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QUESTION:

Why Do My Muscles Get Smaller from Running? I’ve been putting in miles and getting stronger, but my muscles seem to be shrinking—I’m getting skinnier, not fitter-looking.

ANSWER:

Your running muscles get smaller with high-volume endurance training for one simple reason: it’s more efficient to run with smaller muscles.

Most people equate “strength” with bigger muscles. When you think of someone strong, your mind goes to Dwayne “The Rock” Johnson, not Justin Bieber. So if elite distance runners spend so much time strengthening their muscles fibers, why are they all so skinny? Shouldn’t all those miles, hills, sprints drills and exercises be building Muscle Beach bodies?

In a word: No. The world’s top distance runners are defined by almost nonexistent upper bodies, slim thighs, and calves that are smaller than the norm.

When you run—and after you’ve fortified your muscles fibers by weeding out weak myofilaments—your muscle fiber DNA has to make a choice:

a) Use the fiber’s limited adaptive energy to create bigger muscles

b) Harness the fiber’s adaptive energy to forge more powerful aerobic power plants (mitochondria) within the fibers

If you want to be a top competitive distance runner, you can’t have it both ways: the sheer volume of your training triggers a physiological response that shirks the big biceps and embraces increased aerobic power. On the other hand, if you want to be a fit, fast, but far-from emaciated good runner, there are ways around this physiological roadblock.

big muscles
photo: Getty Images

For competitive distance runners, the choice is easy: Build those aerobic power plants! Greater volume (mileage) accomplishes that goal, and it also increases the number of capillaries (your smallest blood vessels) around your muscle fibers. More capillaries mean more oxygen and nutrient delivery for your improved power plants. And the combination of more power plants and more fuel means you’ll be able to produce a lot more energy, which is the key to fatigue resistance.

As your muscles budget less energy for maintaining mass, the fast-twitch fibers in your endurance-trained muscles begin to shrink; at the same time, your slow-twitch fibers do get bigger, but not enough to offset the loss of fast-twitch size. In this battle between fast-twitch atrophy (shrinkage) and slow-twitch hypertrophy (growth), atrophy wins, resulting in smaller, more physiologically-efficient muscles.

Then again, you may not want to sacrifice a stronger build for a slightly faster time in your next 5K or marathon. Never fear. High-intensity training (e.g., weight training or hill sprints) spurs growth in muscle size. As long as the high-intensity training is separated from the endurance stimulus (e.g., instead of doing a workout after a distance run, you do it a few hours later) and as long as your endurance training volume isn’t too high, you’ll be able to live dual roles as a road-running warrior and fitness club standout.

Your running muscles get smaller with high-volume endurance training for one simple reason: It’s more efficient to run with smaller muscles. Your body is no dummy. It goes with what works.

WHO ASKED YOU, ANYWAY?

Pete’s freebee training tip: None of this means that high-volume competitive runners should skip the weights. Resistance training isn’t just about sporting a ripped beach bod. Most strength gains in the first few weeks (sometimes months) of resistance training comes from nervous system adaptations, not muscular growth. So competitive endurance athletes will get stronger regard regardless of whether they get bigger.

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