You added three runs a week in the spring. Six weeks later your squat is still sitting at 100 kg (220 lb) for 5, 5, 4, and the prescription has not moved since April. The obvious suspect is the running. The obvious fix is to stop running.

Both of those are worth examining, because the research on training for strength and endurance at the same time is more specific than "cardio kills gains" — and the thing it says you actually lose is probably not the thing you are watching.

What the two best meta-analyses found

The interference effect has two landmark reviews, published a decade apart, and they disagree in an instructive way.

Wilson, 2012

Wilson and colleagues pooled 21 studies and 422 effect sizes and found a real cost. Averaged across studies, the effect size for hypertrophy was 1.23 for strength training alone against 0.85 for concurrent training. For strength it was 1.76 against 1.44. For power it was 0.91 against 0.55, and power was the only outcome where all three conditions differed significantly from one another.

Two moderator findings from that paper are the ones that stuck in the culture. Resistance training alongside running, but not cycling, produced significant decrements in both hypertrophy and strength. And the correlations ran against volume: frequency of endurance training correlated at −0.26 to −0.35 with resistance outcomes, duration at −0.29 to −0.75. The authors' own conclusion was that interference is a function of the modality, frequency and duration of the endurance work chosen — not of doing any at all.

Schumann, 2022

Ten years later, Schumann and colleagues repeated the exercise across 43 studies in Sports Medicine, comparing concurrent training against an identical strength programme with no aerobic work. The standardised mean differences were −0.06 for maximal strength (95% CI −0.20 to 0.09, p = 0.446), −0.01 for muscle hypertrophy (−0.16 to 0.18, p = 0.919), and −0.28 for explosive strength (−0.48 to −0.08, p = 0.007).

Read that row carefully. Size: no measurable cost. Maximal strength: no measurable cost. Explosive strength — rate of force development, jumps, anything where the speed of the contraction is the point: a small but statistically clear cost.

The subgroup analyses matter just as much. The attenuation of explosive strength was more pronounced when both were done in the same session (p = 0.043) than when the sessions were at least three hours apart, where it was no longer significant. And the moderators everyone argues about on the internet came back empty: cycling versus running, more than five versus fewer than five weekly sessions, trained versus untrained, over 40 versus under 40 — none of them reached significance.

What to take from the disagreement

Two good reviews, ten years apart, landing differently is not a reason to ignore both. It is a reason to hold the overlap confidently and the rest loosely.

• Muscle size is not the thing at risk. The newer and larger review found a hypertrophy effect of essentially zero. If your goal is more muscle, aerobic work is not the reason it is going slowly.

• Explosive strength is the confirmed cost. It is the one outcome where both reviews point the same direction, and it is the outcome most lifters never measure.

• Proximity is a lever you control. Same-session concurrent training was worse for explosive strength than separating the two by three hours or more. That is a scheduling change, not a training sacrifice.

• Dose probably matters, but less than 2012 suggested. Wilson found volume correlations; Schumann found no frequency moderator at the five-session split. The honest reading is that a genuinely large aerobic block is worth accounting for, and three runs a week probably is not the thing blocking your squat.

The cost shows up in your log as missed reps, not lost muscle

Here is the practical problem with watching for interference in the mirror: you cannot see an effect size of −0.01, and neither can anyone else.

What you can see is a working set. If your squat prescription is 4 × 6 at 100 kg and you hit 6, 6, 5, 4 on the Thursday after a hard Wednesday session on the bike, that is not a plateau and it is not lost muscle. It is one fatigued session, and treating it as evidence that your programme has stopped working is how people end up rebuilding a programme that was fine.

The distinction worth holding is between a session you missed because you were tired and a lift that has genuinely stopped progressing. One miss is noise. Three sessions in a row where a working set falls below the bottom of your rep range is a signal — and that signal usually means the load needs to come down about 10% and climb again, whether or not there is any running in the picture.

Four changes worth making

None of these require dropping a modality.

1. Put at least three hours between them, or use different days. This is the one change with direct subgroup evidence behind it. If you have to double up, morning run and evening lift beats back-to-back.

2. Lift first when lifting is the priority. If the two have to share a session and the strength work is what you care about most, do it while you are fresh and accept that the aerobic session is the one running on tired legs.

3. Protect the explosive work specifically. Jumps, throws, Olympic lift variations and speed work are where both reviews agree there is a cost. Put them at the start of a session, early in the week, and as far from hard aerobic work as your schedule allows. Slow grinding compounds are far more robust.

4. During a heavy aerobic block, hold the weight and chase reps. If you are eight weeks out from a half marathon, this is not the month to be adding 2.5 kg a fortnight to everything. Keep the loads where they are and try to add a rep per set. You will maintain your lifts through the block and still be progressing, rather than accumulating missed sessions and a deload.

A week that works

Three runs and three lifting sessions, arranged so nothing important collides:

• Monday — Lower body. Squat, hinge, accessories. No running.

• Tuesday — Easy run, 40 minutes.

• Wednesday — Upper body. Any running here is the easiest of the week, and in the morning if it happens at all.

• Thursday — Intervals or the hard run. No lifting.

• Friday — Lower body, lighter. If Thursday was genuinely hard, this is the session to hold loads rather than push them.

• Saturday — Long run.

• Sunday — Off.

The principle behind it is simply that the hardest aerobic session and the heaviest lower-body session should not be next to each other in either direction. Everything else is negotiable.

When it is not the running

Lifts stall for reasons that have nothing to do with your cardio, and it is worth ruling them out before you cut something you enjoy. A rep range you have quietly stopped honouring, working sets judged by your best set rather than your worst, a load increment too large for the lift, or eight weeks without a backoff are all more common causes of a stalled prescription than three easy runs. Our guide to the 2026 ACSM resistance training guidelines is a reasonable place to audit the programme itself.

And two things are outside the scope of this article and of any training app: if a lift hurts, that is a question for a qualified clinician rather than a programme change; and if you suspect the problem is how much you are eating or sleeping, that is a conversation for a professional who can look at your actual situation.

Where Adensa fits

Adensa is a progressive-overload tracker for lifting, and it reasons from the sets you log — which is exactly why it reads your other training too. It pulls your workouts from Apple Health, so runs, rides and classes appear beside the lifting numbers instead of being invisible to a system that is about to tell you to add weight. Those workouts are read, never stored: nothing goes into Adensa's database, and revoking access in the Health app removes them.

The thresholds are deliberately blunt, because the decision they feed is blunt. Three hours of other training in the last seven days, or two sessions you yourself rated 8 or higher on Apple's effort scale, and Adensa's Insights briefing raises a card saying so — and the action it gives you is the one this article argues for: pick Recover for your next session and hold the reps rather than the weight. Choosing Recover drops the prescribed load to 90% and cuts the set count, and — the part that matters over a training block — a Recover session never moves your progression reference, so an easy week during a hard running block does not reset the weight you had worked up to. That card, and its action, are free.

The cross-training numbers are cited in-app to Wilson 2012 and Schumann 2022 — the same two papers above — alongside every other number the engine uses, under Settings.

If you are running and lifting in the same week and cannot tell which one your last bad session belongs to, connect Apple Health and see whether the weeks your lifts sagged were the weeks your other training spiked. If the picture it draws is wrong, tell us what it got wrong.