strength and endurance sessions are separated by time.
Strength is performed before endurance on the same day.
Total weekly volume is controlled.
Training phases emphasize one primary quality at a time.
Under these conditions, athletes can build both strength and endurance simultaneously without forcing one quality to bleed for the other. Our breakdown of how endurance training affects strength goes deeper on the specific physiological mechanisms that determine when this relationship tips from coexistence into interference.
What This Means for Tactical and Hybrid Athletes
Tactical athletes, military operators, LEOs, firefighters, and anyone whose job demands real-world physical performance, cannot specialize in only one quality. The job itself is the program: load carriage, sustained efforts under fatigue, repeat-effort work, and short bursts of maximal force. No civilian sport demands all of these at once. Tactical and hybrid training does.
They must develop:
Strength for lifting, carrying, and grappling
Endurance for long operations
Work capacity for high-intensity tasks
Durability for repeated stress
Avoiding hybrid training isn’t an option.
Instead, the goal is to manage interference, not eliminate it entirely.
That means:
Structuring sessions intelligently
Controlling fatigue
Periodizing training phases
Building a strong aerobic base
Done well, these four levers are enough. Done poorly, or skipped entirely, they explain almost every stalled program we see in coaching. One thing the tactical population should take from the updated research specifically. Most of the reassuring evidence concerns maximal strength and muscle size, the outcomes the general lifting audience cares about. The outcome that does show reliable interference is explosive strength and power, and that is the one this job actually depends on.
Practically: if you are carrying high aerobic volume and your maximal strength is holding, do not assume everything is fine. Power decays quietly. A deadlift can stay flat while your ability to move fast under load erodes, and no standard test in most units will catch it. If you are in a high-mileage phase, keep a small amount of genuinely explosive work in the program, jumps, throws, short maximal sprints, light and fast lifting, specifically to protect the quality most at risk. It costs very little volume and it defends the thing the evidence says you are most likely to lose.
Practical Programming Strategies
To reduce the interference effect, most effective hybrid programs follow a few simple rules.
Session structure
Lift before conditioning when possible.
Separate sessions by at least 4–6 hours.
Avoid pairing heavy strength with intense intervals.
The lift-before-conditioning rule matters most on heavy strength days. On power or speed-strength days, the rule reverses: those qualities are exquisitely sensitive to even mild pre-fatigue, and putting any conditioning ahead of them is a guaranteed way to under-train the primary stimulus.
Weekly structure
Limit high-intensity endurance sessions.
Use zone 2 work as the aerobic foundation.
Alternate hard and easy days.
Zone 2 work is the single most underused tool in tactical training. It builds the aerobic engine that everything else, strength sessions, intervals, sustained operational efforts, runs on top of, and it does it with minimal interference cost because it sits at intensities the strength pathway largely ignores.
Long-term planning
Use phases that emphasize one primary quality.
Maintain secondary qualities at lower volumes.
Include deload or transition periods.
This approach allows both strength and endurance to improve over time without constant conflict, which is the actual definition of successful hybrid training. The contrast between concurrent training and block periodization explains which structural approach best minimizes interference for different training goals and time horizons.
Signs the Interference Effect Is Becoming a Problem
Watch for these indicators:
Strength numbers declining during endurance blocks
Persistent leg fatigue
Plateaued lifts despite consistent training
Slower recovery between sessions
Reduced power output
If these appear, the issue is often:
Too much intensity
Poor session sequencing
Lack of a clear training priority
The fix is rarely to train less overall. It's almost always to redistribute the same volume across smarter session pairings, longer recovery windows between competing stimuli, and a clearer hierarchy of which quality is the priority in the current training phase.
Frequently Asked Questions
What is the interference effect?
In training science, the interference effect is the phenomenon where concurrent strength and endurance training compete physiologically, reducing adaptation in one or both qualities. It is distinct from the psychological interference effect, which concerns memory rather than training.
Is the interference effect real?
Yes, but it is narrower than its reputation. Current pooled evidence suggests maximal strength and hypertrophy are largely unaffected by concurrent training at sensible volumes, while explosive strength and power are reliably reduced. The older "cardio kills your gains" framing overstates the risk for most people.
Who discovered the interference effect?
Robert C. Hickson, in a 1980 study published in the European Journal of Applied Physiology titled "Interference of strength development by simultaneously training for strength and endurance." He compared strength-only, endurance-only, and concurrent groups over ten weeks and found the concurrent group's strength gains plateaued and then declined in the back half of the study, while endurance adaptations were unaffected.
What did Hickson (1980) actually find?
That concurrent training compromised strength development specifically. The endurance-only group improved aerobically as expected, the strength-only group continued gaining strength throughout, and the concurrent group's strength progress stalled and then reversed late in the ten-week program. The paper effectively founded the concurrent-training literature.
Does the ACSM have a position on concurrent training?
The American College of Sports Medicine publishes position stands covering resistance training and cardiorespiratory training that address program design where both are performed. If you need the authoritative statement for academic or professional purposes, go to the ACSM position stands directly rather than to secondary summaries, including this one.
Does cardio kill your gains?
Not at reasonable volumes with reasonable structure. The pooled evidence does not support meaningful loss of maximal strength or muscle size from adding endurance work. What it does support is reduced explosive strength and power at higher endurance volumes, plus the ordinary problem that excessive total training load compromises recovery for everything.
Does running interfere more than cycling?
It appears to, largely because running involves substantial eccentric loading and muscle damage in the same muscles being trained for strength, while cycling is mostly concentric. If you are prioritising strength, non-impact aerobic modalities carry less interference cost at the same aerobic benefit.
How long should you wait between strength and endurance sessions?
Longer is better. A few hours of separation reduces the acute signaling and fatigue overlap substantially, and separate days reduce it further. Same-session, same-muscle pairing of hard strength and hard endurance work is the highest-interference arrangement available.
Can tactical athletes avoid the interference effect?
Not entirely, and they should not try. The job demands strength, endurance, power, and work capacity simultaneously. The goal is managing interference through session sequencing, controlling total volume, phasing priorities, and building an aerobic base at intensities that carry a low interference cost.
The Bottom Line
The interference effect is real, well-documented, and decades old as a finding, but it's still routinely misunderstood, even by athletes and coaches who know the term.
It doesn’t mean you can’t train strength and endurance together.
It means you must structure hybrid training intelligently.
Well-designed programs:
Control fatigue
Sequence sessions properly
Periodize training phases
Build a strong aerobic base
When done correctly, hybrid athletes can develop high levels of both strength and endurance without major compromises. Two posts that address specific decision points within this system: how strength training affects endurance examines the relationship from the other direction, while when endurance training hurts strength gains helps athletes identify exactly when to pull back on conditioning volume.
References
Hickson, R. C. (1980). Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology and Occupational Physiology, 45(2–3), 255–263. doi:10.1007/BF00421333 · PMID: 7193134
Schumann, M., et al. (2021). Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis. Sports Medicine. PMID: 34757594
Wilson, J. M., et al. (2012). Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. Journal of Strength and Conditioning Research, 26(8).
Petré, H., et al. (2021). Concurrent training and the interference effect - meta-analysis.
