Strength lifters worry adding cardio will wipe out their gains. Endurance athletes worry adding lifting will slow them down. Both fears trace back to a 1980 study by Robert Hickson, who watched runners and cyclists stall on leg strength after weeks of long aerobic work. Forty years on, a 2022 Sports Medicine meta-analysis pulled together 43 trials testing the same question in healthy adults. The result reshapes the conversation around hybrid training and HYROX.
The review, led by Moritz Schumann at the German Sport University, found combining aerobic and strength training did not reduce maximal strength gains or muscle hypertrophy. The standardised mean difference for maximal strength was -0.06 with a confidence interval crossing zero. For hypertrophy, the difference was -0.01. Both effects sit at clinical irrelevance. The one outcome with a measurable hit was explosive strength, the speed you produce force, at a standardised mean difference of -0.28 favouring strength-only training. Timing closed most of that gap: separating cardio and lifting by at least three hours wiped out the deficit.
The numbers that matter
Where the interference comes from
The body senses energy demand. Endurance exercise activates AMPK, an enzyme that switches on fat oxidation and slows cell-building work. Heavy lifting activates mTOR, the signal telling muscle fibres to grow. Stacking a long aerobic session inside the same gym slot as a lifting session leaves AMPK raised when mTOR tries to fire. The hypertrophy signal weakens. That molecular pattern sits behind what coaches call the interference effect, described in a 2018 review in the journal Sports.
The gap between molecular noise and real-world results is wide. A small drop in mTOR signalling does not always reduce muscle growth, because daily protein, sleep, weekly volume, and total recovery matter more than one overlapping session. The Schumann meta-analysis confirmed that gap. The molecular interference is real, the clinical interference for size and maximal strength is not. Explosive strength, the most neural and fast-twitch dependent outcome, is the one feeling the squeeze, especially when lifters run rather than cycle, because running adds eccentric muscle damage on top of the metabolic cost.
Effect sizes at a glance
Who feels the interference most
A 2024 Sports Medicine meta-analysis pulled together 59 trials and 1,346 participants to look at sex and training status. Lower-body strength dropped slightly with concurrent training in males but not in females. Upper-body strength, power, and VO2 max moved similarly across sexes. Trained endurance athletes adding lifting are the most likely to see strength erode if they do not adjust volume. Strength athletes adding low to moderate cardio sit in a safer zone.
A 2021 Sports Medicine systematic review on training status tracked one-rep max leg press and squat across untrained, moderately trained, and trained adults. Highly trained lifters lost ground when lifting and cardio happened back to back in the same session, but not when sessions were separated. Moderately trained and untrained adults gained equally either way. The harder you push at one end of the strength-endurance spectrum, the more session separation matters.
A 4-week concurrent training week
This template fits an adult training for general strength and aerobic capacity, with around five hours of training per week. The structure separates cardio and lifting wherever possible and protects heavy leg days from impact work.
Five rules to protect your strength
- Separate lifting and cardio by at least three hours whenever possible, based on the Schumann 2022 session-spacing finding.
- Pick cycling, rowing, or the elliptical over running on heavy leg days. Running adds impact and eccentric load.
- Keep weekly cardio under four hours if maximal strength is the priority. Higher volumes raise interference risk.
- Eat 1.6 to 2.2 grams of protein per kilogram of body weight daily, the range supported by hypertrophy meta-analyses.
- Sleep at least 7 hours. Recovery is where the adaptations from both training modes happen.
Common mistakes
The most common error is stacking a hard cardio session immediately before lifting. AMPK is fresh, mTOR is suppressed, the lifting session feels heavy, and the body diverts recovery to the cardio. Fix the order: lift first or wait three hours. The second mistake is running on the same day as heavy squats or deadlifts. Running combines impact, eccentric load, and high heart rate, compounding recovery cost. Cycling or rowing on those days protects the legs.
The third mistake is treating hybrid training as twice the volume. Concurrent training works only when total weekly load stays inside what food and sleep recover. Cutting one strength session to add three cardio sessions usually overshoots recovery and stalls both. A fourth pitfall is ignoring the long aerobic session. Most adults benefit from one weekly bout of 60 to 90 minutes at conversational pace, because it builds mitochondrial density without raising the eccentric load that hurts strength. A fifth pitfall is skipping deload weeks. After four to six weeks of concurrent work, a lighter week of half-volume sessions clears accumulated fatigue and lets the next training block produce gains.
Frequently asked questions
Bottom line
Hybrid training works. The 2022 Sports Medicine meta-analysis of 43 trials found combining cardio with strength training did not reduce maximal strength or muscle size in healthy adults. Explosive strength was the only outcome with a measurable gap, and a three-hour buffer between sessions closed most of it. Sex, age, and training type were not strong moderators. Treat concurrent training as the default for most adults aiming to live longer, lift heavier, and move better.
Sources
- 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 Med. 2022, 52(3): 601-612. PubMed 34757594
- Authors et al. A Brief Review on Concurrent Training: From Laboratory to the Field. Sports (Basel). 2018, 6(4): 127. PubMed 30355976
- Authors et al. Concurrent Strength and Endurance Training: A Systematic Review and Meta-Analysis on the Impact of Sex and Training Status. Sports Med. 2024, 54(2): 485-503. PMC10933151
- Authors et al. Concurrent Training: A Meta-Analysis Examining Interference of Aerobic and Resistance Exercises. J Strength Cond Res. 2012, 26(8): 2293-2307. PubMed 22002517
- NHS. Physical activity guidelines for adults aged 19 to 64. nhs.uk
- WHO. Physical activity fact sheet, 2024. who.int