Basically the title. If I go to the gym three times a week, I can hit different muscles each time (meaning each muscle is really only being targeted once) and I'll see noticeable muscle growth over time.

If I run once a week, though, my heart won't improve in any meaningful way. I couldn't run once a week for 20 years and be ready to run a marathon, but I could hit each muscle once a week for 20 years and be a legitimate bodybuilding competitor.

What's the difference? Why is there no way to do one cardio exercise a week and see significant improvement?

  • Your question makes the assumption that running is dependent on development of the heart and only the heart.
    – G__
    Jul 31 '18 at 3:34

Not a cardiologist, but an avid reader of such topics and I like endurance sports...

You have skeletal muscle (biceps, triceps, etc...), smooth muscle (like those in your digestive tract), and cardiac muscle. The cardiac muscle fibers, while similar to skeletal muscle, have a lot more mitochondria in them, allowing them to provide for a lot more ATP. This makes the heart muscle much more efficient and is also one of the reasons humans can do things like Ironman races where they stress the muscle for hours at a time. Try doing a set of weightlifting for 8 straight hours!

So, ultimately, the answer really lies in the heart's ability to be far more efficient than other muscle tissue. It makes it harder to stress the heart to initiate a "muscle response."

Finally, I'd like to note that while not optimal, running once a week is still better than not running at all. That said, you should strive to meet the minimum guidelines for cardio exercise in a week.

  • Just to add to this - You lose fitness within 2 days of previously doing cardio whereas muscle loss usually takes over a week at least so if you're training 3 times a week and hitting a muscle group each week you won't be losing muscle before you train again assuming your diet is correct.
    – Twyxz
    Jul 27 '18 at 11:45

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