well if u still want to use expression then u can ......program ur geometry e.g

like:

if (your foot touches ground )
the ground should effect;

else if (if its leaving the surface)
the ground should effect;



i think u'll not be able to ...produce realistic effects with only expressions
like Rage said blendshape could help....and i think that there will be an attribute that can be keyable and can be controlled through expressions...so...give a try...u know...expression makes your scene clean from raw data that take up extra..resourse cause they excute when conditions are met......

well will be putting....expression for same type of effect soon...which produces swells in softbodies....it will be helpfull to produce perfect result...for ur scene


enjoy ....