Concurrent training and the interference effect.

Our coaches hold the NSCA Certified Strength and Conditioning Specialist (CSCS) credential and have programmed concurrent blocks for hybrid athletes inside Nishaana — from lifters adding their first 5K to runners trying to hold a squat number through a marathon build. If you have ever worried that your Saturday long run is undoing your squat, this is the guide for you. We pulled the actual meta-analyses — Wilson's 2012 pooled analysis [1], Schumann's 2022 update [2], and Murach and Bagley's review of the hypertrophy evidence [3] — and translated them into how you should schedule a week. The short version: the interference effect is real, it is small, and it is largely programmable. You can train both without wrecking either, and you can log the whole thing free in the Nishaana workout tracker.

What is the interference effect?

The interference effect is the reduction in strength, power, or muscle adaptations that can happen when endurance and resistance training are combined in the same program, compared with resistance training done on its own. It was first formally described by Robert Hickson in 1980, and the phenomenon is now studied under the broader heading of concurrent training.

Concurrent training simply means training two opposing qualities in the same block — most often strength and endurance. The worry is that these adaptations pull the body in different directions: resistance training asks muscle to get bigger and more forceful, while aerobic exercise asks it to become more fatigue-resistant and efficient. Hickson's original observation was that athletes doing both stopped gaining strength after several weeks even as their endurance kept climbing.

Two things make the modern picture more reassuring than that 1980s finding. First, the interference is mostly one-directional: cardio can slightly blunt strength, but lifting does not blunt your VO2max. Second, the size of the effect depends heavily on how you arrange the two — modality, order, separation and volume all move the needle. For the wider context on training both at once, start with our hybrid training guide.

What is the molecular basis?

The leading molecular explanation is competition between two signalling pathways: mTOR, activated by heavy lifting to drive muscle protein synthesis, and AMPK, activated by endurance work when cellular energy runs low. AMPK can transiently suppress mTOR, so an endurance stimulus may dampen the growth signal a resistance session just switched on.

Think of mTOR as the build switch and AMPK as the endurance switch. When you lift heavy, mechanical loading turns up mTOR and elevates muscle protein synthesis for a day or more. When you do endurance work, falling energy status and glycogen depletion activate AMPK, which favours mitochondrial biogenesis through PGC-1 alpha — great for aerobic fitness, but AMPK can also inhibit the mTOR complex. Fyfe, Bishop and Stepto's 2014 review lays out this molecular framework and, importantly, stresses that individual training variables determine how much it actually matters. [4]

Here is the honest caveat coaches too often skip: the acute molecular story does not cleanly predict long-term results. Coffey and Hawley's 2017 review, pointedly titled "do opposites distract?", argues that the neat AMPK-versus-mTOR model breaks down over weeks of real training, where fatigue, substrate availability and the athlete's training history matter as much as any single enzyme. [5] The molecules explain a mechanism; they do not sentence you to lost gains.

What do the meta-analyses actually show?

The pooled evidence says interference is real for strength and power but small overall, and it is modality-, volume- and frequency-dependent. Wilson's 2012 meta-analysis found the strongest interference with running and with high-frequency, long-duration endurance, while more recent reviews report that concurrent training does not meaningfully compromise muscle size.

Wilson and colleagues pooled dozens of studies and reported that hypertrophy, strength and power were all attenuated to some degree by concurrent training, but the size of the effect depended on the endurance dose — more sessions, longer sessions and running rather than cycling all made interference worse. [1] That dose-dependence is the single most useful takeaway: interference is not a fixed tax, it is a dial you set with your programming.

Schumann and colleagues' 2022 systematic review and meta-analysis updated the picture with better volume-matched controls. Their conclusion was reassuring — combined training was broadly compatible with gains in muscle size and function, with only limited evidence that endurance compromised strength or power when the resistance stimulus was preserved. [2] In other words, when you do not cut your lifting to make room for cardio, most of the feared interference does not show up.

How does interference differ by outcome?

Interference is not uniform across adaptations. Power and explosiveness are the outcomes most consistently blunted, maximal strength takes a smaller and often programmable hit, hypertrophy is largely spared, and aerobic fitness is not impaired at all by adding resistance training. The effect essentially runs one direction: cardio can touch strength, lifting does not touch endurance.

This table is the practical heart of the topic. If you are a physique-focused lifter, the row that matters most — hypertrophy — shows negligible interference, so you can run steady-state cardio for heart health and fat loss without fear. If you are a strength-speed or explosive athlete, the power row is your warning: guard your recovery and cap your endurance volume, because that is where the effect actually bites.

Why do strength and power suffer most?

Strength and especially power are the most affected because they depend on qualities that endurance training directly opposes: high-threshold motor unit recruitment, rate of force development, and freshness of the nervous system. Residual fatigue and endurance-induced shifts toward slower, more fatigue-resistant muscle characteristics chip away at exactly the explosive edge these outcomes need.

Power is fragile because it is expressed in milliseconds. Rate of force development and jump height depend on a rested nervous system firing fast fibres hard and fast. A heavy dose of endurance leaves the neuromuscular system fatigued and can nudge muscle toward a more oxidative, slower-contracting profile — helpful for a marathon, unhelpful for a vertical jump. That is why sprinters and Olympic lifters ration their cardio carefully.

Maximal strength sits in the middle. It is somewhat blunted in high-volume concurrent programs, but the effect is smaller than for power and it responds well to the programming levers below. Keep your heavy work heavy, keep frequency for strength intact, and cap the endurance dose, and most of the strength interference in the literature disappears. See our running for lifters guide for the practical version.

