Guide
How do you run without losing muscle?
Keep lifting while you run, and put your legs and back in it, not only chest, arms and shoulders. At moderate running volume the studies find little to no cost to muscle size, and the cost grows with how much you run.
Last updated: 2026-10-16
By Ole Fiegenbaum, founder of Loadcast. Before the app, Ole's triathlon training ran through one ChatGPT chat. More on the landing page.
How this page was made: the rules and example weeks come from the Loadcast engine, and every example week runs through the same check as the app. The studies are linked. The text was written with AI help.
What running does to your muscle
The cost runs one way. Adding running to lifting can blunt gains in strength and power, most of all in the lower body. Adding lifting to running does not reduce VO2 max, and it makes runners more economical.
How much it costs depends on the dose. At moderate weekly volume, meta-analyses find little to no average cost to muscle size and only a small one to strength and power. In the research summary behind Loadcast, strength holds up to roughly 6 to 8 hours a week of endurance work, as long as the lifting volume and intensity are kept up and some of that work is cycling or swimming rather than long runs. Beyond 8 to 10 hours a week of mostly running, the risk of losing strength and size goes up.
Running costs more than cycling. In Wilson and colleagues (2012), running on top of lifting showed clear losses in size and strength against lifting alone, and cycling on top of lifting did not. Coffey and Hawley think this is likely down to the heavier eccentric load and muscle damage of running.
Wilson et al. 2012. J Strength Cond Res. Concurrent training meta-analysis. PMID 22002517
Coffey & Hawley 2017. J Physiol. The molecular basis of concurrent training. PMC5407958
Schumann et al. 2022. Sports Med. Updated concurrent training meta-analysis. PMID 34757594
Blagrove et al. 2018. Sports Med. Strength training and middle and long-distance running performance, systematic review. DOI 10.1007/s40279-017-0835-7
Berryman et al. 2018. Int J Sports Physiol Perform. Strength training for middle and long-distance performance.
What one Ironman athlete says happened
In r/HybridAthlete, someone training for an Ironman posted under the title "I lost almost 8kg of muscle with Ironman training". They write that their carb intake is large, that they fuel during sessions and that their protein is right, but "I now look small".
About their lifting they write "I can fit a day or two in the gym depending on the week. I end up with a 'looksmaxing' routine, arms chest and shoulders. Heavy weights and intense sets, not much volume."
The reply with the most points (76 when we read the thread on 13 September 2026) says "You're lifting and eating like an endurance athlete, you're gonna start looking like one". Another reply (59 points) went at the split and says "That's why you're losing muscle, train properly in the gym and the problem will go away." A third (34 points) went at food and says "Are you still lifting? If so then you just have to eat more."
That is one thread and one person's account, and the eight kilos are their own number, which we cannot check. What it shows is what the replies picked on: what was lifted, and what was eaten.
What to do
- If you lift only chest, arms and shoulders, add legs and back, because lower-body strength is the outcome the studies find running touches most.
- If your running weeks get longer, keep the lifting in the plan instead of dropping it first.
- If you run more than 8 to 10 hours a week, mostly running, plan a few weeks where lifting comes first and the running is cut back.
- If you can swap one run for a ride, the ride costs your legs less than the run.
- If you lift legs and back and are still getting smaller, look at what you eat before you look at the plan.
What these lines do not know
The studies compare people who lift and run over weeks. Most of them cover programmes of 12 weeks or less at moderate volumes, and data on years of training at 8 to 12 hours a week is thin. None of them follows someone through an Ironman build.
They do not track food either, so the last line rests on the thread, not on a study. No study tests lifting legs and back as the fix for lost muscle, so the first line is our reading of the research and of the thread, not a result from one paper.
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Loadcast is an iPhone app. See it in the App Store.