Hammer toes can result from intrinsic foot muscle imbalance, specifically weakness in the foot's short flexors and overactivity in the long flexors.

“so on the top of the foot the short toe extensors are doing a lot of work and the long toe extensors are not yeah this is one of those things where if you're listening to us this is very difficult to understand it's it's why watching what you're saying makes a lot of sense and and and again just so folks understand the extensors would be pulling back the flexors would curl forward and so it seems counterintuitive to say how can a hammer toe be in part driven by this extensor phenomenon well if the short extensors the ones that attach with a shorter moment arm are fired up and the long ones are relaxed it actually looks like a hyperflexion”