While designing shoe lifts, there are several factors that are considered by the designers. Any well-designed lift will have a proper length so that it can extend well forward below the arch to provide enough support to the ball of your foot. This will reduce the bridging of the ball of the foot and the heel. A short lift will cause discomfort in walking as well because you will feel the heel under your foot.
Longer the shoe lift it is better as it will also prevent your foot from slipping forward in the shoe while you walk and cause awkward walking. Therefore, the length of the lift should be considered during designing.
Slope of the Heel
The slope of the heel is another aspect that should be considered during designing the shoe lifts. Too steep a slope will make the foot slip forward and exert excess pressure on your toes as well. Any well-designed shoe lift will have a long and constant slope from back to front to support your feet well. Ideally, it should not be tapered at the front and level at the back. The change in taper will result in a hump in the lift which will cause discomfort as well as damage your foot.
Add Elevation Only
Ideal shoe lift will provide elevation only and not compressibility. This will help in preventing the vertical heel motion along with rubbing it with the shoe. The side effects of such up and down rubbing may result in calluses and blisters and even Achilles tendon irritation.
The primary objective of the shoe lift is to provide support, eliminate loss of control and elevate to perform multiple functions like valgus wedging along with elevation for leg length discrepancy or LLD. Any compressible material used in manufacturing the shoe lift should be placed under the insole to provide maximum comfort.