Running helps me to learn my own anatomy one sore muscle at a time. When a particular muscle gets sore, it's exact shape and location become readily apparent. It's as if the entire body is transparent except that one muscle, but instead of seeing it, I can feel it. It feels like a big bruised piece of meat inside my leg. Today that muscle is the fibularis longus. The fibularis longus plantar-flexes the foot. Plantar flexion is movement of the foot that flexes the foot or toes downward toward the sole. This is a muscle that I never knew existed until the non-stop steep ups and downs of the seven sisters trail race the other day. There it is, running down the front side of the leg.
It's nice to know that the soreness matches the obvious cause perfectly. I would imagine that plantar flexion occurs while pushing off when going uphill, and also to catch your fall with each downhill step (assuming a minimalist forefoot landing). The fibularis longus is also the most conveniently located muscle for massaging with a rolling massager. But ouch!
I hope you enjoyed your anatomy lesson for the day!