In the fic It's A Dangerous Business, Going Out Your Door the author headcanoned that Earth ponies have a special connection with the ground that allows them to essentially become a stone wall- immovable.

My guess on how this works is that Earth Ponies are able to collect magical energy under their hooves and convert it into a near infinitely strong adhesive, disallowing any shift of footing. Then, when the pony does not need it, the adhesive dissipates into some other form of energy (likely heat).

This is easily one of the coolest powers a pony could have and I'm fully behind it.