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I often hear the claim that running promotes muscle loss. To what extent is this true? If a person is a triathlete don't they need both cardio and muscle? Does it matter what distance is run and the speed?

I first red such a claim on this article, though it clearly is not scientific.

And while the idea of masculinity varies somewhat from era to era and from one region of the world to another, masculinity is usually associated with superior strength, muscularity, speed, and power. The human body, as it turns out, isn't a very good multi-tasker when it comes to "S.A.I.D." (specific adaptation to imposed demand). It prefers to be either big and strong, or small and weak, (albeit with good endurance). True, some guys can manage to have it both ways, but if you find it challenging to gain muscle or strength, you're not one of them.

This article seems to second it but also isn't scientific.

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This claim is often repeated on lifting forums and among people at the gym. "Cardio kills gains!" is even a running joke, to be found in fitness humour forums like www.reddit.com/r/swoleacceptance. The scientific answer appears to be sort of.

Originally I had a nice list of sources (there's a very large literature on this topic), but by far the best is an enormous 2012 meta-study examining the interaction of running and lifting with muscle mass and strength gains, found here. The study examined results from 21 other studies, with more than 400 reported values. The main conclusion of use in answering this question was "The mean ES for hypertrophy for strength training was 1.23; for endurance training, it was 0.27; and for concurrent training, it was 0.85, with strength and concurrent training being significantly greater than endurance training only.", where ES stands for effect size (in this case, defined the difference in mean value of the variable before and after the application of the different exercise regimes, divided by standard deviation). So we can see that running actually leads to hypertrophy (muscle mass gain), of about 0.27 of a standard deviation, on average. However, this is less than the gain you'd see by lifting weights (about 1.23 standard deviations), and less than the gain you'd see by doing both together (0.85 sd).

So, running does not cause the loss of muscle mass. However, it does appear to slow the gain of muscle mass, when performing a weight lifting regimen, by about a third. This is probably the origin of the claim. You will not lose muscle mass from running, all else being equal, but you might not gain it as quickly!

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  • I confess I haven't looked at your reference. Does it conclude causality, or merely correlation? Either way, I would emphasize that more in the answer.
    – Oddthinking ♦
    Commented Aug 5, 2015 at 1:40
  • @Nomenagentis-The question is about short or long distance running causing muscle loss and not about effects of endurance/concurrent training on strength, power, or hypertrophy. Its already known that some individuals experience strength decrease after concurrent training, whereas others experience substantial increase. This quoted metanalysis talks about concurrent running vs. cycling and also "long distance running causes large increases in muscle damage". A causal relationship between exercise induced muscle loss and muscle hypertrophy is yet to be established.
    – pericles316
    Commented Aug 5, 2015 at 7:19
  • @John Doucette- The same paper quotes "A hockey player wanting to increase leg strength during dry ice training may want to avoid running and instead select a cycling exercise, which more closely approximates the demands of skating." and "Running has a high eccentric component, whereas cycling consists of primarily concentric activity. These differences in contraction types (eccentric vs. concentric) may create greater damage in running than in cycling." This means leg strength of a ice hockey player using running as his exercise will not be increased due to muscle fatigue.
    – pericles316
    Commented Aug 5, 2015 at 7:33
  • @Nomenagentis-I agree that muscle loss and hypertrophy are opposites. However, slow twitch muscle fibers are used for low intensity activities such as jogging and high repetition, low intensity exercise, such as distance running also mainly uses slow twitch fibers. Successful distance runners have a high proportion of slow twitch muscles. So in a way doesn't running short or long distance relate to endurance athletes having more slow twitch fibers in their active muscles and less of fast twitch fibers when compared to strength athletes and sprinters ?
    – pericles316
    Commented Aug 5, 2015 at 8:52
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    In chat, we concluded that pericles316's objections were caused by a misunderstanding about what the question being asked was. The bold text at the end of my answer was deemed a satisfactory answer to the question "Does running cause muscle loss?", which we believe was the asker's intent.
    – John Doucette
    Commented Aug 6, 2015 at 17:21

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