The navicular drop is a clinical assessment measurement or observation of how much the navicular moves in the sagittal plane between either:
1. Subtalar joint neutral and subtalar joint relaxed
2. The foot on the ground rested seated and then full weight bearing standing.
It is basically a measure of how much the arch collapses between the two different positions.
The height of the navicular is either measured (by marking it on a piece of card) or observed between the two positions. The change in height is known as the navicular drop.
Please note that when reading research on the navicular drop, that you note which of the two different positions or methods that are being used for it, ie did they measure it between non-weightbearing and weightbearing; or did the measure it between subtalar joint neutral and subtalar joint relaxed.