Concurrent Training Interference Effect on Hypertrophy

Concurrent training is the simultaneous pursuit of resistance and endurance adaptations within the same training week. Hickson opened the field in 1980: over a ten-week program, the group doing strength work alongside endurance work improved leg strength at the same rate as the strength-only group for seven weeks, then levelled off and actually declined in weeks nine and ten, while VO2 max rose normally.^[7] The phenomenon was named "interference" and the question of how, when, and how badly two opposing stimuli interfere has driven hundreds of trials since.

The 2026 picture is meaningfully different from the Wilson 2012 meta-analysis that anchored a generation of strength coaches. In the newer pooled data the hypertrophy and maximal-strength penalties have largely disappeared, leaving explosive strength as the outcome that still shows a measurable cost. This article walks through Wilson 2012, the Schumann 2022 update, what Markov 2023 does and does not tell us, and the practical splits that fall out.

The Wilson 2012 baseline

Wilson and colleagues pooled 21 concurrent-training trials in 2012, yielding 422 effect sizes across hypertrophy, strength, and power.^[1] The analysis reports mean effect sizes per training group rather than a single pooled interference term. Headline numbers:

• Hypertrophy: mean effect size 1.23 for strength training alone, 0.85 for concurrent training, 0.27 for endurance alone. Concurrent groups still grew, just less than strength-only groups.
• Strength: 1.76 strength-only, 1.44 concurrent, 0.78 endurance-only.
• Power: 0.91 strength-only, 0.55 concurrent, 0.11 endurance-only — the only outcome where all three groups differed significantly from one another, and the largest relative gap.
• Mode interaction: adding running, but not cycling, produced significant decrements in both hypertrophy and strength.
• Dose interaction: endurance frequency (r = -0.26 to -0.35) and endurance session duration (r = -0.29 to -0.75) both correlated negatively with hypertrophy, strength, and power.

The Wilson model became the canonical reference. It was easy to summarise in a single sentence: running hurts hypertrophy more than cycling, more endurance is worse, power is the most fragile outcome.

The Schumann 2022 update

Schumann and colleagues re-ran the question in 2022 across 43 studies, each comparing supervised concurrent training against an identical strength-training prescription with no aerobic work.^[2] Several findings reversed or shrank.

• Muscle hypertrophy: pooled SMD -0.01 (95% CI -0.16 to 0.18, p = 0.919). No detectable interference at all.
• Maximal strength: pooled SMD -0.06 (95% CI -0.20 to 0.09, p = 0.446). Not statistically significant.
• Explosive strength: pooled SMD -0.28 (95% CI -0.48 to -0.08, p = 0.007). The one outcome with a real penalty, and it survives the newer data.
• Session separation: the attenuation of explosive strength was more pronounced when both qualities were trained within the same session (p = 0.043) than when the sessions were separated by at least 3 hours.
• Moderators that did not matter: type of aerobic training (cycling vs running), concurrent frequency (more vs fewer than 5 weekly sessions), training status, and mean age (under vs over 40).

The shift between Wilson and Schumann reflects a real change in the studied populations: newer trials use more carefully controlled resistance programs, with adequate volume and recovery, while older trials often pitted a moderate strength block against a heavy endurance load with insufficient recovery. The interference effect was partly an artefact of poor program design.

Markov 2023: does concurrent training still work in older adults?

Markov, Hauser, and Chaabene 2023 asked a different question from Wilson and Schumann.^[3] Rather than comparing concurrent training against strength training alone, they pooled 15 randomised controlled trials (566 participants, healthy adults aged 50 to 73) comparing concurrent training against a passive control. The question is not how much interference concurrent training costs, but whether the combined model delivers in a population where both strength and aerobic capacity are declining.

