The evidence-led answer
NST verdict
Do whichever you care about most first. When lifting and cardio share a session, doing the weights first gives a small edge to lower body strength, but the order makes no measurable difference to muscle growth, aerobic fitness or fat loss. The reason is simple. Whichever you do first, you do fresh; the second is done tired, so its quality drops a bit. If strength or leg power is the priority, lift first. If a hard run, ride or interval session is the priority because you are training for an event, do that first and accept a slightly lesser lift. Better still, if your schedule allows, split the two onto different times of day or different days. That cuts any interference more than reshuffling a combined session does, and so does keeping the cardio volume from creeping too high.
Does the order matter?
A little, and only for one thing. Two pooled analyses looked at programmes that combined resistance and endurance work in the same session, comparing weights first with cardio first. Weights first produced better lower body dynamic strength, about 7% more change in one analysis and about 4 kg more on a lower body one rep max in the other. Order had no effect on muscle growth, aerobic capacity or body fat percentage in either analysis. So if you care specifically about leg strength, lift before you do cardio. For size, fitness and fat loss, the order does not change your results.
Why, and what matters more
The reason is fatigue, not some metabolic clash. Whichever mode you do first, you do it fresh and well; the second is done tired, so a bit of quality is lost. Contrary to a popular idea, the molecular "interference" between endurance and strength signalling has not been reliably shown in humans after a single combined session. The practical effect is mostly leftover tiredness.
And a few things matter more than order. Separating the modes: putting recovery time between them, several hours apart or on different days, cuts interference more than reordering one session. Cardio dose: the volume, intensity and type of endurance work drive interference more than the within session order does. A lot of hard running eats into strength and size far more than the question of what you did first.
So which order for you?
If your priority is strength or leg power, lift first, then do your cardio. If your priority is a running or cycling event, do the key endurance session first and accept a slightly lesser lift that day. If you have both goals and general fitness in mind, either order is fine, so pick the one you will actually stick to, and split the two across the day or week where you can.
What this means in practice
Put your main goal first in the session, and stop worrying about it beyond that. If you can, do your cardio and your lifting in separate sessions, morning and evening, or different days, which helps more than any ordering trick. Keep an eye on your total cardio load during phases where strength or muscle is the priority. A big block of hard endurance work is what actually eats into those gains, not whether you ran before or after the squats.
Practical meaning
Put your main goal first in a combined session, lift first for strength or leg power, do the key endurance session first if you are training for a running or cycling event. For general fitness with both goals, either order is fine, so pick what you will stick to. If you can, do cardio and lifting in separate sessions across the day or week; that helps more than any ordering trick. Keep total cardio load in check during strength focused phases.
Limitations and uncertainty
Both order pooled analyses cover a small number of studies, and the only significant effect is on lower body dynamic strength, not upper body and not every power measure. The order studies used same session training, so the point that separating the modes helps comes from a wider review, not a direct head to head. The training variables review is from 2014. This is about strength, size and fitness adaptations, not which order burns more fat in one session, since fat loss is total energy balance over time.