Why is muscle size largely spared?

Hypertrophy is largely spared because muscle growth is driven mainly by mechanical tension and total resistance volume, both of which you can keep intact while adding cardio. Murach and Bagley reviewed the evidence and argued that when resistance volume is preserved, concurrent training does not meaningfully reduce muscle cross-sectional area — the contrary evidence for an interference effect on size.

Murach and Bagley's 2016 review title says it plainly: "contrary evidence for an interference effect" on hypertrophy. [3] Their point is that most apparent size interference in older studies came from confounds — endurance simply crowding out lifting volume, or fatigue reducing lifting quality — rather than from a fundamental biological incompatibility. Protect your training volume and your mechanical tension, and the muscle keeps coming.

There is a mechanistic reason this holds up. Endurance training can even support the muscle-building machinery in some ways — improving nutrient delivery, capillary density, and the activity of satellite cells that donate nuclei to growing fibres. So for a bodybuilder or general lifter, moderate cardio is closer to neutral-or-helpful than harmful. The people who lose size to concurrent training are almost always the ones who let running eat their lifting, not the ones who added a couple of easy sessions.

Which programming variables reduce interference?

Five levers control how much interference you actually experience: modality (cycling beats running), intra-session order (lift first for strength), session separation (6+ hours or separate days), endurance frequency (cap at 2-3 for most lifters), and endurance volume (keep most sessions short and easy). Set these well and the measurable interference in the research nearly vanishes.

Modality is the most underrated lever. Wilson's data showed running interfered more than cycling with both strength and size, largely because running's eccentric braking forces beat up the same quadriceps and calves you load in the squat. [1] If you lift for size or strength and just want cardiovascular fitness, a bike, rower or incline walk gives you the aerobic benefit with far less overlap. If you specifically need to run, treat the running as sport-specific work and manage it like any other hard session — with a deload when fatigue stacks up.

Does exercise order and timing matter?

Order matters when the two are done in one session: if strength or power is your priority, lift first. Eddens and colleagues' 2018 meta-analysis of intra-session sequence found that resistance-before-endurance better preserved lower-body strength and power development than the reverse order. If endurance is your main goal, cardio-first is acceptable.

The Eddens review pooled studies that manipulated only the order within a shared session and concluded that doing resistance work first was the safer default for anyone who cares about lower-body strength and power. [6] The logic fits the fatigue story: whichever quality you train second inherits the fatigue of the first, and power tolerates pre-fatigue worst. So lead with the priority.

Better than sequencing, though, is separation. Putting the two workouts six or more hours apart — a morning lift and an evening ride, say — lets the resistance-driven mTOR signal do its work and clears acute fatigue before the endurance session competes. Best of all for pure strength athletes is putting them on separate days entirely. Our how to run and lift guide walks through building that split around your schedule.

How do you program around it?

Program around interference by protecting your resistance training first, then fitting endurance into the gaps: keep hard strength sessions and long endurance sessions on different days, separate any same-day work by six-plus hours, prefer low-impact cardio, and cap most weeks at two to three endurance sessions unless endurance is your sport. Recovery capacity is the real budget you are managing.

Here is a balanced concurrent week for a lifter who also wants solid aerobic fitness. It keeps three strength days fully separated from the hardest endurance work, puts easy intervals on a lower-body day only when they can sit hours after the lift, and leaves genuine rest in place.

Adjust the balance toward whichever quality is your priority — a marathoner flips this to endurance-led with two maintenance lifts, a powerlifter flips it to strength-led with two easy rides. The unifying principle is that total recoverable work is finite, so you manage periodization and monitor readiness rather than piling both to the ceiling at once. Build and track the whole plan in Nishaana programs, and see the deeper sequencing options in hybrid athlete training and our strength and endurance programming guide (landing soon).

Who actually needs to worry?

Most people do not need to worry much. Beginners improve at strength, size and endurance simultaneously because they are far from any ceiling. The athletes who must manage interference carefully are advanced, strength- and power-focused competitors near their genetic limits, where a small percentage loss in explosiveness is competitively meaningful.

Trained status changes the math. A novice can run three times a week and still add to their squat, because there is so much low-hanging adaptation available. A national-level sprinter or weightlifter has none of that slack, so every hour of misplaced cardio can cost measurable power. If you are somewhere in between — a recreational lifter who also likes to run or ride — you are firmly in the zone where good scheduling erases nearly all of the concern.

The honest bottom line: do not let the interference effect scare you away from cardio. Cardiovascular fitness improves your health, your work capacity between sets, and your recovery across a training week. For the vast majority of lifters, the right move is to keep doing cardio, arrange it intelligently using the levers above, and stop worrying that an easy run is stealing your gains. If you want the full both-worlds blueprint, the hybrid training guide ties it together.

References

1. Wilson JM, et al. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises (2012). Journal of Strength and Conditioning Research
2. Schumann M, et al. Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis (2022). Sports Medicine
3. Murach KA, Bagley JR. Skeletal Muscle Hypertrophy with Concurrent Exercise Training: Contrary Evidence for an Interference Effect (2016). Sports Medicine
4. Fyfe JJ, Bishop DJ, Stepto NK. Interference between concurrent resistance and endurance exercise: molecular bases and the role of individual training variables (2014). Sports Medicine
5. Coffey VG, Hawley JA. Concurrent exercise training: do opposites distract? (2017). The Journal of Physiology
6. Eddens L, van Someren K, Howatson G. The Role of Intra-Session Exercise Sequence in the Interference Effect: A Systematic Review with Meta-Analysis (2018). Sports Medicine