Recently, I've been reading a lot about how cardio reduces muscle mass and strength.

The following is an excerpt from crossfit.com that seems to sum up what I've been reading:

Aerobic training benefits cardiovascular function and decreases body fat – all good. Aerobic conditioning allows us to engage in low power extended efforts efficiently (cardio/respiratory endurance and stamina). This is critical to many sports. Athletes engaged in sports or training where a preponderance of the training load is spent in aerobic efforts witness decreases in muscle mass, strength, speed, and power. It is not uncommon to find marathoners with a vertical leap of only several inches! Furthermore, aerobic activity has a pronounced tendency to decrease anaerobic capacity. This does not bode well for most athletes or those interested in elite fitness.

I've also seen some dramatic pictures(if unflattering to endurance runners) that emphasize this:marathoner vs sprinter.

So here is my question. Why is there a decrease in muscle mass etc. with aerobic, but not with anaerobic activities? Is cross training enough to offset this? Or should everyone give up on long distance running and other endurance sports if they want muscles, more power, etc.? Is there any benefit to endurance sports other than better cardiovascular function when compared to strength training (if this in fact the case)?

  • 2
    Not effective as strength training towards what goal?
    – user4644
    Feb 10, 2013 at 6:21
  • Overall fitness and anything other than endurance.
    – ehmhunt
    Feb 10, 2013 at 6:22

5 Answers 5


Cardio training doesn't necessarily lead to muscle loss, but generally, training time is limited, and if you're preparing for a marathon, you don't have the time to spend in the gym, and your body will be busy adapting to the stresses of long distance running, which are different than the adaptations needed for sprinting 100m, dunking a basketball, or moving heavy things around.

So, yes, marathon runners will be less strong, slower, less powerful, have smaller verticals, etc.

This doesn't mean that their "overall fitness" is lower than that of a powerlifter or sprinter. They are just at different points in the athletic spectrum having optimized for different goals.

The reason crossfit says that "this does not bode well for most athletes or those interested in elite fitness" is because most athletes are power athletes.

Some resources on this topic:

  • These links are extremely helpful, thank you.
    – ehmhunt
    Feb 10, 2013 at 7:44

I've used this illustration in a previous answer, and it really does a good job of demonstrating the idea:

enter image description here

The things we care about on the illustration (in relation to your question) are the sarcoplasm and the myofibril. The myofibril is the part of the muscle that actually does the contraction, where the sarcoplasm is the part of the muscle that stores ATP. The myofibril taps into ATP for energy when it is in an anaerobic state. During an aerobic state, the myofibril just uses aerobic glycosis for energy. In this case the blood is able to provide enough sugar and oxygen for regular cellular respiration. See this lecture for more details on these processes.

Training that involves using the muscle during an anaerobic state (weight lifting, sprinting, etc) can cause sarcoplasmic hypertrophy. In other words, the muscle is trained to store more sarcoplasm so that it can provide more ATP.

On the contrary, training that involves using the muscle in an aerobic state (distance running, cycling, etc), doesn't induce (as much) sarcoplasmic hypertrophy, and therefore the muscle does not swell in size (that's not to say myofibril hypertrophy doesn't occur).

So to answer your question, aerobic exercise doesn't lead to muscle loss -- it just doesn't contribute strongly to (sarcoplasmic) muscle growth. An endurance athlete COULD have larger muscles if they also trained in an anaerobic state. This would be detrimental to performance in endurance sports though, since it would be useless, additional weight.

  • So essentially, the muscles that endurance athletes do have, are more myofibril (actual muscle lifting power) and less sarcoplasm (ATP storage)?
    – ehmhunt
    Feb 10, 2013 at 22:01
  • and to compound the visual difference, the actual strength requirements are different for endurance activities?
    – ehmhunt
    Feb 10, 2013 at 22:03
  • 2
    The myofibril should develop relative to the requirements of the activity. Jogging requires less myofibril than sprinting, so if all an athlete does is jog -- never jumps or does a push-up -- unused muscle fiber should atrophy away eventually. If an athlete never performs an activity that requires the muscle to tap into ATP stores, the sarcoplasm will diminish. So, olympic weightlifting athletes have tons and tons of muscle fibers, but since they aren't performing strength feats in an anaerobic state, they don't get the "swollen" appearance that comes with significant sarcoplasmic stores.
    – Daniel
    Feb 10, 2013 at 22:23

Any kind of training can produce increases in muscle mass. Aerobic training simply stops doing so very quickly, since it doesn't require much strength to perform. Aerobic training requires a small degree of strength repeated over a relatively long period of time. The body is more stressed by the requirements of repeating the exercise over a period of time than by the strength required for executing it, therefore recovery is focused on improving the body's ability to perform over a period of time rather than muscle gain.

