Concurrent Training and the Interference Effect: What the Research Shows

Every lifter has heard the warning. Do cardio and you’ll lose your gains. Every runner has heard the flip side. Lift and you’ll get slower. Neither statement is quite right, and the research over the past 40 years has quietly moved on from the version most gyms still repeat.

The scientific label is the interference effect, coined after a 1980 study by Robert Hickson at the University of Illinois. It has never fully disappeared. But it has been steadily narrowed by better designs, larger meta-analyses, and molecular studies that clarified what was actually competing at the cellular level. What’s left is a much smaller signal than the original headline, and one you can program around instead of avoiding.

This piece walks through where the interference effect came from, what four decades of follow-up have and haven’t confirmed, and how to combine cardio and strength training so that both stack rather than fight.

The Research: What Studies Show

Hickson (1980): The Study That Started Everything

Robert Hickson was a competitive powerlifter who joined an endurance physiology lab as a young researcher and started running with his colleagues. His strength started dropping despite continued lifting. The observation prompted a formal study. Hickson (1980), in the European Journal of Applied Physiology and Occupational Physiology, randomized subjects into three groups over 10 weeks: strength only (5 sessions per week of heavy leg work), endurance only (6 sessions per week of running and cycling), and a combined group doing both.

The combined group improved strength and VO₂max in parallel for the first 7 weeks. Then strength plateaued, and by weeks 9 and 10 it actually declined, while VO₂max kept climbing. The endurance-only and strength-only groups continued their gains normally. Hickson concluded that stacking both training modes simultaneously interfered with strength development, not with endurance.

The study was landmark. It was also a program almost nobody runs. Eleven sessions a week of heavy leg work plus daily running is not how most lifters or hybrid athletes train. The finding was real. Its generalizability was the question the field spent the next 40 years answering.

Petré et al. (2021): 27 Studies on Trained vs. Untrained Lifters

The most focused modern meta-analysis on maximal strength is Petré, Hemmingsson, Rosdahl and Psilander (2021) in Sports Medicine, pooling 27 studies. The finding was that training status mattered more than anyone expected. Trained individuals showed a small but statistically significant decrement in strength when combining resistance and endurance training (effect size -0.35, p < 0.01). Moderately trained individuals sat in the middle (-0.20, p = 0.08). Untrained individuals showed essentially zero interference (0.03, p = 0.87).

The bigger driver, though, was session timing. Within the trained group, same-session pairings had an effect size of -0.66 (p < 0.01). When sessions were separated by hours or done on different days, the interference shrank to -0.10 (p = 0.55, statistically indistinguishable from zero). Lower-body compound lifts (squat, leg press) took the biggest hit. Upper-body strength largely held.

Practical read: beginners can basically ignore this. Intermediate and advanced lifters can too, as long as they don’t stack a hard endurance session directly on top of a hard leg day.

Schumann et al. (2022): 43 Studies on Muscle Size and Function

The widest-scope modern review is Schumann et al. (2022) in Sports Medicine, an updated systematic review and meta-analysis pooling 43 studies with 1,090 participants. It measured maximal strength (37 studies), explosive strength (18 studies), and hypertrophy (15 studies).

The headline: concurrent training did not compromise maximal strength (SMD -0.06) or muscle hypertrophy (SMD -0.01) compared with resistance training alone. Both point estimates were tiny and statistically insignificant. The exception was explosive strength, where concurrent training produced a modest negative effect (SMD -0.28), especially when aerobic and strength sessions were combined within the same session with less than a 3-hour gap.

The Schumann paper is the strongest piece of evidence against the “cardio kills gains” framing. The Hickson finding stayed intact for explosive power. It largely evaporated for muscle mass and one-rep max.

Huiberts et al. (2023): Sex and Training-Status Differences

The most recent large synthesis is Huiberts, Wüst and van der Zwaard (2023), published in Sports Medicine, pooling 59 studies and 1,346 participants (679 concurrent training, 270 strength-only, 397 endurance-only). It looked specifically at how sex and training status modified the interference effect.

Two findings stood out. First, lower-body maximal strength adaptation was blunted in males (SMD -0.43) but not in females (SMD 0.08), a statistically significant sex difference (p = 0.03). Second, VO₂max improvements were impaired in untrained participants (SMD -0.35) but not in trained or highly trained ones (p = 0.04). Power gains were mildly blunted in both sexes with no significant sex difference. Upper-body strength adaptations were unchanged.

