Subtalar joint: where the talus rests on and articulates with the calcaneus. This is a synovial joint with a weak capsule supported by medial, lateral, posterior & interosseous talocalcaneal ligaments.
The interosseous talocalcaneal ligament (very strong) lies in the tarsal sinus (separates the anterior & posterior talocalcaneal joints).
Anatomical subtalar joint- functionally a single synovial joint between the slightly concave articular surface of the talus and the convex posterior articular surface of the calcaneus.
2 . Frontal (coronal) plane movement occurs at the subtalar joint
Single Oblique axis
Inversion average range 20°
Eversion average range 10°
(Note 0° occurs where the sole of the foot at perpendicular to the lower leg in the frontal plane)
3. Pronation and supination are complex triplanar movements. Pronation incorporates movement of eversion, dorsiflexion and abduction. Supination incorporates movement of inversion, plantarflexion and adduction. (Lowe p50) 5. The minimal range of hallux extension required at the 1st metatarsophalangeal joint is 65°.
In quiet stance body weight is transmitted through the talus bone and passed in equal measure backwards (into the calcaneum) and forwards. Weight distribution
Biomechanics of arch support Newton's 3rd law: Every action has an equal and opposite reaction. If we think of the simple arch structure of a pitched roof, the weight of the roof is supported at the two ends. The action of the load has an equal and opposite ground reaction force. Place two pencil upright (perpendicular) on the table on front of you, and press down gently onto the top . The force and the resultant force are acting in the same line and the pen should be stable. A steady arch can only be achieved by stabilising the pens with your free hand.
In a stable arch the twisting moments are resisted by other structures, e.g a tie-beam in a pitched roof; pictured left.
All pitched roofs are stabilised and supported, otherwise they would collapse.
Biomechanics of arch support
Maintenance of the longitudinal arches The longitudinal arches are supported and stabilised by:
The muscles whose tendons run into the apex of the arches and tend to increase their height (e.g. tibialis anterior )
The muscles whose tendons run into the sole of the foot where they have a longitudinal course. Like ligaments they will prevent the extremities separating (e.g. peroneus longus and small intrinsic muscles which also run longitudinally)
The shape of the bones which allows them to interlock
A variety of longitudinally arranged ligaments which prevent the extremities separating, for example the long and short plantar ligaments and by the plantar calcaneonavicular ("spring") ligament.
The plantar aponeurosis links the extremities of the arches, and acts as the equivalent of a tie beam in an architectural arch Maintenance of the longitudinal arches
WINDLASS MECHANISM At toe off the MTP joints extend and pull the plantar fascia taut. This tension in the plantar fascia assists in stabilization of the longitudinal arch at toe off and provide a more rigid foot. This mechanism is called the “windlass mechanism.