Country to popular belief the evidence doesn't support the claim that wearing orthotics weakens the foot muscles, this is a concern I often hear from my fitness conscious patients.
What does the evidence say?
Mayer et al (2007) showed an increase in calf muscle strength in the orthotic wearing group.
Jung et al (2011) showed an increase in intrinsic muscle strength in the foot orthotic wearing group
Payne et al (2005) showed no weakness after 4 weeks of foot orthotic use (there was actually a statistically non-significant increase in strength)
It is not fully understood why this is the case but I would hypothesise that as foot orthotics can limit excessive motion and thus are aiding in optimising the foot posture to achieve a mechanical advantage thus better-utilising muscle.
Are flat feet due to weak muscles?
A further misconception is that flat feet are as a result of the weakness of the foot and ankle muscles. There is no evidence that this is the case! A person foot posture is more related genetics which influences the formation of bones and joints in the rearfoot. The way these multiple bones, ligaments and tendons in the rearfoot are put together has an effect on mechanical advantage and disadvantage of tendons and muscle that acts to move the foot & ankle. The position that the centre of force sits in relationship to these joints complexes and a persons ligament flexibility all plays a role in determing foot posture
Doing foot muscle exercises maybe help with foot function but it will not change foot posture