The pattern the review paints: female lifters get largely a free pass on lower-body strength, male lifters lose a small amount, and untrained people take a slight cardiorespiratory hit, likely because their signal-to-noise ratio for aerobic adaptations is different from a fit adult’s. Not a doomsday finding in any subgroup.

Coffey & Hawley (2017): Why It Happens at the Molecular Level

The mechanism was reviewed by Coffey and Hawley (2017), “Concurrent exercise training: do opposites distract?” in The Journal of Physiology. Resistance training activates the mTOR (mechanistic target of rapamycin) pathway, the main driver of muscle protein synthesis. Endurance training activates AMPK (adenosine monophosphate-activated protein kinase), a low-energy sensor that promotes mitochondrial biogenesis and, in some contexts, inhibits mTOR. In simple terms, endurance signals the muscle to become more efficient. Resistance signals the muscle to become bigger. Those two signals can partially compete.

The competition is real acutely. Coffey’s lab showed that high-intensity sprint intervals performed 15 minutes before heavy resistance work suppressed the post-lifting mTOR activation that normally follows a strength session. The signal recovered with time between sessions. That’s the molecular basis for the “separate by 3+ hours” recommendation and for the same-session pattern seen in Petré and Schumann.

What the Interference Actually Costs You

Stacking the four modern meta-analyses, a clear map of the effect emerges. Not everything is affected equally. Here’s the honest ranking, largest cost first.

• Explosive power and rate of force development. The most consistently affected outcome. SMD around -0.28 across Schumann 2022 pooling. If you’re a jumper, sprinter, thrower, or CrossFit competitor, this is the number to respect.
• Lower-body max strength in trained male lifters. SMD -0.35 to -0.43 depending on the review. Real, small, and concentrated in same-session pairings.
• VO₂max in untrained beginners. A slight blunting (SMD -0.35 per Huiberts 2023) that fades once you become moderately fit.
• Hypertrophy. Essentially unaffected in modern reviews (SMD -0.01 in Schumann 2022). Muscle mass appears well protected.
• Maximal strength overall. Also essentially unaffected across pooled reviews (SMD -0.06). The exception is the trained-lifter, same-session subgroup.
• Upper-body strength. Almost no interference signal in any recent review. Bench press and pull-up gains don’t care much about your cycling.

Translation: if your goals are muscle and general strength, you’re fine to do a lot of cardio. If your goal is peak explosive power or a competitive lower-body one-rep max, you’ll want to program the two more carefully.

How to Program Around the Interference Effect

The good news about a small, well-mapped effect is that you can steer it. Six practical rules cover most of the evidence.

Separate the Sessions

Every meta-analysis flags this as the biggest lever. Same-session concurrent training carries the interference. Different-day or same-day-different-time training does not, or does much less. If you have to combine on one day, put at least 3 hours between them, with a full meal in between. If you can, split them across different days.

Priority Goes First

When you must combine same-session, do the quality that matters most first, while you’re fresh. Chasing hypertrophy or a lift PR? Lift first, cardio after. Training for a race? Run first, lift after. The pattern that hurts progress most is doing a fatiguing endurance session immediately before your strength work.

Cap the Endurance Volume

The Hickson study used 6 sessions of running per week, which is a lot. Modern research suggests the interference signal is small at 2 to 3 endurance sessions per week paired with 2 to 4 strength sessions. It grows when endurance frequency dominates the week. If you’re running 5+ days and lifting 3, expect the lifts to feel harder.

Prefer Low-Impact Cardio on Lift Days

Cycling and walking produce less muscle damage than running. Studies including Coffey and Hawley (2017) report that modality matters. Running on a heavy leg day compounds the eccentric load. Zone 2 cycling or an easy walk stresses the aerobic system without adding eccentric recovery cost. If cardio has to share a day with legs, cycle.

Manage Same-Day Order by Modality

Upper-body lifting the same day as any cardio is fine in either order for most people. Legs and hard cardio the same day is the pairing that carries the interference. Reserve heavy leg days for days without high-volume endurance work whenever possible.

Fuel the Overlap

Both AMPK activation and low glycogen amplify the interference signal at the molecular level. Under-fueled sessions concentrate the effect. Eating adequate carbohydrate around training, especially between the two sessions on same-day pairings, dampens it. This is not license to eat anything, just a reminder that chronic underfueling turns a small effect into a bigger one.

