Cusick's Corner

Author: Beverly (Billi) Cusick, PT, MS, NDT, COF/BOC

SUBTALAR JOINT ANATOMY & FUNCTION
The calcaneus (heel bone) is the loading base of the hindfoot. The medial process of the posterior tuberosity contacts the ground.

The talus is the upper bone of the hindfoot. The talar body is boxed between the tibial and fibular malleoli like a mortise forming a mechanical connection between the talus and the leg bones in the frontal (mediolateral) and transverse (rotational) planes.

The calcaneus bears a projection on the upper inner surface known as the sustentaculum tali (ST) which means “sustains or supports the talus”. Using this projection, the calcaneus operates like a doorknob that can “open and close” the hindfoot. With the posterior calcaneus aligned in or near vertical position, the ST supports the medial talus. Calcaneal eversion – rotation of the tuberosity laterally – causes the ST to drop out from under the medial talus. Under bodyweight (BW), the unsupported talus falls forward and medially into the gap, and the “open” foot collapses into pronation, gaining momentary flexibility needed to soften the force of heel strike in gait. Inverting the calcaneus restores foot stability.

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Author: Beverly (Billi) Cusick, PT, MS, NDT, COF/BOC

FOOT FEATURES AND DEVELOPMENTAL FACTORS
Whereby much information is published pertaining to typical changes in the developing foot anatomy and load distribution, the biomechanical and kinesiologic elements and methods that foster those changes are largely unknown. All aspects of this discussion regarding those methods represent my science-based – and working - hypotheses.

In my nearly 50 years as a pediatric PT and student of developmental orthopedics, I have learned the significance of three major factors that appear to me to contribute to early foot development:

1. The presence of the apparently innate drive to achieve and maintain the upright position while awake
2. The organized achievement of neck and trunk muscle strength and control against gravity, beginning with antigravity extension, and
3. The demands on the foot joints and muscles to support the verticality drive.

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