That's the long and short of it! Brittle bones, so he could get injured easily... so he doesn't exercise. A terrible spiral of laziness and weakness!

[This is about the best topic for distracting Papyrus from the ominous whispers. He rants with the free enthusiasm of someone who has hundreds words more on the topic, and only rarely a willing audience for it.]

If he just ate better, he could exercise more... but he enjoys terrible food too much. I despair of him, sometimes.