A Sample Concurrent Training Week

This is one workable template, not the only one. It shows what “program around it” looks like when the goal is both a stronger lower body and a fitter aerobic engine.

Notice what this schedule does. Heavy legs go on Monday and Friday, both separated from the hardest cardio day (Thursday intervals) by 24-48 hours in either direction. Zone 2 sits on the day after a leg day when the aerobic system is fresh but the legs are still recovering. Upper-body lifting can safely share days with an easy walk. The long easy cardio anchors Saturday, when nothing has to be freshly explosive on Sunday.

Common Misconceptions

“Any cardio at all kills muscle growth.”

False. This is the most persistent gym-floor myth, and the modern data flatly disagrees. Schumann et al. 2022 pooled 15 hypertrophy studies and found a standardized mean difference of -0.01, meaning muscle growth was statistically indistinguishable between concurrent trainers and lifting-only trainers. Cardio does not kill hypertrophy in reasonable doses.

“You need to skip cardio to get strong.”

Also false, with a small caveat. For general strength, cardio is fine and often helpful (it improves recovery capacity between lifting sessions). For elite lower-body max strength in trained male lifters, high-volume same-session endurance work does cost you. That’s a specific subgroup, not a rule for anyone who wants to squat 315.

“Hickson proved cardio destroys strength.”

Hickson showed that an extreme daily-double program blunted strength gains after 7 weeks. The program was 5 heavy leg sessions plus 6 running sessions per week, and even that group kept most of its early strength gains. The finding was real. It doesn’t apply to almost any real-world training week you or I would run.

“Women have to worry about interference too.”

The most recent large review (Huiberts 2023) found no lower-body strength interference in women (SMD 0.08) versus a small blunting in men (SMD -0.43). The mechanism is still being worked out. Possible drivers include hormonal differences in mTOR signaling and slightly different baseline recovery capacity. The practical takeaway: women lifting and doing cardio have even less to worry about than the average number suggests.

What the Research Suggests Going Forward

The interference effect has moved from a scary meme to a well-mapped, small phenomenon you can program around. Four decades in, the picture is clearer than most other exercise-science questions. Cardio and strength training can be combined without meaningfully sacrificing muscle mass or general strength. The narrow costs (explosive power, lower-body max strength in trained males, a small aerobic hit in beginners) are real and manageable with session separation, priority ordering, and modality choice.

The direction of ongoing research is toward finer-grained mechanistic and practical questions. What’s the smallest gap between sessions that removes interference (probably around 3-6 hours)? How much do nutrition timing and total energy availability modulate the effect (a lot)? Are there sex-specific programming rules that maximize both adaptations (likely, but the data is still emerging)? None of those are urgent for a person deciding whether to add three cardio sessions per week to their strength program. They should. The costs, honestly measured, are small.

For related programming reads, see our science pieces on rest periods and muscle growth, training frequency, and zone 2 aerobic training. On the practical side, the home workouts guide and the zone 2 cardio at home post cover how to run a low-friction weekly schedule that respects the rules above.

References

1. Hickson RC. Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology and Occupational Physiology. 1980;45(2-3):255-263. DOI: 10.1007/BF00421333 · PMID: 7193134.
2. Coffey VG, Hawley JA. Concurrent exercise training: do opposites distract? The Journal of Physiology. 2017;595(9):2883-2896. DOI: 10.1113/JP272270 · PMID: 27506998.
3. Petré H, Hemmingsson E, Rosdahl H, Psilander N. Development of Maximal Dynamic Strength During Concurrent Resistance and Endurance Training in Untrained, Moderately Trained, and Trained Individuals: A Systematic Review and Meta-analysis. Sports Medicine. 2021;51(5):991-1010. DOI: 10.1007/s40279-021-01426-9 · PMID: 33751469.
4. Schumann M, Feuerbacher JF, Sünkeler M, Freitag N, Rønnestad BR, Doma K, Lundberg TR. Compatibility of Concurrent Aerobic and Strength Training for Skeletal Muscle Size and Function: An Updated Systematic Review and Meta-Analysis. Sports Medicine. 2022;52(3):601-612. DOI: 10.1007/s40279-021-01587-7 · PMID: 34757594.
5. Huiberts RO, Wüst RCI, van der Zwaard S. Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status. Sports Medicine. 2023;54(2):485-503. DOI: 10.1007/s40279-023-01943-9 · PMC10933151.