• Muscle strength: moderate positive effect (SMD 0.74).
• Muscle power: moderate positive effect (SMD 0.50).
• Cardiorespiratory endurance: small-to-moderate positive effect (SMD 0.48).
• Balance: no significant effect.
• Age and sex: adults over 65 (SMD 1.04) and females (SMD 1.05) showed larger strength improvements than adults 65 and under (0.60) and males (0.38), though none of the subgroup differences reached significance.

The authors also ran an independent single-training-factor analysis to guide prescription. It is a set of subgroup comparisons, not a moderator regression, and the differences between subgroups were not statistically significant — so read the following as a hierarchy of plausible prescriptions, not as measured dose-response curves:

• Frequency: three sessions per week produced larger strength effects (SMD 0.91) than four (0.55), while cardiorespiratory endurance only showed a moderate effect at four sessions per week (0.58).
• Session duration: 30 to 60 minutes favoured strength (0.99) and power (0.88) over 60 to 90 minutes (0.40 and 0.29), while endurance favoured the longer sessions (0.61 vs 0.34).
• Program length: 12 weeks produced the largest strength (0.87) and power (0.88) effects; endurance kept improving out to 21 weeks (0.62 vs 0.40).
• Endurance intensity: moderate-to-near-maximal endurance work produced effects on strength (0.64) and endurance (0.49); low-intensity endurance work produced none.
• Order and separation: strength before endurance within the same session gave the largest strength gains (1.00 vs 0.55 for separate sessions), whereas keeping the two in separate sessions favoured endurance (0.58 vs 0.49 for endurance before strength in one session).

Two cautions before carrying any of this into a lifter's program. The population is healthy 50-to-73-year-olds and the comparator is doing nothing — so these numbers say concurrent training works, not that one configuration interferes less than another. For the interference question in trained lifters, Wilson and Schumann remain the relevant pooled evidence.

The mechanism: AMPK-mTOR cross-talk

Atherton 2005 proposed the molecular mechanism that anchors the modern interference framework.^[6] Endurance work activates AMP-activated protein kinase (AMPK), a metabolic sensor that responds to depleted ATP and rising AMP. Resistance work activates the mTOR pathway, the master regulator of muscle protein synthesis. AMPK and mTOR have inhibitory cross-talk: when AMPK is high, mTOR signalling is suppressed.

The clinical consequence is that endurance work performed shortly before a resistance session attenuates the muscle protein synthesis response to that session. The duration of the AMPK suppression is not infinite; AMPK signalling decays over 3 to 6 hours after moderate-intensity endurance work, longer after exhaustive work. This is the molecular basis for the "6-hour rule": separate endurance and resistance by at least 6 hours and the AMPK signal has cleared before the lifting stimulus is presented.

Robineau and colleagues 2016 tested this directly.^[10] Fifty-eight amateur rugby players trained for seven weeks with 0, 6, or 24 hours of recovery between the strength and aerobic sequences, against strength-only and non-training controls. Maximal strength gains in the bench press and half squat were lower in the 0-hour group than in the 6-hour, 24-hour, and strength-only groups, and VO2peak gains were largest with 24 hours of separation. The authors' own recommendation is not to schedule the two qualities less than 6 hours apart.

Order of operations

If both modalities have to happen on the same day, which order is better? Eddens 2018 meta-analysed ten trials of intra-session sequence and found resistance-before-endurance favoured lower-body dynamic strength (weighted mean difference 6.9% change, 95% CI 2.0 to 11.9), with no effect of order on lower-body hypertrophy, static strength, maximal aerobic capacity, or body-fat percentage.^[8] Enright 2015, working with youth elite soccer players over five weeks, found no significant interaction for training order, though the effect-size comparison leaned toward putting strength work before soccer-specific endurance work.^[9]

Practical synthesis: when same-day work is unavoidable, do the priority modality first while the system is fresh. For a hypertrophy-priority block, lift first. For a VO2-max-priority block, run intervals first.