One gets good at things by demanding the body do them. Gaining strength and muscle mass means demanding the body perform at or near maximal strength and muscular exertion. Gaining greater cardiovascular ability means demanding the body perform low-level exertions repeatedly. The Specific Adaptation to Imposed Demand (SAID) principle governs the body's development in the face of these stimuli.


The situation is a bit tricky. Every exercise leads to muscle increase, but only in that amount that is needed to handle the situation. Sprinters need a lot of strength in legs, so their leg muscles are growing bigger. So with mountain runners. Marathon runners don't need a lot of strength, they need endurance.

In theory, you can have both, marathoner's endurance and culturists's body, if you combine endurance training and strength training.

In practice, you will meet the difficulties:

1) Endurance training requires a lot of nutrition. You burn extreme amounts of calories and need much more proteins. In some situations it can lead to muscle burning. This is seen, for example, in long mountain trekkings with tents, where you have the food amount limited by how much you can carry. People are loosing muscles, mostly in upper body parts.

2) The bigger the muscles, the more energy you burn. Having big muscles, you can run the marathon, but the best efforts have those who looks as those runner from people, because they burn less energy. This is the reason why you need to train muscles in order to force them to grow. This is energy and protein saving. Your body will not let you have bigger muscles as you really need. The economy was always the key to survival.

3) Marathons are extremally exhausting. If you train regularly, you can have problem to do strength training, because you will be too exhausted. Your body won't have much energy for more.

  • So this is maybe a dumb follow up question. But how does endurance training burn calories if your muscles aren't that large. Is it that the energy is going to bringing energy and to those muscles over a longer period of time? And supplying your body with oxygen?
    – ehmhunt
    Feb 10, 2013 at 8:33
  • 1
    Smaller bodies are lighter, and the muscles themselves also burn more energy if they are bigger, exactly like car engines. I was one on trekking camp where all were measured for calory burn and the relation between muscle mass and calory burn were very clear. Feb 10, 2013 at 8:52
  • 4
    @user522 - Your muscles use Adenosine Tri Phosphate for energy. Glycogen and glucose are burned to turn adenosine DI phosphate (used up ATP) back into ATP so that it can be used again. This comes from muscle stores, liver stores, and fat stores in varying proportions according to exercise intensity. So in a sense yes, some energy is spent bringing it, but mostly it's a function of how long and at what intensity your muscles work.
    – JohnP
    Feb 10, 2013 at 14:29

Part of the difference that you are noting is genetic; some people are naturally better at aerobic sports, and some are better at putting muscle on. In running for example, you don't see sprinters become marathoners, nor do you see marathoners become sprinters, no matter how hard they train. They don't have the genetics for it.

For a given person, however, the answer to your question is that your body will optimize to keep muscles that are being used and eliminate muscle that is not being used. You see that in strength training regardless of whether you do aerobic training or not.

Long-period aerobic exercise - such as long-distance running or cycling - is uniquely good at tearing down unused muscle. This is mostly because of the nutritional requirements of those sports; you are in the aerobic zone for a long time and burning lots of calories, and some of those will come from protein. Also, after the exercise is over, your body will want to rebuild carbohydrate stores and it may tear down muscle mass to get there. Good nutrition - especially recovery nutrition - will help with this, but as a long-cyclist who also lifts weights, it's hard to do both. You can, however, hit a balance, and if you don't do a ton of cardio, the effect isn't that big.

I will note, however, that pure steady-state cardio isn't the most effective use of training time for most people; HIT training will give them both a larger cardio training effect and improve their anaerobic fitness as well.

I'll also note that in long-distance running and cycling, lightness is an advantage; in running it's more efficient to have lighter legs, and in both you have less weight to carry up hills.

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