This is a clinical test in which the examiner is attempting to assess the strength of one of the arches of the foot.  The patient sits with his feet flat on the floor and the examiner marks the height of the navicular bone.  The patient is asked to stand, and the examiner again measures the height of the navicular.  If the navicular bone “drops” by more than 10mm when the patient stands, it suggests a laxity or weakness in the medial longitudinal arch.