The ADM design emanates from functional foot anatomy and kinematics.  The mean dominant axis of the sub-talar joint is generally accepted as being 42 degrees from the transverse plane and 16 degrees from the sagittal plane.  The ADM has a jointed mechanism aligned to the dominant sub-talar joint axis. The range of motion is limited to 5 degrees of varus to 30 degrees of valgus.

A second ADM joint is aligned to the mean dominant axis of the tibio-talar joint. This axis is generally accepted as running 20 to 30 degrees from the frontal plane and 8 degrees to the transverse plane when the sub-talar joint is in a neutral position.

When attached to either Daytime or Nightime footwear the ADM simultaneously abducts, everts and dorsiflexes the foot. The ADM support a range of motions from 45 degrees of plantarflexion to 30 degrees of dorsiflexion and 5 degrees inversion to 30 degrees eversion.

Each joint mechanism contains a torsion spring.  The springs apply precisely defined torsion forces to rotate the foot about the sub-talar and tibio-talar joints.  Various spring options deliver torsion forces ranging from 70 to 1,330 Nmm.

The effect of the torsion applied by the ADM to the Sub-Talar Joint (STJ) and the Tibio Talar Joint (TTJ) is to apply abducting and dorsiflexing forces to the foot.

The STJ spring will apply abducting forces (F_stj blue arrow) and the TTJ spring will apply dorsiflexing forces (F_ttj green arrow). The net force for a given foot position will be the resultant of the two forces (F_res grey arrow).

To appreciate the magnitude of the net resultant force on the foot it is easiest to consider a point force or weight applied to the MTP1 (big toe area) as you might do when examining a patient to test their range of motion and stiffness.

For a given spring strength the force will reduce as the foot length increases. For this reason we require stronger springs for bigger longer feet.