What the interference effect is, and how big it is
Training strength and endurance in the same programme is called concurrent training. The worry is the interference effect: the running blunts what the lifting builds. It is real, but narrower than its reputation.
The 2012 meta-analysis by Wilson and colleagues pooled 21 studies comparing strength training alone with strength plus endurance work. Adding endurance clearly cut the gains in power, the one difference from strength training alone that was statistically significant; strength and size gains were lower too, but not significantly. When the endurance work was running rather than cycling, strength and size gains were significantly smaller, and the more often and the longer people did endurance work, the smaller their gains.
A decade later, Schumann and colleagues pooled 43 studies of healthy adults. Adding aerobic training made no measurable difference to maximal strength or muscle size. Explosive strength, the jump-and-sprint quality, was blunted, and clearly so only in studies that did both in the same session. In this larger analysis, running and cycling did not differ, and neither did training status or age.
Adding running costs you little or no maximal strength or muscle. What is at stake is explosive strength and the quality of whichever session comes second, and both depend on how you arrange the day. If size is a goal in itself, read HYROX and bodybuilding.
Order: what the sequence studies found
Two 2018 meta-analyses asked the same question: in a session with both, which goes first?
- Eddens, van Someren and Howatson pooled ten studies of at least five weeks. Lifting before endurance improved lower-body dynamic strength by 6.91 percentage points more than the reverse. Order made no difference to muscle size, static strength, maximal aerobic capacity or body fat.
- Murlasits, Kneffel and Thalib found lifting first added 3.96 kg to the lower-body one-rep max, and no difference in aerobic capacity.
So the order changes one thing, lower-body strength, by a few kilos. Your engine improves the same either way. That is why the choice follows your priority rather than a fixed sequence. If a heavier squat is the point of the day, lift first. If the run is the point, your aerobic fitness will not suffer for running first, and the fatigue studies below show that a hard run goes better on fresh legs.
The gap: hours between sessions
A rugby trial set the hours between sessions directly, and Schumann's meta-analysis split its studies by gap. Between them they found effects on more outcomes than the order studies did. These are the gaps they used.
- 0, 6 or 24 hours. In a trial that compared gaps directly, Robineau and colleagues split 58 amateur rugby players into five groups for seven weeks; three of them lifted and then did aerobic work 0, 6 or 24 hours later. With no gap, bench press and half-squat gains were smaller than with 6 or 24 hours, even though lifting came first. Aerobic gains were larger with 24 hours than with 0 or 6.
- At least 3 hours. In Schumann's analysis, explosive strength was clearly blunted in studies that did both in one session, within 20 minutes of each other, whichever came first. In studies that kept the sessions at least 3 hours apart the loss was not statistically clear.
- 6 hours. Doma and Deakin had 15 trained and moderately trained male runners lift, then run 6 hours later. Easy running economy was unchanged after every lifting session. Time to exhaustion at a hard pace fell after the heavy whole-body and heavy lower-body sessions, but not after a light whole-body one.
Nobody has tested every gap in between, so read 3, 6 and 24 hours as landmarks, not a timer. The practical version: a morning session and an evening session beat two back to back, and a full day apart is better again for the engine.
The better order bought a few kilos on the squat. In the rugby trial, no gap at all cost strength gains even with lifting first, and gave smaller aerobic gains than a day apart. In Schumann's analysis, explosive strength suffered when the two were close together, whichever came first. The gap evidence is thinner than the order evidence, but it reaches more outcomes.
Heavy legs and the next run
A hard lifting session leaves fatigue that lasts hours to days. A 2017 review by Doma, Deakin and Bentley lists the proposed causes: altered muscle recruitment, running mechanics that raise the energy cost of each stride, soreness, and lower muscle glycogen. The studies measured it like this.
- The same day. As the 6 hour study above found, the hard run is the one that suffers.
- The next day. 14 trained and moderately trained male runners lifted and ran 6 hours apart, in both orders. The following day, running cost more energy only after the lift-first day, and time to exhaustion at a hard pace was shorter after both: 238 seconds after lifting first and 275 after running first, against 335 at baseline.
- The first sessions hit hardest. In 14 men new to lifting, one heavy lower-body session at a six-rep max impaired easy running for 24 hours. In 12 men new to lifting, easy running felt harder 24 and 48 hours after each of the first two sessions but not after the third, and even then hard running measured 24 and 48 hours later was still 12 to 20% below baseline.
Two lessons follow. Heavy legs cost the next run most when that run is hard and the lifting is new to you. And the body adapts, so a leg session you repeat regularly dents your running less than the first few did.
For HYROX, a run you care about is a threshold session, intervals or a race simulation. Put heavy legs after it, or before a run that is meant to be easy. Easy and hard running are laid out in running for HYROX.
Strength and running in one session for HYROX
The race is itself a concurrent session: eight runs of about 1 km, each followed by a station, with the sled push, burpee broad jumps, sandbag lunges and wall balls all loading the legs. In 24 HYROX Pro men at Madrid 2025, running speed fell from 16.1 km/h on the first run to 13.6 km/h on the eighth. A combined session can train that, or it can train strength. They are different sessions.
A strength session with a run attached
The goal is to get stronger. Lift first and heavy: back squat, Romanian deadlift, split squat, a heavy sled push. Then run easy and short. The sequence research backs this order for lower-body strength, and the run is easy because it is not the point. The lifts that carry over to each station are in the strength guide.
A race-style circuit
The goal is to run well on tired legs: a run, a station at race weight, another run. That is conditioning, and heavy sets rarely belong in it. In the first study of the race, 11 recreational athletes raced a simulated Open course; the authors judged the maximum-strength demands moderate to low compared with other functional fitness formats, and the faster finishers had the bigger aerobic engines and more endurance training. How to train the run off a station is in compromised running.
If you lift on a push, pull and legs split, the studies above measured heavy sessions that included the legs. Push and pull days can sit beside most runs. Put the legs day after a hard run, or hours before an easy one, and not on the day before a threshold, interval or simulation run.
A double-day template (illustrative)
Three ways to arrange a day with both. It is an illustration of the research above, not a prescription; your work and sleep decide the clock.
| Priority | First | Second | Next day |
|---|---|---|---|
| The run | Threshold or interval run | Hours later: upper body and core, or legs if the next run is easy | Easy run or rest |
| Strength | Heavy lower body: squat, Romanian deadlift, lunges | Hours later: an easy conversational run of 20 to 40 minutes | Easy run or rest |
| One session only | Heavy lifting | Straight after: an easy run of 15 to 20 minutes | Easy run or rest |
- Hard run on fresh legs: first in the day, and not the morning after heavy legs.
- Lift first when lower-body strength is the point of the day.
- Leave hours between the two whenever your day allows it.
- Expect the first weeks of a new leg programme to cost your running more than the later ones.
Which days of the week to double up, and how many, is a question for how many days a week. How the whole week fits around your other sports is in hybrid training for HYROX.
Two sessions on one day in Smash Coach
A double day in the app happens when something you already do meets a session from your coach.
Your own gym and strength work is not one of your sports: that is the space your coach programs for you, set under Coach sessions. Add the things you would do whether or not you had a coach, like the club night or the Saturday run with friends. More in Add a sport or class.
When one of your own sports lands on the same day as a coach session, that day has two sessions. Open the day and a strip of tabs appears under the header, one per session, each showing its sport. Tap a tab to switch. More in Read a workout and How your week works.