I've recently started training for a marathon and, for the most part, I'm enjoying the results both in terms of the way I feel and in the shape of my body. But here's one thing I don't understand: Why is my butt deflating? I'm a pear shape, and I've always had big quads and a full, womanly behind. But since I've taken up running, I've noticed that rather than developing a firmer fanny, I feel like I'm carting around an empty, saggy butt. I'm partly concerned about the aesthetics, but I'm also thinking about the inefficiency of my running. A physical therapist told me once that the reason my hamstrings kill so much after a long run is because I don't use my glutes at all. What? How is that even possible? I suspect if they were working for me, they'd make me faster and firmer, reinforcing the reasons I started running in the first place—to feel and look better. So what can I do to engage those butt muscles when I run? - Katie, Bellingham, WA

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