Extensor substitution in runners what is it?

I was just watching an injured runner walk. We were trying to pin down her diagnosis and figure out why she was getting injured. She asked me what I actually saw when analyzing her gait. One of the things I saw was that she had this thing called extensor substitution.  She wanted to know how it contributed to the issues she has been having when she was running. Today on the Doc On The Run podcast, we're talking about extensor substitution in runners.