The short foot exercise was first described by Janda as a means to strengthen the intrinsic muscles of the foot, specifically the abductor hallucis muscle. It is done to raise the arch by sliding the first metatarsal head/hallux posteriorly. The position is held for 5-8 seconds and repeated with more repetitions being added over time. Usually some guidance is needed by the clinician to demonstrate. Watch this video:
There has been some research done on the test, but none of it is very good:
In addition to the above, there is the body of evidence showing no correlation between muscle strength and arch height.
What causes lowering of longitudinal arch? There are many many different causes of flat feet, 'overpronation' or lowering of the medial longitudinal arch. The short foot exercise is going to work really well if a weakness of the abductor hallucis muscle is the cause of it (its not a very common cause).
For example (1): there is no idea how to expect any sort of muscle strengthening to work in those who have 'flat feet' . Its impossible to strengthen the muscles to fix a 'flat foot' due to an forefoot varus (well, if it was possible, they would be walking about with the big toe off the ground, and surely that is not a good thing). The short foot exercise perhaps helping restore the arch in those with a forefoot supinatus, but something is causing that supinatus and it is unlikely to be a weakness of the abductor hallucis. And even then it is probably the action of the exercise in plantar flexing the first ray that helps and not necessarily the strengthening of the muscles.
For example (2): Probably one of the more common causes of 'overpronation', 'flat feet' or a lowering of the medial longitudinal arch is tight calf muscles. As the tibia move over the ankle its required range of motion and if it does not have adequate range at the ankle, the tibia continues to more forward by collapsing the midfoot and no muscle strengthening exercise is going to stop this?
There are literally, 100's of causes of 'flat feet'. The short foot exercise is only going to help those in which a weakness of the muscles are a cause.
In the above table on research done with the short foot exercise, some of the subjects would have had, for eg, forefoot varus or tight calf muscles, so including them would have theoretically lowered the mean response as they would have been 'non-responders'.
Use in plantar fasciitis
We do know that there is an atrophy of the intrinsic muscles in those with plantar fasciitis, so the short foot exercise is something I do use in those with plantar fasciitis as the muscles are weak and its part of the rehab for it. However, its the chicken or egg. Is the weakness a cause of the plantar fasciitis or is the weakness a consequence of altered gait from the pain. Regardless of which one it is, it is still part of the rehab as the muscles are weaker (as long as the issues of load reduction and capacity of the fascia to take load are addressed)
Of course, a stronger muscle is better than a weaker muscle. Foot short exercise is just needs to be used for what it is for to strengthening the abductor hallucis muscle not to fix flat feet.