My chiro would agree with Oak She told me, and I've read, that as we ride more and build more leg muscles, we need to remember to stretch them out, especially the hip flexors. I'm horrible about that, and have really been feeling it lately myself with all the extra riding I've been doing.
If you think about the way our body is set up, it makes sense - we get used to being in that cycling position, kind of bent over, so it gets tight when we're upright. I feel it mostly in the tops of my quads right at my hips. She tells me to "open them up" with these hip flexor stretches. I think I need a sticky note on my computer reminding me to do it!