Frame 1 - foot supinatus at approximately week 8 post-ovulation. The entire plantar surface of the foot is in supinatus relative to the posterior compartment of the leg. This is a structural relationship, not a positional change.
Frame 2 – skeletal supinatus at approximately week 8 post-ovulation (same picture as Frame 1 but with the skin removed). If the foot stops unwinding at this point, the entire foot remains in supinatus. Podiatrists have identified this foot type as a combination rearfoot/forefoot varum. Rothbart refers to this foot structure as the preclinical clubfoot deformity (both the calcaneus and talus remain in supinatus).
Frame 3 – early in the 8th week post ovulation, the calcaneus (and with it, the metatarsals 2-5, adjoining phalanges, middle/lateral cuneiforms and cuboid) torsionally unwind from a supinatus into a plantargrade position. At this point only the medial column of the foot remains in supinatus (e.g., talar supinatus). If the foot stops its torsional development at this point, it is clinically termed Primus Metatarsus Supinatus. Prior to 2002, this foot structure was not recognized.
Frame 4 – later in the 8th week post ovulation, the talus (and with it, the navicular, medial cuneiform, first metatarsal and adjoining phalanges) torsionally unwinds from a supinatus into its adult position. The entire plantar surface of the foot is now plantargrade (perpendicular to the leg).
The animated model above demonstrates the structural unwinding of the foot during the first 8 weeks of development (Fetal Development):
Fetal Ontogenetic Development of the Foot