This is an excerpt from Dynamic Alignment Through Imagery - 2nd Edition by Eric Franklin.
Transverse Tarsal Joint
The transverse tarsal joint (the midtarsal joint) is composed of the talonavicular and the calcaneocuboid articulations (figure 11.38). Together with the subtalar joint, the
transverse tarsal joint creates most of the supination and pronation action of the foot. The two joints can work together to create a large degree of supination and pronation or in opposition to each other, allowing the forefoot and hindfoot to twist in opposite directions.
Working together to create full supination, the subtalar and transverse tarsal joints cause the foot to become more rigid, which is called the closed-pack position and increases the stability of the foot. The foot is now in lever mode and is able to push against the ground with great force. If the foot is fully pronated, the bones unwind and the foot becomes more flexible. This can be described as the foundation mode where the foot is wider on the ground than in the air. The foot should not spend all its time fixated in either one of these positions but be able to change between the two modes depending on the necessities of movement and the terrain.
Without the transverse tarsal joint, it would be difficult to walk across uneven ground because this joint mediates between the forefoot and the hindfoot (figure 11.39). If the forefoot is forced to supinate, the hindfoot need not follow suit because the transverse tarsal joint can counter this action, allowing the heel to remain vertical. The reverse holds true as well: If the heel slips and rolls into a pronated position, for example, the forefoot can remain flat on the ground.
If the forefoot is made to pronate, the hindfoot counters by supinating. If the heel slips and rolls into a supinated position, the forefoot compensates by pronating (see figure 11.32c). This is essential if you like to walk in moccasins or barefoot, but otherwise the rigid shoes worn today all but eliminate the action of the transverse tarsal joint. This is not necessarily advantageous, because it places additional strain on the ankle and knee joints. Also, certain muscles and joints of the foot lose strength because they are not being exercised.
The transverse tarsal joint, also called Chopart's joint, has a wavelike contour. The lateral part of the wave is formed by the articulation between the calcaneus and the cuboid. The cuboid is somewhat triangular in shape and its joint surface facing the calcaneus is shaped like a saddle. The ventral calcaneus is somewhat funnel shaped and faces the cuboid with its matching convex saddle-shaped joint surface. The joints interlock rather firmly, helping to stabilize the heel side of the foot. The medial part of the transverse tarsal joint is between the tarsal bone and the navicular. The navicular offers an oval concave joint surface that is smaller than the opposing facet from the talus, which is convex. Here ample movement is possible. You can imagine an Earth-to-Moon relationship as the flexible navicular swings around its more stable neighbor in supination and pronation.
Two joint axes can be visualized in the transverse tarsal joint, creating supination and pronation with a spiraling feeling in the midfoot and forefoot. The longitudinal axis runs in a slightly oblique fashion through the calcaneus and the nose of the cuboid. Around this axis the forefoot spirals in an inversion-and-eversion fashion. The oblique axis runs from craniomedial to caudolateral and is angled 50 degrees to horizontal. The spiral created by this axis contains adduction and plantar flexion in supination and abduction and dorsiflexion in pronation.