Endurance-mode and intensity interactions

Gergley 2009 compared cycle ergometry against incline treadmill walking as the endurance mode added to a 9-week lower-body strength program in untrained participants.^[11] Both modes blunted leg-press 1RM gains relative to resistance-only training, but the cycling group kept significantly more of the strength gain than the treadmill group. Weight-bearing, eccentrically loaded endurance work costs lower-body recovery in a way cycling avoids.

Blagrove 2018 reviewed the strength-training literature for distance runners and found the inverse: running performance improves with added strength work, especially explosive lifting (heavy loads at high velocity, jump training).^[5] The interference is asymmetric. Strength work improves running economy without harming aerobic capacity; running, especially high volumes, blunts hypertrophy and power.

Practical splits by goal

Synthesising the 2022 to 2023 evidence into programmable splits:

Hypertrophy priority (lifter who runs)
  Resistance:           4 sessions/week, full programmed volume
  Endurance:            2 sessions/week max
  Mode:                 cycling preferred over running
  Intensity:            zone 2 base + 1 short interval session
  Separation:           24 hours from any lifting session
  Expected interference: negligible for hypertrophy and maximal strength
                        (Schumann pooled SMD -0.01 and -0.06, neither
                        statistically significant)

Endurance priority (runner who lifts)
  Resistance:           2 sessions/week, heavy + explosive
  Endurance:            5–6 sessions/week, full volume
  Mode:                 running (the priority specificity)
  Lifting focus:        compound, low-rep, high-intent
  Separation:           lift on quality run days, after the run
                        AM run + PM lift, or vice versa
  Expected interference: small for running performance; explosive
                         strength is the outcome most likely to lag

Hybrid athlete (true concurrent)
  Resistance:           3–4 sessions/week
  Endurance:            3–4 sessions/week
  Mode:                 mix cycling + running, with running for race specificity
  Separation:           same-day if needed, prefer 6+ hours
  Order:                priority adaptation first
  Expected outcome:     cost concentrated in explosive strength rather
                         than size or maximal strength; total work
                         capacity exceeds either single-focus program

Where the meta-analyses still disagree

• Whether endurance modality matters. Wilson 2012 found that adding running, but not cycling, produced significant decrements in hypertrophy and strength; Schumann 2022 found no significant moderation by aerobic modality at all. The same split shows up on training frequency, which Wilson found correlated with worse outcomes and Schumann found non-significant.
• Acute versus chronic interference. Doma 2017 reviewed the effect running the other way: a single resistance session leaves residual fatigue that can degrade the quality of subsequent endurance sessions for hours to days when recovery is short.^[4] The review is explicit that the link between that acute cost and chronic endurance adaptation is not fully established.
• Concurrent training in older adults. Schumann 2022 includes both age groups and reports no significant moderation by age, but the older-adult sample is small.
• Female-specific data. Sex-specific interference estimates remain thin, and none of the three pooled analyses above reports an interference effect broken out by sex.
• Long-duration trials. Most studies run 8 to 12 weeks. The interference effect compounded over a year of concurrent training is largely extrapolated, not measured.

Cross-link tools

• 1RM Calculator tracks the strength outcome that interference is most likely to compromise on power-priority blocks.
• VO2 Max Estimator tracks the endurance outcome on the other side.
• Workout Volume Calculator manages the resistance volume that determines whether the interference window matters.

• Wilson 2012 set the field: concurrent groups gained less than strength-only groups on size, strength, and especially power, with the penalty tracking endurance modality, frequency, and duration.
• Schumann 2022 found no detectable interference for hypertrophy or maximal strength, leaving explosive strength as the one outcome with a significant penalty — largest when both qualities share a session.
• Markov 2023 answers a different question and should not be read as an interference finding: in adults aged 50 to 73, concurrent training beat a passive control on strength, power, and cardiorespiratory endurance.
• The molecular mechanism is AMPK-mTOR cross-talk; separating sessions by 6 to 24 hours largely resolves the conflict.
• Practical splits cap endurance at 2 to 3 sessions for hypertrophy priority, prefer cycling, and put separation between modalities when possible.