How can a 2nd metatarsal stress fracture cause a 5th metatarsal stress reaction?

I was just on a call with a cross-country runner  who had had a second metatarsal stress reaction. 

A stress reaction is basically like a mild stress fracture, but without any crack in the bone. 

She was doing well and her foot had been getting better. 

But then when she went for her first run, she had pain in her foot. 

The pain during that first run was in a completely different bone. The new problem was not in the second metatarsal, it was in the fifth metatarsal on the outside of the foot. 

Let's talk about how that happens.

Today on the Doc on the Run podcast we're talking about how a second metatarsal stress fracture might cause a fifth metatarsal stress reaction.